ALBERT

Description: The combination of Cavity Enhanced Absorption Spectroscopy (CEAS) with broad-band light sources (e.g. Light-Emitting Diodes, LEDs) lends itself to the application of cavity enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) to perform sensitive and selective point measurements of multiple trace gases and aerosol extinction with a single instrument. In contrast to other broad-band CEAS techniques, CE-DOAS relies only on the measurement of relative intensity changes, i.e. does not require knowledge of the light intensity in the absence of trace gases and aerosols (I0). We have built a prototype LED-CE-DOAS instrument in the blue spectral range (420–490 nm) to measure nitrogen dioxide (NO2), glyoxal (CHOCHO), methyl glyoxal (CH3COCHO), iodine oxide (IO), water vapour (H2O) and oxygen dimers (O4). We demonstrate the first direct detection of methyl glyoxal, and the first CE-DOAS detection of CHOCHO and IO. The instrument is further inherently calibrated for light extinction from the cavity by observing O4 or H2O (at 477 nm and 443 nm) and measuring the pressure, relative humidity and temperature independently. This approach is demonstrated by experiments where laboratory aerosols of known size and refractive index were generated and their extinction measured. The measured extinctions were then compared to the theoretical extinctions calculated using Mie theory (3–7 × 10−7cm−1). Excellent agreement is found from both the O4 and H2O retrievals. This enables the first inherently calibrated CEAS measurement at blue wavelengths in open cavity mode, and eliminates the need for sampling lines to supply air to the cavity, i.e., keep the cavity enclosed and/or aerosol free. Measurements in open cavity mode are demonstrated for CHOCHO, CH3COCHO, NO2, H2O and aerosol extinction. Our prototype LED-CE-DOAS provides a low cost, yet research grade innovative instrument for applications in simulation chambers and in the open atmosphere.

Description: The potential of a new generation of ceilometer instruments for aerosol monitoring has been studied in the Ceilometer Lidar Comparison (CLIC) study. The used ceilometer was developed by Jenoptik, Germany, and is designed to find both thin cirrus clouds at tropopause level and aerosol layers at close ranges during day and night-time. The comparison study was performed to determine up to which altitude the ceilometers are capable to deliver particle backscatter coefficient profiles. For this, the derived ceilometer profiles are compared to simultaneously measured lidar profiles at the same wavelength. The lidar used for the comparison was the multi-wavelengths Raman lidar PollyXT. To demonstrate the capabilities and limits of ceilometers for the derivation of particle backscatter coefficient profiles from their measurements two examples of the comparison results are shown. Two cases, a daytime case with high background noise and a less noisy night-time case, are chosen. In both cases the ceilometer profiles compare well with the lidar profiles in atmospheric structures like aerosol layers or the boundary layer top height. However, the determination of the correct magnitude of the particle backscatter coefficient needs a calibration of the ceilometer data with an independent measurement of the aerosol optical depth by a sun photometer. To characterizes the ceilometers signal performance with increasing altitude a comprehensive signal-to-noise ratio study was performed. During daytime the signal-to-noise ratio is higher than 1 up to 4–5 km depending on the aerosol content. In our night-time case the SNR is higher than 1 even up to 8.5 km, so that also aerosol layers in the upper troposphere had been detected by the ceilometer.

Description: In East Asia, satellite observation is important because aerosols from natural and anthropogenic sources have been recognized as a major source of regional and global air pollution. However, retrieving aerosols properties from satellite observations over land can be difficult because of the surface reflection, complex aerosol composition, and aerosol absorption. In this study, a new aerosol retrieval method called as the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite aerosol retrieval (MSTAR) was developed and applied to three different aerosol event cases over East Asia. MSTAR uses a separation technique that can distinguish aerosol reflectance from top-of-atmosphere (TOA) reflectance. The aerosol optical thickness (AOT) was determined by comparing this aerosol reflectance with pre-calculated values. Three case studies show how the methodology identifies discrepancies between measured and calculated values to retrieve more accurate AOT. The comparison between MODIS and the Aerosol Robotic Network (AERONET) showed improvement using the suggested methodology with the cluster-based look-up-tables (LUTs) (linear slope = 0.94, R = 0.92) than using operational MODIS collection 5 aerosol products (linear slope = 0.78, R = 0.87). In conclusion, the suggested methodology is shown to work well with aerosol models acquired by statistical clustering of the observation data in East Asia.

Description: In this study we explore the sensitivity of satellite observations of the Ring effect (at various wavelengths) to atmospheric aerosol properties. Compared to clouds, aerosols have a rather weak influence on the Ring effect, thus the requirements on the accuracy of the measurements and the radiative transfer simulations are high. In this study, we show that for moderate and high aerosol optical depth (AOD), Ring effect observations are sensitive enough to yield information not only on the AOD, but also on the absorbing properties of aerosols and the aerosol layer height. The latter two quantities are especially important for the determination of the radiative effects of aerosols. Our investigations are based on observations by the satellite instrument SCIAMACHY on ENVISAT (2004–2008) and on model simulations using the Monte-Carlo radiative transfer model McArtim. In addition to the Ring effect we investigate the impact of aerosols on the absorptions of the oxygen molecule (O2) and dimer (O4) as well as the radiance. In general good consistency between measured and simulated quantities is found. In some cases also systematic differences occurred, which are probably mainly related to the strong polarisation sensitivity of the SCIAMACHY instrument. Our study indicates that Ring effect observations have important advantages for aerosol retrievals: they can be analysed with high accuracy in various wavelength ranges; and depending on the wavelength range, they show different sensitivities on aerosol properties like single scattering albedo, optical depth or layer height. The results of this study are of particular interest for future aerosol inversion algorithms for satellite instruments with reduced polarisation sensitivity and smaller ground pixels, capable of measuring the Ring effect with higher accuracy.

Description: We describe a continuous-flow isotope ratio mass spectrometry (CF-IRMS) technique for high-precision δD and δ13C measurements of atmospheric methane on 40 mL air samples. CH4 is separated from other air components by utilizing purely physical processes based on temperature, time and mechanical valve switching. Chemical agents are avoided. Trace amounts of interfering compounds can be separated by gas chromatography after pre-concentration of the CH4 sample. The purified sample is then either combusted to CO2 or pyrolyzed to H2 for stable isotope measurement. Apart from connecting samples and refilling liquid nitrogen as coolant the system is fully automated and allows an unobserved, continuous analysis of samples. The analytical system has been used for analysis of air samples with CH4 mixing ratios between ~100 and ~10 000 ppb, for higher mixing ratios samples usually have to be diluted.

Description: Laboratory calibrations of the Cloud Droplet Probe (CDP) sample area and droplet sizing are performed using water droplets of known size, generated at a known rate. Although calibrations with PSL and glass beads were consistent with theoretical instrument response, liquid water droplet calibrations were not, and necessitated a 2 μm shift in the manufacturer's calibration. We show that much of this response shift may be attributable to a misalignment of the optics relative to the axis of the laser beam. Comparison with an independent measure of liquid water content (LWC) during in-flight operation suggests much greater biases in the droplet size and/or droplet concentration measured by the CDP than would be expected based on the laboratory calibrations. Since the bias in CDP-LWC is strongly concentration dependent, we hypothesize that this discrepancy is a result of coincidence, when two or more droplets pass through the CDP laser beam within a very short time. The coincidence error, most frequently resulting from the passage of one droplet outside and one inside the instrument sample area at the same time, is evaluated in terms of an "extended sample area" (SAE), the area in which individual droplets can affect the sizing detector without necessarily registering on the qualifier. SAE is calibrated with standardized water droplets, and used in a Monte-Carlo simulation to estimate the effect of coincidence on the measured droplet size distributions. The simulations show that extended coincidence errors are important for the CDP at droplet concentrations even as low as 200 cm−3, and these errors are necessary to explain the trend between calculated and measured LWC observed in liquid and mixed-phase clouds during the Aerosol, Radiation and Cloud Processes Affecting Arctic Climate (ARCPAC) study. We estimate from the simulations that 60% oversizing error and 50% undercounting error can occur at droplet concentrations exceeding 400 cm−3. Modification of the optical design of the CDP is currently being explored in an effort to reduce this coincidence bias.

Description: A headspace solid-phase microextraction (HS-SPME) and gas chromatography/mass spectrometry (GC/MS) system has been developed for quantifying enantiomeric and nonenantiomeric monoterpenes in plant chamber studies and ambient air. Performance of this system was checked using a capillary diffusion system to produce monoterpene standards. The adsorption efficiency, competitive adsorption and chromatographic peak resolution of monoterpene enantiomer pairs were compared for three SPME fibre coatings: 75 μm Carboxen-PDMS (CAR-PDMS), 50/30 μm divinylbenzene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS) and 65 μm divinylbenzene-polydimethylsiloxane (DVB-PDMS). Key parameters such as the linearity and reproducibility of the SPME system have been investigated in this work. The best compromise between the enantiomeric separation of monoterpenes and competitive adsorption of the isoprenoids on the solid SPME fibre coating was found for DVB-PDMS fibres. The optimum conditions using DVB-PDMS fibres were applied to measure the exchange rates of monoterpenes in the emission of Quercus ilex using a laboratory whole plant enclosure under light and dark conditions, as well as in ambient air. With 592 and 223 ng m−2 s−1 respectively, β-myrcene and limonene were the predominant monoterpenes in the emission of Q. ilex. These values were closely comparable to those obtained using a zNose and cartridge GC-FID systems.

Description: The calibration of the Moderate-resolution Imaging Spectroradiometer (MODIS) 6.8 μm water vapor (WV) channel was assessed by comparing Terra/MODIS measurements with the WV channel brightness temperatures equivalent to Infrared Atmospheric Sounding Interferometer (IASI) measurements for June 2007 and December 2007. IASI spectral information was transferred to the Japanese Multifunctional Transport Satellite (MTSAT)-1R WV channel and then to the MODIS WV channel. Results indicate that the MODIS WV channel is biased low by 2–3 K, possibly caused by the shift of the spectral response function by about +11 cm−1. This finding is particularly important because the bias of −3 K can cause about +30% relative errors in the retrieval of upper tropospheric humidity (UTH) over the tropics. In other words, the current MODIS measurements may result in higher UTH values, showing relative errors up to +30%.

Description: A direct detection Doppler lidar for measuring wind speed in the middle atmosphere up to 80 km with 2 h resolution was implemented in the ALOMAR Rayleigh/Mie/Raman lidar (69° N, 16° E). The random error of the line of sight wind is about 0.6 m/s and 10 m/s at 49 km and 80 km, respectively. We use a Doppler Rayleigh Iodine Spectrometer (DoRIS) at the iodine line 1109 (~532.260 nm). DoRIS uses two branches of intensity cascaded channels to cover the dynamic range from 10 to 100 km altitude. The wind detection system was designed to extend the existing multi-wavelength observations of aerosol and temperature performed at wavelengths of 355 nm, 532 nm and 1064 nm. The lidar uses two lasers with a mean power of 14 W at 532 nm each and two 1.8 m diameter tiltable telescopes. Below about 49 km altitude the accuracy and time resolution is limited by the maximum count rate of the detectors used and not by the number of photons available. We report about the first simultaneous Rayleigh temperature and wind measurements by lidar in the strato- and mesosphere on 17 and 23 January 2009.

Description: In June 2009, 22 spectrometers from 14 institutes measured tropospheric and stratospheric NO2 from the ground for more than 11 days during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI), at Cabauw, NL (51.97° N, 4.93° E). All visible instruments used a common wavelength range and set of cross sections for the spectral analysis. Most of the instruments were of the multi-axis design with analysis by differential spectroscopy software (MAX-DOAS), whose non-zenith slant columns were compared by examining slopes of their least-squares straight line fits to mean values of a selection of instruments, after taking 30-min averages. Zenith slant columns near twilight were compared by fits to interpolated values of a reference instrument, then normalised by the mean of the slopes of the best instruments. For visible MAX-DOAS instruments, the means of the fitted slopes for NO2 and O4 of all except one instrument were within 10% of unity at almost all non-zenith elevations, and most were within 5%. Values for UV MAX-DOAS instruments were almost as good, being 12% and 7%, respectively. For visible instruments at zenith near twilight, the means of the fitted slopes of all instruments were within 5% of unity. This level of agreement is as good as that of previous intercomparisons, despite the site not being ideal for zenith twilight measurements. It bodes well for the future of measurements of tropospheric NO2, as previous intercomparisons were only for zenith instruments focussing on stratospheric NO2, with their longer heritage.

Description: We present ground-based FTIR (Fourier Transform Infrared) water vapour analyses performed in four different spectral regions: 790–880, 1090–1330, 2650–3180, and 4560–4710 cm−1. All four regions allow the retrieval of lower, middle, and upper tropospheric water vapour amounts with a vertical resolution of about 3, 6, and 10 km, respectively. In addition the analyses at 1090–1330 and 2650–3180 cm−1 allow the retrieval of lower and middle/upper tropospheric δD values with vertical resolutions of 3 and 10 km, respectively. A theoretical and empirical error assessment – taking coincident Vaisala RS92 radiosonde measurements as a reference – suggests that the H2O data retrieved at high wavenumbers are slightly more precise than those retrieved at low wavenumbers. We deduce an H2O profile precision and accuracy of generally better than 20% except for the low wavenumber retrieval at 790–880 cm−1, where the assessed upper precision limit of middle/upper tropospheric H2O is 35%. The scatter between the H2O profiles produced by the four different retrievals is generally below 20% and the bias below 10%, except for the boundary layer, where it can reach 24%. These values well confirm the theoretical and empirical error assessment and are rather small compared to the huge tropospheric H2O variability of about one order of magnitude thereby demonstrating the large consistency between the different H2O profile retrievals. By comparing the two δD profile versions we deduce a precision of about 8 and 17‰ for the lower and middle/upper troposphere, respectively. However, at the same time we observe a systematic difference between the two retrievals of up to 40‰ in the middle/upper troposphere which is a large value compared to the typical tropospheric δD variability of only 80‰.

Description: We present here the aerosol retrieval technique FAR that uses radiative transfer computations in the process of retrieval rather than look-up tables (LUT). This approach provides operational satellite data processing due to the use of the accurate and extremely fast radiative transfer code RAY previously developed by authors along with approximate analytical solutions of the radiative transfer theory. The model of the stratified atmosphere is taken as two coupled layers. Both layers include aerosol scattering and absorption, molecular scattering and gas absorption. The atmosphere parameters are assumed to change from pixel to pixel in the lower atmosphere layer, but the upper stratified layer of the atmosphere over 2–3 km is supposed to be horizontally homogenous for the frame under retrieval. The model of the land spectral albedo is taken as a weighted sum of two a priory chosen basic spectra. The aerosol optical thickness (AOT), Angström exponent and the weight in the land spectral albedo are optimized in the iteration process using the least-squares technique with fast computations of the derivatives of radiative characteristics with respect to retrieved values. The aerosol model and, hence, the aerosol phase function and single scattering albedo, is predefined and does not change in the iteration process. The presented version of FAR is adjusted to process the MERIS data. But it is important that the developed technique can be adapted for processing data of various satellite instruments (including any spectral multi-angle polarization-sensitive sensors). The use of approximate analytical radiative transfer solutions considerably speeds up data processing but may lead to about 15–20% increase of AOT retrieval errors. This approach is advantageous when just the satellite data processing time rather than high accuracy of the AOT retrieval is crucial. A good example is monitoring the trans-boundary transfer of aerosol impurities, particularly in the case of emergencies such as volcano eruptions, or various industrial disasters. Beside, two important problems that determine the accuracy of the AOT retrieval are considered. The first one is the effect of the preliminary choice of the aerosol model, particularly for the retrieval from satellite instruments providing only spectral data (MERIS, MODIS). The second problem is the influence of clouds in adjacent pixels. As for our knowledge, this problem has not been given the required attention up to now and it should be properly accounted for in the AOT retrieval algorithms.

Description: Remotely operable compact instruments for measuring atmospheric CO2 and CH4 column densities were developed in two independent systems: one utilizing a grating-based desktop optical spectrum analyzer (OSA) with a resolution enough to resolve rotational lines of CO2 and CH4 in the regions of 1565–1585 and 1674–1682 nm, respectively; the other is an application of an optical fiber Fabry-Perot interferometer (FFPI) to obtain the CO2 column density. Direct sunlight was collimated via a small telescope installed on a portable sun tracker and then transmitted through an optical fiber into the OSA or the FFPI for optical analysis. The near infrared spectra of the OSA were retrieved by a least squares spectral fitting algorithm. The CO2 and CH4 column densities deduced were in excellent agreement with those measured by a Fourier transform spectrometer with high resolution. The rovibronic lines in the wavelength region of 1570–1575 nm were analyzed by the FFPI. The I0 and I values in the Beer-Lambert law equation to obtain CO2 column density were deduced by modulating temperature of the FFPI, which offered column CO2 with the statistical error less than 0.2% for six hours measurement.

Description: A new method to detect cloud-free snow-covered areas has been developed using the measurements by the Advanced Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite in order to discriminate clear snow fields for the retrieval of aerosol optical thickness or snow properties. The algorithm uses seven AATSR channels from visible (VIS) to thermal infrared (TIR) and analyses the spectral behaviour of each pixel in order to recognize the spectral signature of snow. The algorithm includes a set of relative thresholds and combines all seven channels into one flexible criterion, which allows us to filter out all the pixels with spectral behaviour similar to that of snow. The algorithm does not use any kind of morphological criteria and does not require the studied surface to have any special structure. The snow spectral shape criterion was determined by a comprehensive theoretical study, which included radiative transfer simulations for various atmospheric conditions as well as studying existing models and measurements of optical and physical properties of snow in different spectral bands. The method has been optimized to detect cloud-free snow-covered areas, and does not produce cloud/land/ocean/snow mask. However, the algorithm can be extended and able to discriminate various kinds of surfaces. The presented method has been validated against Micro Pulse Lidar data and compared to Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask over snow-covered areas, showing quite good correspondence to each other. Comparison of both MODIS cloud mask and presented snow mask to AATSR operational cloud mask showed that in some cases of snow surface the accuracy of AATSR operational cloud mask is questionable.

Description: In this paper, we discuss the development of the inversion algorithm for the GOMOS (Global Ozone Monitoring by Occultation of Star) instrument on board the Envisat satellite. The proposed algorithm takes accurately into account the wavelength-dependent modeling errors, which are mainly due to the incomplete scintillation correction in the stratosphere. The special attention is paid to numerical efficiency of the algorithm. The developed method is tested on a large data set and its advantages are demonstrated. Its main advantage is a proper characterization of the uncertainties of the retrieved profiles of atmospheric constituents, which is of high importance for data assimilation, trend analyses and validation.

Description: In this study aerosol optical properties are studied over the Po Valley from June 2006 to February 2009 using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations/Cloud-Aerosol LIdar with Orthogonal Polarization (CALIPSO/CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra. The choice of the Po valley has been driven by the numerous occurrences of pollutant events leading to a mean MODIS-derived aerosol optical depth (AOD) of 0.27 (±0.17) at 550 nm over a large area of ~120 000 km2. AOD derived from MODIS, AERONET and CALIOP have been compared. The comparison with AERONET sun-photometers has highlighted an overestimation of AOD from MODIS radiometers of 0.047 for Aqua and 0.088 for Terra. A systematic underestimation of AOD derived from CALIOP Level-2 products has been observed in comparison to Aqua (0.060) and Terra (0.075) MODIS values. Considering those discrepancies a synergistic approach combining CALIOP level-1 data and MODIS AOD has been developed for the first time over land to retrieve the equivalent extinction-to-backscatter ratio at 532 nm (LR). MODIS-derived AOD were indeed used to constrain CALIOP profiles inversion. A significant number of CALIOP level-1 vertical profiles have been averaged (~200 individual laser shots) in the Po Valley, leading to a signal-to-noise ratio (SNR) higher than 10 in the planetary boundary layer (PBL), which is sufficient to invert the mean lidar profiles. The mean LR (together with the associated variabilities) over the Po Valley retrieved from the coupling between CALIOP/MODIS-Aqua and CALIOP/MODIS-Terra are ~78±22 sr and ~86±27 sr, respectively. The total uncertainty on LR retrieval has been assessed to be ~12 sr using a Monte Carlo approach. The mean LR determined from a look-up table through a selection algorithm in CALIOP level 2 operational products (~63±8 sr) show a good agreement for daytime inversion (70±11 sr for Aqua and 74±14 sr for Terra). These values appear close to what is expected for pollution aerosols in an urban area. Contrarily large differences are observed when considering nighttime CALIOP profiles inverted with daytime AOD from MODIS (63±7 sr for CALIOP level-2 compared with 89±28 sr for CALIOP/Aqua and 103±32 sr for CALIOP/Terra synergies). They can be explained by a significant evolution of AOD between lidar and radiometer passing times. In most of cases, the mean aerosol extinction coefficient in the PBL significantly differs between the level-2 operational products and the result CALIPSO/MODIS synergy results. Mean differences of 0.10 km−1 (~50%) and 0.13 km−1 (~60%) have indeed been calculated using MODIS-Aqua/CALIOP and MODIS-Terra/CALIOP coupling studies, respectively. Such differences may be due to the identification of the aerosol model by the operational algorithm and thus to the choice of the LR.

Description: A model of the sea surface bidirectional reflectance distribution function (BRDF) is presented for the visible and near-IR channels (over the spectral range 550 nm to 1.6 μm) of the dual-viewing Along-Track Scanning Radiometers (ATSRs). The intended application is as part of the Oxford-RAL Aerosols and Clouds (ORAC) retrieval scheme. The model accounts for contributions to the observed reflectance from whitecaps, sun-glint and underlight. Uncertainties in the parametrisations used in the BRDF model are propagated through into the forward model and retrieved state. The new BRDF model offers improved coverage over previous methods, as retrievals are possible into the sun-glint region, through the ATSR dual-viewing system. The new model has been applied in the ORAC aerosol retrieval algorithm to process Advanced ATSR (AATSR) data from September 2004 over the south-eastern Pacific. The assumed error budget is shown to be generally appropriate, meaning the retrieved states are consistent with the measurements and a priori assumptions. The resulting field of aerosol optical depth (AOD) is compared with colocated MODIS-Terra observations, AERONET observations at Tahiti, and cruises over the oceanic region. MODIS and AATSR show similar spatial distributions of AOD, although MODIS reports values which are larger and more variable. It is suggested that assumptions in the MODIS aerosol retrieval algorithm may lead to a positive bias in MODIS AOD of order 0.01 at 550 nm over ocean regions where the wind speed is high.

Description: A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution. The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions.

Description: Methods of UV/VIS absorption spectroscopy to determine the constituents in the Earth's atmosphere from measurements of scattered light are often based on the Beer-Lambert law, like e.g. Differential Optical Absorption Spectroscopy (DOAS). While the Beer-Lambert law is strictly valid for a single light path only, the relation between the optical depth and the concentration of any absorber can be approximated as linear also for scattered light observations at a single wavelength if the absorption is weak. If the light path distribution is approximated not to vary with wavelength, also linearity between the optical depth and the product of the cross-section and the concentration of an absorber can be assumed. These assumptions are widely made for DOAS applications for scattered light observations. For medium and strong absorption of scattered light (e.g. along very long light-paths like in limb geometry) the relation between the optical depth and the concentration of an absorber is no longer linear. In addition, for broad wavelength intervals the differences in the travelled light-paths at different wavelengths become important, especially in the UV, where the probability for scattering increases strongly with decreasing wavelength. However, the DOAS method can be extended to cases with medium to strong absorptions and for broader wavelength intervals by the so called air mass factor modified (or extended) DOAS and the weighting function modified DOAS. These approaches take into account the wavelength dependency of the slant column densities (SCDs), but also require a priori knowledge for the air mass factor or the weighting function from radiative transfer modelling. We describe an approach that considers the fitting results obtained from DOAS, the SCDs, as a function of wavelength and vertical optical depth and expands this function into a Taylor series of both quantities. The Taylor coefficients are then applied as additional fitting parameters in the DOAS analysis. Thus the variability of the SCD in the fit window is determined by the retrieval itself. This new approach provides a description of the SCD the exactness of which depends on the order of the Taylor expansion, and is independent from any assumptions or a priori knowledge of the considered absorbers. In case studies of simulated and measured spectra in the UV range (332–357 nm), we demonstrate the improvement by this approach for the retrieval of vertical profiles of BrO from the SCIAMACHY limb observations. The results for BrO obtained from the simulated spectra are closer to the true profiles, when applying the new method for the SCDs of ozone, than when the standard DOAS approach is used. For the measured spectra the agreement with validation measurements is also improved significantly, especially for cases with strong ozone absorption. While the focus of this article is on the improvement of the BrO profile retrieval from the SCIAMACHY limb measurements, the novel approach may be applied to a wide range of DOAS retrievals.

Description: A new ambient air monitor, the Measurement of Ozone Production Sensor (MOPS), measures directly the rate of ozone production in the atmosphere. The sensor consists of two 11.3 L environmental chambers made of UV-transmitting Teflon film, a unit to convert NO2 to O3, and a modified ozone monitor. In the sample chamber, flowing ambient air is exposed to the sunlight so that ozone is produced just as it is in the atmosphere. In the second chamber, called the reference chamber, a UV-blocking film over the Teflon film prevents ozone formation but allows other processes to occur as they do in the sample chamber. The air flows that exit the two chambers are sampled by an ozone monitor operating in differential mode so that the difference between the two ozone signals, divided by the exposure time in the chambers, gives the ozone production rate. High-efficiency conversion of NO2 to O3 prior to detection in the ozone monitor accounts for differences in the NOx photostationary state that can occur in the two chambers. The MOPS measures the ozone production rate, but with the addition of NO to the sampled air flow, the MOPS can be used to study the sensitivity of ozone production to NO. Preliminary studies with the MOPS on the campus of the Pennsylvania State University show the potential of this new technique.

Description: Laboratory measurements of the Electrochemical Concentration Cell (ECC) ozone sonde cell current using ozone free air as well as defined amounts of ozone reveal that background current measurements during sonde preparation are neither constant as a function of time, nor constant as a function of ozone concentration. Using a background current, measured at a defined timed after exposure to high ozone may often overestimate the real background, leading to artificially low ozone concentrations in the upper tropical troposphere, and may frequently lead to operator dependent uncertainties. Based on these laboratory measurements an improved cell current to partial pressure conversion is proposed, which removes operator dependent variability in the background reading and possible artifacts in this measurement. Data from the Central Equatorial Pacific Experiment (CEPEX) have been reprocessed using the improved background treatment based on these laboratory measurements. In the reprocessed data set near-zero ozone events no longer occur. At Samoa, Fiji, Tahiti, and San Cristóbal, nearly all near-zero ozone concentrations occur in soundings with larger background currents. To a large extent, these events are no longer observed in the reprocessed data set using the improved background treatment.

Description: We report on the development of an accurate, portable, dynamic calibration system for volatile organic compounds (VOCs). The Mobile Organic Carbon Calibration System (MOCCS) combines the production of gas-phase VOC standards using permeation or diffusion sources with quantitative total organic carbon (TOC) conversion on a palladium surface to CO2 in the presence of oxygen, and the subsequent CO2 measurement. MOCCS was validated using three different comparisons: (1) TOC of high accuracy methane standards compared well to expected concentrations (3% relative error), (2) a gas-phase benzene standard was generated using a permeation source and measured by TOC and gas chromatography mass spectrometry (GC-MS) with excellent agreement (

Description: We describe a new instrument (μDirac) capable of measuring halocarbons in the atmosphere. Portability, power efficiency and autonomy were critical design requirements and the resulting instrument can be readily deployed unattended on a range of platforms: long duration balloon, aircraft, ship and ground-based stations. The instrument is a temperature programmed gas chromatograph with electron capture detector (GC-ECD). The design requirements led to μDirac being built in-house with several novel features. It currently measures a range of halocarbons (including short-lived tracers having biogenic and anthropogenic sources) with measurement precision relative standard deviations ranging from ± 1% (CCl4) to ± 9% (CH3I). The prototype instrument was first tested in 2005 and the instrument has been proved in the field on technically challenging aircraft and ground-based campaigns. Results from an aircraft and a ground-based deployment are described.

Description: The accurate determination of nitrogen dioxide (NO2) tropospheric vertical columns from satellite measurements depends strongly on the airmass factor (AMF) used. A sensitivity study was performed with the radiative transfer model SCIATRAN to better understand the impact of aerosols on the calculation of NO2 AMFs. This influence was studied by varying the NO2 and aerosol vertical distributions, as well as physical and optical properties of the particles. In terms of aerosol definitions, the key factors for these calculations were identified as the relation between trace gas and aerosol vertical profiles, the optical depth of the aerosol layer, and single scattering albedo. In addition, surface albedo also has a large impact on the calculations. Overall it was found that particles mixed with the trace gas increases the measurements' sensitivity, but only when the aerosol is not very absorbing. The largest change, a factor of ~2 relative to the situation without aerosols, was found when a low layer of aerosol (600 m) was combined with a homogenous NO2 layer of 1.0 km. A layer of aerosol above the NO2 usually reduces the sensitivity of the satellite measurement. This situation is found mostly for runs with discrete elevated aerosol layers (representative for long-range transport) that can generate a decrease of the AMF values of up to 70%. The use of measured aerosol profiles and modelled NO2 resulted, generally, in much smaller changes of AMF relative to the pure Rayleigh case. Exceptions are some events of elevated layers with high aerosol optical depth that lead to a strong decrease of the AMF values. These results highlight the importance of aerosols in the retrieval of tropospheric NO2 columns from space and indicate the need for detailed information on aerosol properties and vertical distribution.

Description: We have developed a High-Temperature Proton-Transfer-Reaction Mass Spectrometer (HT-PTR-MS) in which both the ion source and the ion drift tube can be continuously operated at temperatures up to 250 °C. The instrument was characterized in a high E/N-mode (130 Td) and in a low E/N-mode (87 Td) at an operating temperature of 200 °C. Instrumental sensitivities and 2σ-detection limits were on the order of 50–110 cps/ppb and 100 ppt (1 s signal integration time), respectively. The HT-PTR-MS is primarily intended for measuring "sticky" or semi-volatile trace gases. Alternatively, it may be coupled to a particle collection/thermal desorption apparatus to measure particle-bound organics in near real-time. In view of these applications, we have measured instrumental response times for a series of reference compounds. 1/e2-response times for dimethyl sulfoxide, ammonia and monoethanolamine were in the sub-second to second regime. 1/e2-response times for levoglucosan, oxalic acid and cis-pinonic acid ranged from 8 to 370 s.

Description: A compact, fast-response Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of ammonia (NH3) has been evaluated under both laboratory and field conditions. Absorption of radiation from a pulsed, thermoelectrically cooled QC laser occurs at reduced pressure in a 0.5 L multiple pass absorption cell with an effective path length of 76 m. Detection is achieved using a thermoelectrically-cooled Mercury Cadmium Telluride (HgCdTe) infrared detector. A novel sampling inlet was used, consisting of a short, heated, quartz tube with a hydrophobic coating to minimize the adsorption of NH3 to surfaces. The inlet contains a critical orifice that reduces the pressure, a virtual impactor for separation of particles, and additional ports for delivering NH3-free background air and calibration gas standards. The level of noise in this instrument has been found to be 0.23 ppb at 1 Hz. The sampling technique has been compared to the results of a conventional lead salt Tunable Diode Laser Absorption Spectrometer (TDLAS) during a laboratory intercomparison. The effect of humidity and heat on the surface interaction of NH3 with sample tubing was investigated at mixing ratios ranging from 30–1000 ppb. Humidity was seen to worsen the NH3 time response and considerable improvement was observed when using a heated sampling line. A field intercomparison of the QC-TILDAS with a modified Thermo 42CTL chemiluminescence-based analyzer was also performed at Environment Canada's Centre for Atmospheric Research Experiments (CARE) in the rural town of Egbert, ON between May–July 2008. Background tests and calibrations using two different permeation tube sources and an NH3 gas cylinder were regularly carried out throughout the study. Results indicate a very good correlation at 1 min time resolution (R2 = 0.93) between the two instruments at the beginning of the study, when regular background subtraction was applied to the QC-TILDAS. An overall good correlation of R2 = 0.85 was obtained over the entire two month data set, where the majority of the spread can be attributed to differences in inlet design and background subtraction methods.

Description: Quasi-continuous, in-situ measurements of atmospheric CO2, O2/N2, CH4, CO, N2O, and SF6 have been performed since August 2005 at the tall tower station near Bialystok, in Eastern Poland, from five heights up to 300 m. Besides the in-situ measurements, flask samples are filled approximately weekly and measured at Max-Planck Institute for Biogeochemistry for the same species and, in addition, for H2, Ar/N2 and the stable isotopes 13C and 18O in CO2. The in-situ measurement system was built based on commercially available analysers: a LiCor 7000 for CO2, a Sable Systems "Oxzilla" FC-2 for O2, and an Agilent 6890 gas chromatograph for CH4, CO, N2O and SF6. The system was optimized to run continuously with very little maintenance and to fulfill the precision requirements of the CHIOTTO project. The O2/N2 measurements in particular required special attention in terms of technical setup and quality assurance. The evaluation of the performance after more than three years of operation gave overall satisfactory results, proving that this setup is suitable for long term remote operation with little maintenance. The precision achieved for all species is within or close to the project requirements. The comparison between the in-situ and flask sample results, used to verify the accuracy of the in-situ measurements, showed no significant difference for CO2, O2/N2, CH4 and N2O, and a very small difference for SF6. The same comparison however revealed a statistically significant difference for CO, of about 6.5 ppb, for which the cause could not be fully explained. From more than three years of data, the main features at Bialystok have been characterized in terms of variability, trends, and seasonal and diurnal variations. CO2 and O2/N2 show large short term variability, and large diurnal signals during the warm seasons, which attenuate with the increase of sampling height. The trends calculated from this dataset, over the period August 2005 to December 2008, are 2.02±0.46 ppm/year for CO2 and −23.2±2.5 per meg/year for O2/N2. CH4, CO and N2O show also higher variability at the lower sampling levels, which in the case of CO is strongly seasonal. Diurnal variations in CH4, CO and N2O mole fractions can be observed during the warm season, due to the periodicity of vertical mixing combined with the diurnal cycle of anthropogenic emissions. We calculated increase rates of 10.1±4.4 ppb/year for CH4, (−8.3)±5.3 ppb/year for CO and 0.67±0.08 ppb/year for N2O. SF6 shows only few events, and generally no vertical gradients, which suggests that there are no significant local sources. A weak SF6 seasonal cycle has been detected, which most probably is due to the seasonality of atmospheric circulation. SF6 increased during the time of our measurement at an average rate of 0.29±0.01 ppt/year.

Description: The recently developed PTR-TOF instrument was evaluated to measure methanol fluxes emitted from grass land using the eddy covariance method. The high time resolution of the PTR-TOF allowed storing full mass spectra up to m/z 315 with a frequency of 10 Hz. Three isobaric ions were found at a nominal mass of m/z 33 due to the high mass resolving power of the PTR-TOF. Only one of the three peaks contributed to eddy covariance fluxes. The exact mass of this peak agrees well with the exact mass of protonated methanol (m/z 33.0335). The eddy covariance methanol fluxes measured with PTR-TOF were compared to virtual disjunct eddy covariance methanol fluxes simultaneously measured with a conventional PTR-MS. The methanol fluxes from both instruments show excellent agreement.

Description: High-accuracy continuous measurements of greenhouse gases (CO2 and CH4) during the BARCA (Balanço Atmosférico Regional de Carbono na Amazônia) phase B campaign in Brazil in May 2009 were accomplished using a newly available analyzer based on the cavity ring-down spectroscopy (CRDS) technique. This analyzer was flown without a drying system or any in-flight calibration gases. Water vapor corrections associated with dilution and pressure-broadening effects for CO2 and CH4 were derived from laboratory experiments employing measurements of water vapor by the CRDS analyzer. Before the campaign, the stability of the analyzer was assessed by laboratory tests under simulated flight conditions. During the campaign, a comparison of CO2 measurements between the CRDS analyzer and a nondispersive infrared (NDIR) analyzer on board the same aircraft showed a mean difference of 0.22±0.09 ppm for all flights over the Amazon rain forest. At the end of the campaign, CO2 concentrations of the synthetic calibration gases used by the NDIR analyzer were determined by the CRDS analyzer. After correcting for the isotope and the pressure-broadening effects that resulted from changes of the composition of synthetic vs. ambient air, and applying those concentrations as calibrated values of the calibration gases to reprocess the CO2 measurements made by the NDIR, the mean difference between the CRDS and the NDIR during BARCA was reduced to 0.05±0.09 ppm, with the mean standard deviation of 0.23±0.05 ppm. The results clearly show that the CRDS is sufficiently stable to be used in flight without drying the air or calibrating in flight and the water corrections are fully adequate for high-accuracy continuous airborne measurements of CO2 and CH4.

Description: PREMIER is one of three candidates for ESA's 7th Earth Explorer mission that are currently undergoing feasibility studies. The main mission objective of PREMIER is to quantify processes controlling atmospheric composition in the mid/upper troposphere and lower stratosphere, a region of particular importance for climate change. To achieve this objective, PREMIER will employ the first satellite Fourier transform infrared limb-imager with a 2-D detector array combined with a millimetre-wave limb-sounder. The infrared limb-imager can be operated in a high spatial resolution mode ("dynamics mode") for observations of small-scale structures in atmospheric temperatures and trace gas fields with unprecedented 3-D sampling (0.5 km in the vertical direction, 50 km along track, 25 km across track). In this paper, a fast tomographic retrieval scheme is presented, which is designed to fully exploit the high-resolution radiance observations of the dynamics mode. Based on a detailed analysis of the "observational filter", we show that the dynamics mode provides unique information on global distributions of gravity waves (GW). The achievable vertical resolution for GW observations has values between the vertical sampling (0.5 km) of the dynamics mode and the vertical field of view (about 0.75 km). The horizontal across track resolution corresponds to the horizontal across track sampling of 25 km. Since the achievable along track horizontal resolution is about 70 km, the dynamics mode will provide GW limb-observations with a horizontal resolution comparable to nadir sounders. Compared to previous observations, PREMIER will therefore considerably extend the range of detectable GWs in terms of horizontal and vertical wavelength.

Description: At the Izaña Observatory, water vapour amounts have been measured routinely by different techniques for many years. We intercompare the total precipitable water vapour (PWV) amounts measured between 2005 and 2009 by a Fourier Transform Infrared (FTIR) spectrometer, a Multifilter Rotating Shadow-band Radiometer (MFRSR), a Cimel sunphotometer, a Global Positioning System (GPS) receiver, and daily radiosondes (Vaisala RS92). The long-term characteristics of our study allows a reliable and extensive empirical quality assessment of long-term validity, which is an important prerequisite when applying the data to climate research. We estimate a PWV precision of 1% for the FTIR, about 10% for the MFRSR, Cimel, and GPS (when excluding rather dry conditions), and significantly better than 15% for the RS92 (the detection of different airmasses avoids a better constrained estimation). We show that the MFRSR, Cimel and GPS data quality depends on the atmospheric conditions (humid or dry) and that the restriction to clear-sky observations introduces a significant dry bias in the FTIR and Cimel data. In addition, we intercompare the water vapour profiles measured by the FTIR and the Vaisala RS92, which allows the conclusion that both experiments are able to detect lower to upper tropospheric water vapour mixing ratios with a precision of better than 15%.

Description: Four hydroxynitrates (R(OH)R'ONO2) representative of atmospheric volatile organic compound (VOC) oxidation products were synthesized, nebulized and sampled into an Aerodyne High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS). The resulting mass spectrum was used to evaluate calibration factors for elemental analysis of organic nitrates by AMS, and to determine the distribution of nitrogen in the detected fragments in a search for an AMS signature of organic nitrates. We found that 30% of the detected nitrogen mass is in the NO+ and NO2+ fragments, 12% at NHx+ fragments, 5% at CxHyOzN+ fragments, and 53% at various CxHyN+ fragments. Elemental analysis indicated that nitrogen was detected with higher efficiency than carbon and hydrogen, but oxygen was detected with reduced efficiency compared to previously reported results for a suite of organics which did not include organic nitrates. The results are used to suggest the maximum corrections to ambient O:C and N:C ratios based on AMS measurements.

Description: The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) is a long-term monitoring program making regular atmospheric measurements from an instrument container installed monthly aboard a passenger aircraft. Typical cruising altitudes of the aircraft allow for the study of the free troposphere and the extra-tropical upper troposphere as well as the lowermost stratosphere. CARIBIC measurements include a number of real time analyses as well as the collection of aerosol and whole air samples. These whole air samples are analyzed post-flight for a suite of trace gases, which includes non-methane hydrocarbons (NMHC). The NMHC measurement system and its analytical performance are described here. Precision was found to vary slightly by compound, and is less than 2% for the C2–C6 alkanes and ethyne, and between 1% and 6% for C7–C8 alkanes and aromatic compounds. Preliminary results from participation in a Global Atmospheric Watch (WMO) VOC audit indicate accuracies within the precision of the system. Limits of detection are 1 pptv for most compounds, and up to 3 pptv for some aromatics. These are sufficiently low to measure mixing ratios typically observed in the upper troposphere and lowermost stratosphere for the longer-lived NMHC, however, in air samples from these regions many of the compounds with shorter lifetimes (

Description: The SCIAMACHY instrument onboard the ESA satellite ENVISAT allows measurements of various atmospheric trace gases, such as NO2. A unique feature of SCIAMACHY is that measurements are made alternately in limb and nadir mode. The limb measurements provide an opportunity for directly determining stratospheric column densities (CDs), which are needed to extract tropospheric CDs from the total CD measurements performed in (quasi simultaneous) nadir geometry. Here we discuss the potential and limitations of SCIAMACHY limb measurements for estimating stratospheric CDs of NO2 in comparison to a simple reference sector method, and the consequences for the resulting tropospheric CDs. A direct, absolute limb correction scheme is presented that improves spatial patterns of tropospheric NO2 column densities at high latitudes, but results in artificial zonal stripes at low latitudes. Subsequently, a relative limb correction scheme is introduced that successfully reduces stratospheric artefacts in the tropospheric data product without introducing new ones. This relative limb correction scheme is rather simple, robust, and, in essence, based on measurements alone. The effects of the different stratospheric estimation schemes on tropospheric CDs are discussed with respect to zonal and temporal dependencies. In addition, we define error quantities from the nadir/limb measurements that indicate remaining systematic errors as a function of latitude and day. Our new suggested stratospheric estimation scheme, the relative limb correction, improves mean tropospheric slant CDs significantly, e.g. from −1×1015 molec/cm2 (using a reference sector method) to ≈0 in the Atlantic ocean, and from +1×1015 molec/cm2 to ≈0 over Siberia, at 50° N in January 2003–2008.

Description: In this work, we modelled activation and growth of silver particles in the water-operated TSI model 3785 water condensation particle counter (WCPC). Our objective was to investigate theoretically how various effects influence the counting efficiency of this CPC. Coupled fluid and particle dynamic processes were modelled with the computational fluid dynamics (CFD) code FLUENT in combination with the Fine Particle Model (FPM) to obtain profiles of temperature, vapour concentration, nucleation rate, and particle size. We found that the counting efficiency of the TSI 3785 for small particles might be affected by the presence of larger particles. Moreover, homogeneous nucleation processes can significantly influence counting efficiency.

Description: In this work we present the setup and first tests of our new BIO IN detector. This detector was constructed to classify atmospheric ice nuclei (IN) for their biological content. It is designed to be coupled to the Fast Ice Nucleus CHamber FINCH. If one particle acts as an ice nucleus, it will be at least partly covered with ice at the end of the development section of the FINCH chamber. The device combines an auto-fluorescence detector and a circular depolarization detector for simultaneous detection of biological material and discrimination between water droplets, ice crystals and non activated large aerosol particles. The excitation of biological material with UV light and analysis of auto-fluorescence is a common principle used for flow cytometry, fluorescence microscopy, spectroscopy and imaging. The detection of auto-fluorescence of airborne single particles demands some more experimental effort. However, expensive commercial sensors are available for special purposes, e.g. size distribution measurements. But these sensors will not fit the specifications needed for the FINCH IN counter (e.g. high sample flow of up 10 LPM). The newly developed -low cost- BIO IN sensor uses a single high-power UV LED for the electronic excitation instead of much more expensive UV lasers. Other key advantages of the new sensor are the low weight, compact size, and the little effect on the aerosol sample, which allows it to be coupled with other instruments for further analysis. The instrument will be flown on one of the first missions of the new German research aircraft "HALO" (High Altitude and LOng range).

Description: A new retrieval method has been developed to derive water vapour number density profiles from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). This method is intentionally kept simple and based on a combination of an onion peeling approach with a modified DOAS (Differential Optical Absorption Spectroscopy) fit in the wavelength region around 940 nm. Reasonable resulting water vapour profiles are currently obtained in the altitude range 15–45 km. Comparisons of the SCIAMACHY profiles with water vapour data provided by the Atmospheric Chemistry Explorer Fourier Transform Spectrometer (ACE-FTS) show an average agreement within about 5% between 20 and 45 km. SCIAMACHY water vapour data tend to be systematically higher than ACE-FTS. These results are in principal confirmed by comparisons with water vapour profiles derived from model data of the European Centre for Medium Range Weather Forecasts (ECMWF), although ECMWF concentrations are systematicly lower than both corresponding SCIAMACHY and ACE-FTS data at all altitudes.

Description: The recently increasing development of whole sky imagers enables temporal and spatial high-resolution sky observations. One application already performed in most cases is the estimation of fractional sky cover. A distinction between different cloud types, however, is still in progress. Here, an automatic cloud classification algorithm is presented, based on a set of mainly statistical features describing the color as well as the texture of an image. The k-nearest-neighbour classifier is used due to its high performance in solving complex issues, simplicity of implementation and low computational complexity. Seven different sky conditions are distinguished: high thin clouds (cirrus and cirrostratus), high patched cumuliform clouds (cirrocumulus and altocumulus), stratocumulus clouds, low cumuliform clouds, thick clouds (cumulonimbus and nimbostratus), stratiform clouds and clear sky. Based on the Leave-One-Out Cross-Validation the algorithm achieves an accuracy of about 97%. In addition, a test run of random images is presented, still outperforming previous algorithms by yielding a success rate of about 75%, or up to 88% if only "serious" errors with respect to radiation impact are considered. Reasons for the decrement in accuracy are discussed, and ideas to further improve the classification results, especially in problematic cases, are investigated.

Description: A buoy based instrument platform (the "O-buoy") was designed, constructed, and field tested for year-round measurement of ozone, bromine monoxide, carbon dioxide, and meteorological variables over Arctic sea ice. The O-buoy operated in an autonomous manner with daily, bi-directional data transmissions using Iridium satellite communication. The O-buoy was equipped with three power sources: primary lithium-ion battery packs, rechargeable lead acid packs, and solar panels that recharge the lead acid packs, and can fully power the O-buoy during summer operation. This system was designed to operate under the harsh conditions present in the Arctic, with minimal direct human interaction, to aid in our understanding of the atmospheric chemistry that occurs in this remote region of the world. The current design requires approximately yearly maintenance limited by the lifetime of the primary power supply. The O-buoy system was field tested in Elson Lagoon, Barrow, Alaska from February to May 2009, and deployed in the Beaufort Sea in October 2009. Here, we describe the design and present preliminary data.

Description: A fast response ozone analyzer based on the ozone-nitric oxide chemiluminescence method was integrated into the NOAA-ESRL flux system to achieve the first ship-borne, direct ozone flux measurements over the open ocean. Air was collected from an inlet at 18 m height over the ocean surface mounted to the bow-jackstaff and via a 30 m-long sampling line to the ozone instrument on the ship deck. A "puff" system was used for accurate and regular determination of the sample transport time (lag) between the inlet and the chemical analyzer. A Nafion-membrane dryer facilitated removal of fast water vapor fluctuations, which eliminated the need for quenching and density correction of the ozone signal. The sampling-analyzer system was found to have a ~0.25–0.40 s response time at a sensitivity of ~2800 counts s−1 per ppbv of ozone. Quality control and data filtering procedures for eliminating data that did not meet measurement requirements were critically evaluated. The new ozone flux system was deployed aboard the NOAA Ship Ronald H. Brown, and evaluated using results obtained during several research cruises off the coasts of the North and South America continents, yielding ozone deposition velocities (mean ± standard error) ranging from 0.009±0.001 cm s−1 to 0.24±0.020 cm s−1.

Description: The aerosol light absorption coefficient is an essential parameter involved in atmospheric radiation budget calculations. The Aethalometer (AE) has the great advantage of measuring the aerosol light absorption coefficient at several wavelengths, but the derived absorption coefficients are systematically too high when compared to reference methods. Up to now, four different correction algorithms of the AE absorption coefficients have been proposed by several authors. A new correction scheme based on these previously published methods has been developed, which accounts for the optical properties of the aerosol particles embedded in the filter. All the corrections have been tested on six datasets representing different aerosol types and loadings and include multi-wavelength AE and white-light AE. All the corrections have also been evaluated through comparison with a Multi-Angle Absorption Photometer (MAAP) for four datasets lasting between 6 months and five years. The modification of the wavelength dependence by the different corrections is analyzed in detail. The performances and the limits of all AE corrections are determined and recommendations are given.

Description: Multiple Axis Differential Optical Absorption Spectrometer (MAX-DOAS) systems are inherently very simple instruments, which have been shown to provide extremely useful information about a wide variety of environmental parameters. In order to exploit the potential of the technique we have developed a new field-deployable, passive MAX-DOAS system that is automated and uses little power (

Description: From Multi-Axis- (MAX-) DOAS observations, information on tropospheric trace gases close to the surface and up to the free troposphere can be obtained. Usually MAX-DOAS observations are performed at fixed locations, which allows to retrieve the diurnal variation of tropospheric species at that location. Alternatively, MAX-DOAS observations can also be made on mobile platforms like cars, ships or aircrafts. Then, in addition to the vertical (and temporal) distribution, also the horizontal variation of tropospheric trace gases can be measured. Such information is important for the quantitative comparison with model simulations, study of transport processes, and for the validation of tropospheric trace gas products from satellite observations. However, for MAX-DOAS observations from mobile platforms, the standard analysis techniques for MAX-DOAS observations can usually not be applied, because the probed airmasses can change rapidly between successive measurements. In this study we introduce a new technique which overcomes these problems and allows the exploitation of the full information content of mobile MAX-DOAS observations. Our method can also be applied to MAX-DOAS observations made at fixed locations in order to improve the accuracy especially in cases of strong winds. We apply the new technique to MAX-DOAS observations made during an automobile trip from Brussels to Heidelberg.

Description: A new atmospheric measurement station has been established on the North Sea oil and gas production platform F3, 200 km north off the Dutch coast (5°51' N, 4°44' E). Atmospheric concentrations of O2 and CO2 are continuously measured using fuel cell technology and compact infrared absorption instruments, respectively. Furthermore, the station includes an automated air flask sampler for laboratory analysis of the atmospheric concentrations of CO2, CH4, CO and O2 and isotope measurements of δ13C, δ18O and Δ14C from CO2. This station is the first fixed sea based station with on-site continuous O2 and CO2 measurements and therefore yields valuable information about the CO2 uptake in coastal marine regions, specifically the North Sea. This paper presents the measurement station and the used methodologies in detail. In comparison to land-based stations, the data show low day-to-day variability, as they are practically free of nightly inversions as well as human influences, due to the station's remoteness. Therefore, the data set collected at this measurement station serves directly as background data for the coastal northwest European region. Additionally, the first data are presented showing the seasonal cycle as expected during August 2008 through June 2009. Furthermore, some short-term O2 and CO2 signals are presented. The observations at the platform include several large and fast changing negative atmospheric O2 excursions without an accompanying change in the CO2 signal, which most likely indicate marine O2 uptake.

Description: A range of bubble and sea spray aerosol generators has been tested in the laboratory and compared with oceanic measurements from the literature. We have shown that the method of generation has a significant influence on the properties of the aerosol particles produced. Hence, the validity of a generation system to mimic atmospheric aerosol is dependent on its capacity for generating bubbles and particles in a realistic manner. A bubble-bursting aerosol generator which produces bubbles by water impingement was shown to best reproduce the oceanic bubble spectral shapes, which confirms previous findings. Two porous bubblers and a plunging-water jet system were tested as bubble-bursting aerosol generators for comparison with a standard nebulizer. The methods for aerosol production were evaluated by analysing the bubble spectrum generated by the bubble-bursting systems and the submicron size distribution, hygroscopicity and cloud condensation nucleus activity of the aerosols generated by the different techniques. Significant differences in the bubble spectrum and aerosol properties were observed when using different aerosol generators. The aerosols generated by the different methods exhibited similar hygroscopicity and cloud condensation nucleus activity behaviour when a sample of purely inorganic salts was used as a parent seawater solution; however, significant differences in the aerosol properties were found when using samples of filtered natural seawater enriched with biogenic organics. The presence of organics in the aerosol caused suppression of the growth factor at humidities above 75% RH and an increase in the critical supersaturation with respect to the generation from artificial seawater devoid of organics. The extent of the effect of organics on the aerosol properties varied depending on the method of particle production. The results of this work indicate that the aerosol generation mechanism affects the particles organic enrichment, thus the behaviour of the produced aerosols strongly depends on the laboratory aerosol generator employed. Comparison between bubble lifetimes in several laboratory simulations and the oceanic conditions indicated that it would require a considerable extension of the dimensions of the currently used bubble-bursting laboratory systems in order to replicate the characteristic oceanic bubble lifetimes. We analyzed the implications derived from the reduced bubble residence times in scaled systems, regarding marine surfactants adsorption on rising bubbles, and found that adsorption equilibrium is reached on a timescale much shorter than the bubble lifetime in small-scale laboratory generators. This implies that adsorption of marine surface-active material is not limited by surfactant transport to the bubble surface.

Description: The detection of multiple cloud layers using satellite observations is important for retrieval algorithms as well as climate applications. In this paper, we describe a relatively simple algorithm to detect multiple cloud layers and distinguish them from vertically-extended clouds. The algorithm can be applied to coincident passive sensors that derive both cloud-top pressure from the thermal infrared observations and an estimate of solar photon pathlength from UV, visible, or near-IR measurements. Here, we use data from the A-train afternoon constellation of satellites: cloud-top pressure, cloud optical thickness, the multi-layer flag from the Aqua MODerate-resolution Imaging Spectroradiometer (MODIS) and the optical centroid cloud pressure from the Aura Ozone Monitoring Instrument (OMI). For the first time, we use data from the CloudSat radar to evaluate the results of a multi-layer cloud detection scheme. The cloud classification algorithms applied with different passive sensor configurations compare well with each other as well as with data from CloudSat. We compute monthly mean fractions of pixels containing multi-layer and vertically-extended clouds for January and July 2007 at the OMI spatial resolution (12 km×24 km at nadir) and at the 5 km×5 km MODIS resolution used for infrared cloud retrievals. There are seasonal variations in the spatial distribution of the different cloud types. The fraction of cloudy pixels containing distinct multi-layer cloud is a strong function of the pixel size. Globally averaged, these fractions are approximately 20% and 10% for OMI and MODIS, respectively. These fractions may be significantly higher or lower depending upon location. There is a much smaller resolution dependence for fractions of pixels containing vertically-extended clouds (~20% for OMI and slightly less for MODIS globally), suggesting larger spatial scales for these clouds. We also find higher fractions of vertically-extended clouds over land as compared with ocean, particularly in the tropics and summer hemisphere.

Description: We present a detailed assessment of a commercially available water vapor isotope analyzer (WVIA, Los Gatos Research, Inc.) for simultaneous in-situ measurements of δ2H and δ18O in water vapor. This method, based on off-axis integrated cavity output spectroscopy, is an alternative to the conventional water trap/isotope ratio mass spectrometry (IRMS) techniques. We evaluate the analyzer in terms of precision, memory effects, concentration dependence, temperature sensitivity and long-term stability. A calibration system based on a droplet generator is used to characterize the performance and to calibrate the analyzer. Our results show that the precision at an averaging time of 15 s is 0.16‰ for δ2H and 0.08‰ for δ18O. The isotope ratios are strongly dependent on the water mixing ratio of the air. Taking into account this concentration dependence as well as the temperature sensitivity of the instrument we obtained a long-term stability of the water isotope measurements of 0.38‰ for δ2H and 0.25‰ for δ18O. The accuracy of the WVIA was further assessed by comparative measurements using IRMS and a dew point generator indicating a linear response in isotopic composition and H2O concentrations. The WVIA combined with a calibration system provides accurate high resolution water vapor isotope measurements and opens new possibilities for hydrological and ecological applications.

Description: The Canadian Arctic Atmospheric Chemistry Experiment Validation Campaigns have been carried out at Eureka, Nunavut (80.05° N, 86.42° W) during the polar sunrise period since 2004. During the International Polar Year (IPY) springs of 2007 and 2008, three ground-based Fourier transform infrared (FTIR) spectrometers were operated simultaneously. This paper presents a comparison of trace gas measurements of stratospherically important species involved in ozone depletion, namely O3, HCl, ClONO2, HNO3 and HF, recorded with these three spectrometers. Total column densities of the gases measured with the new Canadian Network for the Detection of Atmospheric Change (CANDAC) Bruker 125HR are shown to agree to within 3.5% with the existing Environment Canada Bomem DA8 measurements. After smoothing both of these sets of measurements to account for the lower spectral resolution of the University of Waterloo Portable Atmospheric Research Interferometric Spectrometer for the Infrared (PARIS-IR), the measurements were likewise shown to agree with PARIS-IR to within 7%. Concurrent measurements of these gases were also made with the satellite-based Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) during overpasses of Eureka during these time periods. While one of the mandates of the ACE satellite mission is to study ozone depletion in the polar spring, previous validation exercises have identified the highly variable polar vortex conditions of the spring period to be a challenge for validation efforts. In this work, comparisons between the CANDAC Bruker 125HR and ACE-FTS have been used to develop strict criteria that allow the ground- and satellite-based instruments to be confidently compared. When these criteria are taken into consideration, the observed biases between the ACE-FTS and ground-based FTIR spectrometer are not persistent for both years and are generally insignificant, though small positive biases of ~5%, comparable in magnitude to those seen in previous validation exercises, are observed for HCl and HF in 2007, and negative biases of −15.3%, −4.8% and −1.5% are seen for ClONO2, HNO3 and O3 in 2008.

Description: Relative humidity (RH) influences the water content of aerosol particles and therefore has an important impact on the particles' ability to scatter visible light. The RH dependence of the particle light scattering coefficient (σsp is therefore an important measure for climate forcing calculations. We built a humidification system for a nephelometer which allows the measurement of σsp at a defined RH in the range of 40–90%. This RH conditioner consists of a humidifier followed by a dryer, which enables us to measure the hysteresis behavior of deliquescent aerosol particles. In this paper we present the set-up of a new humidified nephelometer, a detailed characterization with well defined laboratory generated aerosols, and a first application in the field by comparing our instrument to another humidified nephelometer. Monodisperse ammonium sulfate and sodium chloride particles were measured at four different dry particle sizes. Agreement between measurement and prediction based on Mie theory was found for both σsp and f(RH)=σsp(RH)/σsp(dry) within the range of uncertainty. The two humidified nephelometers measuring at a rural site in the Black Forest (Germany) often detected different f(RH), probably caused by the aerosol hysteresis behavior: when the aerosol was metastable, therefore was scattering more light, only one instrument detected the higher f(RH).

Description: NO2 concentrations were measured by various instruments during the NO3Comp campaign at the atmosphere simulation chamber SAPHIR at Forschungszentrum Jülich, Germany, in June 2007. Analytical methods included photolytic conversion with chemiluminescence (PC-CLD), broadband cavity ring-down spectroscopy (BBCRDS), pulsed cavity ring-down spectroscopy (CRDS), incoherent broadband cavity-enhanced absorption spectroscopy (IBB-CEAS), and laser-induced fluorescence (LIF). All broadband absorption spectrometers were optimized for the detection of the main target species of the campaign, NO3, but were also capable of detecting NO2 simultaneously with reduced sensitivity. NO2 mixing ratios in the chamber were within a range characteristic of polluted, urban conditions, with a maximum mixing ratio of approximately 75 ppbv. The overall agreement between measurements of all instruments was excellent. Linear fits of the combined data sets resulted in slopes that differ from unity only within the stated uncertainty of each instrument. Possible interferences from species such as water vapor and ozone were negligible under the experimental conditions.

Description: The presence of molecular iodine in the atmosphere is thought to have implications for both climate and human nutritional health, but measurement of the gas at low concentrations requires technically demanding techniques that are not widely accessible. Here, amylose coated denuder tubes and solvent traps coupled with spectrophotometric detection are evaluated and compared as relatively cheap and straightforward methods to measure gaseous molecular iodine at environmentally relevant concentrations. Denuder tubes were found to give unacceptably low and highly variable recoveries of molecular iodine from a test gas source, with values ranging from 1 to 62%. Blank concentrations were also high, being equivalent to a gas phase concentration of 5 pptv under typical operating conditions. Ethanol and hexane solvent traps gave much better performance. Optimisation of the hexane solvent trap method gave 100% recovery and an atmospheric limit of detection of 70 pptv, which is within the range of concentrations observed in the coastal marine atmosphere.

Description: Thermal-optical analysis is a conventional method for determining the carbonaceous aerosol fraction and for classifying it into organic carbon, OC, and elemental carbon, EC. Unfortunately, the different thermal evolution protocols in use can result in a wide elemental carbon-to-total carbon variation by up to a factor of five. In Europe, there is currently no standard procedure for determining the carbonaceous aerosol fraction which implies that data from different laboratories at various sites are of unknown accuracy and cannot be considered comparable. In the framework of the EU-project EUSAAR (European Supersites for Atmospheric Aerosol Research), a comprehensive study has been carried out to identify the causes of differences in the EC measured using different thermal evolution protocols; thereby the major positive and negative biases affecting thermal-optical analysis have been isolated and minimised to define an optimised protocol suitable for European aerosols. Our approach to improve the accuracy of the discrimination between OC and EC was essentially based on four goals. Firstly, charring corrections rely on faulty assumptions – e.g. pyrolytic carbon is considered to evolve completely before native EC throughout the analysis –, thus we have reduced pyrolysis to a minimum by favoring volatilisation of OC. Secondly, we have minimised the potential negative bias in EC determination due to early evolution of light absorbing carbon species at higher temperatures in the He-mode, including both native EC and combinations of native EC and pyrolytic carbon potentially with different specific attenuation cross section values. Thirdly, we have minimised the potential positive bias in EC determination resulting from the incomplete evolution of OC during the He-mode which then evolves during the He/O2-mode, potentially after the split point. Finally, we have minimised the uncertainty due to the position of the OC/EC split point on the FID response profile by introducing multiple desorption steps in the He/O2-mode. Based on different types of carbonaceous PM encountered across Europe, we have defined an optimised thermal evolution protocol, the EUSAAR_2 protocol, as follows: step 1 in He, 200 °C for 120 s; step 2 in He 300 °C for 150 s; step 3 in He 450 °C for 180 s; step 4 in He 650 °C for 180 s. For steps 1–4 in He/O2, the conditions are 500 °C for 120 s, 550 °C for 120 s, 700 ° C for 70 s, and 850 °C for 80 s, respectively.

Description: Mass spectrometric measurement of DMS by atmospheric pressure ionization with an isotopically labeled standard (APIMS-ILS) is a sensitive method with sufficient bandpass for direct flux measurements by eddy correlation. Use of an isotopically labeled internal standard greatly reduces instrumental drift, improving accuracy and precision. APIMS-ILS has been used in several recent campaigns to study ocean-atmosphere gas transfer and the chemical budget of DMS in the marine boundary layer. This paper provides a comprehensive description of the method and errors associated with DMS flux measurement from ship platforms. The APIMS-ILS instrument used by most groups today has a sensitivity of 100–200 counts s−1 pptv−1, which is shown to be more than sufficient for flux measurement by eddy covariance. Mass spectral backgrounds (blanks) are determined by stripping DMS from ambient air with gold. The instrument is found to exhibit some high frequency signal loss, with a half-power frequency of ≈1 Hz, but a correction based on an empirically determined instrument response function is presented. Standard micrometeorological assumptions of steady state and horizontal uniformity are found to be appropriate for DMS flux measurement, but rapid changes in mean DMS mixing ratio may serve as a warning that measured flux does not represent the true surface flux. In addition, bias in surface flux estimates arising from the flux divergence is not generally significant in the surface layer, but under conditions of lowered inversion and high flux may become so. The effects of error in motion corrections and of vertical motion within the surface layer concentration gradient are discussed and the estimated maximum error from these effects is ≤18%.

Description: The aerodynamic gradient method is widely used for flux measurements of ammonia, nitric acid, particulate ammonium nitrate (the NH3-HNO3-NH4NO3 triad) and other water-soluble reactive trace compounds. The surface exchange flux is derived from a measured concentration difference and micrometeorological quantities (turbulent exchange coefficient). The significance of the measured concentration difference is crucial for the significant determination of surface exchange fluxes. Additionally, measurements of surface exchange fluxes of ammonia, nitric acid and ammonium nitrate are often strongly affected by phase changes between gaseous and particulate compounds of the triad, which make measurements of the four individual species (NH3, HNO3, NH4+, NO3− necessary for a correct interpretation of the measured concentration differences. We present here a rigorous analysis of results obtained with a multi-component, wet-chemical instrument, able to simultaneously measure gradients of both gaseous and particulate trace substances. Basis for our analysis are two field experiments, conducted above contrasting ecosystems (grassland, forest). Precision requirements of the instrument as well as errors of concentration differences and micrometeorological exchange parameters have been estimated, which, in turn, allows the establishment of thorough error estimates of the derived fluxes of NH3, HNO3, NH4+, and NO3−. Derived median flux errors for the grassland and forest field experiments were: 39% and 50% (NH3), 31% and 38% (HNO3), 62% and 57% (NH4+), and 47% and 68% (NO3−), respectively. Additionally, we provide the basis for using field data to characterize the instrument performance, as well as subsequent quantification of surface exchange fluxes and underlying mechanistic processes under realistic ambient measurement conditions.

Description: We examine volume mixing ratios (vmr) retrieved from limb emission spectra recorded with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board Envisat. In level 2 (L2) data products of three different retrieval processors, which perform one dimensional (1-D) retrievals, we find significant differences between species' profiles from ascending and descending orbit parts. The relative differences vary systematically with time of the year, latitude, and altitude. In the lower stratosphere their monthly means can reach maxima of 20% for CFC-11, CFC-12, HNO3, H2O, 10% for CH4 and N2O. Relative differences between monthly means of 1-D retrieval results and of the true atmospheric state can be expected to reach half of these percentage values, while relative differences in single vmr profiles might well exceed those numbers. Often there are no physical or chemical reasons for these differences, so they are an indicator for a problem in the data processing. The differences are generally largest at locations where the meridional temperature gradient of the atmosphere is strong. On the contrary, when performing the retrieval with a tomographic two dimensional (2-D) retrieval, L2 products generally do not show these differences. This suggests that inhomogeneities in the temperature field, and possibly in the species' fields, which are accounted for in the 2-D algorithm and not in standard 1-D processors, may cause significant deviations in the results. Inclusion of an externally given adequate temperature gradient in the forward model of a 1-D processor helps to reduce the observed differences. However, only the full tomographic 2-D approach is suitable to resolve the horizontal inhomogeneities. Implications for the use of the 1-D data, e.g. for validation, are discussed. The dependence of the ascending/descending differences on the observation strategy suggests that this problem may affect 1-D retrievals of infrared limb sounders, if the line of sight of the instrument has a significant component in the direction of the horizontal temperature variation.

Description: The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere – New Frontiers (CRISTA-NF) instrument participated in the AMMA-SCOUT-O3 measurement campaign in July and August 2006. The instrument is mounted on the high-flying Russian research aircraft M55-Geophysica and measures limb-emissions in the mid-infrared region in the tangent altitude range of about 6 to 21 km. We present a new retrieval setup which is based on 9 integrated spectral windows allowing to retrieve an extended set of trace gases and temperature fields with high vertical resolution (up to 500 m). Retrieval results are shown for temperature, water vapor (H2O), ozone (O3), nitric acid (HNO3), peroxyacetyl nitrate (PAN), carbon tetrachloride (CCl4), and aerosol extinction. Comparisons of temperature, O3, and H2O observations with corresponding in situ measurements on-board M55-Geophysica show reasonable agreement. In particular, CRISTA-NF observations in the vicinity of the aircraft resemble horizontal variations found in the in situ measurements better than corresponding ECMWF fields.

Description: Fast response optical analyzers based on laser absorption spectroscopy are the preferred tools to measure field-scale mixing ratios and fluxes of a range of trace gases. Several state-of-the-art instruments have become commercially available and are gaining in popularity. This paper aims for a critical field evaluation and intercomparison of two compact, cryogen-free and fast response instruments: a quantum cascade laser based absorption spectrometer from Aerodyne Research, Inc., and an off-axis integrated cavity output spectrometer from Los Gatos Research, Inc. In this paper, both analyzers are characterized with respect to precision, accuracy, response time and also their sensitivity to water vapour. The instruments were tested in a field campaign to assess their behaviour under various meteorological conditions. The instrument's suitability for eddy covariance flux measurements was evaluated by applying an artificial flux of CH4 generated above a managed grassland with otherwise very low methane exchange. This allowed an independent verification of the flux measurements accuracy, including the overall eddy covariance setup and data treatment. The retrieved fluxes were in good agreement with the known artificial emission flux, which is more than satisfactory, given that the analyzers were attached to separate sonic anemometers placed on individual eddy towers with different data acquisition systems but similar data treatment that are specific to the best practice used by the involved research teams.

Description: This article introduces a new fully automated FTIR system that is part of the Total Carbon Column Observing Network (TCCON). It will provide continuous ground-based measurements of column-averaged volume mixing ratio for CO2, CH4 and several other greenhouse gases in the tropics. Housed in a 20-foot shipping container it was developed as a transportable system that could be deployed almost anywhere in the world. We describe the automation concept which relies on three autonomous subsystems and their interaction. Crucial components like a sturdy and reliable solar tracker dome are described in detail. The automation software employs a new approach relying on multiple processes, database logging and web-based remote control. First results of total column measurements at Jena, Germany show that the instrument works well and can provide parts of the diurnal as well as seasonal cycle for CO2. Instrument line shape measurements with an HCl cell suggest that the instrument stays well-aligned over several months. After a short test campaign for side by side intercomaprison with an existing TCCON instrument in Australia, the system will be transported to its final destination Ascension Island.

Description: A technique for the selective measurement of atmospheric HO2 and RO2 using peroxy radical chemical amplification coupled to laser-induced fluorescence NO2 detection (PERCA-LIF) is demonstrated. By pulling the air through a filled pre-inlet advantage can be taken of the higher heterogeneous loss rate of HO2 relative to CH3O2. Pre-inlet conditions have been found where ca. 90% of HO2 was removed whereas the comparable CH3O2 loss was 15%. The dependence of loss rate on humidity and peroxy radical concentration has been investigated. When using glass beads as the surface for peroxy radical removal, the influence of the relative humidity on the removal efficiency becomes negligible. It may therefore be possible to apply this technique to the measurement of absolute concentrations of solely RO2 as well as the sum of HO2 and RO2. The practical utility of the PERCA-LIF coupled to a denuder has been demonstrated with atmospheric measurements.

Description: Commercial microwave radio links forming cellular communication networks are known to be a valuable instrument for measuring near-surface rainfall. However, operational communication links are more uncertain relatively to the dedicated installations since their geometry and frequencies are optimized for high communication performance rather than observing rainfall. Quantification of the uncertainties for measurements that are non-optimal in the first place is essential to assure usability of the data. In this work we address modeling of instrumental impairments, i.e. signal variability due to antenna wetting, baseline attenuation uncertainty and digital quantization, as well as environmental ones, i.e. variability of drop size distribution along a link affecting accuracy of path-averaged rainfall measurement and spatial variability of rainfall in the link's neighborhood affecting the accuracy of rainfall estimation out of the link path. Expressions for root mean squared error (RMSE) for estimates of path-averaged and point rainfall have been derived. To verify the RMSE expressions quantitatively, path-averaged measurements from 21 operational communication links in 12 different locations have been compared to records of five nearby rain gauges over three rainstorm events. The experiments show that the prediction accuracy is above 90% for temporal accumulation less than 30 min and lowers for longer accumulation intervals. Spatial variability in the vicinity of the link, baseline attenuation uncertainty and, possibly, suboptimality of wet antenna attenuation model are the major sources of link-gauge discrepancies. In addition, the dependence of the optimal coefficients of a conventional wet antenna attenuation model on spatial rainfall variability and, accordingly, link length has been shown. The expressions for RMSE of the path-averaged rainfall estimates can be useful for integration of measurements from multiple heterogeneous links into data assimilation algorithms.

Description: The World Wide Lightning Location Network (WWLLN) uses globally-distributed Very Low Frequency (VLF) receivers in order to observe lightning around the globe. Its objective is to locate as many global lightning strokes as possible, with high temporal and spatial (〈 10 km) accuracy. Since detection is done in the VLF range, signals from high peak current lightning strokes are able to propagate up to ~104 km before being detected by the WWLLN sensors, allowing for receiving stations to be sparsely spaced. Through a comparison with measurements made by the Canadian Lightning Detection Network (CLDN) between May and August 2008 over a 4° latitude by 4° longitude region centered on Toronto, Canada, this study found that WWLLN detection was most sensitive to high peak current lightning strokes. Events were considered shared between the two networks if they fell within 0.5 ms of each other. Using this criterion, 19 128 WWLLN strokes (analyzed using the Stroke_B algorithm) were shared with CLDN lightning strokes, producing a detection efficiency of 2.8%. The peak current threshold for WWLLN detection is found to be ~20 kA, with its detection efficiency increasing from 11.3% for peak currents greater than 20 kA to 75.8% for peak currents greater than 120 kA. The detection efficiency is seen to have a clear diurnal dependence, with a higher detection efficiency at local midnight than at local noon; this is attributed to the difference in the thickness of the ionospheric D-region between night and day. The mean time difference (WWLLN − CLDN) between shared events was −6.44 μs with a standard deviation of 35 μs, and the mean absolute location accuracy was 7.24 km with a standard deviation of 6.34 km. These results are generally consistent with previous comparison studies of the WWLLN with other regional networks around the world. Additional receiver stations are continuously being added to the network, acting to improve this detection efficiency.

Description: Remote sensing of aerosol from space is a challenging and typically underdetermined retrieval task, requiring many assumptions to be made with respect to the aerosol and surface models. Therefore, the quality of a priori information plays a central role in any retrieval process (apart from the cloud screening procedure and the forward radiative transfer model, which to be most accurate should include the treatment of light polarization and molecular-aerosol coupling). In this paper the performance of various algorithms with respect to the of spectral aerosol optical thickness determination from optical spaceborne measurements is studied. The algorithms are based on various types of measurements (spectral, angular, polarization, or some combination of these). It is confirmed that multiangular spectropolarimetric measurements provide more powerful constraints compared to spectral intensity measurements alone, particularly those acquired at a single view angle and which rely on a priori assumptions regarding the particle phase function in the retrieval process.

Description: An automated Fourier Transform Spectroscopic (FTS) solar observatory was established in Darwin, Australia in August 2005. The laboratory is part of the Total Carbon Column Observing Network, and measures atmospheric column abundances of CO2 and O2 and other gases. Measured CO2 columns were calibrated against integrated aircraft profiles obtained during the TWP-ICE campaign in January–February 2006, and show good agreement with calibrations for a similar instrument in Park Falls, Wisconsin. A clear-sky low airmass relative precision of 0.1% is demonstrated in the CO2 and O2 retrieved column-averaged volume mixing ratios. The 1% negative bias in the FTS XCO2 relative to the World Meteorological Organization (WMO) calibrated in situ scale is within the uncertainties of the NIR spectroscopy and analysis.

Description: A new "Bayesian" minimization algorithm for the retrieval of the diurnal variation of UV/vis absorbing radicals (O3, NO2, BrO, OClO and HONO) from balloon-borne limb scattered skylight observations is discussed. The method evaluates spectroscopic measurements in combination with radiative transfer calculations to drive a mathematical inversion on a discrete time and height grid. Here, the proposed method is applied to data obtained during two deployments of the mini-DOAS instrument on different balloon payloads in northern Brazil in June 2005. The retrieval is tested by comparing the inferred profiles to in-situ ozone sounding data and to measurements of the ENVISAT/SCIAMACHY satellite instrument performed during a collocated overpass. The comparison demonstrates the strength and validity of our approach. In particular for time-varying radical concentrations, photochemical corrections due to temporal mismatch of the corresponding observations are rendered dispensable. Thus, limb scanning UV/vis spectrometry from balloon platforms offers a more direct and concise approach for satellite validation of radical measurements than solar occultation measurements. Furthermore, monitoring of the diurnal variation of stratospheric radicals allows us to constrain photochemical parameters which are critical for stratospheric ozone chemistry, such as the photolysis frequency of N2O5 by observations of the diurnal variation of NO2.

Description: Principal component (PC) analysis has received considerable attention as a technique for the extraction of meteorological signals from hyperspectral infra-red sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder (AIRS). In addition to achieving substantial bit-volume reductions for dissemination purposes, the technique can also be used to generate reconstructed radiances in which random instrument noise has been reduced. Studies on PC analysis of hyperspectral infrared sounder data have been undertaken in the context of numerical weather prediction, instrument monitoring and geophysical variable retrieval, as well as data compression. This study examines the potential of PC analysis for chemistry applications. A major concern in the use of PC analysis for chemistry is that the spectral features associated with trace gases may not be well represented in the reconstructed spectra, either due to deficiencies in the training set or due to the limited number of PC scores used in the radiance reconstruction. In this paper we show examples of reconstructed IASI radiances for several trace gases: ammonia, sulphur dioxide, methane and carbon monoxide. It is shown that care must be taken in the selection of spectra for the initial training set: an iterative technique, in which outlier spectra are added to a base training set, gives the best results. For the four trace gases examined, key features of the chemical signatures are retained in the reconstructed radiances, whilst achieving a substantial reduction in instrument noise. A new regional re-transmission service for IASI is scheduled to start in 2010, as part of the EUMETSAT Advanced Retransmission Service (EARS). For this EARS-IASI service it is intended to include PC scores as part of the data stream. The paper describes the generation of the reference eigenvectors for this new service.

Description: Differential Optical Absorption Spectroscopy (DOAS) is a well established spectroscopic method to determine trace gases in the atmosphere. During the last decade, passive DOAS, which uses solar radiation scattered in the atmosphere as a light source, has become a standard tool to determine SO2 column densities and emission fluxes from volcanoes and other large sources by ground based as well as satellite measurements. For the determination of SO2 column densities, the structured absorption of the molecule in the 300–330 nm region (due to the A1B1 ← X1A1 transition) is used. However, there are several problems limiting the accuracy of the technique in this particular application. Here we propose to use an alternative wavelength region (360–390 nm) due to the spin-forbidden a3B2 ← X1A1 transition for the DOAS evaluation of SO2 in conditions where high SO2 column densities prevail. We show this range to have considerable advantages in such cases, in particular when the particle content of the plume is high and when measurements are performed at large distances from the area of interest.

Description: A negative-ion proton-transfer chemical ionization mass spectrometric technique (NI-PT-CIMS), using acetate as the reagent ion, was applied to the measurement of volatile inorganic acids of atmospheric interest: hydrochloric (HCl), nitrous (HONO), nitric (HNO3), and isocyanic (HNCO) acids. Gas phase calibrations through the sampling inlet showed the method to be intrinsically sensitive (6–16 cts/pptv), but prone to inlet effects for HNO3 and HCl. The ion chemistry was found to be insensitive to water vapor concentrations, in agreement with previous studies of carboxylic acids. The inlet equilibration times for HNCO and HONO were 2 to 4 s, allowing for measurement in biomass burning studies. Several potential interferences in HONO measurements were examined: decomposition of HNO3·NO3− clusters within the CIMS, and NO2-water production on inlet surfaces, and were quite minor (≤1%, 3.3%, respectively). The detection limits of the method were limited by the instrument backgrounds in the ion source and flow tube, and were estimated to range between 16 and 50 pptv (parts per trillion by volume) for a 1 min average. The comparison of HONO measured by CIMS and by in situ FTIR showed good correlation and agreement to within 17%. The method provided rapid and accurate measurements of HNCO and HONO in controlled biomass burning studies, in which both acids were seen to be important products.

Description: An important new challenge in the field of multi-angle photo-polarimetric satellite remote sensing is the retrieval of aerosol properties under cloudy conditions. In this paper the possibility has been explored to perform a simultaneous retrieval of aerosol and cloud properties for partly cloudy scenes and for fully cloudy scenes where the aerosol layer is located above the cloud, using multi-angle photo-polarimetric measurements. Also, for clear sky conditions a review is given of the capabilities of multi-angle photo-polarimetric measurements in comparison with other measurement types. It is shown that already for clear sky conditions polarization measurements are highly important for the retrieval of aerosol optical and microphysical properties over land surfaces with unknown reflection properties. Furthermore, it is shown that multi-angle photo-polarimetric measurements have the capability to distinguish between aerosols and clouds, and thus facilitate a simultaneous retrieval of aerosol and cloud properties. High accuracy (0.002–0.004) of the polarimetric measurements plays an essential role here.

Description: We present car Multi-Axis (MAX-) DOAS observations of tropospheric NO2 carried out on circles around the cities of Mannheim and Ludwigshafen (Germany) on 24 August 2006. Together with information on wind speed and direction, the total emissions of the encircled source(s) are quantified from these measurements. In contrast to recent similar studies based on zenith scattered sun light (elevation angle of 90°), we use a MAX-DOAS instrument mounted on a car, which observes scattered sun light under different elevation angles (here 45°, and 90°). Compared to simple zenith sky observations, MAX-DOAS observations have higher sensitivity and reduced uncertainty, and avoid systematic offsets in the determination of the vertically integrated trace gas concentration. The determination of the absolute value of the integrated tropospheric trace gas concentrations is especially important for the calculation of absolute trace gas fluxes through arbitrary transects. However, even if emission sources are completely surrounded, systematic offsets in the measured vertically integrated trace gas concentration can lead to errors in the determined total emissions, especially for observations around extended areas. In this study we discuss and quantify different error sources. In most cases, the largest error source is the variability and imperfect knowledge of the wind field. In addition – depending on the trace species observed - also chemical transformations between the emission sources and the measurement location have to be considered. For that purpose we use local observations within the encircled area to quantify and/or correct these errors. From our observations we derive a total NOx emission from the Mannheim/Ludwigshafen area of (7.4±1.8)×1024 molec/sec, which if assumed to be constant throughout the year would correspond to a total emission of 17 830±4340 t/yr (calculated with the mass of NO2 t/yr, consistent with existing emission estimates. From our observations it is also possible to separately determine the average influx into the Mannheim/Ludwigshafen area (5.4±0.9×1024 molec/sec or 13 010±2170 t/yr) and the average outflux (12.8±1.8×1024 molec/sec or 13 010±4340 t/yr).

Description: An innovative temperature sonde, equipped with an ultra thin tungsten wire (10 μm in diameter), has been developed to meet the scientific requirements suitable for climate change research. The response time, shorter than 40 ms achieved at the altitude of 30 km, enables the temperature observations with the radiation correction of less than 0.5 K in the whole observation range. Test flights during the development stage reveal significant artificial perturbations in the observed temperature profiles. They are identified as the thermal contamination arising primarily from radiosonde package box with some additional effect from the launching balloon. The modification of the sensor mount successfully removed the contribution from the former effect. On the other hand, some filtering procedure need to be applied to remove the latter, although the use of a long suspension line will be effective. There remain unavoidable small fluctuations (less than 0.4 K) that are brought about by the solid angle modulation of the illumination against the sensor body in the daytime. While conventional radiation correction may unintentionally have taken a part of such contaminations into account, they may not be properly corrected in existing radiosonde data, as the origin of errors has not been identified. Our tungsten sonde that scarcely relies on the ambiguous correction procedures will be ideal for serving as a kind of an international reference.

Description: We show that the near infrared solar absorption spectra recorded in the framework of the Total Carbon Column Observing Network (TCCON) can be used to derive the vertical distribution of tropospheric water vapour. The resolution of the TCCON spectra of 0.02 cm−1 is sufficient for retrieving lower and middle/upper tropospheric water vapour concentrations with a vertical resolution of about 3 and 8 km, respectively. We document the good quality of the remotely-sensed profiles by comparisons with coincident in-situ Vaisala RS92 radiosonde measurements. Due to the high measurement frequency, the TCCON water vapour profile data offer novel opportunities for estimating the water vapour variability at different timescales and altitudes.

Description: GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) is a new remote sensing instrument essentially combining a Fourier transform infrared spectrometer with a two-dimensional (2-D) detector array in combination with a highly flexible gimbal mount. It will be housed in the belly pod of the German research aircraft HALO (High Altitude and Long Range Research Aircraft). It is unique in its combination of high spatial and state-of-the art spectral resolution. Furthermore, the horizontal view angle with respect to the aircraft flight direction can be varied from 45° to 135°. This allows for tomographic measurements of mesoscale events for a wide variety of atmospheric constituents. In this paper, a tomographic retrieval scheme is presented, which is able to fully exploit the manifold radiance observations of the GLORIA limb sounder. The algorithm is optimized for massive 3-D retrievals of several hundred thousands of measurements and atmospheric constituents on common hardware. The new scheme is used to explore the capabilities of GLORIA to sound the atmosphere in full 3-D with respect to the choice of the flightpath and to different measurement modes of the instrument using ozone as a test species. It is demonstrated that the achievable resolution should approach 200 m vertically and 20 km–30 km horizontally. Finally, a comparison of the 3-D inversion with conventional 1-D inversions using the assumption of a horizontally homogeneous atmosphere is performed.

Description: A growing number of space-borne instruments measures nitrogen dioxide (NO2) concentrations in the troposphere, but validation of these instruments is hampered by the lack of ground-based and in situ profile measurements. The Royal Netherlands Meteorological Institute (KNMI) has developed a working NO2 sonde. The sonde is attached to a small meteorological balloon and measures a tropospheric NO2 profile. The NO2 sonde has a vertical resolution of 5 m and a measurement range between 1 and 100 ppbv. The instrument is light in weight (0.7 kg), cheap (disposable), energy efficient and not harmful to the environment or the person who finds the package after use. The sonde uses the chemiluminescent reaction of NO2 in an aqueous luminol solution. The NO2-luminol reaction produces faint blue/purple light (at about 425 nm), which is detected by an array of silicon photodiodes. The luminol solution is optimised to be specific to NO2. An on-ground comparison with measurements from a Photolytic Analyser of The National Institute for Public Health and the Environment (RIVM) shows that both instruments measure similar NO2 variations in ambient air. During the Cabauw Intercomparison campaign of Nitrogen Dioxide measuring instruments (CINDI) in June/July 2009, six vertical profiles of NO2 from the ground to a 5 km altitude were measured, which clearly show that the largest amount of NO2 is measured in the boundary layer. The measured boundary layer heights of the NO2 sonde are in good agreement with boundary layer heights determined by a LD40 Ceilometer at Cabauw.

Description: CO mixing ratios for the lowermost 2-km atmospheric layer were retrieved from downwelling infrared (IR) radiance spectra of the clear sky measured between 2002 and 2009 by a zenith-viewing Atmospheric Emitted Radiance Interferometer (AERI) deployed at the Southern Great Plains (SGP) observatory of the Atmospheric Radiation Measurements (ARM) Program near Lamont, Oklahoma. A version of a published earlier retrieval algorithm was improved and validated. Archived temperature and water vapor profiles retrieved from the same AERI spectra through automated ARM processing were used as input data for the CO retrievals. We found the archived water vapor profiles required additional constraint using SGP Microwave Radiometer retrievals of total precipitable water vapor. A correction for scattered solar light was developed as well. The retrieved CO was validated using simultaneous independently measured CO profiles from an aircraft. These tropospheric CO profiles were measured from the surface to altitudes of 4572 m a.s.l. once or twice a week between March 2006 and December 2008. The aircraft measurements were supplemented with ground-based CO measurements using a non-dispersive infrared gas correlation instrument at the SGP and retrievals from the Atmospheric IR Sounder (AIRS) above 5 km to create full tropospheric CO profiles. Comparison of the profiles convolved with averaging kernels to the AERI CO retrievals found a squared correlation coefficient of 0.57, a standard deviation of ±11.7 ppbv, a bias of -16 ppbv, and a slope of 0.92. Averaged seasonal and diurnal cycles measured by the AERI are compared with those measured continuously in situ at the SGP in the boundary layer. Monthly mean CO values measured by the AERI between 2002 and 2009 are compared with those measured by the AIRS over North America, the Northern Hemisphere mid-latitudes, and over the tropics.

Description: In 2006 bi-directional optical inter-satellite communication experiments were conducted between the Japan Aerospace Exploration Agency (JAXA) Optical Inter-orbit Communications Engineering Test Satellite (OICETS) and the European Space Agency (ESA) multi-purpose telecommunications and technology demonstration satellite (Advanced Relay and Technology MISsion) ARTEMIS. On 5 April 2006, an experiment was successfully carried out by maintaining the inter-satellite link during OICETS's setting behind the Earth limb until the signal was lost. This setup resembles an occultation observation where the influence of Earth's atmosphere is evident in the power fluctuations recorded at ARTEMIS's (and OICETS's) receiver. These fluctuations do not exist or are at a low level at a link path above the atmosphere and steadily increase as OICETS sets behind the horizon until the tracking of the signal is lost. This specific experiment was performed only once since atmospheric science was not the goal of this demonstration. Nevertheless, this kind of data, if available more frequently in future, can help to study atmospheric turbulence and validate models. The data present here were recorded at ARTEMIS.

Description: The methodology for the operation and calibration of a hygroscopicity tandem differential mobility analyser (HTDMA) and cloud condensation nuclei counter (CCNc) for size-resolved measurements of aerosol water uptake are presented. A state of the science aerosol thermodynamic model is used to benchmark the performance of the instruments. The performance, calibration and operation of the instruments is then demonstrated in the field.

Description: To monitor the continental carbon cycle, a fully automated low maintenance measurement system is installed at the Zotino Tall Tower Observatory in Central Siberia (ZOTTO, 60°48' N, 89°21' E) since April 2009. A cavity ring-down spectroscopy (CRDS) analyzer continuously measures carbon dioxide (CO2) and methane (CH4) from six heights up to 301 m a.g.l. Buffer volumes in each air line remove short term CO2 and CH4 mixing ratio fluctuations associated with turbulence, and allow continuous, near-concurrent measurements from all tower levels. Instead of drying the air sample, the simultaneously measured water vapor is used to correct the dilution and pressure-broadening effects for the accurate determination of dry air CO2 and CH4 mixing ratios. The stability of the water vapor correction was demonstrated by repeated laboratory and field tests. The effect of molecular adsorption in the wet air lines was shown to be negligible. The low consumption of four calibration tanks that need recalibration only on decadal timescale further reduces maintenance. The measurement precision (accuracy) of 0.04 ppm (0.09 ppm) for CO2 and 0.3 ppb (1.5 ppb) for CH4 is compliant with the WMO recommendations. The data collected so far (until April 2010) reveals a seasonal cycle amplitude for CO2 of 30.4 ppm at the 301 m level.

Description: This study presents a chemical ionization mass spectrometer (CIMS) for fast response, in-situ measurements of gas phase ammonia (NH3). The NH3 background level detected with the CIMS ranged between 0.3–1 ppbv, with an uncertainty of 30 pptv under optimized conditions. The instrument sensitivity varied from 4–25 Hz/pptv for 〉1 MHz of reagent ion signals (protonated ethanol ions), with a 30% uncertainty estimated based on variability in calibration signals. The CIMS detection limit for NH3 was ~60 pptv at a 1 min integration time (3 sigma). The CIMS time response was

Description: A method designed to track the life cycle of contrail-cirrus using satellite data with high temporal and spatial resolution, from its formation to the final dissolution of the aviation-induced cirrus cloud is presented. The method follows the evolution of contrails from their linear stage until they are undistinguishable from natural cirrus clouds. Therefore, the study of the effect of aircraft-induced clouds in the atmosphere is no longer restricted to linear contrails and can include contrail-cirrus. The method takes advantage of the high spatial resolution of polar orbiting satellites and the high temporal resolution of geostationary satellites to identify the pixels that belong to an aviation induced cloud. The high spatial resolution data of the MODIS sensor is used for contrail detection, and the high temporal resolution of the SEVIRI sensor in the Rapid Scan mode is used for contrail tracking. An example is included in which the method is applied to the study of a long lived contrail over the bay of Biscay.

Description: A particle-into-liquid sampler (PILS) was coupled with a total organic carbon analyzer (TOC) and two ion chromatographs (IC) to enable high time-resolution measurements of water-soluble ions and water-soluble organic carbon (WSOC) by a single sampling and analytical set-up. The new high time-resolution measurement system, the PILS-TOC-IC, was able to provide essential chemical and physical information about fast changes in composition, concentrations and likely sources of the water-soluble fraction of atmospheric aerosol. The concentrations of major water-soluble ions and WSOC were measured by the PILS-TOC-IC system from 25 April to 28 May 2009. The data of the PILS-TOC-IC setup was compared with the data from the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) data measured from 25 April to 8 May 2009. The measured water-soluble particulate organic matter (WSPOM) concentration varied typically from 0.10 to 8.8 μg m−3 (on average 1.5 μg m−3). The WSPOM contributed on average 51% to particulate organic matter (POM) measured with the AMS. The correlation between the data of all the online measurement devices (AMS, PILS-TOC-IC, semicontinuous EC/OC carbon analyzer and TEOM) was excellent. For sulfate, nitrate and ammonium the correlations between the PILS-TOC-IC and AMS were 0.93, 0.96 and 0.96, respectively. The correlation between WSPOM and POM was also strong (r = 0.88). The identified sources of WSPOM were long-range transported biomass burning and secondary organic aerosol (SOA) formation. WSPOM and oxalate produced in biomass burning were clearly correlated with carbon monoxide.

Description: In this paper we present recent achievements on developing and testing a tool to detect the composition of ambient ions in the mass/charge range up to 2000 Th. The instrument is an Atmospheric Pressure Interface Time-of-Flight Mass Spectrometer (APi-TOF, Tofwerk AG). Its mass accuracy is better than 0.002%, and the mass resolving power is 3000 Th/Th. In the data analysis, a new efficient Matlab based set of programs (tofTools) were developed, tested and used. The APi-TOF was tested both in laboratory conditions and applied to outdoor air sampling in Helsinki at the SMEAR III station. Transmission efficiency calibrations showed a throughput of 0.1–0.5% in the range 100–1300 Th for positive ions, and linearity over 3 orders of magnitude in concentration was determined. In the laboratory tests the APi-TOF detected sulphuric acid-ammonia clusters in high concentration from a nebulised sample illustrating the potential of the instrument in revealing the role of sulphuric acid clusters in atmospheric new particle formation. The APi-TOF features a high enough accuracy, resolution and sensitivity for the determination of the composition of atmospheric small ions although the total concentration of those ions is typically only 400–2000 cm−3. The atmospheric ions were identified based on their exact masses, utilizing Kendrick analysis and correlograms as well as narrowing down the potential candidates based on their proton affinities as well isotopic patterns. In Helsinki during day-time the main negative ambient small ions were inorganic acids and their clusters. The positive ions were more complex, the main compounds were (poly)alkyl pyridines and – amines. The APi-TOF provides a near universal interface for atmospheric pressure sampling, and this key feature will be utilized in future laboratory and field studies.

Description: The possibility of measuring aerosol optical absorption properties in the UV spectral range such as single scattering albedo (SSA), using remote sensing techniques, is currently an open scientific issue. We investigate the limitations on calculating column average SSA using a combination of global UV spectral measurements (that are comon in various UV monitoring stations worldwide) with radiative transfer modeling. To point out the difficulties in such a retrieval we have used the travelling reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) results from 27 visits to UV monitoring stations around Europe. We have used the QASUME instrument as relative reference, analyzing absolute differences and also temporal and spectral deviations of UV irraidances, that are used as basic input for the SSA retrieval. The results comparing the mean SSA derived by all instruments, measuring synchronous UV spectra, showed that 5 were within ± 0.02 difference from the SSA calculated from the QASUME instrument, while 17 were within ± 0.04, for the Solar zenith angle of 60 degrees. As for the uncertainty that has been calculated using the 2σ standard deviation of the spectral measurements, a mean 0.072 and 0.10 (2σ) uncertainties have been calculated for 60° and 30°, respectively. Based on the fact that additional uncertainties would be introduced in the SSA retrieval from AOD model input accuracy, assymetry parameter assumptions, we show that only very few instrumnents could be able to detect long term SSA changes. However, such measurements/results ar useful in order to retrieve SSA at UV wavelengths, a product needed for various applications such as, inputs for modeling radiative forcing studies and satellite retrieval algorithms.

Description: A new method, near-infrared laser desorption/ionization aerosol mass spectrometry (NIR-LDI-AMS), is described for the real time analysis of organic aerosols at atmospherically relevant total mass loadings. Particles are sampled with an aerodynamic lens onto an aluminum probe. A moderate energy NIR laser pulse at 1064 nm is directed onto the probe to vaporize and ionize particle components. Delayed pulse extraction is then used to sample the ions into a reflectron time of flight mass spectrometer for chemical analysis. The soft ionization afforded by the NIR photons results in minimal fragmentation (loss of a hydrogen atom) producing intact pseudo-molecular anions at [M-H]−. The limit of detection measured for pure oleic acid particles (geometric mean diameter and standard deviation of 180 nm and 1.3, respectively) was 140 fg (or 1.7 ng m−3 per minute sampling time). As an example of the utility of NIR-LDI-AMS to measurements of atmospheric importance, the method was applied to laboratory chamber measurements of the secondary organic aerosol formation from ozonolysis of α-pinene. High quality mass spectra were recorded with a 2-min time resolution for total aerosol mass loadings ranging from 1.5 to 8.7 μg m−3. These results demonstrate the potential of NIR-LDI-AMS to allow for more accurate measurements of the organic fraction of atmospheric particulate at realistic mass loadings. Measurements at ambient-levels of SOA mass loading are important to improve parameterizations of chamber-based SOA formation for modeling regional and global SOA fluxes and to aid in remediating the discrepancy between modeled and observed atmospheric total SOA production rates and concentrations.

Description: Formaldehyde measurements can provide useful information about photochemical activity in ambient air, given that HCHO is formed via numerous oxidation processes. Proton transfer reaction mass spectrometry (PTR-MS) is an online technique that allows measurement of VOCs at the sub-ppbv level with good time resolution. PTR-MS quantification of HCHO is hampered by the humidity dependence of the instrument sensitivity, with higher humidity leading to loss of PTR-MS signal. In this study we present an analytical, first principles approach to correct the PTR-MS HCHO signal according to the concentration of water vapor in sampled air. The results of the correction are validated by comparison of the PTR-MS results to those from a Hantzsch fluorescence monitor which does not have the same humidity dependence. Results are presented for an intercomparison made during a field campaign in rural Ontario at Environment Canada's Centre for Atmospheric Research Experiments.

Description: Surface reflectance is a key parameter in satellite trace gas retrievals in the UV/visible range and in particular for the retrieval of nitrogen dioxide (NO2) vertical tropospheric columns (VTCs). Current operational retrievals rely on coarse-resolution reflectance data and do not account for the generally anisotropic properties of surface reflectance. Here we present a NO2 VTC retrieval that uses MODIS bi-directional reflectance distribution function (BRDF) data at high temporal (8 days) and spatial (1 km × 1 km) resolution in combination with the LIDORT radiative transfer model to account for the dependence of surface reflectance on viewing and illumination geometry. The method was applied to two years of NO2 observations from the Ozone Monitoring Instrument (OMI) over Europe. Due to its wide swath, OMI is particularly sensitive to BRDF effects. Using representative BRDF parameters for various land surfaces, we found that in July (low solar zenith angles) and November (high solar zenith angles) and for typical viewing geometries of OMI, differences between MODIS black-sky albedos and surface bi-directional reflectances are of the order of 0–10% and 0–40%, respectively, depending on the position of the OMI pixel within the swath. In the retrieval, black-sky albedo was treated as a Lambertian (isotropic) reflectance, while for BRDF effects we used the kernel-based approach in the MODIS BRDF product. Air Mass Factors were computed using the LIDORT radiative transfer model based on these surface reflectance conditions. Differences in NO2 VTCs based on the Lambertian and BRDF approaches were found to be of the order of 0–3% in July and 0–20% in November with the extreme values found at large viewing angles. The much larger differences in November are mainly due to stronger BRDF effects at higher solar zenith angles. To a smaller extent, they are also caused by the typically more pronounced maximum of the NO2 a priori profiles in the boundary layer during the cold season, which make the retrieval more sensitive to radiation changes near the surface. However, BRDF impacts vary considerably across Europe due to differences in land surface type and increasing solar zenith angles at higher latitude. Finally, we compare BRDF-based NO2 VTCs with those retrieved using the GOME/TOMS Lambertian equivalent reflectance (LER) data set. The relative differences are mostly below 15% in July but in November the NO2 VTCs from TOMS/GOME are lower by 20–60%. Our results indicate that the specific choice of albedo data set is even more important than accounting for surface BRDF effects, and this again demonstrates the strong requirement for more accurate surface reflectance data sets.

Description: Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a technique to measure trace gas amounts in the lower troposphere from ground-based scattered sunlight observations. MAX-DOAS observations are especially suitable for validation of tropospheric trace gas observations from satellite, since they have a representative range of several kilometers, both in the horizontal and in the vertical dimension. A two-step retrieval scheme is presented here, to derive aerosol corrected tropospheric NO2 columns from MAX-DOAS observations. In a first step, boundary layer aerosols, characterized in terms of aerosol optical thickness (AOT), are estimated from relative intensity observations, which are defined as the ratio of the sky radiance at elevation α and the sky radiance in the zenith. Relative intensity measurements have the advantage of a strong dependence on boundary layer AOT and almost no dependence on boundary layer height. In a second step, tropospheric NO2 columns are derived from differential slant columns, based on AOT-dependent air mass factors. This two-step retrieval scheme was applied to cloud free periods in a twelve month data set of observations in De Bilt, The Netherlands. In a comparison with AERONET (Cabauw site) a mean difference in AOT (AERONET minus MAX-DOAS) of −0.01±0.08 was found, and a correlation of 0.85. Tropospheric-NO2 columns were compared with OMI-satellite tropospheric NO2. For ground-based observations restricted to uncertainties below 10%, no significant difference was found, and a correlation of 0.88.

Description: Toluene was measured using both a gas chromatographic system (GC), with a flame ionization detector (FID), and a proton transfer reaction-mass spectrometer (PTR-MS) at the AIRMAP atmospheric monitoring station Thompson Farm (THF) in rural Durham, NH during the summer of 2004. Simultaneous measurements of monoterpenes, including α- and β-pinene, camphene, Δ 3-carene, and d-limonene, by GC-FID demonstrated large enhancements in monoterpene mixing ratios relative to toluene, with median and maximum enhancement ratios of ~2 and ~30, respectively. A detailed comparison between the GC-FID and PTR-MS toluene measurements was conducted to test the specificity of PTR-MS for atmospheric toluene measurements under conditions often dominated by biogenic emissions. We derived quantitative estimates of potential interferences in the PTR-MS toluene measurements related to sampling and analysis of monoterpenes, including fragmentation of the monoterpenes and some of their primary carbonyl oxidation products via reactions with H3O+, O2+ and NO+ in the PTR-MS drift tube. The PTR-MS and GC-FID toluene measurements were in good quantitative agreement and the two systems tracked one another well from the instrumental limits of detection to maximum mixing ratios of ~0.5 ppbv. A correlation plot of the PTR-MS versus GC-FID toluene measurements was described by the least squares regression equation y=(1.13± 0.02)x−(0.008±0.003) ppbv, suggesting a small ~13% positive bias in the PTR-MS measurements. The bias corresponded with a ~0.055 ppbv difference at the highest measured toluene level. The two systems agreed quantitatively within the combined 1σ measurement precisions for 60% of the measurements. Discrepancies in the measured mixing ratios were not well correlated with enhancements in the monoterpenes. Better quantitative agreement between the two systems was obtained by correcting the PTR-MS measurements for contributions from monoterpene fragmentation in the PTR-MS drift tube; however, the improvement was minor (

Description: Limb measurements provided by the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on the ENVISAT satellite allow retrieving stratospheric profiles of various trace gases on a global scale. Combining measurements of the same air volume from different viewing positions along the orbit, a tomographic approach can be applied and 2-D distribution fields of stratospheric trace gases can be acquired in one inversion. With this approach, it is possible to improve the accounting for the effect of horizontal gradients in the trace gas distribution on the profile retrieval. This was shown in a previous study for the retrieval of NO2 and OClO profiles in the Arctic region near the polar vortex boundary. In this study, the tomographic retrieval is applied on measurements during special limb-only orbits performed on 14 December 2008. For these orbits the distance between consecutive limb scanning sequences was reduced to ~3.3° of the orbital circle (i.e. more than two times with respect to the nominal operational mode). Thus, the same air volumes are scanned successively by more than one scanning sequence also for midlatitudes and the tropics. It is found that the profiles obtained by the tomographic 2-D approach show significant differences to those obtained by the 1-D approach. In particular, for regions close to stratospheric transport barriers (i.e. near to the edge of the polar vortex and subtropical transport barrier) up to 50% larger or smaller NO2 number densities (depending on the sign of the gradient along the line of sight) for altitudes below the peak of the profile (around 20 km) are obtained. The limb-only measurements allow examining the systematic error if the horizontal gradient is not accounted for, and studying the impact of the gradient strength on the profile retrieval on a global scale. The findings for the actual SCIAMACHY observations are verified by sensitivity studies for simulated data for which the NO2 distributions to be retrieved are known in advance. In addition, the impact of the horizontal distance between consecutive limb scanning sequences on the quality of the tomographic 2-D retrieval is investigated and a possibility to take into account the horizontal gradients by an interpolation approach is studied.

Description: An information content analysis for multi-wavelength SYNergetic AErosol Retrieval algorithm SYNAER was performed to quantify the number of independent pieces of information that can be retrieved. In particular, the capability of SYNAER to discern various aerosol types is assessed. This information content depends on the aerosol optical depth, the surface albedo spectrum and the observation geometry. The theoretical analysis is performed for a large number of scenarios with various geometries and surface albedo spectra for ocean, soil and vegetation. When the surface albedo spectrum and its accuracy is known under cloud-free conditions, reflectance measurements used in SYNAER is able to provide for 2–4° of freedom that can be attributed to retrieval parameters: aerosol optical depth, aerosol type and surface albedo. The focus of this work is placed on an information content analysis with emphasis to the aerosol type classification. This analysis is applied to synthetic reflectance measurements for 40 predefined aerosol mixtures of different basic components, given by sea salt, mineral dust, biomass burning and diesel aerosols, water soluble and water insoluble aerosols. The range of aerosol parameters considered through the 40 mixtures covers the natural variability of tropospheric aerosols. After the information content analysis performed in Holzer-Popp et al. (2008) there was a necessity to compare derived degrees of freedom with retrieved aerosol optical depth for different aerosol types, which is the main focus of this paper. The principle component analysis was used to determine the correspondence between degrees of freedom for signal in the retrieval and derived aerosol types. The main results of the analysis indicate correspondence between the major groups of the aerosol types, which are: water soluble aerosol, soot, mineral dust and sea salt and degrees of freedom in the algorithm and show the ability of the SYNAER to discern between this aerosol types. The results of the work will be further used for the development of the promising methodology of the construction error covariance matrices in the assimilation system.

Description: Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and thermal desorption Gas Chromatography-Mass Spectrometry (GC-MS) allow for absolute quantification of a wide range of atmospheric volatile organic compounds (VOCs) with concentrations in the ppbv to pptv range. Although often neglected, routine calibration is necessary for accurate quantification of VOCs by PTR-MS and GC-MS. Several gas calibration methods currently exist, including compressed gas cylinders, permeation tubes, diffusion tubes, and liquid injection. While each method has its advantages and limitations, no single technique has emerged that is capable of dynamically generating known concentrations of complex mixtures of VOCs over a large concentration range (ppbv to pptv) and is technically simple, field portable, and affordable. We present the development of a new VOC calibration technique based on liquid injection with these features termed Dynamic Solution Injection (DSI). This method consists of injecting VOCs (0.1–0.5 mM) dissolved in cyclohexane (PTR-MS) or methanol (GC-MS) into a 1.0 slpm flow of purified dilution gas in an unheated 25 ml glass vial. Upon changes in the injection flow rate (0.5–4.0 μl min−1), new VOC concentrations are reached within seconds to minutes, depending on the compound, with a liquid injection flow rate accuracy and precision of better than 7% and 4% respectively. We demonstrate the utility of the DSI technique by calibrating a PTR-MS to seven different cyclohexane solutions containing a total of 34 different biogenic compounds including volatile isoprenoids, oxygenated VOCs, fatty acid oxidation products, aromatics, and dimethyl sulfide. We conclude that because of its small size, low cost, and simplicity, the Dynamic Solution Injection method will be of great use to both laboratory and field VOC studies.

Description: The Ozone Profile Algorithm (OPERA), developed at KNMI, retrieves the vertical ozone distribution from nadir spectral satellite measurements of back scattered sunlight in the ultraviolet and visible wavelength range. To produce consistent global datasets the algorithm needs to have good global performance, while short computation time facilitates the use of the algorithm in near real time applications. To test the global performance of the algorithm we look at the convergence behaviour as diagnostic tool of the ozone profile retrievals from the GOME instrument (on board ERS-2) for February and October 1998. In this way, we uncover different classes of retrieval problems, related to the South Atlantic Anomaly, low cloud fractions over deserts, desert dust outflow over the ocean, and the intertropical convergence zone. The influence of the first guess and the external input data including the ozone cross-sections and the ozone climatologies on the retrieval performance is also investigated. By using a priori ozone profiles which are selected on the expected total ozone column, retrieval problems due to anomalous ozone distributions (such as in the ozone hole) can be avoided. By applying the algorithm adaptations the convergence statistics improve considerably, not only increasing the number of successful retrievals, but also reducing the average computation time, due to less iteration steps per retrieval. For February 1998, non-convergence was brought down from 10.7% to 2.1%, while the mean number of iteration steps (which dominates the computational time) dropped 26% from 5.11 to 3.79.