ALBERT

Description: The Level 2 research product algorithms for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) Atmospheric Measurement Techniques Discussions, 4, 3593-3645, 2011 Author(s): P. Baron, J. Urban, H. Sagawa, J. Möller, D. P. Murtagh, J. Mendrok, E. Dupuy, T. O. Sato, S. Ochiai, K. Suzuki, T. Manabe, T. Nishibori, K. Kikuchi, R. Sato, M. Takayanagi, Y. Murayama, M. Shiotani, and Y. Kasai This paper describes the algorithms of the level-2 research (L2r) processing chain developed for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES). The chain has been developed in parallel to the operational chain for conducting researches on calibration and retrieval algorithms. L2r chain products are available to the scientific community. The objective of version 2 is the retrieval of the vertical distribution of trace gases in the altitude range of 18–90 km. An theoretical error analysis is conducted to estimate the retrieval feasibility of key parameters of the processing: line-of-sight elevation tangent altitudes (or angles), temperature and O 3 profiles. The line-of-sight tangent altitudes are retrieved between 20 and 50 km from the strong ozone (O 3 ) line at 625.371 GHz, with low correlation with the O 3 volume-mixing ratio and temperature retrieved profiles. Neglecting the non-linearity of the radiometric gain in the calibration procedure is the main systematic error. It is large for the retrieved temperature (between 5–10 K). Therefore, atmospheric pressure can not be derived from the retrieved temperature, and, then, in the altitude range where the line-of-sight tangent altitudes are retrieved, the retrieved trace gases profiles are found to be better represented on pressure levels than on altitude levels. The error analysis for the retrieved HOCl profile demonstrates that best results for inverting weak lines can be obtained by using narrow spectral windows. Future versions of the L2r algorithms will improve the temperature/pressure retrievals and also provide information in the upper tropospheric/lower stratospheric region (e.g., water vapor, ice content, O 3 ) and on stratospheric and mesospheric line-of-sight winds.

Description: Near infrared nadir sounding of vertical column densities: methodology and application to SCIAMACHY Atmospheric Measurement Techniques Discussions, 4, 3685-3737, 2011 Author(s): S. Gimeno García, F. Schreier, G. Lichtenberg, and S. Slijkhuis Nadir observations with the shortwave infrared channels of SCIAMACHY onboard the ENVISAT satellite can be used to derive information on atmospheric gases such as CO, CH 4 , N 2 O, CO 2 , and H 2 O. For the operational level 1b–2 processing of SCIAMACHY data a new retrieval code BIRRA (Beer InfraRed Retrieval Algorithm) has been developed: BIRRA performs a nonlinear least squares fit of the measured radiance, where molecular concentration vertical profiles are scaled to fit the observed data. Here we present the forward modeling (radiative transfer) and inversion (least squares optimization) fundamentals of the code along with the further processing steps required to generate higher level products such as global distributions and time series. Moreover, various aspects of level 1 (observed spectra) and auxiliary input data relevant for successful retrievals are discussed. BIRRA is currently used for operational analysis of carbon monoxide vertical column densities from SCIAMACHY channel 8 observations, and is being prepared for methane retrievals using channel 6 spectra. A set of representative CO retrievals and first CH 4 results are presented to demonstrate BIRRA's capabilities.

Description: Volatilizable biogenic organic compounds (VBOCs) with two dimensional gas chromatography-time of flight mass spectrometry (GC × GC-TOFMS): sampling methods, VBOC complexity, and chromatographic retention data Atmospheric Measurement Techniques Discussions, 4, 3647-3684, 2011 Author(s): J. F. Pankow, W. Luo, A. N. Melnychenko, K. C. Barsanti, L. M. Isabelle, C. Chen, A. B. Guenther, and T. N. Rosenstiel Two dimensional gas chromatography (GC × GC) with detection by time-of-flight mass spectrometry (TOFMS) was applied in the rapid analysis of air samples containing highly complex mixtures of volatilizable biogenic organic compounds (VBOCs). VBOC analytical methodologies are briefly reviewed, and optimal conditions are discussed for sampling with both adsorption/thermal desorption (ATD) cartridges and solid-phase microextraction (SPME) fibers. Air samples containing VBOC emissions from leaves of two tree species ( Cedrus atlantica and Calycolpus moritzianus ) were obtained by both ATD and SPME. The optimized gas chromatographic conditions utilized a 45 m, 0.25 mm I.D. low-polarity primary column (DB-VRX, 1.4 μm film) and a 1.5 m, 0.25 mm I.D. polar secondary column (Stabilwax® 0.25 μm film). Excellent separation was achieved in a 36 min temperature programmed GC × GC chromatogram. Thousands of VBOC peaks were present in the sample chromatograms; hundreds of tentative identifications by NIST mass spectral matching are provided. Very few of the tentatively identified compounds are currently available as authentic standards. Method detection limit values for a 5 l ATD sample were 3.5 pptv (10 ng m −3 ) for isoprene, methyl vinyl ketone, and methacrolein, and ~1.5 pptv (~10 ng m −3 ) for monoterpenes and sesquiterpenes. Kovats-type chromatographic retention index values on the primary column and relative retention time values on the secondary column are provided for 21 standard compounds and for 417 tentatively identified VBOCs. 19 of the 21 authentic standard compounds were found in one of the Cedrus atlantica SPME samples. In addition, easily quantifiable levels of at least 13 sesquiterpenes were found in an ATD sample obtained from a branch enclosure of Calycolpus moritzianus . Overall, the results obtained via GC × GC-TOFMS highlight an extreme, and largely uncharacterized diversity of VBOCs, consistent with the hypothesis that sesquiterpenes and other compounds beyond the current list of typically determined VBOC analytes may well be important contributors to global atmospheric levels of organic particulate matter.

Description: Sensing Hadley cell with space lidar Atmospheric Chemistry and Physics Discussions, 11, 16599-16610, 2011 Author(s): W. Sun and B. Lin This letter shows that the extent of the Hadley cell could reliably be estimated by measuring the height of the uppermost super-thin clouds in the troposphere with space-borne lidar. Through consecutive multi-year measurements of the height of the uppermost super-thin clouds, a good estimation of the expansion of the Hadley cell could be obtained.

Description: Domestic wells have high probability of pumping septic tank leachate Hydrology and Earth System Sciences Discussions, 8, 5701-5732, 2011 Author(s): J. E. Horn and T. Harter Onsite wastewater treatment systems such as septic systems are common in rural and semi-rural areas around the world; in the US, about 25–30 % of households are served by a septic system and a private drinking water well. Site-specific conditions and local groundwater flow are often ignored when installing septic systems and wells. Particularly in areas with small lots, thus a high septic system density, these typically shallow wells are prone to contamination by septic system leachate. Typically, mass balance approaches are used to determine a maximum septic system density that would prevent contamination of the aquifer. In this study, we estimate the probability of a well pumping partially septic system leachate. A detailed groundwater and transport model is used to calculate the capture zone of a typical drinking water well. A spatial probability analysis is performed to assess the probability that a capture zone overlaps with a septic system drainfield depending on aquifer properties, lot and drainfield size. We show that a high septic system density poses a high probability of pumping septic system leachate. The hydraulic conductivity of the aquifer has a strong influence on the intersection probability. We conclude that mass balances calculations applied on a regional scale underestimate the contamination risk of individual drinking water wells by septic systems. This is particularly relevant for contaminants released at high concentrations, for substances which experience limited attenuation, and those being harmful even in low concentrations.

Description: CARIBIC aircraft measurements of Eyjafjallajökull volcanic plumes in April/May 2010 Atmospheric Chemistry and Physics Discussions, 11, 16693-16744, 2011 Author(s): A. Rauthe-Schöch, A. Weigelt, M. Hermann, B. G. Martinsson, A. K. Baker, K.-P. Heue, C. A. M. Brenninkmeijer, A. Zahn, D. Scharffe, S. Eckhardt, A. Stohl, and P. F. J. van Velthoven The Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) project investigates physical and chemical processes in the Earth's atmosphere using a Lufthansa Airbus long-distance passenger aircraft. After the beginning of the explosive eruption of the Eyjafjallajökull volcano on Iceland on 14 April 2010, the first CARIBIC volcano-specific measurement flight was carried out over the Baltic Sea and Southern Sweden on 20 April. Two more flights followed: one over Ireland and the Irish Sea on 16 May and the other over the Norwegian Sea on 19 May 2010. During these three special mission flights the CARIBIC container proved its merits as a versatile and comprehensive flying laboratory. The elemental composition of particles collected over the Baltic Sea during the first flight (20 April) indicated the presence of volcanic ash. Over Northern Ireland and the Irish Sea (16 May), the DOAS system detected SO 2 and BrO co-located with volcanic ash particles that increased the aerosol optical depth. Over the Norwegian Sea (19 May), the optical particle counter detected a strong increase of particles larger than 400 nm diameter in a region where ash clouds were predicted by aerosol dispersion models. Aerosol particle samples collected over the Irish Sea and the Norwegian Sea showed large relative enhancements of the elements silicon, iron, titanium and calcium. Non-methane hydrocarbon concentrations in whole air samples collected on 16 May and 19 May 2010 showed a pattern of removal of several hydrocarbons that is typical for chlorine chemistry in the plumes. Comparisons of measured ash concentrations and simulations with the FLEXPART dispersion model demonstrate the difficulty of detailed volcanic ash dispersion modelling due to the large variability of the volcanic plume sources, extent and patchiness as well as the thin ash layers formed in the volcanic plumes.

Description: Explicit modeling of organic chemistry and secondary organic aerosol partitioning for Mexico City and its outflow plume Atmospheric Chemistry and Physics Discussions, 11, 17013-17070, 2011 Author(s): J. Lee-Taylor, S. Madronich, B. Aumont, M. Camredon, A. Hodzic, G. S. Tyndall, E. Apel, and R. A. Zaveri The evolution of organic aerosols (OA) in Mexico City and its outflow is investigated with the nearly explicit gas phase photochemistry model GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere), wherein precursor hydrocarbons are oxidized to numerous intermediate species for which vapor pressures are computed and used to determine gas/particle partitioning in a chemical box model. Precursor emissions included observed C3–10 alkanes, alkenes, and light aromatics, as well as larger n -alkanes (up to C25) not directly observed but estimated by scaling to particulate emissions according to their volatility. Conditions were selected for comparison with observations made in March 2006 (MILAGRO). The model successfully reproduces the magnitude and diurnal shape for both primary (POA) and secondary (SOA) organic aerosols, with POA peaking in the early morning at 15–20 μg m −3 , and SOA peaking at 10–15 μg m −3 during mid-day. The majority (≥75 %) of the model SOA stems from the large n -alkanes, with the remainder mostly from the light aromatics. Simulated OA elemental composition reproduces observed H/C and O/C ratios reasonably well, although modeled ratios develop more slowly than observations suggest. SOA chemical composition is initially dominated by δ-hydroxy ketones and nitrates from the large alkanes, with contributions from peroxy acyl nitrates and, at later times when NO x is lower, organic hydroperoxides. The simulated plume-integrated OA mass continues to increase for several days downwind despite dilution-induced particle evaporation, since oxidation chemistry leading to SOA formation remains strong. In this model, the plume SOA burden several days downwind exceeds that leaving the city by a factor of 〉3. These results suggest significant regional radiative impacts of SOA.

Description: Nudging technique for scale bridging in air quality/climate atmospheric composition modelling Atmospheric Chemistry and Physics Discussions, 11, 17177-17199, 2011 Author(s): A. Maurizi, F. Russo, M. D'Isidoro, and F. Tampieri The interaction between air quality and climate involves dynamical scales that cover an immensely wide range. Bridging these scales in numerical simulations is fundamental in studies devoted to megacity/hot-spot impacts on climate. The nudging technique is proposed as a bridging method that can couple different models at different scales. Here, nudging is used to force low resolution chemical composition models using a high resolution run on critical areas. A one-year numerical experiment focused on the Po Valley hot spot is performed using the BOLCHEM model to asses the method. The results show that the model response is stable to perturbation induced by the nudging and that, if a high resolution run is taken as a reference, there is an increase in model skills of low resolution run when the technique is applied. This improvement depends on the species and the season. The effect spreads outside the forcing area and remains noticeable over an extension about 9 times larger.

Description: Use of ENVISAT ASAR Global Monitoring Mode to complement optical data in the mapping of rapid broad-scale flooding in Pakistan Hydrology and Earth System Sciences Discussions, 8, 5769-5809, 2011 Author(s): D. O'Grady, M. Leblanc, and D. Gillieson Envisat ASAR Global Monitoring Mode (GM) data are used to produce maps of the extent of the flooding in Pakistan which are made available to the rapid response effort within 24 h of acquisition. The high temporal frequency and independence of the data from cloud-free skies makes GM data a viable tool for mapping flood waters during those periods where optical satellite data is unavailable, which may be crucial to rapid response disaster planning, where thousands of lives are affected. Image differencing techniques are used, with pre-flood baseline image backscatter values being deducted from target values to eliminate regions with a permanent flood-like radar response due to volume scattering and attenuation, and to highlight the low response caused by specular reflection by open flood water. The effect of local incidence angle on the received signal is mitigated by ensuring that the deducted image is acquired from the same orbit track as the target image. Poor separability of the water class with land in areas beyond the river channels is tackled using a region-growing algorithm which seeks threshold-conformance from seed pixels at the center of the river channels. The resultant mapped extents are tested against MODIS SWIR data where available, with encouraging results.

Description: Soil weathering rates in 21 catchments of the Canadian Shield Hydrology and Earth System Sciences Discussions, 8, 5743-5768, 2011 Author(s): D. Houle, P. Lamoureux, N. Bélanger, M. Bouchard, C. Gagnon, S. Couture, and A. Bouffard Soil mineral weathering represents an essential source of nutrient base cation (Ca, Mg and K) for forest growth in addition to provide a buffering power against precipitation acidity for soils and surface waters. Weathering rates of base cations were obtained for 21 catchments located within the temperate and the boreal forest of the Canadian Shield with the geochemical model PROFILE. Weathering rates ranged from 0.58 to 4.46 kmol c ha −1 yr −1 and their spatial variation within the studied area was mostly in agreement with spatial variations in soil mineralogy. Weathering rates of Ca and Mg were significantly correlated ( r = 0.80 and 0.64) with their respective lake concentrations. Weathering rates of K and Na did not correlate with lake concentrations of K and Na. The modeled weathering rates for each catchment were also compared with estimations of net catchment exportations. The result show that modeled weathering rates of Ca were not significantly different than the net catchment exportations while modeled weathering rates of Mg were higher by 51 %. Larger differences were observed for K and Na weathering rates that were significantly different than net catchment exportations being 6.9 and 2.2 times higher than net exportations, respectively. The results for K were expected given its high reactivity with biotic compartments and suggest that most of the K produced by weathering reactions was retained within soil catchments and/or above ground biomass. This explanation does not apply to Na, however, which is a conservative element in forest ecosystems because of the insignificant needs of Na for soil microorganisms and above ground vegetations. It raises concern about the liability of the PROFILE model to provide reliable values of Na weathering rates. Overall, we concluded that the PROFILE model is powerful enough to reproduce spatial geographical gradients in weathering rates for relatively large areas as well as adequately predict absolute weathering rates values for the sum of base cations, Ca and Mg.

Description: Measurements of ozone and its precursors in Beijing during summertime: impact of urban plumes on ozone pollution in downwind rural areas Atmospheric Chemistry and Physics Discussions, 11, 17337-17373, 2011 Author(s): J. Xu, J. Z. Ma, X. L. Zhang, X. B. Xu, X. F. Xu, W. L. Lin, Y. Wang, W. Meng, and Z. Q. Ma Sea-land and mount-valley circulations are the dominant mesoscale synoptic systems affecting the Beijing area during summertime. Under the influence of these two circulations, the prevailing wind is southwesterly from afternoon to midnight, and then changes to northeasterly till forenoon. In this study, surface ozone (O 3 ), carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO 2 ), nitrogen oxide (NO x ) and non-methane hydrocarbons (NMHCs) were measured at four sites located along the route of prevailing wind, including two upwind urban sites (Fengtai (FT) and Baolian (BL)), an upwind suburban site (Shunyi (SY)) and a downwind rural site (Shangdianzi (SDZ)) during 20 June–16 September 2007. The purpose is to improve our understanding of ozone photochemistry in urban and rural areas of Beijing and the influence of urban plumes on ozone pollution in downwind rural areas. It is found that ozone pollution was synchronism in the urban and rural areas of Beijing, coinciding with the regional-scale synoptic processes. Due to the high traffic density and local emissions, the average levels of reactive gases NO x and NMHCs at the non-rural sites were much higher than those at SDZ. The level of long-lived gas CO at SDZ was comparable to and slightly lower than it was at other sites. The daily-averaged ozone concentration at SDZ was much higher than at other sites due to weak titration. Ranking by OH loss rate coefficient ( L OH ), alkenes played a dominant role in total NMHCs reactivity at both urban and rural sites during the experiment, accounting for 48.6 % and 52.1 % of total L OH , respectively. The NMHCs data were also used to estimate the ozone potential formation (OFP) in Beijing. The leading contributors to ozone formation were aromatics at both urban and rural sites during the experiment, which accounts for 55.5 % and 49.4 % of total OFP, respectively. The ozone peak values are found to lag behind one site after another along the route of prevailing wind from SW to NE. Intersection analyses of trace gases reveal that polluted air masses arriving at SDZ were more aged with both higher O 3 and O x concentrations than those at BL. The results indicate that urban plume can transport not only O 3 but its precursors, the latter leading more photochemical O 3 production when being mixed with background atmosphere in the downwind rural area.

Description: Correcting orbital drift signal in the time series of AVHRR derived convective cloud fraction using rotated empirical orthogonal function Atmospheric Measurement Techniques Discussions, 4, 3877-3890, 2011 Author(s): A. Devasthale, K. Karlsson, J. Quaas, and H. Grassl The AVHRRs instruments onboard the series of NOAA satellites offer the longest available meteorological data records from space. These satellites have drifted in orbit resulting in shifts in the local time sampling during the life span of sensors onboard. Depending on the amplitude of a diurnal cycle of the geophysical parameters derived, orbital drift may cause spurious trends in their time series. We investigate tropical deep convective clouds, which show pronounced diurnal cycle amplitude, to bracket an upper bound of the impact of orbital drift on their time series. We carry out a rotated empirical orthogonal function analysis and show that the REOFs are useful in delineating orbital drift signal and, more importantly, in correcting this signal in the time series of convective cloud amount. These results will help facilitate the derivation of homogenized data series of cloud amount from NOAA satellite sensors and ultimately analyzing trends from them. However, we suggest detailed comparison of various methods and their rigorous testing before applying final orbital drift corrections.

Description: Ice nucleation properties of volcanic ash from Eyjafjallajökull Atmospheric Chemistry and Physics Discussions, 11, 17201-17243, 2011 Author(s): C. R. Hoyle, V. Pinti, A. Welti, B. Zobrist, C. Marcolli, B. Luo, Á. Höskuldsson, H. B. Mattsson, T. Thorsteinsson, G. Larsen, and T. Peter The ice nucleation ability of volcanic ash particles collected close to the Icelandic volcano Eyjafjallajökull during its eruptions in April and May 2010 is investigated experimentally, in the immersion and deposition modes, and applied to atmospheric conditions by comparison with airborne measurements and microphysical model calculations. The number of ash particles which are active as ice nuclei (IN) is strongly temperature dependent, with a very small minority being active in the immersion mode at temperatures of 250–263 K. Average ash particles show only a moderate effect on ice nucleation, by inducing freezing at temperatures between 236 K and 240 K (i.e. approximately 3–4 K higher than temperatures required for homogeneous ice nucleation, measured with the same instrument). By scaling the results to aircraft and lidar measurements of the conditions in the ash plume days down wind of the eruption and by applying a simple microphysical model, it was found that the IN active in the immersion mode in the range 250–263 K generally occurred in atmospheric number densities at the lower end of those required to have an impact on ice cloud formation. However, 3–4 K above the homogeneous freezing point, immersion mode IN number densities a few days down wind of the eruption were sufficiently high to have a moderate influence on ice cloud formation. The efficiency of IN in the deposition mode was found to be poor except at very cold conditions ( 〈 238 K), when they reach an efficiency similar to that of mineral dust with the onset of freezing at 10 % supersaturation with respect to ice, and with the frozen fraction nearing its maximum value at a supersaturation 20 %. In summary, these investigations suggest volcanic ash particles to have only moderate effects on atmospheric ice formation.

Description: Novel application of satellite and in-situ measurements to map surface-level NO 2 in the Great Lakes region Atmospheric Chemistry and Physics Discussions, 11, 17245-17287, 2011 Author(s): C. J. Lee, J. R. Brook, G. J. Evans, R. V. Martin, and C. Mihele Ozone Monitoring Instrument (OMI) tropospheric NO 2 vertical column density data were used in conjunction with in-situ NO 2 concentrations collected by permanently installed monitoring stations to infer 24 h surface-level NO 2 concentrations at 0.1° (~ 11 km) resolution. The region examined included rural and suburban areas, and the highly industrialised area of Windsor, Ontario, which is situated directly across the US-Canada border from Detroit, MI. Photolytic NO 2 monitors were collocated with standard NO 2 monitors to provide qualitative data regarding NO z interference during the campaign. To test the accuracy of the OMI-inferred concentrations, two-week integrative NO 2 measurements were collected using passive monitors at 18 locations, approximating a 15 km grid across the region, for 7 consecutive two-week periods. When compared with these passive results, satellite-inferred concentrations showed an 18 % positive bias. The correlation of the passive monitor and OMI-inferred concentrations ( R = 0.69, n = 115) was stronger than that for the passive monitor concentrations and OMI column densities ( R = 0.52), indicating that using a sparse network of monitoring sites to estimate concentrations improves the direct utility of the OMI observations. OMI-inferred concentrations were then calculated for four years to show an overall declining trend in surface NO 2 concentrations in the region. Additionally, by separating OMI-inferred surface concentrations by wind direction, clear patterns in emissions and affected down-wind regions, in particular around the US-Canada border, were revealed.

Description: Variable lifetimes and loss mechanisms for NO 3 and N 2 O 5 during the DOMINO campaign: contrasts between marine, urban and continental air Atmospheric Chemistry and Physics Discussions, 11, 17825-17877, 2011 Author(s): J. N. Crowley, J. Thieser, M. Tang, G. Schuster, H. Bozem, Z. Hosaynali Beygi, H. Fischer, J. Diesch, F. Drewnick, S. Borrmann, W. Song, N. Yassaa, J. Williams, D. Pöhler, U. Platt, and J. Lelieveld Nighttime mixing ratios of boundary layer N 2 O 5 were determined using cavity-ring-down spectroscopy during the DOMINO campaign. Observation of N 2 O 5 was intermittent, with mixing ratios ranging from below the detection limit (~5 ppt) to ~500 ppt. A steady-state analysis constrained by measured mixing ratios of NO 2 and O 3 was used to derive NO 3 lifetimes and compare them to calculated rates of loss via gas-phase and heterogeneous reactions of both NO 3 and N 2 O 5 . Three distinct types of air masses were encountered, which were largely marine (Atlantic), continental or urban-industrial in origin. NO 3 lifetimes were longest in the Atlantic sector (up to ~30 min) but were very short (a few seconds) in polluted, air masses from the local city and petroleum-related industrial complex of Huelva. Air from the continental sector was an intermediate case. The high reactivity to NO 3 of the urban air mass was not accounted for by gas-phase and heterogeneous reactions, rates of which were constrained by measurements of NO, volatile organic species and aerosol surface area. In general, high NO 2 mixing ratios resulted in low NO 3 lifetimes, though heterogeneous processes (e.g. reaction of N 2 O 5 on aerosol) were generally less important than direct gas-phase losses of NO 3 . The presence of SO 2 at levels above ~2 ppb in the urban air sector was always associated with very low N 2 O 5 mixing ratios indicating either very short NO 3 lifetimes in the presence of combustion-related emissions or an important role for reduced sulphur species in urban, nighttime chemistry. High production rates coupled with low lifetimes of NO 3 imply an important contribution of nighttime chemistry to removal of both NO x and VOC.

Description: The role of catchment classification in rainfall-runoff modeling Hydrology and Earth System Sciences Discussions, 8, 6113-6153, 2011 Author(s): Y. He, A. Bárdossy, and E. Zehe A sound catchment classification scheme is a fundamental step towards improved catchment hydrology science and prediction in ungauged basins. Two categories of catchment classification methods are presented in the paper. The first one is based directly on physiographic properties and climatic conditions over a catchment and regarded as a Linnaean type or natural classification scheme. The second one is based on numerical clustering and regionalization methods and considered as a statistical or arbitrary classification scheme. This paper reviews each category including what has been done since recognition of the intrinsic value of catchment classification, what is being done in the current research, as well as what is to be done in the future.

Description: General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales Atmospheric Chemistry and Physics Discussions, 11, 17941-18160, 2011 Author(s): M. Kulmala, A. Asmi, H. K. Lappalainen, U. Baltensperger, J.-L. Brenguier, M. C. Facchini, H.-C. Hansson, Ø. Hov, C. D. O'Dowd, U. Pöschl, A. Wiedensohler, R. Boers, O. Boucher, G. de Leeuw, H. Denier van den Gon, J. Feichter, R. Krejci, P. Laj, H. Lihavainen, U. Lohmann, G. McFiggans, T. Mentel, C. Pilinis, I. Riipinen, M. Schulz, A. Stohl, E. Swietlicki, E. Vignati, M. Amann, M. Amann, C. Alves, S. Arabas, P. Artaxo, D. C. S. Beddows, R. Bergström, J. P. Beukes, M. Bilde, J. F. Burkhart, F. Canonaco, S. Clegg, H. Coe, S. Crumeyrolle, B. D'Anna, S. Decesari, S. Gilardoni, M. Fischer, A. M. Fjæraa, C. Fountoukis, C. George, L. Gomes, P. Halloran, T. Hamburger, R. M. Harrison, H. Herrmann, T. Hoffmann, C. Hoose, M. Hu, U. Hõrrak, Y. Iinuma, T. Iversen, M. Josipovic, M. Kanakidou, A. Kiendler-Scharr, A. Kirkevåg, G. Kiss, Z. Klimont, P. Kolmonen, M. Komppula, J.-E. Kristjánsson, L. Laakso, A. Laaksonen, L. Labonnote, V. A. Lanz, K. E. J. Lehtinen, R. Makkonen, G. McMeeking, J. Merikanto, A. Minikin, S. Mirme, W. T. Morgan, E. Nemitz, D. O'Donnell, T. S. Panwar, H. Pawlowska, A. Petzold, J. J. Pienaar, C. Pio, C. Plass-Duelmer, A. S. H. Prévôt, S. Pryor, C. L. Reddington, G. Roberts, D. Rosenfeld, J. Schwarz, Ø. Seland, K. Sellegri, X. J. Shen, M. Shiraiwa, H. Siebert, B. Sierau, D. Simpson, J. Y. Sun, D. Topping, P. Tunved, P. Vaattovaara, V. Vakkari, J. P. Veefkind, A. Visschedijk, H. Vuollekoski, R. Vuolo, B. Wehner, J. Wildt, S. Woodward, D. R. Worsnop, G.-J. van Zadelhoff, A. A. Zardini, K. Zhang, P. G. van Zyl, V.-M. Kerminen, K. S. Carslaw, and S. N. Pandis In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) the first comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. This work enabled EUCAARI to improve our understanding of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

Description: A global climatology of tropospheric and stratospheric ozone derived from Aura OMI and MLS measurements Atmospheric Chemistry and Physics Discussions, 11, 17879-17911, 2011 Author(s): J. R. Ziemke, S. Chandra, G. Labow, P. K. Bhartia, L. Froidevaux, and J. C. Witte A global climatology of tropospheric and stratospheric column ozone is derived by combining six years of Aura Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) ozone measurements for the period October 2004 through December 2010. The OMI/MLS tropospheric ozone climatology exhibits large temporal and spatial variability which includes ozone accumulation zones in the tropical south Atlantic year-round and in the subtropical Mediterranean/Asia region in summer months. High levels of tropospheric ozone in the Northern Hemisphere also persist in mid-latitudes over the Eastern North American and Asian continents extending eastward over the Pacific Ocean. For stratospheric ozone climatology from MLS, largest ozone abundance lies in the Northern Hemisphere in the latitude range 70° N–80° N in February–April and in the Southern Hemisphere around 40° S–50° S during months August–October. The largest stratospheric ozone abundances in the Northern Hemisphere lie over North America and Eastern Asia extending eastward across the Pacific Ocean and in the Southern Hemisphere south of Australia extending eastward across the dateline. With the advent of many newly developing 3-D chemistry and transport models it is advantageous to have such a dataset for evaluating the performance of the models in relation to dynamical and photochemical processes controlling the ozone distributions in the troposphere and stratosphere. The OMI/MLS ozone gridded climatology data, both calculated mean values and RMS uncertainties are made available to the science community via the NASA total ozone mapping spectrometer (TOMS) website http://toms.gsfc.nasa.gov .

Description: Infiltration-soil moisture redistribution under natural conditions: experimental evidence as a guideline for realizing simulation models Hydrology and Earth System Sciences Discussions, 8, 6199-6225, 2011 Author(s): R. Morbidelli, C. Corradini, C. Saltalippi, A. Flammini, and E. Rossi The evolution in time, t , of the experimental soil moisture vertical profile under natural conditions is investigated in order to address the corresponding simulation modelling. The measurements were conducted in a plot with a bare silty loam soil. The soil water content, θ, was continuously monitored at different depths, z , using a Time Domain Reflectometry (TDR) system. For each profile four buriable three-rod waveguides were inserted horizontally at different depths (5, 15, 25 and 35 cm). In addition, we used sensors of air temperature and relative humidity, wind speed, solar radiation, evaporation and rain as supports for the application of selected simulation models, as well as for the detection of elements leading to their improvement. The results indicate that, under natural conditions, very different trends of the θ( z , t ) function can be observed in the given fine-textured soil, where the formation of a sealing layer over the parent soil requires an adjustment of the simulation modelling commonly used for hydrological applications. In particular, because of the considerable variations in the shape of the moisture content vertical profile as a function of time, a generalization of the existing models should incorporate a representation of the variability in time of the saturated hydraulic conductivity of the uppermost soil. This conclusion is supported by the fact that the observed shape of θ( z ) can be appropriately reproduced by adopting this approach, however the observed rainfall rate and the occurrence of freeze-thaw cycles with high soil moisture contents have to be explicitly incorporated.

Description: Scattering and absorption by aerosols during EUCAARI-LONGREX: can airborne measurements and models agree? Atmospheric Chemistry and Physics Discussions, 11, 18487-18525, 2011 Author(s): E. J. Highwood, M. J. Northway, G. R. McMeeking, W. T. Morgan, D. Liu, S. Osborne, K. Bower, H. Coe, C. Ryder, and P. Williams Scattering and absorption by aerosol in anthropogenically perturbed air masses over Europe has been measured using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM) on 14 flights during the EUCAARI-LONGREX campaign in May 2008. The geographical and temporal variations of the derived shortwave optical properties of aerosol are presented. Values of single scattering albedo of dry aerosol at 550 nm varied considerably over the data set from 0.86 to near unity. Dry aerosol optical depths ranged from 0.03 to 0.24. An optical properties closure study comparing calculations from composition data and Mie scattering code with the measured properties is presented. Very good agreement (to within 30 %) can be achieved for scattering, but the modelling of absorption is shown to be sensitive to the refractive indices chosen for organic aerosols, and to a lesser extent black carbon. Agreement with the measured absorption can only be achieved if organic carbon is assumed to be only weakly absorbing. Hygroscopic growth curves derived from the wet nephelometer indicate moderate water uptake by the aerosol with a campaign mean f (RH) value (change in scattering) of 1.3 at 80 % relative humidity. This value is consistent with the major chemical components of the aerosol measured by the aerosol mass spectrometer (AMS), which are primarily mixed organics and nitrate and some sulphate. As expected the effect of humidity is to raise the single scattering albedo, and to increase the aerosol optical depth. This study represents an important new body of data regarding European aerosol amounts, composition and optical properties and additionally demonstrates the importance of airborne measurements of black carbon mass and aerosol hygroscopicity.

Description: Sensitivity studies for a space-based methane lidar mission Atmospheric Measurement Techniques Discussions, 4, 3545-3592, 2011 Author(s): C. Kiemle, M. Quatrevalet, G. Ehret, A. Amediek, A. Fix, and M. Wirth Methane is the third most important greenhouse gas in the atmosphere after water vapour and carbon dioxide. A major handicap to quantify the emissions at the Earth's surface in order to better understand biosphere-atmosphere exchange processes and potential climate feedbacks is the lack of accurate and global observations of methane. Space-based integrated path differential absorption (IPDA) lidar has potential to fill this gap, and a Methane Remote Lidar Mission (MERLIN) on a small satellite in Polar orbit was proposed by DLR and CNES in the frame of a German-French climate monitoring initiative. System simulations are used to identify key performance parameters and to find an advantageous instrument configuration, given the environmental, technological, and budget constraints. The sensitivity studies use representative averages of the atmospheric and surface state to estimate the measurement precision, i.e. the random uncertainty due to instrument noise. Key performance parameters for MERLIN are average laser power, telescope size, orbit height, surface reflectance, and detector noise. A modest-size lidar instrument with 0.45 W average laser power and 0.55 m telescope diameter on a 506 km orbit could provide 50-km averaged methane column measurement along the sub-satellite track with a precision of about 1 % over vegetation. The use of a methane absorption trough at 1.65 μm improves the near-surface measurement sensitivity and vastly relaxes the wavelength stability requirement that was identified as one of the major technological risks in the pre-phase A studies for A-SCOPE, a space-based IPDA lidar for carbon dioxide at the European Space Agency. Minimal humidity and temperature sensitivity at this wavelength position will enable accurate measurements in tropical wetlands, key regions with largely uncertain methane emissions. In contrast to actual passive remote sensors, measurements in Polar Regions will be possible and biases due to aerosol layers and thin ice clouds will be minimised.

Description: Biological residues define the ice nucleation properties of soil dust Atmospheric Chemistry and Physics Discussions, 11, 16585-16598, 2011 Author(s): F. Conen, C. E. Morris, J. Leifeld, M. V. Yakutin, and C. Alewell Soil dust is a major driver of ice nucleation in clouds leading to precipitation. It consists largely of mineral particles with a small fraction of organic matter constituted mainly of remains of micro-organisms that participated in degrading plant debris before their own decay. Some micro-organisms have been shown to be much better ice nuclei than the most efficient soil mineral. Yet, current aerosol schemes in global climate models do not consider a difference between soil dust and mineral dust in terms of ice nucleation activity. Here, we show that particles from the clay and silt size fraction of four different soils naturally associated with 0.7 to 11.8 % organic carbon (w/w) can have up to four orders of magnitude more ice nuclei per unit mass active in the immersion freezing mode at −12 °C than montmorillonite, the most efficient pure clay mineral. Most of this activity was lost after heat treatment. Removal of biological residues reduced ice nucleation activity to, or below that of montmorillonite. Desert soils, inherently low in organic content, are a large natural source of dust in the atmosphere. In contrast, agricultural land use is concentrated on fertile soils with much larger organic matter contents than found in deserts. It is currently estimated that the contribution of agricultural soils to the global dust burden is less than 20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere of a very different and much more potent kind than mineral dusts.

Description: ACE-FTS measurements of trace species in the characterization of biomass burning plumes Atmospheric Chemistry and Physics Discussions, 11, 16611-16637, 2011 Author(s): K. A. Tereszchuk, G. González Abad, C. Clerbaux, D. Hurtmans, P.-F. Coheur, and P. F. Bernath To further our understanding of the effects of biomass burning emission on atmospheric composition, we report measurements of trace species from biomass burning plumes made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the SCISAT-1 satellite. An extensive set of 15 molecules, C 2 H 2 , C 2 H 6 , CH 3 OH, CH 4 , CO, H 2 CO, HCN, HCOOH, HNO 3 , NO, NO 2 , N 2 O 5 , O 3 , OCS and SF 6 are used in our analysis. Even though most biomass burning smoke is typically confined to the boundary layer, much of these emissions are injected directly into the free troposphere via fire-related convective processes and transported away from the emission region. Further knowledge of the aging of biomass burning emission in the free troposphere is needed. Tracer-tracer correlations are made between known pyrogenic species in these plumes in an effort to classify them and follow their chemical evolution. Criteria such as age and type of biomass material burned are considered. Emission factors are derived and compared to airborne measurements of biomass burning from numerous ecosystems to validate the ACE-FTS data.

Description: Emission controls versus meteorological conditions in determining aerosol concentrations in Beijing during the 2008 Olympic Games Atmospheric Chemistry and Physics Discussions, 11, 16655-16691, 2011 Author(s): Y. Gao, X. Liu, C. Zhao, M. Zhang, and Y. Wang A series of emission control measures were undertaken in Beijing and the adjacent provinces in China during the 2008 Beijing Olympic Games on 8–24 August 2008. This provides a unique opportunity for investigating the effectiveness of emission controls on air pollution in Beijing. We conducted a series of numerical experiments over East Asia for the period of July to September 2008 using a coupled meteorology-chemistry model (WRF-Chem). Model can generally reproduce the observed variation of aerosol concentrations. Consistent with observations, modeled concentrations of aerosol species (sulfate, nitrate, ammonium, black carbon, organic carbon, total particulate matter) in Beijing were decreased by 30–50 % during the Olympic period compared to the other periods in July and August in 2008 and the same period in 2007. Model results indicate that emission controls were effective in reducing the aerosol concentrations by comparing simulations with and without emission controls. However, our analysis suggests that meteorological conditions (e.g., wind direction and precipitation) are at least as important as emission controls in producing the low aerosol concentrations appearing during the Olympic period. Transport from the regions surrounding Beijing determines the temporal variation of aerosol concentrations in Beijing. Based on the budget analysis, we suggest that to improve the air quality over Beijing, emission control strategy should focus on the regional scale instead of the local scale.

Description: Information operator approach applied to the retrieval of the vertical distribution of atmospheric constituents from ground-based high-resolution FTIR measurements Atmospheric Measurement Techniques Discussions, 4, 3739-3785, 2011 Author(s): C. Senten, M. De Mazière, G. Vanhaelewyn, and C. Vigouroux The analysis of high spectral resolution Fourier Transform infrared (FTIR) solar absorption spectra is an important issue in remote sensing. If this is done carefully, one can obtain information, not only about the total column abundances, but also about the vertical distribution of various constituents in the atmosphere. This work introduces the application of the information operator approach for extracting vertical profile information from ground-based FTIR measurements. The algorithm is implemented and tested within the well-known retrieval code SFIT2, adapting the optimal estimation method such as to take into account only the significant contributions to the solution. In particular, we demonstrate the feasibility of the method in an application to ground-based FTIR spectra taken in the frame of the Network for the Detection of Atmospheric Composition Change (NDACC) at Ile de La Réunion (21° S, 55° E). A thorough comparison is made between the original optimal estimation method and this alternative retrieval algorithm, regarding information content, retrieval robustness and corresponding full error budget evaluation for the target species ozone (O 3 ), nitrous oxide (N 2 O), methane (CH 4 ), and carbon monoxide (CO). For O 3 and CH 4 , a comparison with the Tikhonov regularization method has also been made. It is shown that the information operator approach performs well and in most cases yields both a better accuracy and stability than the optimal estimation method. Additionally, the information operator approach has the advantage of being less sensitive to the choice of a priori information. The Tikhonov regularization results seem to be situated between both methods' results, as to profile retrievals, error budgets and column stability.

Description: Opportunistic validation of sulfur dioxide in the Sarychev Peak volcanic eruption cloud Atmospheric Measurement Techniques Discussions, 4, 3861-3875, 2011 Author(s): S. A. Carn and T. M. Lopez We report attempted validation of Ozone Monitoring Instrument (OMI) sulfur dioxide (SO 2 ) retrievals in the stratospheric volcanic cloud from Sarychev Peak (Kurile Islands) in June 2009, through opportunistic deployment of a ground-based ultraviolet (UV) spectrometer (FLYSPEC) as the volcanic cloud drifted over Central Alaska. The volcanic cloud altitude (~12–14 km) was constrained using coincident CALIPSO lidar observations. By invoking some assumptions about the spatial distribution of SO 2 , we derive averages of FLYSPEC vertical SO 2 columns for comparison with OMI SO 2 measurements. Despite limited data, we find minimum OMI-FLYSPEC differences of ~5–6 % which support the validity of the operational OMI SO 2 algorithm. These measurements represent the first attempt to validate SO 2 in a stratospheric volcanic cloud using a mobile ground-based instrument, and demonstrate the need for a network of rapidly deployable instruments for validation of space-based volcanic SO 2 measurements.

Description: Validity of satellite measurements used for the monitoring of UV radiation risk on health Atmospheric Chemistry and Physics Discussions, 11, 17375-17421, 2011 Author(s): F. Jégou, S. Godin-Beekman, M. P. Corrêa, C. Brogniez, F. Auriol, V. H. Peuch, M. Haeffelin, A. Pazmino, P. Saiag, F. Goutail, and E. Mahé In order to test the validity of ultraviolet index (UVI) satellite products and UVI model simulations for general public information, intercomparison involving three satellite instruments (SCIAMACHY, OMI and GOME-2), the Chemistry and Transport Model, Modélisation de la Chimie Atmosphérique Grande Echelle (MOCAGE), and ground-based instruments was performed in 2008 and 2009. The intercomparison highlighted a systematic high bias of ~1 UVI in the OMI clear-sky products compared to the SCIAMACHY and TUV model clear-sky products. The OMI and GOME-2 all-sky products are close to the ground-based observations with a low 6 % positive bias, comparable to the results found during the satellite validation campaigns. This result shows that OMI and GOME-2 all-sky products are well appropriate to evaluate the UV-risk on health. The study has pointed out the difficulty to take into account either in the retrieval algorithms or in the models, the large spatial and temporal cloud modification effect on UV radiation. This factor is crucial to provide good quality UV information. OMI and GOME-2 show a realistic UV variability as a function of the cloud cover. Nevertheless these satellite products do not sufficiently take into account the radiation reflected by clouds. MOCAGE numerical forecasts show good results during periods with low cloud covers, but are actually not adequate for overcast conditions; this is why Météo-France currently uses human-expertised cloudiness (rather than direct outputs from Numerical Prediction Models) together with MOCAGE clear-sky UV indices for its operational forecasts. From now on, the UV monitoring could be done using free satellite products (OMI, GOME-2) and operational forecast for general public by using modelling, as long as cloud forecasts and the parametrisation of the impact of cloudiness on UV radiation are adequate.

Description: Aerosol concentration and size distribution measured below, in, and above cloud from the DOE G-1 during VOCALS-REx Atmospheric Chemistry and Physics Discussions, 11, 17289-17336, 2011 Author(s): L. I. Kleinman, P. H. Daum, Y.-N. Lee, E. R. Lewis, A. J. Sedlacek III, G. I. Senum, S. R. Springston, J. Wang, J. Hubbe, J. Jayne, Q. Min, S. S. Yum, and G. Allen During the VOCALS Regional Experiment, the DOE G-1 aircraft was used to sample a varying aerosol environment pertinent to properties of stratocumulus clouds over a longitude band extending 800 km west from the Chilean coast at Arica. Trace gas and aerosol measurements are presented as a function of longitude, altitude, and dew point in this study. Spatial distributions are consistent with an upper atmospheric source for O 3 and South American coastal sources for marine boundary layer (MBL) CO and aerosol, most of which is acidic sulfate in agreement with the dominant pollution source being SO 2 from Cu smelters and power plants. Pollutant layers in the free troposphere (FT) can be a result of emissions to the north in Peru or long range transport from the west. At a given altitude in the FT (up to 3 km), dew point varies by 40 °C with dry air descending from the upper atmospheric and moist air having a BL contribution. Ascent of BL air to a cold high altitude results in the condensation and precipitation removal of all but a few percent of BL water along with aerosol that served as CCN. Thus, aerosol volume decreases with dew point in the FT. Aerosol size spectra have a bimodal structure in the MBL and an intermediate diameter unimodal distribution in the FT. Comparing cloud droplet number concentration (CDNC) and pre-cloud aerosol ( D p 〉 100 nm) gives a linear relation up to a number concentration of ~150 cm −3 , followed by a less than proportional increase in CDNC at higher aerosol number concentration. A number balance between below cloud aerosol and cloud droplets indicates that ~25 % of aerosol in the PCASP size range are interstitial (not activated). One hundred and two constant altitude cloud transects were identified and used to determine properties of interstitial aerosol. One transect is examined in detail as a case study. Approximately 25 to 50 % of aerosol with D p 〉 110 nm were not activated, the difference between the two approaches possibly representing shattered cloud droplets or unknown artifact. CDNC and interstitial aerosol were anti-correlated in all cloud transects, consistent with the occurrence of dry in-cloud areas due to entrainment or circulation mixing.

Description: Interannual variability of ozone and carbon monoxide at the Whistler high elevation site: 2002–2006 Atmospheric Chemistry and Physics Discussions, 11, 17621-17664, 2011 Author(s): A. M. Macdonald, K. G. Anlauf, W. R. Leaitch, and E. Chan In spring 2002, an atmospheric measurement site was established at the peak of Whistler Mountain in British Columbia, Canada to measure trace gases, particle chemistry and physics, and meteorology. This paper uses continuous measurements from March 2002 to December 2006 to investigate the influence of trans-Pacific transport and North American forest fires on both O 3 and CO at Whistler. Annual mean mixing ratios of O 3 and CO were 41 ppbv (monthly means of 35–48 ppbv) and 145 ppbv (monthly means of 113–177 ppbv) respectively with both species exhibiting an annual cycle of late-winter to early-spring maxima and summer minima. The absence of a broad summer O 3 peak differs from previously-reported high altitude sites in the western US. The highest monthly-averaged O 3 and CO mixing ratios relative to the 5-year monthly means were seen in fall 2002 and spring 2003 with increased O 3 and CO of 10 % and 25 % respectively. These increases correspond to anomalously-high values reported at other Northern Hemisphere sites and are attributed to fires in the Russian Federation. Air mass back trajectory analysis is used to associate the mean enhancements of O 3 and CO with trans-Pacific transported or North American air masses relative to the Pacific background. Mean values of the enhancements for March to June were 6 ppbv and 16 ppbv for O 3 and CO respectively. In summers 2002–2006, higher CO and O 3 mixing ratios were always observed in North American air masses and this relative enhancement co-varied for each year with the western US and Canada total wildfire area. The greatest enhancements in O 3 and CO were seen in 2004, a record year for forest fires in Alaska and the Yukon Territory. In August 2004, average O 3 and CO mixing ratios were 13 and 44 ppbv above background values.

Description: Atmospheric dust modeling from meso to global scales with the online NMMB/BSC-Dust model – Part 1: Model description, annual simulations and evaluation Atmospheric Chemistry and Physics Discussions, 11, 17551-17620, 2011 Author(s): C. Pérez, K. Haustein, Z. Janjic, O. Jorba, N. Huneeus, J. M. Baldasano, T. Black, S. Basart, S. Nickovic, R. L. Miller, J. P. Perlwitz, M. Schulz, and M. Thomson We describe and evaluate the NMMB/BSC-Dust, a new dust aerosol cycle model embedded online within the NCEP Non-hydrostatic Multiscale Model (NMMB). NMMB is a further evolution of the operational Non-hydrostatic Mesoscale Model (WRF-NMM), which together with other upgrades has been extended from meso to global scales. Its unified non-hydrostatic dynamical core is prepared for regional and global simulation domains. The new NMMB/BSC-Dust is intended to provide short to medium-range weather and dust forecasts from regional to global scales and represents a first step towards the development of a unified chemical-weather model. This paper describes the parameterizations used in the model to simulate the dust cycle including sources, transport, deposition and interaction with radiation. We evaluate monthly and annual means of the global configuration of the model against the AEROCOM dust benchmark dataset for year 2000 including surface concentration, deposition and aerosol optical depth (AOD), and we evaluate the daily AOD variability in a regional domain at high resolution covering Northern Africa, Middle East and Europe against AERONET AOD for year 2006. The NMMB/BSC-Dust provides a good description of the horizontal distribution and temporal variability of the dust. Daily AOD correlations at the regional scale are around 0.6–0.7 on average without dust data assimilation. At the global scale the model lies within the top range of AEROCOM dust models in terms of performance statistics for surface concentration, deposition and AOD. This paper discusses the current strengths and limitations of the modeling system and points towards future improvements.

Description: Sources and seasonality of atmospheric methanol based on tall tower measurements in the US Upper Midwest Atmospheric Chemistry and Physics Discussions, 11, 17473-17505, 2011 Author(s): L. Hu, M. J. Mohr, K. C. Wells, T. J. Griffis, D. Helmig, and D. B. Millet We present over one year of continuous atmospheric methanol measurements from the University of Minnesota tall tower Trace Gas Observatory (KCMP tall tower; 244 m a.g.l.), and interpret the dataset in terms of constraints on regional methanol sources and seasonality. The seasonal cycle of methanol concentrations observed at the KCMP tall tower is generally similar to that simulated by a global 3-D chemical transport model (GEOS-Chem, driven with MEGANv2.0 biogenic emissions) except the seasonal peak occurs ~1 month earlier in the observations, apparently reflecting a model underestimate of emission rates for younger versus older leaves. Based on a source tracer approach, which we evaluate using GEOS-Chem and with multiple tracers, we estimate that anthropogenic emissions account for approximately 40 % of ambient methanol abundance during winter and 10 % during summer. During daytime in summer, methanol concentrations increase exponentially with temperature, reflecting the temperature sensitivity of the biogenic source, and the observed temperature dependence is statistically consistent with that in the model. Nevertheless, summertime concentrations are underestimated by on average 35 % in the model for this region. The seasonal importance of methanol as a source of formaldehyde (HCHO) and carbon monoxide (CO) is highest in spring through early summer, when biogenic methanol emissions are high but isoprene emissions are still relatively low. During that time observed methanol concentrations account for on average 20 % of the total CO and HCHO production rates as simulated by GEOS-Chem, compared to 12 % later in the summer and 12 % on an annual average basis. The biased seasonality in the model means that the photochemical role for methanol early in the growing season is presently underestimated.

Description: Inversion of tropospheric profiles of aerosol extinction and HCHO and NO 2 mixing ratios from MAX-DOAS observations in Milano during the summer of 2003 and comparison with independent data sets Atmospheric Measurement Techniques Discussions, 4, 3891-3964, 2011 Author(s): T. Wagner, S. Beirle, T. Brauers, T. Deutschmann, U. Frieß, C. Hak, J. D. Halla, K. P. Heue, W. Junkermann, X. Li, U. Platt, and I. Pundt-Gruber We present aerosol and trace gas profiles derived from MAX-DOAS observations. Our inversion scheme is based on simple profile parameterisations used as input for an atmospheric radiative transfer model (forward model). From a least squares fit of the forward model to the MAX-DOAS measurements, two profile parameters are retrieved including integrated quantities (aerosol optical depth or trace gas vertical column density), and parameters describing the height and shape of the respective profiles. From these results, the aerosol extinction and trace gas mixing ratios can also be calculated. We apply the profile inversion to MAX-DOAS observations during a measurement campaign in Milano, Italy, September 2003, which allowed simultaneous observations from three telescopes (directed to north, west, south). Profile inversions for aerosols and trace gases were possible on 23 days. Especially in the middle of the campaign (17–20 September 2003), enhanced values of aerosol optical depth and NO 2 and HCHO mixing ratios were found. The retrieved layer heights were typically similar for HCHO and aerosols. For NO 2 , lower layer heights were found, which increased during the day. The MAX-DOAS inversion results are compared to independent measurements: (1) aerosol optical depth measured at an AERONET station at Ispra; (2) near-surface NO 2 and HCHO (formaldehyde) mixing ratios measured by long path DOAS and Hantzsch instruments at Bresso; (3) vertical profiles of HCHO and aerosols measured by an ultra light aircraft. Depending on the viewing direction, the aerosol optical depths from MAX-DOAS are either smaller or larger than those from AERONET observations. Similar comparison results are found for the MAX-DOAS NO 2 mixing ratios versus long path DOAS measurements. In contrast, the MAX-DOAS HCHO mixing ratios are generally higher than those from long path DOAS or Hantzsch instruments. The comparison of the HCHO and aerosol profiles from the aircraft showed reasonable agreement with the respective MAX-DOAS layer heights. From the comparison of the results for the different telescopes, it was possible to investigate the internal consistency of the MAX-DOAS observations. As part of our study, a cloud classification algorithm was developed (based on the MAX-DOAS zenith viewing directions), and the effects of clouds on the profile inversion were investigated. Different effects of clouds on aerosols and trace gas retrievals were found: while the aerosol optical depth is systematically underestimated and the HCHO mixing ratio is systematically overestimated under cloudy conditions, the NO 2 mixing ratios are only slightly affected. These findings are in basic agreement with radiative transfer simulations.

Description: Modeling the ascent of sounding balloons: derivation of the vertical air motion Atmospheric Measurement Techniques Discussions, 4, 3965-4012, 2011 Author(s): A. Gallice, F. G. Wienhold, C. R. Hoyle, F. Immler, and T. Peter A new model to describe the ascent of sounding balloons in the troposphere and lower stratosphere (up to ~30–35 km altitude) is presented. Contrary to previous models, detailed account is taken of both the variation of the drag coefficient with altitude and the heat imbalance between the balloon and the atmosphere. To compensate for the lack of data on the drag coefficient of sounding balloons, a reference curve for the relationship between drag coefficient and Reynolds number is derived from a dataset of flights launched during the Lindenberg Upper Air Methods Intercomparisons (LUAMI) campaign. The transfer of heat from the surrounding air into the balloon is accounted for by solving the radial heat diffusion equation inside the balloon. The potential applications of the model include the forecast of the trajectory of sounding balloons, which can be used to increase the accuracy of the match technique, and the derivation of the air vertical velocity. The latter is obtained by subtracting the ascent rate of the balloon in still air calculated by the model from the actual ascent rate. This technique is shown to provide an approximation for the vertical air motion with an uncertainty error of 0.5 m s −1 in the troposphere and 0.2 m s −1 in the stratosphere. An example of extraction of the air vertical velocity is provided in this paper. We show that the air vertical velocities derived from the balloon soundings in this paper are in general agreement with small-scale atmospheric velocity fluctuations related to gravity waves, mechanical turbulence, or other small-scale air motions measured during the SUCCESS campaign (Subsonic Aircraft: Contrail and Cloud Effects Special Study) in the orographically unperturbed mid-latitude middle troposphere.

Description: Impact of lightning-NO on Eastern United States photochemistry during the summer of 2006 as determined using the CMAQ model Atmospheric Chemistry and Physics Discussions, 11, 17699-17757, 2011 Author(s): D. J. Allen, K. E. Pickering, R. W. Pinder, B. H. Henderson, K. W. Appel, and A. Prados A lightning-nitrogen oxide (NO) algorithm is developed for the regional Community Multiscale Air Quality Model (CMAQ) and used to evaluate the impact of lightning-NO emissions (LNO x ) on tropospheric photochemistry over the Eastern United States during the summer of 2006. The scheme assumes flash rates are proportional to the model convective precipitation rate but then adjusts the flash rates locally to match monthly average observations. Over the Eastern United States, LNO x is responsible for 20–25 % of the tropospheric nitrogen dioxide (NO 2 ) column. This additional NO 2 reduces the low-bias of simulated NO 2 columns with respect to satellite-retrieved Dutch Ozone Monitoring Instrument NO 2 (DOMINO) columns from 41 to 14 %. It also adds 10–20 ppbv to upper tropospheric ozone and 1.5–4.5 ppbv to 8-h maximum surface layer ozone, although, on average, the contribution of LNO x to surface ozone is 1–2 ppbv less on poor air quality days. Biases between modeled and satellite-retrieved tropospheric NO 2 columns vary greatly between urban and rural locations. In general, CMAQ overestimates columns at urban locations and underestimates columns at rural locations. These biases are consistent with in situ measurements that also indicate that CMAQ has too much NO 2 in urban regions and not enough in rural regions. However, closer analysis suggests that most of the differences between modeled and satellite-retrieved urban to rural ratios are likely a consequence of the horizontal and vertical smoothing inherent in columns retrieved by the Ozone Monitoring Instrument (OMI). Within CMAQ, LNO x increases wet deposition of nitrate by 50 % and total deposition of nitrogen by 11 %. This additional deposition reduces the magnitude of the CMAQ low-bias in nitrate wet deposition with respect to National Atmospheric Deposition monitors to near zero. In order to obtain an upper bound on the contribution of uncertainties in chemistry to upper tropospheric NO x low biases, sensitivity calculations with updated chemistry were run for the time period of the Intercontinental Chemical Transport Experiment (INTEX-A) field campaign (summer 2004). After adjusting for possible interferences in NO 2 measurements and averaging over the entire campaign, these updates reduced 7–9 km biases from 32 to 17 % and 9–12 km biases from 57 to 46 %. While these changes lead to better agreement, a considerable NO 2 low-bias remains in the uppermost troposphere.

Description: Interpreting elevated space-borne HCHO columns over the Mediterranean Sea using the OMI and SCIAMACHY sensors Atmospheric Chemistry and Physics Discussions, 11, 17913-17940, 2011 Author(s): A. Sabolis, N. Meskhidze, G. Curci, P. I. Palmer, and B. Gantt Formaldehyde (HCHO) is an oxidation product of a wide range of volatile organic compounds (VOCs) and important atmospheric constituent found in both the polluted urban atmosphere and remote background sites. In this study, remotely sensed data of HCHO vertical column densities are analyzed over the Mediterranean Sea using the Ozone Monitoring Instrument (OMI). Data analysis indicates a marked seasonal cycle with a summer maximum and winter minimum confined to the marine environment during a three year period (2005–2007) examined. A possible retrieval artifact associated with Saharan dust transport over the region is explored by changing intensity of Saharan dust sources in GEOS-Chem following the recommendation of Generoso et al. (2008). Recalculated air mass factors (AMF), based on the new values of aerosol loadings, lead to a reduction of the summertime "hot spot" in OMI retrieval of HCHO columns over the Mediterranean Sea; however, even after the correction, enhanced values are still present in this region. To explain these values, marine biogenic sources of VOCs are examined. Calculations indicate that emissions of phytoplankton-produced isoprene and monoterpenes are not likely to explain the enhanced HCHO columns over the Mediterranean Sea. To further understand spatial and seasonal variation of HCHO over the Mediterranean Sea, OMI HCHO columns are compared to those of the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) sensor. Unlike OMI retrievals, over the Mediterranean Sea SCIAMACHY HCHO columns did not reveal clear seasonality during the three years and the two sensors did not agree within their retrieval uncertainty. Overall, comparison of OMI and SCIAMACHY HCHO columns were inconclusive. Moreover, retrievals of HCHO columns over other water bodies showed that the two sensors agree reasonably well over the Equatorial Pacific region, Gulf of Mexico, and the North Sea, but do not show similar magnitudes or seasonal variations over oligotrophic water bodies such as Mediterranean Sea, Northwestern and Southern Pacific Oceans. Model simulations in conjunction with measurements studies may be required to fully explore the complex mechanism of HCHO formation over the Mediterranean and its implications for the air quality in the region.

Description: Sand box experiments to evaluate the influence of subsurface temperature probe design on temperature based water flux calculation Hydrology and Earth System Sciences Discussions, 8, 6155-6197, 2011 Author(s): M. Munz, S. E. Oswald, and C. Schmidt Quantification of subsurface water fluxes based on the one dimensional solution to the heat transport equation depends on the accuracy of measured subsurface temperatures. The influence of temperature probe setup on the accuracy of vertical water flux calculation was systematically evaluated in this experimental study. Four temperature probe setups were installed into a sand box experiment to measure temporal highly resolved vertical temperature profiles under controlled water fluxes in the range of ±1.3 m d −1 . Pass band filtered time series provided amplitude and phase of the diurnal temperature signal varying with depth depending on water flux. Amplitude ratios of setups directly installed into the saturated sediment significantly varied with sand box hydraulic gradients. Amplitude ratios provided an accurate basis for the analytical calculation of water flow velocities, which matched measured flow velocities. Calculated flow velocities were sensitive to thermal properties of saturated sediment and to probe distance, but insensitive to thermal dispersivity equal to solute dispersivity. Amplitude ratios of temperature probe setups indirectly installed into piezometer pipes were influenced by thermal exchange processes within the pipes and significantly varied with water flux direction only. Temperature time lags of small probe distances of all setups were found to be insensitive to vertical water flux.

Description: Rate coefficients for the reaction of methylglyoxal (CH 3 COCHO) with OH and NO 2 and glyoxal (HCO) 2 with NO 3 Atmospheric Chemistry and Physics Discussions, 11, 18211-18248, 2011 Author(s): R. K. Talukdar, L. Zhu, K. J. Feierabend, and J. B. Burkholder Rate coefficients, k , for the gas-phase reaction of CH 3 COCHO (methylglyoxal) with the OH and NO 3 radicals and (CHO) 2 (glyoxal) with the NO 3 radical are reported. Rate coefficients for the OH + CH 3 COCHO ( k 1 ) reaction were measured under pseudo-first-order conditions in OH as a function of temperature (211–373 K) and pressure (100–220 Torr, He and N 2 bath gases) using pulsed laser photolysis to produce OH radicals and laser induced fluorescence to measure its temporal profile. k 1 was found to be independent of the bath gas pressure with k 1 (295 K) = (1.29 ± 0.13) × 10 −11 cm 3 molecule −1 s −1 and a temperature dependence that is well represented by the Arrhenius expression k 1 (T) = (1.74 ± 0.20) × 10 −12 exp[(590 ± 40)/T] cm 3 molecule −1 s −1 where the uncertainties are 2σ and include estimated systematic errors. Rate coefficients for the NO 3 + (CHO) 2 ( k 3 ) and NO 3 + CH 3 COCHO ( k 4 ) reactions were measured using a relative rate technique to be k 3 (296 K) = (3.7 ± 1.0) × 10 −16 cm 3 molecule −1 s −1 and k 4 (296 K) = (4.1 ± 1.2) × 10 −16 cm 3 molecule −1 s −1 . k 3 (T) was also measured using an absolute rate coefficient method under pseudo-first-order conditions at 296 and 353 K to be (4.2 ± 0.8) × 10 −16 and (7.9 ± 3.6) × 10 −16 cm 3 molecule −1 s −1 , respectively, in agreement with the relative rate result obtained at room temperature. The atmospheric implications of the OH and NO 3 reaction rate coefficients measured in this work are discussed.

Description: The use of LIDAR as a data source for digital elevation models – a study of the relationship between the accuracy of digital elevation models and topographical attributes in northern peatlands Hydrology and Earth System Sciences Discussions, 8, 5497-5522, 2011 Author(s): A. Hasan, P. Pilesjö, and A. Persson It is important to study the factors affecting estimates of wetness since wetness is crucial in climate change studies. The availability of digital elevation models (DEMs) generated with high resolution data is increasing, and their use is expanding. LIDAR earth elevation data have been used to create several DEMs with different resolutions, using various interpolation parameters, in order to compare the models with collected surface data. The aim is to study the accuracy of DEMs in relation to topographical attributes such as slope and drainage area, which are normally used to estimate the wetness in terms of topographic wetness indices. Evaluation points were chosen from the high-resolution LIDAR dataset at a maximum distance of 10 mm from the cell center for each DEM resolution studied, 0.5, 1, 5, 10, 30 and 90 m. The interpolation method used was inverse distance weighting method with four search radii: 1, 2, 5 and 10 m. The DEM was evaluated using a quantile-quantile test and the normalized median absolute deviation. The accuracy of the estimated elevation for different slopes was tested using the DEM with 0.5 m resolution. Drainage areas were investigated at three resolutions, with coinciding evaluation points. The ability of the model to generate the drainage area at each resolution was obtained by pairwise comparison of three data subsets. The results show that the accuracy of the elevations obtained with the DEM model are the same for different resolutions, but vary with search radius. The accuracy of the values (NMAD of errors) varies from 29.7 mm to 88.9 mm, being higher for flatter areas. It was also found that the accuracy of the drainage area is highly dependent on DEM resolution. Coarse resolution yielded larger estimates of the drainage area but lower slope values. This may lead to overestimation of wetness values when using a coarse resolution DEM.

Description: Multivariate design via Copulas Hydrology and Earth System Sciences Discussions, 8, 5523-5558, 2011 Author(s): G. Salvadori, C. De Michele, and F. Durante Calculating return periods and design quantiles in a multivariate framework is a difficult problem: essentially, this is due to the lack of a natural total order in multi-dimensional Euclidean spaces. This paper tries to make the issue clear. First, we outline a possible way to introduce a coherent notion of multivariate total order, and discuss its consequences on the calculation of multivariate return period: in particular, the latter is based on Copulas and the Kendall's measure, which provides a consistent notion of multivariate quantile. Secondly, we introduce several approaches for the identification of critical design events: these latter quantities are of utmost importance in practical applications, but their calculation is yet limited, due to the lack of a suitable theoretical setting where to embed the problem. Throughout the paper, a case study involving the behavior of a dam is used to illustrate the new concepts outlined in this work.

Description: Photochemical production of ozone in Beijing during the 2008 Olympic Games Atmospheric Chemistry and Physics Discussions, 11, 16553-16584, 2011 Author(s): C. C.-K. Chou, C.-Y. Tsai, C.-C. Chang, P.-H. Lin, S. C. Liu, and T. Zhu As a part of the CAREBeijing-2008 campaign, observations of O 3 , oxides of nitrogen (NO x and NO y ), CO, and hydrocarbons (NMHCs) were carried out at the air quality observatory of the Peking University in Beijing, China during August 2008, including the period of the 29th Summer Olympic Games. The measurements were compared to those of the CAREBeijing-2006 campaign to evaluate the effectiveness of the air pollution control measures, which were conducted for improving the air quality in Beijing during the Olympics. The results indicate that significant reduction in the emissions of primary air pollutants had been achieved; the monthly averages of NO x , NO y , CO, and NMHCs reduced by 42.2, 56.5, 27.8, and 49.7 %, respectively. In contrast to the primary pollutants, the averaged mixing ratio of O 3 increased by 42.2 %. Nevertheless, it was revealed that the ambient levels of total oxidants (O x =O 3 +NO 2 +1.5NO z ) and NO z reduced by 21.3 and 77.4 %, respectively. The contradictions between O 3 and O x were further examined in two case studies. Ozone production rates of 30–70 ppbv hr −1 and OPEx of ~8 mole mole −1 were observed on a clear-sky day in spite of the reduced levels of precursors. In that case, it was found that the concentrations of O 3 increased with the increasing NO 2 /NO ratio, whereas the NO z concentrations leveled off when NO 2 /NO〉8. Consequently, the ratio of O 3 to NO z increased to above 10, indicating the shift from VOC-sensitive regime to NO x -sensitive regime. However, in the other case, it was found that the O 3 production was inhibited significantly due to substantial reduction in the ambient levels of NMHCs. According to the observations, it was suggested that the O 3 /O x production rates in Beijing should have been reduced for the reduction in the emissions of precursors during the Olympic period; however, the nighttime O 3 levels were increased for decline in the NO-O 3 titration, and the midday O 3 peak levels were elevated for the shift in the photochemical regime and the inhibition of NO z formation.

Description: ARCTAS-A ground-based observational campaign and meteorological context, interior Alaska, April 2008 Atmospheric Chemistry and Physics Discussions, 11, 16499-16552, 2011 Author(s): D. E. Atkinson, K. Sassen, M. Hayashi, C. F. Cahill, G. Shaw, D. Harrigan, and H. Fuelberg Arctic aerosol loading in interior Alaska displays a strong seasonality, with pristine conditions generally prevailing during winter months and increasing frequency of midlatitude air intrusions occurring in spring. By summer, local aerosol sources, like boreal forest fire smoke, may come into prominence. Long term aerosol research from the University of Alaska Fairbanks indicates that the period around April typically marks the beginning of the retreat of the Polar Front, opening the free exchange of midlatitude air. In April 2008 the NASA ARCTAS field campaign was conducted, supported in Fairbanks by comprehensive polarization (0.693 μm) lidar, surface and balloon-borne aerosol measurements, and synoptic weather analyzes. The data provided information on the vertical distribution and type of aerosol, the size distributions and chemical nature of the surface aerosol, as well as the large scale view of aerosol transport conditions to Alaska. We found evidence to suggest four major aerosol loading events in the 25 March–30 April 2008 timeframe: a typical Arctic haze event, several days of extremely clear conditions, rapid onset of a period dominated by Asian dust with some smoke, and a period dominated by Asian smoke. A focused case study analysis conducted on 19 April 2008 using a balloon-borne optical particle counter suggested that, on this day, the majority of the suspended particulate matter consisted of Asian dust although a contribution from Asian smoke cannot be ruled out on the basis of backtrack analysis. In the last week of April concentrations gradually decreased as synoptic conditions shifted away from favoring transport to Alaska.

Description: Influence of initial heterogeneities and recharge limitations on the evolution of aperture distributions in carbonate aquifers Hydrology and Earth System Sciences Discussions, 8, 5631-5666, 2011 Author(s): B. Hubinger and S. Birk Karst aquifers evolve where the dissolution of soluble rocks causes the enlargement of discrete pathways along fractures or bedding planes, thus creating highly conductive solution conduits. To identify general interrelations between hydrogeological conditions and the properties of the evolving conduit systems the aperture-size frequency distributions resulting from generic models of conduit evolution are analysed. For this purpose, a process-based numerical model coupling flow and rock dissolution is employed. Initial protoconduits are represented by tubes with log-normally distributed aperture sizes with a mean of 0.5 mm. Apertures are spatially uncorrelated and widen up to the metre range due to dissolution by chemically aggressive waters. Several examples of conduit development are examined focussing on influences of the initial heterogeneity and the available amount of recharge. If the available recharge is sufficiently high the evolving conduits compete for flow and those with large apertures and high hydraulic gradients attract more and more water. As a consequence, the positive feedback between increasing flow and dissolution causes the breakthrough of a conduit pathway connecting the recharge and discharge sides of the modelling domain. Under these competitive flow conditions dynamically stable bimodal aperture distributions are found to evolve, i.e. a certain percentage of tubes continues to be enlarged while the remaining tubes stay small-sized. The percentage of strongly widened tubes is found to be independent of the breakthrough time and decreases with increasing heterogeneity of the initial apertures and decreasing amount of available water. If the competition for flow is suppressed because the availability of water is strongly limited breakthrough of a conduit pathway is inhibited and the conduit pathways widen very slowly. The resulting aperture distributions are found to be unimodal covering some orders of magnitudes in size. Under these suppressed flow conditions the entire range of apertures continues to be enlarged. Hence, the number of tubes reaching aperture sizes in the order of centimetres or decimetres continues to increase with time and in the long term may exceed the number of large-sized tubes evolving under competitive flow conditions. This suggests that conduit development under suppressed flow conditions may significantly enhance the permeability of the formation e.g. in deep-seated carbonate settings.

Description: Skewness as measure of the invariance of instantaneous renormalized drop diameter distributions – Part 1: Convective vs. stratiform precipitation Hydrology and Earth System Sciences Discussions, 8, 5605-5629, 2011 Author(s): M. Ignaccolo and C. De Michele We investigate the variability of the instantaneous distribution shape of the renormalized drop diameter making use of the third order central moment: the skewness . Disdrometer data, collected at Darwin Australia, are considered either as whole or as divided in convective and stratiform precipitation intervals. We show that in all cases the distribution of the skewness is strongly peaked around 0.64. This allows to identify a most common distribution of renormalized drop diameters and two main variations, one with larger and one with smaller skewness. The distributions' shapes are independent from the stratiform vs. convective classification.

Description: The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties Hydrology and Earth System Sciences Discussions, 8, 5667-5699, 2011 Author(s): H. Norouzi, M. Temimi, W. B. Rossow, C. Pearl, M. Azarderakhsh, and R. Khanbilvardi Microwave observations at low frequencies exhibit more sensitivity to surface and subsurface properties with little interference from the atmosphere. The objective of this study is to develop a global land emissivity product using passive microwave observations from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and to investigate its sensitivity to land surface properties. The developed product complements existing land emissivity products from SSM/I and AMSU by adding land emissivity estimates at two lower frequencies, 6.9 and 10.65 GHz (C- and X-band, respectively). Observations at these low frequencies penetrate deeper into the soil layer. Ancillary data used in the analysis, such as surface skin temperature and cloud mask, are obtained from International Satellite Cloud Climatology Project (ISCCP). Atmospheric properties are obtained from the TIROS Operational Vertical Sounder (TOVS) observations to determine the small upwelling and downwelling atmospheric emissions as well as the atmospheric transmission. A sensitivity test confirms the small effect of the atmosphere but shows that skin temperature accuracy can significantly affect emissivity estimates. Retrieved emissivities at C- and X-bands and their polarization differences exhibit similar patterns of variation with changes in land cover type, soil moisture, and vegetation density as seen at SSM/I-like frequencies (Ka and Ku bands). The emissivity maps from AMSR-E at these higher frequencies agree reasonably well with the existing SSM/I-based product. The inherent but small discrepancy introduced by the difference between SSM/I and AMSR-E frequencies and incidence angles has been examined and found to be small. Large differences between emissivity estimates from ascending and descending overpasses were found at the lower frequencies due to the inconsistency between the thermal IR skin temperatures and passive microwave brightness temperatures which can come from below the surface. This issue must be addressed in future studies to improve the accuracy of the emissivity estimates at lower frequencies.

Description: Influence of soil parameters on the skewness coefficient of the annual maximum flood peaks Hydrology and Earth System Sciences Discussions, 8, 5559-5604, 2011 Author(s): A. Gioia, V. Iacobellis, S. Manfreda, and M. Fiorentino Understanding the spatial variability of key parameters of flood probability distributions represents a strategy to provide insights on hydrologic similarity and building probabilistic models able to reduce the uncertainty in flood prediction in ungauged basins. In this work, we exploited the theoretically derived distribution of floods TCIF (Gioia et al., 2008), based on two different threshold mechanisms associated respectively to ordinary and extraordinary events. The model is based on the hypotheses that ordinary floods are generally due to rainfall events exceeding a threshold infiltration rate in a small source area, while the so-called outlier events, responsible of the high skewness of flood distributions, are triggered when severe rainfalls exceed a storage threshold over a large portion of the basin. Within this scheme, a sensitivity analysis was performed in order to analyze the effects of climatic and geomorphologic parameters on the skewness coefficient. In particular, the analysis was conducted investigating the influence on flood distribution of physical factors such as rainfall intensity, soil infiltration capacity, and basin area, in order to provide insights in catchment classification and process conceptualization.

Description: Technical note: Towards a continuous classification of climate using bivariate colour mapping Hydrology and Earth System Sciences Discussions, 8, 5733-5742, 2011 Author(s): A. J. Teuling Climate is often defined in terms of discrete classes. Here I use bivariate colour mapping to show that the global distribution of Köppen-Geiger climate classes can largely be reproduced by combining the simple means of two key states of the climate system (i.e., air temperature and relative humidity). This allows for a classification that is not only continuous in space, but can be applied at and transferred between timescales ranging from minutes to decades.

Description: Theory of isotope fractionation on facetted ice crystals Atmospheric Chemistry and Physics Discussions, 11, 17423-17445, 2011 Author(s): J. Nelson Present models of the differential incorporation of isotopic water molecules into vapor-grown ice omit surface processes that may be important in temperature reconstructions. This article introduces a model that includes such surface processes and shows that differences in deposition coefficients for water isotopes can produce isotope fractionation coefficients that significantly differ from those of existing theory. For example, if the deposition coefficient of H 2 18 O differs by just 5 % from that of ordinary water (H 2 16 O), the resulting fractionation coefficient at 20 % supersaturation may deviate from the kinetic fractionation (KF) prediction by up to about ±17 ‰. Like the KF model, this "surface-kinetic" fractionation model generally predicts greater deviation from the equilibrium prediction at higher supersaturations; indeed, the sensitivity to supersaturation far exceeds that to temperature. Moreover, the model introduces possible new temperature dependencies from the deposition coefficients. These parameters need to be constrained by new laboratory measurements; nevertheless, the theory suggests that observed δ 18 O changes in ice samples are unlikely to be due solely to temperature changes.

Description: The importance of parameter resampling for soil moisture data assimilation into hydrologic models using the particle filter Hydrology and Earth System Sciences Discussions, 8, 5849-5890, 2011 Author(s): D. A. Plaza, R. De Keyser, G. J. M. De Lannoy, L. Giustarini, P. Matgen, and V. R. N. Pauwels The Ensemble Kalman filter (EnKF) and the Sequential Importance Resampling (SIR) particle filter are evaluated for their performance in soil moisture assimilation and the consequent effect on discharge. With respect to the resulting soil moisture time series, both filters perform similarly. However, both filters have a negative effect on the discharge due to inconsistency between the parameter values and the states after the assimilation. In order to overcome this inconsistency, parameter resampling is applied along with the SIR filter, to obtain consistent parameter values with the analyzed soil moisture state. Extreme parameter replication, which could lead to a particle collapse, is avoided by the perturbation of the parameters with white noise. Both the modelled soil moisture and discharge are improved if the complementary parameter resampling is applied. The SIR filter with parameter resampling offers an efficient way to deal with biased observations. The robustness of the methodology is evaluated for 3 model parameter sets and 3 assimilation frequencies. Overall, the results in this paper indicate that the particle filter is a promising tool for hydrologic modelling purposes, but that an additional parameter resampling may be necessary to consistently update all state variables and fluxes within the model.

Description: Spatial moments of catchment rainfall: rainfall spatial organisation, basin morphology, and flood response Hydrology and Earth System Sciences Discussions, 8, 5811-5847, 2011 Author(s): D. Zoccatelli, M. Borga, A. Viglione, G. B. Chirico, and G. Blöschl This paper provides a general analytical framework for assessing the dependence existing between spatial rainfall organisation, basin morphology and runoff response. The analytical framework builds upon a set of spatial rainfall statistics (termed " spatial moments of catchment rainfall ") which describe the spatial rainfall organisation in terms of concentration and dispersion statistics as a function of the distance measured along the flow routing coordinate. The introduction of these statistics permits derivation of a simple relationship for the quantification of storm velocity at the catchment scale. The paper illustrates the development of the analytical framework and explains the conceptual meaning of the statistics by means of application to five extreme flash floods occurred in various European regions in the period 2002–2007. High resolution radar rainfall fields and a distributed hydrologic model are employed to examine how effective are these statistics in describing the degree of spatial rainfall organisation which is important for runoff modelling. This is obtained by quantifying the effects of neglecting the spatial rainfall variability on flood modelling, with a focus on runoff timing. The size of the study catchments ranges between 36 to 982 km 2 . The analysis reported here shows that the spatial moments of catchment rainfall can be effectively employed to isolate and describe the features of rainfall spatial organization which have significant impact on runoff simulation. These statistics provide essential information on what space-time scales rainfall has to be monitored, given certain catchment and flood characteristics, and what are the effects of space-time aggregation on flood response modeling.

Description: Flight-based chemical characterization of biomass burning aerosols within two prescribed burn smoke plumes Atmospheric Chemistry and Physics Discussions, 11, 17507-17550, 2011 Author(s): K. A. Pratt, S. M. Murphy, R. Subramanian, P. J. DeMott, G. L. Kok, T. Campos, D. C. Rogers, A. J. Prenni, A. J. Heymsfield, J. H. Seinfeld, and K. A. Prather Biomass burning represents a major global source of aerosols impacting direct radiative forcing and cloud properties. Thus, the goal of a number of current studies involves developing a better understanding of how the chemical composition and mixing state of biomass burning aerosols evolve during atmospheric aging processes. During the Ice in Cloud Experiment – Layer Clouds (ICE-L) in fall of 2007, smoke plumes from two small Wyoming Bureau of Land Management prescribed burns were measured by on-line aerosol instrumentation aboard a C-130 aircraft, providing a detailed chemical characterization of the particles. After ~2–4 min of aging, submicron smoke particles, produced primarily from sagebrush combustion, consisted predominantly of organics by mass, but were comprised primarily of internal mixtures of organic carbon, elemental carbon, potassium chloride, and potassium sulfate. Significantly, 100 % of the fresh biomass burning particles contained minor mass fractions of nitrate and sulfate, suggesting that hygroscopic material is incorporated very near or at the point of emission. The mass fractions of ammonium, sulfate, and nitrate increased with aging up to ~81–88 min and resulted in acidic particles, with both nitric acid and sulfuric acid present. Decreasing black carbon mass concentrations occurred due to dilution of the plume. Increases in the fraction of oxygenated organic carbon and the presence of dicarboxylic acids, in particular, were observed with aging. Cloud condensation nuclei measurements suggested all particles 〉100 nm were active at 0.5 % water supersaturation in the smoke plumes, confirming the relatively high hygroscopicity of the freshly emitted particles. For immersion/condensation freezing, ice nuclei measurements at −32 °C suggested activation of ~0.03–0.07 % of the particles with diameters greater than 500 nm.

Description: Wind tunnel experiments on the retention of trace gases during riming: nitric acid, hydrochloric acid, and hydrogen peroxide Atmospheric Chemistry and Physics Discussions, 11, 17447-17472, 2011 Author(s): N. von Blohn, K. Diehl, S. K. Mitra, and S. Borrmann Laboratory experiments were carried out in a vertical wind tunnel to study the retention of different atmospheric trace gases during riming. In the experiments, the rimed ice particles floated in a laminar air stream carrying a cloud of supercooled droplets with radii between 10 and 20 μm. Ice particles, dendritic ice crystals, and snow flakes with diameters between 6 mm and 1.5 cm were allowed to rime at temperatures between −5 and −12 °C where riming mainly proceeds in the atmosphere and with cloud liquid water contents between 1 and 1.5 g m −3 which are values typically found in atmospheric mixed phase clouds. Three trace species were investigated, nitric and hydrochloric acid, and hydrogen peroxide. They were present in the supercooled liquid droplets in concentrations from 1 to 120 ppmv, i.e. similar to the ones measured in cloud drops. The chemical analyses of the rimed ice particles allow to determine the trace species concentration in the ice phase. Together with the known liquid phase concentration the retention coefficients were calculated in terms of the amount of the species which remained in the ice phase after freezing. It was found that the highly soluble trace gases nitric and hydrochloric acid were retained nearly completely (98.6 ± 8 % and 99.7 ± 9 %, respectively) while for hydrogen peroxide a retention coefficient of 64.3 ± 11 % was determined. No influence of the riming temperature on the retention was found which can be explained by the fact that in the observed range of temperature and liquid water content riming proceeded in the dry growth regime.

Description: Evaluation of the transferability of hydrological model parameters for simulations under changed climatic conditions Hydrology and Earth System Sciences Discussions, 8, 5891-5915, 2011 Author(s): S. Bastola, C. Murphy, and J. Sweeney Conceptual hydrological models are widely used for climate change impact assessment. The implicit assumption in most such work is that the parameters estimated from observations remain valid for future climatic conditions. This paper evaluates a simple threshold based approach for testing this assumption, where a set of behavioural simulators are identified for different climatic conditions for the future simulation i.e. wet, average and dry conditions. These simulators were derived using three different data sets that are generated by sampling a block of one year of data without replacement from the observations such that they define the different climatic conditions. The simulators estimated from the wet climatic data set showed the tendency to underestimate flow when applied to dry data set and vice versa. However, the performances of the three sets of basin simulators on chronologically coherent data are identical to the simulators identified from a sufficiently long data series that contains both wet and dry climatic conditions. The results presented suggest that the issue of time invariance in the value of parameters has a minimal effect on the simulation if the change in precipitation is less than 10 % of the data used for calibration.

Description: Mass transfer effects in 2-D dual-permeability modeling of field preferential bromide leaching with drain effluent Hydrology and Earth System Sciences Discussions, 8, 5917-5967, 2011 Author(s): H. H. Gerke, J. Dusek, and T. Vogel Subsurface drained experimental fields are frequently used for studying preferential flow (PF) in structured soils. Considering two-dimensional (2-D) transport towards the drain, however, the relevance of mass transfer coefficients, apparently reflecting small-scale soil structural properties, for the water and solute balances of the entire drained field is largely unknown. This paper reviews and analyzes effects of mass transfer reductions on Br − leaching for a subsurface drained experimental field using a numerical 2-D dual-permeability model (2D-DPERM). The sensitivity of the "diffusive" mass transfer component on bromide (Br − ) leaching patterns is discussed. Flow and transport is simulated in a 2-D vertical cross-section using parameters, boundary conditions (BC), and data of a Br − tracer irrigation experiment on a subsurface drained field (5000 m 2 area) at Bokhorst (Germany), where soils have developed from glacial till sediments. The 2D-DPERM simulation scenarios assume realistic irrigation and rainfall rates, and Br-application in the soil matrix (SM) domain. The mass transfer reduction controls preferential tracer movement and can be related to physical and chemical properties at the interface between flow path and soil matrix in structured soil. A reduced solute mass transfer rate coefficient allows a better match of the Br − mass flow observed in the tile drain discharge. The results suggest that coefficients of water and solute transfer between PF and SM domains have a clear impact on Br − effluent from the drain. Amount and composition of the drain effluent is analyzed as a highly complex interrelation between temporally and spatially variable mass transfer in the 2-D vertical flow domain that depends on varying "advective" and "diffusive" transfer components, the spatial distribution of residual tracer concentrations, and the lateral flow fields in both domains from plots of the whole subsurface drained field. The local-scale soil structural effects (e.g., such as macropore wall coatings), here conceptualized as changes in mass transfer coefficients, can have a clear effect on leaching at the plot and field-scales.

Description: Evaluation and bias correction of satellite rainfall data for drought monitoring in Indonesia Hydrology and Earth System Sciences Discussions, 8, 5969-5997, 2011 Author(s): R. R. E. Vernimmen, A. Hooijer, Mamenun, and E. Aldrian The accuracy of satellite rainfall data from different sources, TRMM 3B42RT, CMORPH and PERSIANN, was investigated through comparison with reliable ground station rainfall data in Indonesia, with a focus on their ability to detect patterns of low rainfall that may lead to drought conditions. It was found that all sources underestimated rainfall in dry season months. The CMORPH and PERSIANN data differed most from ground station data and are also very different from the TRMM data. However, it proved possible to improve TRMM data to yield sufficiently accurate estimates, both for dry periods ( R 2 0.65–0.92) and annually ( R 2 0.84–0.96), applying a single parameterized bias correction equation that is constant in space and time. It is proposed that these bias corrected TRMM data be used in real-time drought monitoring, in Indonesia and probably in other countries where similar conditions exist. This will yield major advantages, in terms of accuracy, spatial coverage, timely availability and cost efficiency, over drought monitoring with only ground stations.

Description: Predicting the relative humidities of liquid-liquid phase separation, efflorescence, and deliquescence of mixed particles of ammonium sulfate, organic material, and water using the organic-to-sulfate mass ratio of the particle and the oxygen-to-carbon elemental ratio of the organic component Atmospheric Chemistry and Physics Discussions, 11, 17759-17788, 2011 Author(s): A. K. Bertram, S. T. Martin, S. J. Hanna, M. L. Smith, A. Bodsworth, Q. Chen, M. Kuwata, A. Liu, Y. You, and S. R. Zorn Individual particles that on a mass basis consist dominantly of the components ammonium sulfate, organic material, and water are a common class of submicron particles found in today's atmosphere. Here we use (1) the organic-to-sulfate (org:sulf) mass ratio of the overall particle and (2) the oxygen-to-carbon (O:C) elemental ratio of the organic component as input variables in parameterisations that predict the critical relative humidity of several different types of particle phase transitions. These transitions include liquid-liquid phase separation (SRH), efflorescence (ERH), and deliquescence (DRH). Experiments were conducted by optical microscopy for 11 different oxygenated organic-ammonium sulfate systems covering the range 0.1 〈 org:sulf 〈12.8 and 0.29 〈 O:C 〈 1.33. These new data, in conjunction with other data already available in the literature, were used to develop the parameterisations SRH(org:sulf, O:C), ERH(org:sulf, O:C), and DRH(org:sulf, O:C). The parameterisations correctly predicted SRH within 15 % RH for 86 % of the measurements, ERH within 5 % for 86 % of the measurements, and DRH within 5 % for 95 % of the measurements. The applicability of the derived parameterisations beyond the training data set was tested against observations for organic-sulfate particles produced in an environmental chamber. The organic component consisted of secondary organic material produced by the oxidation of isoprene, α-pinene, and β-caryophyllene. The predictions of the parameterisations were also tested against data from the Southern Great Plains, Oklahoma, USA. The observed ERH and DRH values for both the chamber and field data agreed within 5 % RH with the value predicted by the parameterisations using the measured org:sulf and O:C ratios as the input variables.

Description: Accounting for non-linear chemistry of ship plumes in the GEOS-Chem global chemistry transport model Atmospheric Chemistry and Physics Discussions, 11, 17789-17823, 2011 Author(s): G. C. M. Vinken, K. F. Boersma, D. J. Jacob, and E. W. Meijer We present a computationally efficient approach to account for the non-linear chemistry occurring during the dispersion of ship exhaust plumes in a global 3-D model of atmospheric chemistry (GEOS-Chem). We use a plume-in-grid formulation where ship emissions age chemically for 5 h before being released in the global model grid. Besides reducing the original ship NO x emissions in GEOS-Chem, our approach also releases the secondary compounds ozone and HNO 3 , produced in the 5 h after the original emissions, into the model. We applied our improved method and also the widely used "instant dilution" approach to a 1-yr GEOS-Chem simulation of global tropospheric ozone-NO x -VOC-aerosol chemistry. We also ran simulations with the standard model, and a model without any ship emissions at all. Our improved GEOS-Chem model simulates up to 0.1 ppbv (or 90 %) more NO x over the North Atlantic in July than GEOS-Chem versions without any ship NO x emissions at all. "Instant dilution" overestimates NO x concentrations by 50 % (0.1 ppbv) and ozone by 10–25 % (3–5 ppbv) over this region. These conclusions are supported by comparing simulated and observed NO x and ozone concentrations in the lower troposphere over the Pacific Ocean. The comparisons show that the improved GEOS-Chem model simulates NO x concentrations in between the instant diluting model and the model with no ship emissions, and results in lower O 3 concentrations than the instant diluting model. The relative differences in simulated NO x and ozone between our improved approach and instant dilution are smallest over strongly polluted seas (e.g. North Sea), suggesting that accounting for in-plume chemistry is most relevant for pristine marine areas.

Description: Potential and limitations of the MAX-DOAS method to retrieve the vertical distribution of tropospheric nitrogen dioxide Atmospheric Measurement Techniques Discussions, 4, 4013-4072, 2011 Author(s): T. Vlemmix, A. J. M. Piters, A. J. C. Berkhout, L. F. L. Gast, P. Wang, and P. F. Levelt Muliple Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments can measure from the ground the absorption by nitrogen dioxide (NO 2 ) of scattered sunlight seen in multiple viewing directions. This paper studies the potential of this technique to derive the vertical distribution of NO 2 in the troposphere. Such profile information is essential in validation studies in which MAX-DOAS retrievals play a role. The retrieval algorithm used is based on a pre-calculated look-up table and assumes homogeneous mixing of aerosols and NO 2 in layers extending from the surface to a variable height. Two retrieval models are compared: one including and one excluding an elevated NO 2 layer at a fixed altitude in the free troposphere. An ensemble technique is applied to derive retrieved model uncertainties. Sensitivity studies demonstrate that MAX-DOAS based retrievals can make a distinction between an NO 2 layer that extends from the surface to a certain height (having a constant mixing ratio, or a mixing ratio that decreases with altitude) and an elevated NO 2 layer. The height of the elevated NO 2 layer can only be retrieved accurately when the aerosol extinction profile is known and the measurement noise is low. The uncertainty in this elevated NO 2 layer height provides the main source of uncertainty in the retrieval of the free tropospheric contribution to the tropospheric NO 2 column. A comparison was performed with independent data, based on observations done at the CINDI campaign, held in the Netherlands in 2009. Comparison with lidar partial tropospheric NO 2 columns showed a correlation of 0.78, and an average difference of 0.1× 10 15 molec cm −2 . The diurnal evolution of the NO 2 volume mixing ratio measured by in-situ monitors at the surface and averaged over five days with cloud-free mornings, compares quite well to the MAX-DOAS retrieval: a correlation was found of 0.8, and an average difference of 0.2 ppb.

Description: A Lagrangian view of convective sources for transport of air across the Tropical Tropopause Layer: distribution, times and the radiative influence of clouds Atmospheric Chemistry and Physics Discussions, 11, 18161-18209, 2011 Author(s): A. Tzella and B. Legras The Tropical Tropopause Layer (TTL) is a key region controlling transport between the troposphere and the stratosphere. The efficiency of transport across the TTL depends on the continuous interaction between the large-scale advection and the small-scale intermittent convection that reaches the Level of Zero radiative Heating (LZH). The wide range of scales involved presents a significant challenge to determine the sources of convection and quantify transport across the TTL. Here, we use a simple Lagrangian model, termed TTL detrainment model , that combines a large ensemble of 200-day back trajectory calculations with high-resolution fields of brightness temperatures (provided by the CLAUS dataset) in order to determine the ensemble of trajectories that are detrained from convective sources. The trajectories are calculated using the ECMWF ERA-Interim winds and radiative heating rates, derived both under all-sky and clear-sky conditions, so that the radiative influence of clouds is established. We show that most trajectories are detrained near the mean LZH with the horizontal distributions of convective sources being highly-localized, even within the space defined by deep convection. As well as modifying the degree of source localization, the radiative heating from clouds facilitates the rapid upwelling of air across the TTL. However, large-scale motion near the fluctuating LZH can lead a significant proportion of trajectories to alternating clear-sky and cloudy regions, thus generating a large dispersion in the vertical transport times. The distributions of vertical transport times are wide and skewed and are largely insensitive to a bias of about ±1 km (∓5 K) in the altitude of cloud top heights (the main sensitivity appearing in the times to escape the immediate neighbourhood of the LZH) while seasonal and regional transport characteristics are only apparent at small time-scales. The strong horizontal mixing that characterizes the TTL ensures that most air of convective origin is well-mixed within the tropical and eventually within the extra-tropical lower-stratosphere.

Description: Water isotopic ratios from a continuously melted ice core sample Atmospheric Measurement Techniques Discussions, 4, 4073-4104, 2011 Author(s): V. Gkinis, T. J. Popp, T. Blunier, M. Bigler, S. Schüpbach, and S. J. Johnsen A new technique for on-line high resolution isotopic analysis of liquid water, tailored for ice core studies is presented. We build an interface between an Infra Red Cavity Ring Down Spectrometer (IR-CRDS) and a Continuous Flow Analysis (CFA) system. The system offers the possibility to perform simultaneuous water isotopic analysis of δ 18 O and δD on a continuous stream of liquid water as generated from a continuously melted ice rod. Injection of sub μl amounts of liquid water is achieved by pumping sample through a fused silica capillary and instantaneously vaporizing it with 100 % efficiency in a home made oven at a temperature of 170 °C. A calibration procedure allows for proper reporting of the data on the VSMOW scale. We apply the necessary corrections based on the assessed performance of the system regarding instrumental drifts and dependance on humidity levels. The melt rates are monitored in order to assign a depth scale to the measured isotopic profiles. Application of spectral methods yields the combined uncertainty of the system at below 0.1 ‰ and 0.5 ‰ for δ 18 O and δD, respectively. This performance is comparable to that achieved with mass spectrometry. Dispersion of the sample in the transfer lines limits the resolution of the technique. In this work we investigate and assess these dispersion effects. By using an optimal filtering method we show how the measured profiles can be corrected for the smoothing effects resulting from the sample dispersion. Considering the significant advantages the technique offers, i.e. simultaneuous measurement of δ 18 O and δD, potentially in combination with chemical components that are traditionally measured on CFA systems, notable reduction on analysis time and power consumption, we consider it as an alternative to traditional isotope ratio mass spectrometry with the possibility to be deployed for field ice core studies. We present data acquired in the framework of the NEEM deep ice core drilling project in Greenland, during the 2010 field season.

Description: Probability density functions of long-lived tracer observations from satellite in the subtropical barrier region: data intercomparison Atmospheric Chemistry and Physics Discussions, 11, 18385-18432, 2011 Author(s): E. Palazzi, F. Fierli, G. P. Stiller, and J. Urban Past studies have shown that a clear relationship exists between the field of a passive tracer and the Probability Distribution Function (PDF) of tracer concentrations, which can be exploited to identify the position and variability of stratospheric barriers to isentropic mixing. In the present study, we focus on the dynamical barrier located in the subtropics. We calculate PDFs of the long-lived tracers nitrous oxide (N 2 O) and methane (CH 4 ) from different satellite instruments: the Microwave Limb Sounder (MLS) on board Aura, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board Envisat, the Sub-Millimetre Radiometre (SMR) on board Odin and the Halogen Occultation Experiment (HALOE) on board UARS, overall covering the time period of 1992–2009. An analysis of the consistency among the different sets of data and their capability of identifying mixing regions and barrier to transport regions in the stratosphere and the subtropical barrier location is a prime aim of the present study. This is done looking at the morphological structure of the one- and two-dimensional PDFs of tracer concentrations measured by the different instruments. The latter differ in their spatial and temporal sampling and resolution, and there are some systematic differences in the determination of the subtropical barrier position that have been highlighted. However, the four satellite instruments offer an overall consistent picture of the subtropical barrier annual cycle. There is a strong seasonality consistently represented, characterized by the wintertime shift of the subtropical edge toward the summer hemisphere. However, the influence of the Quasi Biennial Oscillation (QBO) on isentropic transport and mixing, and by consequence, on the position of the subtropical barrier, is not equally represented in all satellite data using the methodology proposed.

Description: Column-integrated aerosol microphysical properties from AERONET Sun photometer over Southwestern Spain Atmospheric Chemistry and Physics Discussions, 11, 18349-18384, 2011 Author(s): N. Prats, V. E. Cachorro, A. Berjón, C. Toledano, and A. M. De Frutos The aim of the present work is to carry out a detailed analysis of columnar microphysical properties obtained from Cimel sun-photometer measurements in the Southwest of Spain within the frame of the AERONET-RIMA network. AERONET inversion products are analysed, in particular the particle size distribution together with their associated microphysical parameters for both fine and coarse modes: concentration, effective radius and the fine mode volume fraction. This work complements previous works based on aerosol optical depth (AOD) and the Ångström exponent (AE) for a global characterization of atmospheric aerosol in this representative area of Spain and Europe. The analysed dataset spans between February 2000 and October 2008. Time series and statistical analysis has been carried out for these parameters in order to assess their typical values and seasonality together with their relationships with the AOD and AE. Mean values of volume particle concentration are 0.06 ± 0.07 μm 3 μm −2 for total, 0.019 ± 0.015 μm 3 μm −2 for fine and 0.04 ± 0.06 μm 3 μm −2 for coarse mode; and of effective radius are 0.040 ± 0.19 μm for total, 0.14 ± 0.02 μm for fine and 1.96 ± 0.41 μm for coarse mode. The most relevant features are the clear bimodality of the volume particle size distribution, with a slight dominance of the coarse mode for the total climatology and under the prevailing atmospheric conditions of the site (coastal marine). There is a clear prevalence of the coarse mode in summer months, September and March in coincidence with the occurrence of desert dust intrusions and highest AOD values. During aerosol desert dust arrivals, the particle size distribution is practically mono-modal with strong prevalence of the coarse mode which also shows a shift of the modal radius to lower values. The size particle predominance defines the characteristic of the site and it has been analysed under two different approaches: with respect to particle number, using the Ångström exponent and with respect to particle volume, where the fine mode volume fraction V f / V t is taken.

Description: Effects of antecedent soil moisture on runoff modeling in small semiarid watersheds of southeastern Arizona Hydrology and Earth System Sciences Discussions, 8, 6227-6256, 2011 Author(s): Y. Zhang, H. Wei, and M. A. Nearing Antecedent soil moisture prior to a rain event influences the rainfall-runoff relationship. Very few studies have looked at the effects of antecedent soil moisture on runoff modeling sensitivities in arid and semi-arid areas. This study examines the influence of initial soil moisture on model runoff prediction capability in small semiarid watersheds using model sensitivity and by comparing the use of antecedent vs. average long term soil water content for defining the model initial conditions for the modified Green-Ampt Mein-Larson model within the Rangeland Hydrology and Erosion Model (RHEM). Measured rainfall, runoff, and soil moisture data from four semiarid rangeland watersheds ranging in size from 0.34 to 4.53 ha on the Walnut Gulch Experimental Watershed in southeastern Arizona, USA, were used. Results showed that: (a) there were no significant correlations between measured runoff ratio and antecedent soil moisture in any of the four watersheds; (b) average sensitivities of simulated runoff amounts and peaks to antecedent soil moisture were 0.05 mm and 0.18 mm h −1 , respectively, with each 1 % change in antecedent soil moisture; (c) runoff amounts and peaks simulated with long term average soil moisture were statistically equivalent to those simulated with measured antecedent soil moisture. The relative lack of sensitivity of modeled runoff to antecedent soil moisture in this case is contrary to results reported in other studies, and is largely due to the fact that the surface soil is nearly always very dry in this environment.

Description: Uncertainty of the stratospheric/tropospheric temperature trends in 1979–2008: Multiple Satellite MSU, radiosonde, and reanalysis datasets Atmospheric Chemistry and Physics Discussions, 11, 16639-16654, 2011 Author(s): J. Xu and A. M. Powell Jr. The trends and spreads of tropospheric and stratospheric temperature are discussed in terms of three groups of datasets in 1979–2008. These datasets include (a) three satellite observations of Microwave Sounding Units (MSU) measurements, (b) five radiosonde observations and (c) five reanalysis products. The equivalent tropospheric and stratospheric temperature from radiosonde and reanalyses are calculated based on the vertical weighting function of the MSU channel 2 (CH 2 ) and channel 4 (CH 4 ) measurements, respectively. The results show that both cooling in the stratosphere and warming in troposphere significantly depends on the datasets and latitudes.

Description: Contrasting organic aerosol particles from boreal and tropical forests during HUMPPA-COPEC-2010 and AMAZE-08 using coherent vibrational spectroscopy Atmospheric Chemistry and Physics Discussions, 11, 16933-16966, 2011 Author(s): C. J. Ebben, I. S. Martinez, M. Shrestha, A. M. Buchbinder, A. L. Corrigan, A. Guenther, T. Karl, T. Petäjä, W. W. Song, S. R. Zorn, P. Artaxo, M. Kulmala, S. T. Martin, L. M. Russell, J. Williams, and F. M. Geiger We present the vibrational sum frequency generation spectra of organic particles collected in a boreal forest in Finland and a tropical forest in Brazil. These spectra are compared to those of secondary organic material produced in the Harvard Environmental Chamber. By comparing coherent vibrational spectra of a variety of terpene and olefin reference compounds, along with the secondary organic material synthesized in the environmental chamber, we show that submicron aerosol particles sampled in Southern Finland during HUMPPA-COPEC-2010 are composed to a large degree of material similar in chemical composition to synthetic α-pinene-derived material. For material collected in Brazil as part of AMAZE-08, the organic component is found to be chemically complex in the coarse mode but highly uniform in the fine mode. When combined with histogram analyses of the isoprene and monoterpene abundance recorded during the HUMPPA-COPEC-2010 and AMAZE-08 campaigns, the findings presented here indicate that if air is rich in monoterpenes, submicron-sized secondary aerosol particles that form under normal OH and O 3 concentration levels can be described in terms of their hydrocarbon content as being similar to α-pinene-derived model secondary organic aerosol particles. If the isoprene concentration dominates the chemical composition of organic compounds in forest air, then the hydrocarbon component of secondary organic material in the submicron size range is not simply well-represented by that of isoprene-derived model secondary organic aerosol particles but is more complex. Throughout the climate-relevant size range of the fine mode, however, we find that the chemical composition of the secondary organic particle material from such air is invariant with size, suggesting that the particle growth does not change the chemical composition of the hydrocarbon component of the particles in a significant way.

Description: Organic carbon and non-refractory aerosol over the remote Southeast Pacific: oceanic and combustion sources Atmospheric Chemistry and Physics Discussions, 11, 16895-16932, 2011 Author(s): L. M. Shank, S. Howell, A. D. Clarke, S. Freitag, V. Brekhovskikh, V. Kapustin, C. McNaughton, T. Campos, and R. Wood Submicron aerosol physical and chemical properties in remote marine air were measured from aircraft over the Southeast Pacific during VOCALS-REx in 2008 and the North Pacific during IMPEX in 2006, and aboard a ship in the Equatorial Pacific in 2009. A High Resolution – Particle Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS) measured non-refractory submicron aerosol composition during all campaigns. Sulfate (SO 4 ) and organics (Org), during VOCALS and the cruise show lower absolute values than those reported for previous "clean air" studies. In the marine boundary layer, average concentrations for SO 4 were 0.52 μg m −3 for the VOCALS region and 0.85 μg m −3 for the equatorial region while average Org concentrations were 0.10 and 0.07 μg m −3 , respectively. Campaign average Org/SO 4 ratios were 0.19 (VOCALS) and 0.08 (Equatorial Pacific), while previous studies report "clean marine" Org/SO 4 ratios between 0.25 and 0.40, and in some cases as high as 3.5. CO and black carbon (BC) measurements over the Southeast Pacific provided sensitive indicators of pollution, and were used to identify the least polluted air, which had average concentrations of SO 4 and Org of 0.14 and 0.01 μg m −3 , respectively, with an average Org/SO 4 of 0.10. Furthermore, under cleanest MBL conditions, identified by CO below 60 ppbv, we found a robust linear relationship between Org and combustion derived BC concentrations between 2 and 15 ng m −3 , suggesting little to no marine source of submicrometer Org to the atmosphere over the Eastern South Pacific. This suggests that identification of Org in clean marine air may require a BC threshold below 4 ng m −3 , an order of magnitude lower than has been used in prior studies. Data from IMPEX was constrained to similar clean air criterion, and resulted in an average Org/SO 4 ratio of 0.19.

Description: A 3-D tomographic trajectory retrieval for the air-borne limb-imager GLORIA Atmospheric Measurement Techniques Discussions, 4, 3805-3859, 2011 Author(s): J. Ungermann, J. Blank, J. Lotz, K. Leppkes, T. Guggenmoser, M. Kaufmann, P. Preusse, U. Naumann, and M. Riese Infrared limb sounding from aircraft can provide 2-D curtains of multiple trace gas species. However, conventional limb sounders view perpendicular to the aircraft axis and are unable to resolve the observed airmass along their line-of-sight. GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) is a new remote sensing instrument able to adjust its horizontal view angle with respect to the aircraft flight direction from 45° to 135°. This will allow for tomographic measurements of mesoscale structures for a wide variety of atmospheric constituents. Many flights of the GLORIA instrument will not follow closed curves that allow measuring an airmass from all directions. Consequently, it is examined by means of simulations, what results can be expected from tomographic evaluation of measurements made during a straight flight. It is demonstrated that the achievable resolution and stability is enhanced compared to conventional retrievals. In a second step, it is shown that the incorporation of channels exhibiting different optical depth can greatly enhance the 3-D retrieval quality enabling the exploitation of previously unused spectral samples. A second problem for tomographic retrievals is that advection, which can be neglected for conventional retrievals, plays an important role for the time-scales involved in a tomographic measurement flight. This paper presents a method to diagnose the effect of a time-varying atmosphere on a 3-D retrieval and demonstrates an effective way to compensate for effects of advection by incorporating wind-fields from meteorological datasets as a priori information.

Description: Future impact of non-land based traffic emissions on atmospheric ozone and OH – an optimistic scenario and a possible mitigation strategy Atmospheric Chemistry and Physics Discussions, 11, 16801-16859, 2011 Author(s): Ø. Hodnebrog, T. K. Berntsen, O. Dessens, M. Gauss, V. Grewe, I. S. A. Isaksen, B. Koffi, G. Myhre, D. Olivié, M. J. Prather, J. A. Pyle, F. Stordal, S. Szopa, Q. Tang, P. van Velthoven, J. E. Williams, and K. Ødemark The impact of future emissions from aviation and shipping on the atmospheric chemical composition has been estimated using an ensemble of six different atmospheric chemistry models. This study considers an optimistic emission scenario (B1) taking into account e.g. rapid introduction of clean and resource-efficient technologies, and a mitigation option for the aircraft sector (B1 ACARE), assuming further technological improvements. Results from sensitivity simulations, where emissions from each of the transport sectors were reduced by 5 %, show that emissions from both aircraft and shipping will have a larger impact on atmospheric ozone and OH in near future (2025; B1) and for longer time horizons (2050; B1) compared to recent time (2000). However, the ozone and OH impact from aircraft can be reduced substantially in 2050 if the technological improvements considered in the B1 ACARE will be achieved. Shipping emissions have the largest impact in the marine boundary layer and their ozone contribution may exceed 4 ppb (scaled to 100 %) over the North Atlantic Ocean in the future (2050; B1) during northern summer (July). In the zonal mean, ship-induced ozone relative to the background levels may exceed 12 % near the surface. Corresponding numbers for OH are 6.0 × 10 5 molecules cm −3 and 30 %, respectively. This large impact on OH from shipping leads to a relative methane lifetime reduction of 3.92(±0.48) % on the global average in 2050 B1 (ensemble mean CH 4 lifetime is 8.0(±1.0) yr), compared to 3.68(±0.47) % in 2000. Aircraft emissions have about 4 times higher ozone enhancement efficiency (ozone molecules enhanced relative to NO x molecules emitted) than shipping emissions, and the maximum impact is found in the UTLS region. Zonal mean aircraft-induced ozone could reach up to 5 ppb at northern mid- and high latitudes during future summer (July 2050; B1), while the relative impact peaks during northern winter (January) with a contribution of 4.2 %. Although the aviation-induced impact on OH is lower than for shipping, it still causes a reduction in the relative methane lifetime of 1.68(±0.38) % in 2050 B1. However, for B1 ACARE the perturbation is reduced to 1.17(±0.28) %, which is lower than the year 2000 estimate of 1.30(±0.30) %. Based on the fully scaled perturbations we calculate net radiative forcings from the six models taking into account ozone, methane (including stratospheric water vapour), and methane-induced ozone changes. For the B1 scenario, shipping leads to a net cooling with radiative forcings of −28.0(±5.1) and −30.8(±4.8) mW m −2 in 2025 and 2050, respectively, due to the large impact on OH and thereby methane lifetime reductions. Corresponding values for the aviation sector shows a net warming effect with 3.8(±6.1) and 1.9(±6.3) mW m −2 , respectively, but with a small net cooling of −0.6(±4.6) mW m −2 for B1 ACARE in 2050.

Description: Formation of hydroxyl radical from San Joaquin Valley particles extracted in a cell-free solution Atmospheric Chemistry and Physics Discussions, 11, 16861-16894, 2011 Author(s): H. Shen and C. Anastasio Previous studies have suggested that the adverse health effects from ambient particulate matter (PM) are linked to the formation of reactive oxygen species (ROS) by PM. While hydroxyl radical ( • OH) is the most reactive of the ROS species, there are few quantitative studies of • OH generation from PM. Here we report on • OH formation from PM collected at an urban (Fresno) and rural (Westside) site in the San Joaquin Valley (SJV) of California. We quantified • OH in PM extracts using a cell-free, phosphate-buffered saline (PBS) solution with or without 50 μM ascorbate (Asc). The results show that generally the urban Fresno PM generates much more • OH than the rural Westside PM. The presence of Asc at a physiologically relevant concentration in the extraction solution greatly enhances • OH formation from all the samples. Fine PM (PM 2.5 ) generally makes more • OH than the corresponding coarse PM (PM cf , i.e., 2.5 to 10 μm) normalized by air volume collected, while the coarse PM typically generates more • OH normalized by PM mass. • OH production by SJV PM is reduced on average by (97 ± 6) % when the transition metal chelator desferoxamine (DSF) is added to the extraction solution, indicating a dominant role of transition metals. By measuring calibration curves of • OH generation from copper and iron, and quantifying copper and iron concentrations in our particle extracts, we find that PBS-soluble copper is primarily responsible for • OH production by the SJV PM, while iron often makes a significant contribution. Extrapolating our results to expected burdens of PM-derived • OH in human lung lining fluid suggests that typical daily PM exposures in the San Joaquin Valley are unlikely to result in a high amount of pulmonary • OH, although high PM events could produce much higher levels of • OH, which might lead to cytotoxicity.

Description: Singular vector decomposition for sensitivity analyses of tropospheric chemical scenarios Atmospheric Chemistry and Physics Discussions, 11, 16745-16799, 2011 Author(s): N. Goris and H. Elbern Observations of the chemical state of the atmosphere typically provide only sparse snapshots of the state of the system due to their insufficient temporal and spatial density. Therefore the measurement configurations need to be optimised to get a best possible state estimate. One possibility to optimise the state estimate is provided by observation targeting of sensitive system states, to identify measurement configurations of best value for forecast improvements. In recent years, numerical weather prediction adapted singular vector analysis with respect to initial values as a novel method to identify sensitive states. In the present work, this technique is transferred from meteorological to chemical forecast. Besides initial values, emissions are investigated as controlling variables. More precisely uncertainties in the amplitude of the diurnal profile of emissions are analysed, yielding emission factors as target variables. Singular vector analysis is extended to allow for projected target variables not only at final time but also at initial time. Further, special operators are introduced, which consider the combined influence of groups of chemical species. As a preparation for targeted observation calculations, the concept of adaptive observations is studied with a chemistry box model. For a set of six different scenarios, the VOC versus NO x limitation of the ozone formation is investigated. Results reveal, that the singular vectors are strongly dependent on start time and length of the simulation. As expected, singular vectors with initial values as target variables tend to be more sensitive to initial values, while emission factors as target variables are more sensitive to simulation length. Further, the particular importance of chemical compounds differs strongly between absolute and relative error growth.

Description: High-resolution air quality monitoring from space: a fast retrieval scheme for CO from hyperspectral infrared measurements Atmospheric Measurement Techniques Discussions, 4, 3787-3803, 2011 Author(s): N. Smith, H.-L. Huang, E. Weisz, H. J. Annegarn, and R. B. Pierce The first results of the Fast Linear Inversion Trace gas System (FLITS) retrieval scheme are presented here for CO from IASI (Infrared Atmospheric Sounding Interferometer) measurements using RAQMS (Real time Air Quality Modelling System) as atmospheric background. FLITS is a simple linear inversion scheme with a stable performance that retrieves total column CO concentrations (molec cm −2 ) at single field-of-view (FOV) irrespective of cloud cover. A case study is presented here for a biomass burning plume over the Pacific on 29 March 2010. For each FOV a single tropospheric CO density, vertically integrated over 200–800 hPa, is retrieved with 12 channels in the spectral range 2050–2225 cm −1 . Despite variations in cloud cover and temperature, the degrees of freedom for signal (DFS) of the solution ranges between 0.8 and 0.95. In addition, the retrieval error is at least half the background error of 10 %, with dominant contribution from uncertainty in the measurement and temperature. With its stability and processing speed, FLITS meet two of the key requirements for operational processing. We conclude that the linear combination of space-borne measurements with a chemical transport model in the FLITS retrieval scheme holds potential for real-time air quality monitoring and evaluation of pollutant transport at high spatial resolution.

Description: Chemical characteristics of inorganic ammonium salts in PM 2.5 $ in the atmosphere of Beijing (China) Atmospheric Chemistry and Physics Discussions, 11, 17127-17176, 2011 Author(s): A. Ianniello, F. Spataro, G. Esposito, I. Allegrini, M. Hu, and T. Zhu The atmospheric concentrations of gaseous HNO 3 , HCl and NH 3 and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part of CAREBEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique was used with integration times of 2 and 24 h to collect inorganic and soluble PM 2.5 without interferences from gas-particle and particle-particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl − , NH 4 + and SO 4 2− exhibited distinct temporal variations, while fine particulate NO 3 − did not show much variation with respect to season. Daily mean concentrations of fine particulate NH 4 + and SO 4 2− were higher during summer (12.30 μg m −3 and 18.24 μg m −3 , respectively) than during winter (6.51 μg m −3 and 7.50 μg m −3 , respectively). Instead, daily mean concentrations of fine particulate Cl − were higher during winter (2.94 μg m −3 ) than during summer (0.79 μg m −3 ), while fine particulate NO 3 − showed similar variations both in winter (8.38 μg m −3 ) and in summer (9.62 μg m −3 ) periods. However, the presence of large amounts of fine particulate NO 3 − even in summer are due to higher local and regional concentrations of NH 3 in the atmosphere available to neutralize H 2 SO 4 and HNO 3 , which is consistent with the observation that the measured particulate species were neutralized. Indeed, the composition of fine particulate matter indicated the domination of (NH 4 ) 2 SO 4 during winter and summer periods. In addition, the high relative humidity conditions in summer period seemed to dissolve a significant fraction of HNO 3 and NH 3 enhancing fine particulate NO 3 − and NH 4 + in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected nearly by wind direction suggesting regional and local source influences. Indeed, the fine particulate species were also correlated with NO x and PM 2.5 , supporting the hypothesis that the traffic may be also an important source of secondary particles.

Description: The effects of atmospheric waves on the amounts of polar stratospheric clouds Atmospheric Chemistry and Physics Discussions, 11, 16967-17012, 2011 Author(s): M. Kohma and K. Sato A quantitative analysis on the relationship between atmospheric waves and polar stratospheric clouds (PSCs) in the 2008 austral winter and the 2007/2008 boreal winter is made using CALIPSO, COSMIC and Aura MLS observation data and reanalysis data. A longitude-time section of the frequency of PSC occurrence in the Southern Hemisphere indicates that PSC frequency is not regionally uniform and that high PSC frequency regions propagate eastward at different speeds from the background zonal wind. These features suggest a significant influence of atmospheric waves on PSC behavior. Next, three temperature thresholds for PSC existence are calculated using HNO 3 and H 2 O mixing ratios. Among the three, the T STS (a threshold for super cooled ternary solution)-based estimates of PSC frequency accord best with the observations in terms of the amount, spatial and temporal variation, in particular for the latitude range of 55° S–70° S in the Southern Hemisphere and for 55° N–85° N in the Northern Hemisphere. Moreover, the effects of planetary waves, synoptic-scale waves and gravity waves on PSC areal extent are separately examined using the T STS -based PSC estimates. The latitude range of 55° S–70° S is analyzed because the T STS -based estimates are not consistent with observations at higher latitudes ( 〈 75° S) above 18 km, and PSCs in lower latitudes are more important to the ozone depletion because of the earlier arrival of solar radiation in spring. It is shown that nearly 100 % of PSCs between 55° S and 70° S at altitudes of 16–24 km are formed by temperature modulation, which is influenced by planetary waves during winter. Although the effects of synoptic-scale waves on PSCs are limited, around an altitude of 12 km more than 60 % of the total PSC areal extent is formed by synoptic-scale waves. The effects of gravity waves on PSC areal extent are not large in the latitude range of 55° S–70° S. However, at higher latitudes, gravity waves act to increase PSC areal extent at an altitude of 15 km by about 30 % in September. Similar analyses are performed for the Northern Hemisphere. It is shown that almost all PSCs observed in the Northern Hemisphere are attributable to low temperature anomalies associated with planetary waves.

Description: Origins and composition of fine atmospheric carbonaceous aerosol in the Sierra Nevada Mountains, California Atmospheric Chemistry and Physics Discussions, 11, 17071-17125, 2011 Author(s): D. R. Worton, A. H. Goldstein, D. K. Farmer, K. S. Docherty, J. L. Jimenez, J. B. Gilman, W. C. Kuster, J. de Gouw, B. J. Williams, N. M. Kreisberg, S. V. Hering, G. Bench, M. McKay, K. Kristensen, M. Glasius, J. D. Surratt, and J. H. Seinfeld In this paper we report chemically resolved measurements of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In situ measurements of particle phase molecular markers were dominated by secondary compounds and could be categorized into three factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidation products of temperature-driven local biogenic emissions and (3) local light-driven emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g., temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher due to more substantial emissions of isoprene and enhanced photochemistry. Methyl chavicol oxidation contributed similarly to OA during both identified meteorological periods. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during cooler conditions, even though emissions of the precursors were lower. Following the first precipitation event of the fall the abundances of the monoterpene oxidation products increased dramatically, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (〉75 %). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction ( 〈 1 %) of the total OA mass and suggest that other mechanisms, e.g., NO x enhancement of oxidant levels, are more likely to be responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site.

Description: Ice nucleation properties of fine ash particles from the Eyjafjallajökull eruption in April 2010 Atmospheric Chemistry and Physics Discussions, 11, 17665-17698, 2011 Author(s): I. Steinke, O. Möhler, A. Kiselev, M. Niemand, H. Saathoff, M. Schnaiter, J. Skrotzki, C. Hoose, and T. Leisner During the eruption of the Eyjafjallajökull volcano in the south of Iceland in April/May 2010, about 40 Tg of ash mass were emitted into the atmosphere. However, it was unclear whether volcanic ash particles with d 〈 10 μm facilitate the glaciation of clouds. Thus, ice nucleation properties of volcanic ash particles were investigated in AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber experiments simulating atmospherically relevant conditions. The ash sample that was used for our experiments had been collected at a distance of 58 km from the Eyjafjallajökull during the eruption period in April 2010. The temperature range covered by our ice nucleation experiments extended from 219 to 264 K, and both ice nucleation via immersion freezing and deposition nucleation could be observed. Immersion freezing was first observed at 252 K, whereas the deposition nucleation onset lay at 242 K and RH ice = 126 %. About 0.1 % of the volcanic ash particles were active as immersion freezing nuclei at a temperature of 249 K. For deposition nucleation, an ice fraction of 0.1 % was observed at around 233 K and RH ice = 116 %. Taking ice-active surface site densities as a measure for the ice nucleation efficiency, volcanic ash particles are similarly efficient ice nuclei in immersion freezing mode ( n s, imm ~ 10 9 m −2 at 247 K) compared to certain mineral dusts. For deposition nucleation, the observed ice-active surface site densities n s, dep were found to be 10 11 m −2 at 224 K and RH ice = 116 %. Thus, volcanic ash particles initiate deposition nucleation more efficiently than Asian and Saharan dust but appear to be poorer ice nuclei than ATD particles. Based on the experimental data, we have derived ice-active surface site densities as a function of temperature for immersion freezing and of relative humidity over ice and temperature for deposition nucleation.

Description: Integrated versus isolated scenario for prediction dissolved oxygen at progression of water quality monitoring stations Hydrology and Earth System Sciences Discussions, 8, 6069-6112, 2011 Author(s): A. A. Najah, A. El-Shafie, O. A. Karim, and O. Jaafar This study examined the potential of Multi-layer Perceptron Neural Network (MLP-NN) in predicting dissolved oxygen (DO) at Johor River Basin. The river water quality parameters were monitored regularly each month at four different stations by the Department of Environment (DOE) over a period of ten years, i.e. from 1998 to 2007. The following five water quality parameters were selected for the proposed MLP-NN modelling, namely; temperature (Temp), water pH, electrical conductivity (COND), nitrate (NO 3 ) and ammonical nitrogen (NH 3 –NL). In this study, two scenarios were introduced; the first scenario (Scenario 1) was to establish the prediction model for DO at each station based on five input parameters, while the second scenario (Scenario 2) was to establish the prediction model for DO based on the five input parameters and DO predicted at previous station (upstream). The model needs to verify when output results and the observed values are close enough to satisfy the verification criteria. Therefore, in order to investigate the efficiency of the proposed model, the verification of MLP-NN based on collection of field data within duration 2009–2010 is presented. To evaluate the effect of input parameters on the model, the sensitivity analysis was adopted. It was found that the most effective inputs were oxygen-containing (NO 3 ) and oxygen demand (NH 3 –NL). On the other hand, Temp and pH were found to be the least effective parameters, whereas COND contributed the lowest to the proposed model. In addition, 17 neurons were selected as the best number of neurons in the hidden layer for the MLP-NN architecture. To evaluate the performance of the proposed model, three statistical indexes were used, namely; Coefficient of Efficiency (CE), Mean Square Error (MSE) and Coefficient of Correlation (CC). A relatively low correlation between the observed and predicted values in the testing data set was obtained in Scenario 1. In contrast, high coefficients of correlation were obtained between the observed and predicted values for the test sets of 0.98, 0.96 and 0.97 for all stations after adopting Scenario 2. It appeared that the results for Scenario 2 were more adequate than Scenario 1, with a significant improvement for all stations ranging from 4 % to 8 %.

Description: The causes of flow regime shifts in the semi-arid Hailiutu River, Northwest China Hydrology and Earth System Sciences Discussions, 8, 5999-6030, 2011 Author(s): Z. Yang, Y. X. Zhou, J. Wenninger, and S. Uhlenbrook Identifying the causes (climate vs. human activities) for hydrological variability is a major challenge in hydrology. This paper examines the flow regime shifts, changes in the climatic variables such as precipitation, evaporation, temperature, and crop area in the semi-arid Hailiutu catchment in the middle section of the Yellow River by performing several statistical analyses. The Pettitt test, cumulative sum charts (CUSUM), regime shift index (RSI) method, and harmonic analysis were carried out on annual, monthly, and daily discharges. Four major shifts in the flow regime have been detected in 1968, 1986, 1992 and 2001. Characteristics of the flow regime were analyzed in the five periods: 1957–1967, 1968–1985, 1986–1991, 1992–2000, and 2001–2007. From 1957 to 1967, the flow regime reflects quasi natural conditions with high variability and larger amplitude of 6 months periodic fluctuations. The river flow had been affected by the construction of two reservoirs in the period 1968–1985. In the period of 1986–1991, the river discharge decreased due to the combined influence of river diversions and increase of groundwater extractions for irrigation. In the fourth period of 1992–2000, the river discharge reached lowest flow values and variations corresponding to a large increase in crop area. The flow regime recovered, but not yet to natural status in the fifth period of 2001–2007. Climatic factors are not responsible for all these changes in the flow regime, but the changes are corresponding well to human activities.

Description: Integrating coarse-scale uncertain soil moisture data into a fine-scale hydrological modelling scenario Hydrology and Earth System Sciences Discussions, 8, 6031-6067, 2011 Author(s): H. Vernieuwe, B. De Baets, J. Minet, V. R. N. Pauwels, S. Lambot, M. Vanclooster, and N. E. C. Verhoest In a hydrological modelling scenario, often the modeller is confronted with external data, such as remotely-sensed soil moisture observations, that become available to update the model output. However, the scale triplet (spacing, extent and support) of these data is often inconsistent with that of the model. Furthermore, the external data can be cursed with epistemic uncertainty. Hence, a method is needed that not only integrates the external data into the model, but that also takes into account the difference in scale and the uncertainty of the observations. In this paper, a synthetic hydrological modelling scenario is set up in which a high-resolution distributed hydrological model is run over an agricultural field. At regular time steps, coarse-scale field-averaged soil moisture data, described by means of possibility distributions (epistemic uncertainty), are retrieved by synthetic aperture radar and assimilated into the model. A method is presented that allows to integrate the coarse-scale possibility distribution of soil moisture content data with the fine-scale model-based soil moisture data. To this end, a scaling relationship between field-averaged soil moisture content data and its corresponding standard deviation is employed.

Description: Primary versus secondary contributions to particle number concentrations in the European boundary layer Atmospheric Chemistry and Physics Discussions, 11, 18249-18318, 2011 Author(s): C. L. Reddington, K. S. Carslaw, D. V. Spracklen, M. G. Frontoso, L. Collins, J. Merikanto, A. Minikin, T. Hamburger, H. Coe, M. Kulmala, P. Aalto, H. Flentje, C. Plass-Dülmer, W. Birmili, A. Wiedensohler, B. Wehner, T. Tuch, A. Sonntag, C. D. O'Dowd, S. G. Jennings, R. Dupuy, U. Baltensperger, E. Weingartner, H.-C. Hansson, P. Tunved, P. Laj, K. Sellegri, J. Boulon, J.-P. Putaud, C. Gruening, E. Swietlicki, P. Roldin, J. S. Henzing, M. Moerman, N. Mihalopoulos, G. Kouvarakis, V. Ždímal, N. Zíková, A. Marinoni, P. Bonasoni, and R. Duchi It is important to understand the relative contribution of primary and secondary particles to regional and global aerosol so that models can attribute aerosol radiative forcing to different sources. In large-scale models, there is considerable uncertainty associated with treatments of particle formation (nucleation) in the boundary layer (BL) and in the size distribution of emitted primary particles, leading to uncertainties in predicted cloud condensation nuclei (CCN) concentrations. Here we quantify how primary particle emissions and secondary particle formation influence size-resolved particle number concentrations in the BL using a global aerosol microphysics model and observations made during the May 2008 campaign of the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions (EUCAARI). Observations are available from the DLR Falcon 20 aircraft and from 15 ground sites of the European Supersites for Atmospheric Aerosol Research (EUSAAR) and the German Ultrafine Aerosol Network (GUAN). Measurements include total and non-volatile particle number concentrations and the particle size distribution between ~3 nm and ~1 μm. We tested four different parameterisations for BL nucleation and two assumptions for the emission size distribution of anthropogenic and wildfire carbonaceous particles. When we emit small carbonaceous particles (recommended by the Aerosol Intercomparison project, AEROCOM), the spatial distributions of campaign-mean number concentrations 〉50 nm ( N 50 ) and 〉100 nm ( N 100 ) dry diameter were well captured by the model ( R 2 ~0.9) and the normalised mean bias (NMB) was also small (−5 % for N 50 and 12 % for N 100 ). Emission of larger particles, which we consider to be more realistic for global models, results in equally good correlation but larger bias ( R 2 ~0.8, NMB = −51 % and −21 %), which could be partly but not entirely compensated by BL nucleation. The model also predicts the particle concentration frequency distribution fairly well, with an overlap of modelled and observed N 50 hourly histograms of ~60 % across all sites. However, the model-observation temporal correlation on an hourly time scale is poor ( R 2 ≤0.1) for this period. These comparisons show that caution is required when drawing conclusions about model realism from time or site-averaged data or frequency histograms when deterministic behaviour is not captured at individual sites. From this 1-month intensive European dataset it is not possible to determine a reliable estimate of the fraction of CCN-sized particles from primary and secondary sources, although the size of primary emitted particles is shown to be a major source of uncertainty.

Description: Mechanisms leading to oligomers and SOA through aqueous photooxidation: insights from OH radical oxidation of acetic acid Atmospheric Chemistry and Physics Discussions, 11, 18319-18347, 2011 Author(s): Y. Tan, Y. B. Lim, K. E. Altieri, S. P. Seitzinger, and B. J. Turpin Previous experiments have demonstrated that the aqueous OH radical oxidation of methylglyoxal produces low volatility products including oxalate and oligomers. These products are found predominantly in the particle phase in the atmosphere, suggesting that methylglyoxal is a precursor of secondary organic aerosol (SOA). Acetic acid is an important intermediate in aqueous methylglyoxal oxidation and a ubiquitous product of gas phase photochemistry, making it a potential "aqueous" SOA precursor in its own right. Altieri et al. (2008) proposed that acetic acid was the precursor of oligoesters observed in methylglyoxal oxidation. However, the fate of acetic acid upon aqueous-phase oxidation is not well understood. In this research, acetic acid at concentrations relevant to atmospheric waters (20 μM–10 mM) was oxidized by OH radical. Products were analyzed by ion chromatography (IC), electrospray ionization mass spectrometry (ESI-MS), and IC-ESI-MS. The formation of glyoxylic, glycolic, and oxalic acids were observed. In contrast to methylglyoxal oxidation, succinic acid and oligomers were not detected. Using results from these and methylglyoxal + OH radical experiments, radical mechanisms responsible for oligomer formation from methylglyoxal oxidation in clouds and wet aerosols are proposed. The importance of acetic acid/acetate as an SOA precursor is also discussed. We hypothesize that this and similar chemistry is central to the daytime formation of oligomers in wet aerosols.

Description: Scaling and trends of hourly precipitation extremes in two different climate zones – Hong Kong and the Netherlands Hydrology and Earth System Sciences Discussions, 8, 4701-4719, 2011 Author(s): G. Lenderink, H. Y. Mok, T. C. Lee, and G. J. van Oldenborgh Hourly precipitation extremes in very long time series from the Hong Kong Observatory and the Netherlands are investigated. Using the 2 m dew point temperature from 4 h before the rainfall event as a measure of near surface absolute humidity, hourly precipitation extremes closely follow a 14 % per degree dependency – a scaling twice as large as following from the Clausius-Clapeyron relation. However, for dew point temperatures above 23 °C no significant dependency on humidity was found. Strikingly, in spite of the large difference in climate, results are almost identical in Hong Kong and the Netherlands for the dew point temperature range where both observational sets have sufficient data. Trends in hourly precipitation extremes show substantial increases over the last century for both De Bilt (the Netherlands) and Hong Kong. For De Bilt, not only the long term trend, but also variations in hourly precipitation extremes on a inter-decadal timescale of 30 yr and longer, can be linked very well to the above scaling; there is a very close resemblance between variations in dew point temperature and precipitation intensity with an inferred dependency of hourly precipitation extremes of 10 to 14 % per degree. For Hong Kong there is no connection between variations in humidity and those in precipitation intensity in the wet season, May to September, consistent with the found zero-dependency of preciptation intensity on humidity for dew points above 23 °C. Yet, outside the wet season humidity changes do appear to explain the positive trend in hourly precipitation extremes, again following a dependency close to twice the Clausius-Clapeyron relation.

Description: Amine reactivity with charged sulfuric acid clusters Atmospheric Chemistry and Physics Discussions, 11, 14637-14658, 2011 Author(s): B. R. Bzdek, D. P. Ridge, and M. V. Johnston The distribution of ionic species produced by electrospray of an ammonium sulfate solution in both positive and negative polarities is examined using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Positively-charged ammonium bisulfate cluster composition differs significantly from negatively-charged cluster composition. For positively-charged clusters all sulfuric acid is neutralized to bisulfate, whereas for negatively-charged clusters the degree of sulfuric acid neutralization is cluster size-dependent. With increasing cluster size (and, therefore, a decreasing role of charge), both positively- and negatively-charged cluster compositions converge toward ammonium bisulfate. The reactivity of negatively-charged sulfuric acid-ammonia clusters with dimethylamine and ammonia are also investigated by FTICR-MS. Two series of negatively-charged clusters are investigated: [(HSO 4 )(H 2 SO 4 ) x ] − and [(NH 4 ) x (HSO 4 ) x +1 (H 2 SO 4 ) 3 ] − . Dimethylamine substitution for ammonia in [(NH 4 ) x (HSO 4 ) x +1 (H 2 SO 4 ) 3 ] − clusters is nearly collision-limited, and subsequent addition of dimethylamine to neutralize H 2 SO 4 is within one order of magnitude of the substitution rate. Dimethylamine addition to [(HSO 4 )(H 2 SO 4 ) x ] − clusters is either not observed or very slow. The results of this study indicate that amine chemistry will be evident and important only in large ambient negative ions (〉 m/z 400), whereas amine chemistry may be evident in small ambient positive ions. Addition of ammonia to unneutralized clusters occurs at a rate that is ~2–3 orders of magnitude slower than incorporation of dimethylamine either by substitution or addition. Therefore, in locations where amine levels are within a few orders of magnitude of ammonia levels, amine chemistry may compete favorably with ammonia chemistry.

Description: Seasonal variation in vertical volatile compounds air concentrations within a remote hemiboreal mixed forest Atmospheric Chemistry and Physics Discussions, 11, 14607-14635, 2011 Author(s): S. M. Noe, K. Hüve, Ü. Niinemets, and L. Copolovici The vertical distribution of ambient biogenic volatile organic compounds (BVOC) concentrations within a hemiboreal forest canopy was investigated over a period of one year. Variability in temporal and spatial isoprene concentrations can be mainly explained by biogenic emissions from deciduous trees, ranging from 0.1 to 7.5 μg m −3 . Monoterpene concentrations exceeded isoprene largely and ranged from 0.01 to 140 μg m −3 and during winter time anthropogenic contributions are likely. Variation in monoterpene concentrations found to be largest right above the ground and the vertical profile suggest a weak mixing leading to terpene accumulation in the lower canopy. Exceptionally high values were recorded during a heat wave in July 2010 with very high midday temperatures above 30 °C for several weeks. During summer months, monoterpene exceeded isoprene concentrations 6-fold and during winter 12-fold. The relative contribution of diverse monoterpene species to the ambient concentrations revealed a dominance of α-pinene in the lower and of limonene in the upper part of the canopy, both accounting for up to 70 % of the total monoterpene concentration during summer months. The main contributing monoterpene during wintertime was Δ 3 -carene accounting for 60 % of total monoterpene concentration in January. Possible biogenic monoterpene sources beside the foliage are the leaf litter, the soil and also resins exuding from stems. In comparison, the hemiboreal mixed forest canopy showed similar isoprene but higher monoterpene concentrations than the boreal forest and lower isoprene but substantially higher monoterpene concentrations than the temperate mixed forest canopies. These results have major implications for simulating air chemistry and secondary organic aerosol formation within and above hemiboreal forest canopies.

Description: The HNO 3 forming branch of the HO 2 + NO reaction: pre-industrial-to-present trends in atmospheric species and radiative forcings Atmospheric Chemistry and Physics Discussions, 11, 14801-14835, 2011 Author(s): O. A. Søvde, C. R. Hoyle, G. Myhre, and I. S. A. Isaksen Recent laboratory measurements have shown the existence of a HNO 3 forming branch of the HO 2 + NO reaction. This reaction is the main source of atmospheric O 3 , through the subsequent photolysis of NO 2 , as well as being a major source of OH. The branching of the reaction to HNO 3 reduces the formation of these species significantly, affecting O 3 abundances, climate and the oxidation capacity of the troposphere. The Oslo CTM2, a three-dimensional chemistry transport model, is used to calculate atmospheric composition and trends with and without the new reaction branch. Results for the present day atmosphere, when both temperature and pressure effects on the branching ratio are accounted for, show an increase of the global, annual mean methane lifetime by 10.9 %, resulting from a 14.1 % reduction in the global, annual mean OH concentration. Comparisons with measurements show that including the new branch improves the modelled O 3 , but that it is not possible to conclude whether the NO y distribution improves. We model an approximately 11 % reduction in the tropical tropospheric O 3 increase since pre-industrial times, as well as an 8 % decrease in the trend of OH concentration, when the new branch is accounted for. The radiative forcing due to changes in O 3 over the industrial era was calculated as 0.33 W m −2 , reducing to 0.26 W m −2 with the new reaction branch. These results are significant, and it is important that this reaction branching is confirmed by other laboratory groups.

Description: Early in-flight detection of SO 2 via Differential Optical Absorption Spectroscopy: a feasible aviation safety measure to prevent potential encounters with volcanic plumes Atmospheric Measurement Techniques Discussions, 4, 2827-2881, 2011 Author(s): L. Vogel, B. Galle, C. Kern, H. Delgado Granados, V. Conde, P. Norman, S. Arellano, O. Landgren, P. Lübcke, J. M. Alvarez Nieves, L. Cárdenas Gonzáles, and U. Platt Volcanic ash constitutes a risk to aviation, mainly due to its ability to cause jet engines to fail. Other risks include the possibility of abrasion of windshields and potentially serious damage to avionic systems. These hazards have been widely recognized since the early 1980s, when volcanic ash provoked several incidents of engine failure in commercial aircraft. In addition to volcanic ash, volcanic gases also pose a threat. Prolonged and/or cumulative exposure to sulphur dioxide (SO 2 ) or sulphuric acid (H 2 SO 4 ) aerosols potentially affects e.g. windows, air frame and may cause permanent damage to engines. SO 2 receives most attention among the gas species commonly found in volcanic plumes because its presence above the lower troposphere is a clear proxy for a volcanic cloud and indicates that fine ash could also be present. Up to now, remote sensing of SO 2 via Differential Optical Absorption Spectroscopy (DOAS) in the ultraviolet spectral region has been used to measure volcanic clouds from ground based, airborne and satellite platforms. Attention has been given to volcanic emission strength, chemistry inside volcanic clouds and measurement procedures were adapted accordingly. Here we present a set of experimental and model results, highlighting the feasibility of DOAS to be used as an airborne early detection system of SO 2 in two spatial dimensions. In order to prove our new concept, simultaneous airborne and ground-based measurements of the plume of Popocatépetl volcano, Mexico, were conducted in April 2010. The plume extended at an altitude around 5250 m above sea level and was approached and traversed at the same altitude with several forward looking DOAS systems aboard an airplane. These DOAS systems measured SO 2 in the flight direction and at ± 40 mrad (2.3°) angles relative to it in both, horizontal and vertical directions. The approaches started at up to 25 km distance to the plume and SO 2 was measured at all times well above the detection limit. In combination with radiative transfer studies, this study indicates that an extended volcanic cloud with a concentration of 10 12 molecules cm −3 at typical flight levels of 10 km can be detected unambiguously at distances of up to 80 km away. This range provides enough time (approx. 5 min) for pilots to take action to avoid entering a volcanic cloud in the flight path, suggesting that this technique can be used as an effective aid to prevent dangerous aircraft encounters with potentially ash rich volcanic clouds.

Description: Aerosol composition and sources in the Central Arctic Ocean during ASCOS Atmospheric Chemistry and Physics Discussions, 11, 14837-14881, 2011 Author(s): R. Y.-W. Chang, C. Leck, M. Graus, M. Müller, J. Paatero, J. F. Burkhart, A. Stohl, L. H. Orr, K. Hayden, S.-M. Li, A. Hansel, M. Tjernström, W. R. Leaitch, and J. P. D. Abbatt Measurements of submicron aerosol chemical composition were made in the Central Arctic Ocean from 5 August to 8 September 2008 as a part of the Arctic Summer Cloud Ocean Study (ASCOS) using an aerosol mass spectrometer (AMS). The median levels of sulphate and organics for the entire study were 0.042 and 0.046 μg m −3 , respectively. Positive matrix factorisation was performed on the entire mass spectral time series and this enabled marine biogenic and continental sources of particles to be separated. These factors accounted for 33 % and 36 % of the sampled ambient aerosol mass, respectively, and they were both predominantly composed of sulphate, with 47 % of the sulphate apportioned to marine biogenic sources and 48 % to continental sources, by mass. Within the marine biogenic factor, the ratio of methane sulphonate to sulphate was 0.25 ± 0.02, consistent with values reported in the literature. The organic component of the continental factor was more oxidised than that of the marine biogenic factor, suggesting that it was more processed and had been present longer in the atmosphere than the organics in the marine biogenic factor. The remaining ambient aerosol mass was apportioned to an organic-rich factor that could have arisen from a combination of marine and continental sources.

Description: Forest cover influence on flood assessment in Italian catchments Hydrology and Earth System Sciences Discussions, 8, 4891-4926, 2011 Author(s): F. Preti, G. Forzieri, and G. B. Chirico The paper aims at evaluating to what extent forest cover could affect the flood peak frequency and magnitude in Italian catchments. The analysis is restricted to evaluating the component of the runoff coefficient which cannot be captured by the catchment lithology alone. A preliminary data mining is performed on data of 75 catchments distributed from South to Central Italy. Cluster and correlation structure analyses are conducted for distinguishing forest cover effects within sample sets of catchments characterized by hydro-morphological similarities. We propose a method for correcting the bias of the runoff coefficient estimated from the catchment lithology only, by accounting for the effect of forest cover. The bias correction becomes significant for small mountainous catchments, characterised by larger forest cover fraction and lower critical rainfall depth. Consistently with what suggested in previous studies, the bias correction decreases as the rainfall depth and return period increase.

Description: Black carbon aerosol mixing state, organic aerosols and aerosol optical properties over the UK Atmospheric Chemistry and Physics Discussions, 11, 14991-15027, 2011 Author(s): G. R. McMeeking, W. T. Morgan, M. Flynn, E. J. Highwood, K. Turnbull, J. Haywood, and H. Coe Black carbon (BC) aerosols absorb sunlight thereby leading to a positive radiative forcing and a warming of climate and can also impact human health through their impact on the respiratory system. The state of mixing of BC with other aerosol species, particularly the degree of internal/external mixing, has been highlighted as a major uncertainty in assessing its radiative forcing and hence its climate impact, but few in situ observations of mixing state exist. We present airborne single particle soot photometer (SP2) measurements of refractory BC (rBC) mass concentrations and mixing state coupled with aerosol composition and optical properties measured in urban plumes and regional pollution over the UK. All data were obtained using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM). We measured sub-micron aerosol composition using an aerosol mass spectrometer (AMS) and used positive matrix factorization to separate hydrocarbon-like (HOA) and oxygenated organic aerosols (OOA). We found a higher number fraction of thickly coated rBC particles in air masses with large OOA relative to HOA, higher ozone-to-nitrogen oxides (NO x ) ratios and large concentrations of total sub-micron aerosol mass relative to rBC mass concentrations. The more ozone- and OOA-rich air masses were associated with transport from continental Europe, while plumes from UK cities had higher HOA and NO x and fewer thickly coated rBC particles. We did not observe any significant change in the rBC mass absorption efficiency calculated from rBC mass and light absorption coefficients measured by a particle soot absorption photometer despite observing significant changes in aerosol composition and rBC mixing state. The contributions of light scattering and absorption to total extinction (quantified by the single scattering albedo; SSA) did change for different air masses, with lower SSA observed in urban plumes compared to regional aerosol (0.85 versus 0.9–0.95). We attribute these differences to the presence of relatively rapidly formed secondary aerosol, primarily OOA and ammonium nitrate, which must be taken into account in radiative forcing calculations.

Description: Technical Note: Development of an automated lysimeter for the calculation of peat soil actual evapotranspiration Hydrology and Earth System Sciences Discussions, 8, 5009-5033, 2011 Author(s): S. Proulx-McInnis, A. St-Hilaire, A. N. Rousseau, S. Jutras, G. Carrer, and G. Levrel A limited number of publications in the literature deal with the measurement of actual evapotranspiration (AET) from a peat soil. AET is an important parameter in the description of water pathways of an ecosystem. In peatlands, where the water table is near the surface and the vegetation is composed of nonvascular plants without stomatal resistance, the AET measurement represents a challenge. This paper discusses the development of an automated lysimeter installed between 12 and 27 July 2010, at a 11-ha bog site, Pont-Rouge (42 km west of Quebec City, Canada). This system was made of an isolated block of peat, maintained at the same water level as the surrounding water table by a system of submersible pressure transmitters and pumps. The change in water level in millimetres in the isolated block of peat was used to calculate the water lost through evapotranspiration (ET) while accounting the precipitation. The rates of AET were calculated for each day of the study period. Temperature fluctuated between 17.2 and 23.3 °C and total rainfall was 43.76 mm. AET rates from 0.6 to 6.9 mm day −1 were recorded, with a ΣAET/ΣP ratio of 1.38. The estimated potential ET (PET) resulting from Thornthwaite's semi-empirical formula suggested values between 2.8 and 3.9 mm day −1 . The average AET/PET ratio was 1.13. According to the literature, the results obtained are plausible. This system, relatively inexpensive and simple to install, may eventually be used to calculate AET on peaty soils in the years to come.

Description: Technical Note: In-situ derivatization thermal desorption GC-TOFMS for direct analysis of particle-bound non-polar and polar organic species Atmospheric Chemistry and Physics Discussions, 11, 15255-15295, 2011 Author(s): J. Orasche, J. Schnelle-Kreis, G. Abbaszade, and R. Zimmermann An in-situ derivatization thermal desorption method followed by gas chromatography and time-of-flight mass spectrometry (IDTD-GC-TOFMS) was developed for determination of polar organic compounds. Hydroxyl and carboxyl groups of compounds such as anhydrous sugars, alcohols and phenols, fatty acids and resin acids are targets of the derivatization procedure. Derivatization is based on silylation with N-Methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA) during the step of thermal desorption. The high temperature of 300 °C during desorption is utilized for the in-situ derivatization on the collection substrate (quartz fibre filters) accelerating the reaction rate. Thereby, the analysis time is as short as without derivatization. At first the filter surface is dampened with derivatization reagent before insertion of the sample into the thermal desorption unit. To ensure ongoing derivatization during thermal desorption the carrier gas is saturated with MSTFA until the desorption procedure is finished. The method introduced here was compared with direct thermal desorption gas chromatography time-of-flight-mass spectrometry (DTD-GC-TOFMS) and with solvent extraction (SE) procedures followed by gas chromatography and mass spectrometry. Comparisons were carried out with field samples originating from ambient aerosol collected on quartz fibre filters. Moreover, the methods have been applied on NIST Standard Reference Material Urban Dust (SRM 1649a).

Description: Impacts of changes in land use and land cover on atmospheric chemistry and air quality over the 21st century Atmospheric Chemistry and Physics Discussions, 11, 15469-15495, 2011 Author(s): S. Wu, L. J. Mickley, J. O. Kaplan, and D. J. Jacob The effects of future land use and land cover change on the chemical composition of the atmosphere and air quality are largely unknown. To investigate the potential effects associated with future changes in vegetation driven by atmospheric CO 2 concentrations, climate, and anthropogenic land use over the 21st century, we performed a series of model experiments combining a general circulation model with a dynamic global vegetation model and an atmospheric chemical-transport model. Our results indicate that climate- and CO 2 -induced changes in vegetation composition and density could lead to decreases in summer afternoon surface ozone of up to 10 ppb over large areas of the northern mid-latitudes. This is largely driven by the substantial increases in ozone dry deposition associated with changes in the composition of temperate and boreal forests where conifer forests are replaced by those dominated by broadleaf tree types, as well as a CO 2 -driven increase in vegetation density. Climate-driven vegetation changes over the period 2000–2100 lead to general increases in isoprene emissions, globally by 15 % in 2050 and 36 % in 2100. These increases in isoprene emissions result in decreases in surface ozone concentrations where the NO x levels are low, such as in remote tropical rainforests. However, over polluted regions, such as the northeastern United States, ozone concentrations are calculated to increase with higher isoprene emissions in the future. Increases in biogenic emissions also lead to higher concentrations of secondary organic aerosols, which increase globally by 10 % in 2050 and 20 % in 2100. Surface concentrations of secondary organic aerosols are calculated to increase by up to 1 μg m −3 for large areas in Eurasia. When we use a scenario of future anthropogenic land use change, we find less increase in global isoprene emissions due to replacement of higher-emitting forests by lower-emitting cropland. The global atmospheric burden of secondary organic aerosols changes little by 2100 when we account for future land use change, but both secondary organic aerosols and ozone show large regional changes at the surface.

Description: A comprehensive numerical study of aerosol-cloud-precipitation interactions in marine stratocumulus Atmospheric Chemistry and Physics Discussions, 11, 15497-15550, 2011 Author(s): Y.-C. Chen, L. Xue, Z. J. Lebo, H. Wang, R. M. Rasmussen, and J. H. Seinfeld Three-dimensional large-eddy simulations (LES) with detailed bin-resolved microphysics are performed to explore the diurnal variation of marine stratocumulus (MSc) clouds under clean and polluted conditions. The sensitivity of the aerosol-cloud-precipitation interactions to variation of sea surface temperature, free tropospheric humidity, large-scale divergence rate, and wind speed is assessed. The comprehensive set of simulations corroborates previous studies that (1) with moderate/heavy drizzle, an increase in aerosol leads to an increase in cloud thickness; and (2) with non/light drizzle, an increase in aerosol results in a thinner cloud, due to the pronounced effect on entrainment. It is shown that for higher SST, stronger large-scale divergence, drier free troposphere, or lower wind speed, the cloud thins and precipitation decreases. The sign and magnitude of the Twomey effect, droplet dispersion effect, cloud thickness effect, and overall cloud optical depth susceptibility to aerosol perturbations are evaluated by LES experiments and compared with analytical formulations. The Twomey effect emerges as dominant in total cloud susceptibility to aerosol perturbations. The dispersion effect, that of aerosol perturbations on the cloud droplet size spectrum, is positive (i.e., increase in aerosol leads to spectral narrowing) and accounts for 3 % to 10 % of the total cloud susceptibility at nighttime, with the largest influence in heavier drizzling clouds. The cloud thickness effect is negative (i.e., increase in aerosol leads to thinner cloud) for non/light drizzling cloud and positive for moderate/heavy drizzling clouds; the cloud thickness effect contributes 5 % to 22 % of the nighttime cloud susceptibility. The range of magnitude for each effect is more variable in the daytime owing to cloud thinning and decoupling. Overall, the cloud susceptibility is ~0.28 to 0.53 at night; an increase in aerosol concentration enhances cloud optical depth, especially with heavier precipitation and in a more pristine environment. The good agreement between LES experiments and analytical formulations suggests that the latter may be useful in evaluations of cloud susceptibility. The ratio of the magnitude of the cloud thickness effect to that of the Twomey effect depends on cloud base height and cloud thickness in unperturbed (clean) clouds.

Description: Technical Note: Demonstrating a 24/7 solution for monitoring water quality loads in rivers Hydrology and Earth System Sciences Discussions, 8, 5035-5050, 2011 Author(s): P. Jordan and R. Cassidy Quantifying nutrient and sediment loads in catchments is difficult owing to diffuse controls related to storm hydrology. Coarse sampling and interpolation methods are prone to very high uncertainties due to under-representation of high discharge, short duration events. Additionally, important low-flow processes such as diurnal signals linked to point source impacts are missed. Here we demonstrate a solution based on a time-integrated approach to sampling with a standard 24 bottle autosampler configured to take a sample every 7 h over a week. This is evaluated with a number of other sampling strategies using a two-year dataset of sub-hourly discharge and phosphorus concentration data. The 24/7 solution is shown to be the least uncertain in estimating load (inter-quartile range is 96 % to 110 % of actual load in year 1 and 97 % to 104 % in year 2) due to the increased frequency raising the probability of sampling storm events and point source signals. The 24/7 solution would appear to be most parsimonious in terms of technology requirements, the ability to be widely deployed and to represent important nutrient transfer processes in complex catchments.

Description: Analysis of ΔO 2 /ΔCO 2 ratios for the pollution events observed at Hateruma Island, Japan Atmospheric Chemistry and Physics Discussions, 11, 15631-15657, 2011 Author(s): C. Minejima, M. Kubo, Y. Tohjima, H. Yamagishi, Y. Koyama, S. Maksyutov, K. Kita, and H. Mukai In-situ observations of atmospheric CO 2 and O 2 concentrations at Hateruma Island (HAT, 24° N, 124° E) often show synoptic scale pollution events when air masses are transported from East Asian source regions. We calculate the regression slopes (-ΔO 2 /ΔCO 2 molar ratios) of the correlation plots between O 2 and CO 2 for selected pollution events observed between October 2006 and December 2008. The observed -ΔO 2 /ΔCO 2 ratios vary from 1.0 to 1.7. Categorizing the air mass origins for the pollution events by using back trajectory analysis, we find that there is a significant difference in the average -ΔO 2 /ΔCO 2 ratios between events from China (1.14±0.12, n = 25) and Japan/Korea (1.37±0.15, n = 16). These values are comparable to the -O 2 :CO 2 molar exchange ratios, which are estimated from the national fossil fuel inventories from CDIAC. Simulations using a particle dispersion model reveal that the pollution events at HAT are predominantly CO 2 emissions from the combustion of fossil fuels in East Asian countries, which is consistent with the above observational results. Although the average value of the model-predicted -ΔO 2 /ΔCO 2 ratios for Japan/Korea origin is underestimated in comparison with the observation, that for China origin agree well with the observation. The sensitivity experiment suggests that the -ΔO 2 /ΔCO 2 ratio at HAT reflects about 90% of the change in the -O 2 :CO 2 exchange ratio for the fossil carbon emissions from China.

Description: Cloud-system resolving model simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment Atmospheric Chemistry and Physics Discussions, 11, 15573-15629, 2011 Author(s): H. Morrison and W. W. Grabowski This paper presents results from 240-member ensemble simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment. Simulations using a two-dimensional cloud system-resolving model are run with pristine, polluted, or highly polluted aerosol conditions and large-scale forcing from a 6-day period of active monsoon conditions during the 2006 Tropical Warm Pool – International Cloud Experiment (TWP-ICE). Domain-mean surface precipitation is insensitive to aerosols primarily because the large-scale forcing is prescribed and dominates the water and static energy budgets. The spread of the top-of-atmosphere (TOA) shortwave and longwave radiative fluxes among different ensemble members for the same aerosol loading is surprisingly large, exceeding 25 W m −2 even when averaged over the 6-day period. This variability is caused by random fluctuations in the strength and timing of individual deep convective events. The ensemble approach demonstrates a small weakening of convection averaged over the 6-day period in the polluted simulations compared to pristine. Despite this weakening, the cloud top heights and anvil ice mixing ratios are higher in polluted conditions. This occurs because of the larger concentrations of cloud droplets that freeze, leading directly to higher ice particle concentrations, smaller ice particle sizes, and smaller fall velocities compared to simulations with pristine aerosols. Weaker convection in polluted conditions is a direct result of the changes in anvil ice characteristics and subsequent upper-tropospheric radiative heating and weaker tropospheric destabilization. Such a conclusion offers a different interpretation of recent satellite observations of tropical deep convection in pristine and polluted environments compared to the hypothesis of aerosol-induced convective invigoration. Sensitivity tests using the ensemble approach with modified microphysical parameters or domain configuration (horizontal gridlength, domain size) produce results that are similar to baseline, although there are quantitative differences in estimates of aerosol impacts on TOA radiative fluxes.

Description: Coastal precipitation formation and discharge based on TRMM observations Atmospheric Chemistry and Physics Discussions, 11, 15659-15696, 2011 Author(s): R. H. Heiblum, I. Koren, and O. Altaratz The interaction between breezes and synoptic gradient winds creates persistent convergence zones nearby coastlines. The low level convergence of moist air promotes the dynamical and microphysical processes responsible for the formation of clouds and precipitation. Our work focuses on the winter seasons of 1998–2011 in the Eastern Mediterrenean. During the winter the Mediterrenean sea is usually warmer than the adjacent land, resulting in frequent occurence of land breeze that opposes the commom synoptic winds. Using rain-rate vertical profiles from the Tropical Rainfall Measurement Mission (TRMM) satellite, we examined the spatial and temporal distribution of average hydrometeor mass in clouds as a funtion of the distance from coastlines. Results show that coastalines in the Eastern Mediterrenean are indeed favored areas for precipitation formation and discharge. The intra-seasonal and diurnal changes in the distribution of hydrometeor mass indicate that the land breeze is most likely the main responsible mechanism behind our results.

Description: Catalytic oxidation of H 2 on platinum: a method for in situ calibration of hygrometers Atmospheric Measurement Techniques Discussions, 4, 3083-3095, 2011 Author(s): A. W. Rollins, T. D. Thornberry, R.-S. Gao, B. D. Hall, and D. W. Fahey Standard reference samples of water vapor suitable for in situ calibration of atmospheric hygrometers are not currently widespread, leading to difficulties in unifying the calibrations of these hygrometers and potentially contributing to measurement discrepancies. We describe and evaluate a system for reliably and quantitatively converting mixtures of H 2 in air to H 2 O on a heated platinum (Pt) surface, providing a compact, portable, adjustable source of water vapor. The technique is shown to be accurate and can be used to easily and predictably produce a wide range of water vapor concentrations (≈1 ppm−2 %) on demand. The result is a H 2 O standard that is suitable for in situ calibration of hygrometers, with an accuracy nearly that of the available H 2 standards (≈±2 %).

Description: A versatile and reproducible automatic injection system for liquid standard introduction: application to in-situ calibration Atmospheric Measurement Techniques Discussions, 4, 3233-3249, 2011 Author(s): G. Isaacman, N. M. Kreisberg, D. R. Worton, S. V. Hering, and A. H. Goldstein The quantitation of trace organic compounds in ambient organic aerosol is difficult due to the chemical complexity of these mixtures, but is needed to provide insight into their sources and formation processes. Compound-level characterization of organic aerosols is typically performed through sample collection followed by gas or liquid chromatography. With these methods, introduction of liquid standards has long been used as an effective means of quantifying trace compounds, but automating this technique for use with in-situ instrumentation has not previously been achieved. Here we develop an automatic injection system (AutoInject) for the introduction of liquids into a custom collection and analysis cell for improved quantitation in chromatographic measurements. The system consists of chilled reservoirs containing liquid standards from which a sample loop is loaded and then injected into the cell. The AutoInject is shown to have reproducibility over 106 injections with a relative standard deviation of 1.5 %, and have negligible injection-to-injection carryover. A 6-port selector allows injection of different liquid standards separately or simultaneously. Additionally, automatic injection of multiple sample loops is shown to generate a linear multi-point calibration curve. Tests conducted in this work focus on use with the T hermal desorption A erosol G as chromatograph (TAG), but the flexibility of the system allows it to be used for a variety of applications.

Description: TES ammonia retrieval strategy and global observations of the spatial and seasonal variability of ammonia Atmospheric Chemistry and Physics Discussions, 11, 16023-16074, 2011 Author(s): M. W. Shephard, K. E. Cady-Pereira, M. Luo, D. K. Henze, R. W. Pinder, J. T. Walker, C. P. Rinsland, J. O. Bash, L. Zhu, V. H. Payne, and L. Clarisse Presently only limited sets of tropospheric ammonia (NH 3 ) measurements in the Earth's atmosphere have been reported from satellite and surface station measurements, despite the well-documented negative impact of NH 3 on the environment and human health. Presented here is a detailed description of the satellite retrieval strategy and analysis for the Tropospheric Emission Spectrometer (TES) using simulations and measurements. These results show that: (i) the level of detectability for a representative boundary layer TES NH 3 mixing ratio value is ~0.3 ppbv, which typically corresponds to a profile that contains a maximum level value of ~1 ppbv; (ii) TES NH 3 retrievals provide at most one degree of freedom for signal (DOFS), with peak sensitivity between 700 and 900 mbar; (iii) TES NH 3 retrievals show significant spatial and seasonal variability of NH 3 globally; (iv) Initial comparisons of TES observations with GEOS-CHEM estimates show TES values being higher overall. Important differences and similarities between modeled and observed seasonal and spatial trends are noted, with discrepancies indicating areas where the timing and magnitude of modeled NH 3 emissions from agricultural sources, and to lesser extent biomass burning sources, need further study.

Description: Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland Atmospheric Chemistry and Physics Discussions, 11, 15975-16021, 2011 Author(s): C. Buizert, P. Martinerie, V. V. Petrenko, J. P. Severinghaus, C. M. Trudinger, E. Witrant, J. L. Rosen, A. J. Orsi, M. Rubino, D. M. Etheridge, L. P. Steele, C. Hogan, J. C. Laube, W. T. Sturges, V. A. Levchenko, A. M. Smith, I. Levin, T. J. Conway, E. J. Dlugokencky, P. M. Lang, K. Kawamura, T. M. Jenk, J. W. C. White, T. Sowers, J. Schwander, and T. Blunier Compacted snow (firn) preserves a continuous record of atmospheric composition up to a century back in time. Firn air transport modeling is essential for interpretation of firn gas records. Each site needs to be characterised individually through a tuning procedure, in which the effective diffusivity at each depth is adjusted to optimise the agreement between modeled and measured mixing ratios of a selected reference gas (usually CO 2 ). We present the characterisation of the NEEM site, Northern Greenland (77.45° N 51.06° W), where an ensemble of ten reference tracers is used to constrain the diffusivity reconstruction. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the model tuning, and define a root mean square criterion that is minimised in the tuning. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. The modern day Δage, i.e. the difference between gas age and ice age, is calculated to be 182 ± 8 yr. We find evidence that diffusivity does not vanish completely in the firn lock-in zone, as is commonly assumed. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

Description: Isotope effects in N 2 O photolysis from first principles Atmospheric Chemistry and Physics Discussions, 11, 16075-16105, 2011 Author(s): J. A. Schmidt, M. S. Johnson, and R. Schinke For the first time, accurate potential energy surfaces allow N 2 O cross sections and isotopic fractionation spectra to be derived that are in agreement with available experimental data (without ad hoc shifting), extending knowledge to a much broader range of conditions. Absorption spectra of rare N- and O-isotopologues ( 15 N 14 N 16 O, 14 N 15 N 16 O, 15 N 2 16 O, 14 N 2 17 O and 14 N 2 18 O) calculated using wavepacket propagation are compared to the most abundant isotopologue ( 14 N 2 16 O). The fractionation constants as a function of wavelength and temperature are in excellent agreement with experimental data. The study shows that excitations from the 3rd excited bending state, (0,3,0), and the first combination band, (1,1,0), are important for explaining the isotope effect at wavelengths longer than 210 nm. Only a small amount of the mass independent oxygen isotope anomaly observed in atmospheric N 2 O samples can be explained as arising from photolysis.