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  • Copernicus  (138,207)
  • 1
    Publication Date: 2019-05-02
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2019-04-08
    Description: In this study, we present results obtained from modelling the mid-Pliocene warm period using the Community Earth System Models (COSMOS, version: COSMOS-landveg r2413, 2009) with the two different sets of boundary conditions prescribed for the two phases of the Pliocene Model Intercomparison Project (PlioMIP). Boundary conditions, model forcing, and modelling methodology of the two phases of PlioMIP, tagged PlioMIP1 and PlioMIP2,differ considerably in palaeogeography, in particular with regards to the state of ocean gateways, ice-masks, vegetation and topography. Further differences between model setups as suggested for PlioMIP1 and PlioMIP2 consider updates to the concentration of atmospheric carbon dioxide (CO2), that is specified as 405 and 400 parts per million by volume (ppmv) for PlioMIP1 and PlioMIP2, respectively, as well as minor differences in the concentrations of methane (CH4) and nitrous oxide (N2O) due to changes in the protocol of the Paleoclimate Model Intercomparison Project (PMIP) from phase 3 to phase 4. With this manuscript, we bridge the gap between our contributions to PlioMIP1 (Stepanek and Lohmann, 2012) and PlioMIP2 (Stepanek et al., 2019). We highlight some of the effects that differences in the chosen Mid-Pliocene model setup (PlioMIP2 vs. PlioMIP1) have on the climate state as derived with the COSMOS, as this information will be valuable in the framework of the model-model and model-data-comparison within PlioMIP2. We evaluate the model sensitivity to improved mid-Pliocene boundary conditions using PlioMIP’s core mid-Pliocene experiments for PlioMIP1 and PlioMIP2, and present further simulations where we test model sensitivity to variations in palaeogeography, orbit and concentration of CO2. Firstly,we highlight major changes in boundary conditions from PlioMIP1 to PlioMIP2 and also the limitations recorded from the initial effort. The results derived from of our simulations show that COSMOS simulates a mid-Pliocene climate state that is 0.08 K colder in PlioMIP2, if compared to PlioMIP1. On one hand, high-latitude warming,which is supported by proxy evidence of the mid-Pliocene, is underestimated in simulations of both PlioMIP1 andPlioMIP2. On the other hand, spatial variations in surface air temperature (SAT), sea surface temperature (SST) as well as the distribution of sea ice suggest improvement of simulated SAT and SST in PlioMIP2 if employing the updated palaeogeography. The PlioMIP2 Mid-Pliocene simulation produces warmer SSTs in the Arctic and North Atlantic Ocean than derived from the respective PlioMIP1 climate state. The difference in prescribed CO2accountsfor 1.1 K of warming in the Arctic, leading to an ice-free summer in the PlioMIP1 simulation, and a quasi-ice-free summer in PlioMIP2. Furthermore, employing different orbital forcings in simulating the Mid-Pliocene lead to pronounced annual and seasonal variations, which is not accounted for by marine and terrestrial reconstruction of the time-slice.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2019-05-13
    Description: The Antarctic temperature changes over the past millennia remain more uncertain than in many other continental regions. This has several origins: (1) the number of high-resolution ice cores is small, in particular on the East Antarctic plateau and in some coastal areas in East Antarctica; (2) the short and spatially sparse instrumental records limit the calibration period for reconstructions and the assessment of the methodologies; (3) the link between isotope records from ice cores and local climate is usually complex and dependent on the spatial scales and timescales investigated. Here, we use climate model results, pseudo-proxy experiments and data assimilation experiments to assess the potential for reconstructing the Antarctic temperature over the last 2 millennia based on a new database of stable oxygen isotopes in ice cores compiled in the frame- work of Antarctica2k (Stenni et al., 2017). The well-known covariance between δ18O and temperature is reproduced in the two isotope-enabled models used (ECHAM5/MPI-OM and ECHAM5-wiso), but is generally weak over the different Antarctic regions, limiting the skill of the reconstructions. Furthermore, the strength of the link displays large variations over the past millennium, further affecting the potential skill of temperature reconstructions based on statistical methods which rely on the assumption that the last decades are a good estimate for longer temperature reconstructions. Using a data assimilation technique allows, in theory, for changes in the δ18O–temperature link through time and space to be taken into account. Pseudoproxy experiments confirm the benefits of using data assimilation methods instead of statistical methods that provide reconstructions with unrealistic variances in some Antarctic subregions. They also confirm that the relatively weak link between both variables leads to a limited potential for reconstructing temperature based on δ18O. However, the reconstruction skill is higher and more uniform among reconstruction methods when the reconstruction target is the Antarctic as a whole rather than smaller Antarctic subregions. This consistency between the methods at the large scale is also observed when reconstructing temperature based on the real δ18O regional composites of Stenni et al. (2017). In this case, temperature reconstructions based on data assimilation confirm the long-term cooling over Antarctica during the last millennium, and the later onset of anthropogenic warming compared with the simulations without data assimilation, which is especially visible in West Antarctica. Data assimilation also allows for models and direct observations to be reconciled by reproducing the east–west contrast in the recent temperature trends. This recent warming pattern is likely mostly driven by internal variability given the large spread of individual Paleoclimate Modelling Intercomparison Project (PMIP)/Coupled Model Intercomparison Project (CMIP) model realizations in simulating it. As in the pseudoproxy framework, the reconstruction methods perform differently at the subregional scale, especially in terms of the variance of the time series produced. While the potential benefits of using a data assimilation method instead of a statistical method have been highlighted in a pseudoproxy framework, the instrumental series are too short to confirm this in a realistic setup.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 4
    Publication Date: 2019-05-13
    Description: A new 21.3m firn core was drilled in 2015 at a coastal Antarctic high-accumulation site in Adélie Land (66.78◦ S; 139.56◦ E, 602 m a.s.l.), named Terre Adélie 192A (TA192A). The mean isotopic values (−19.3 ‰ ± 3.1 ‰ for δ18O and 5.4 ‰±2.2 ‰ for deuterium excess) are consistent with other coastal Antarctic values. No significant isotope–temperature relationship can be evidenced at any timescale. This rules out a simple interpretation in terms of local temperature. An observed asymmetry in the δ18O seasonal cycle may be explained by the precipitation of air masses coming from the eastern and western sectors in autumn and winter, recorded in the d-excess signal showing outstanding values in austral spring versus autumn. Significant positive trends are observed in the annual d-excess record and local sea ice extent (135–145◦ E) over the period 1998–2014. However, process studies focusing on resulting isotopic compositions and particularly the deuterium excess–δ18O relationship, evidenced as a potential fingerprint of moisture origins, as well as the collection of more isotopic measurements in Adélie Land are needed for an accurate interpretation of our signals.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
    Publication Date: 2019-06-03
    Description: Ice-wedge polygons are common features of lowland tundra in the continuous permafrost zone and prone to rapid degradation through melting of ground ice. There are many interrelated processes involved in ice-wedge thermokarst and it is a major challenge to quantify their influence on the stability of the permafrost underlying the landscape. In this study we used a numerical modelling approach to investigate the degradation of ice wedges with a focus on the influence of hydrological conditions. Our study area was Samoylov Island in the Lena River delta of northern Siberia, for which we had in situ measurements to evaluate the model. The tailored version of the CryoGrid 3 land surface model was capable of simulating the changing microtopography of polygonal tundra and also regarded lateral fluxes of heat, water, and snow. We demonstrated that the approach is capable of simulating ice-wedge degradation and the associated transition from a low-centred to a high-centred polygonal microtopography. The model simulations showed ice-wedge degradation under recent climatic conditions of the study area, irrespective of hydrological conditions. However, we found that wetter conditions lead to an earlier onset of degradation and cause more rapid ground subsidence. We set our findings in correspondence to observed types of ice-wedge polygons in the study area and hypothesized on remaining discrepancies between modelled and observed ice-wedge thermokarst activity. Our quantitative approach provides a valuable complement to previous, more qualitative and conceptual, descriptions of the possible pathways of ice-wedge polygon evolution. We concluded that our study is a blueprint for investigating thermokarst landforms and marks a step forward in understanding the complex interrelationships between various processes shaping ice-rich permafrost landscapes.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 6
    Publication Date: 2019-07-09
    Description: Increasing industrial metal demands due to rapid technological developments may drive the prospection and exploitation of deep-sea mineral resources such as polymetallic nodules. To date, the potential environmental consequences of mining operations in the remote deep sea are poorly known. Experimental studies are scarce, especially with regard to the effect of sediment and nodule debris depositions as a consequence of seabed mining. To elucidate the potential effects of the deposition of crushed polymetallic nodule particles on abyssal meiobenthos communities, a short (11 d) in situ experiment at the seafloor of the Peru Basin in the south-east Pacific Ocean was conducted in 2015. We covered abyssal, soft sediment with approx. 2 cm of crushed nodule particles and sampled the sediment after 11 d of incubation at 4200 m water depth. Short-term ecological effects on the meiobenthos community were studied including changes in their composition and vertical distribution in the sediment as well as nematode genus composition. Additionally, copper burden in a few similar-sized but randomly selected nematodes was measured by means of micro X-ray fluorescence (µXRF). At the end of the experiment, 46±1 % of the total meiobenthos occurred in the added crushed nodule layer, while abundances decreased in the underlying 2 cm compared to the same depth interval in undisturbed sediments. Densities and community composition in the deeper 2–5 cm layers remained similar in covered and uncovered sediments. The migratory response into the added nodule material was particularly seen in polychaetes (73±14 %, relative abundance across all depth layers) copepods (71±6 %), nauplii (61±9 %) and nematodes (43±1 %). While the dominant nematode genera in the added nodule material did not differ from those in underlying layers or the undisturbed sediments, feeding type proportions in this layer were altered, with a 9 % decrease of non-selective deposit feeders and an 8 % increase in epistrate feeders. Nematode tissue copper burden did not show elevated copper toxicity resulting from burial with crushed nodule particles. Our results indicate that burial with a 2 cm layer of crushed nodule particles induces changes in the vertical structure of meiobenthos inside the sediment and an alteration of nematode feeding type proportions within a short time frame of 11 d, while nematode tissue copper burden remains unchanged. These findings considerably contribute to the understanding of the short-term responses of meiobenthos to physical disturbances in the deep sea.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2019-07-22
    Description: The atmospheric CO2 concentration increased by about 20ppm from 6000BCE to the pre-industrial period (1850CE). Several hypotheses have been proposed to explain mechanisms of this CO2 growth based on either ocean or land carbon sources. Here, we apply the Earth system model MPI-ESM-LR for two transient simulations of climate and carbon cycle dynamics during this period. In the first simulation, atmospheric CO2 is prescribed following ice-core CO2 data. In response to the growing atmospheric CO2 concentration, land carbon storage increases until 2000BCE, stagnates afterwards, and decreases from 1CE, while the ocean continuously takes CO2 out of the atmosphere after 4000BCE. This leads to a missing source of 166Pg of carbon in the ocean-land-atmosphere system by the end of the simulation. In the second experiment, we applied a CO2 nudging technique using surface alkalinity forcing to follow the reconstructed CO2 concentration while keeping the carbon cycle interactive. In that case the ocean is a source of CO2 from 6000 to 2000BCE due to a decrease in the surface ocean alkalinity. In the prescribed CO2 simulation, surface alkalinity declines as well. However, it is not sufficient to turn the ocean into a CO2 source. The carbonate ion concentration in the deep Atlantic decreases in both the prescribed and the interactive CO2 simulations, while the magnitude of the decrease in the prescribed CO2 experiment is underestimated in comparison with available proxies. As the land serves as a carbon sink until 2000BCE due to natural carbon cycle processes in both experiments, the missing source of carbon for land and atmosphere can only be attributed to the ocean. Within our model framework, an additional mechanism, such as surface alkalinity decrease, for example due to unaccounted for carbonate accumulation processes on shelves, is required for consistency with ice-core CO2 data. Consequently, our simulations support the hypothesis that the ocean was a source of CO2 until the late Holocene when anthropogenic CO2 sources started to affect atmospheric CO2.
    Type: Article , PeerReviewed
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  • 8
    Publication Date: 2019-08-12
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 9
    Publication Date: 2019-09-30
    Description: The Global Ocean Data Analysis Project (GLODAP) is a synthesis effort providing regular compilations of surface to bottom ocean biogeochemical data, with an emphasis on seawater inorganic carbon chemistry and related variables determined through chemical analysis of water samples. This update of GLODAPv2, v2.2019, adds data from 116 cruises to the previous version, extending its coverage in time from 2013 to 2017, while also adding some data from prior years. GLODAPv2.2019 includes measurements from more than 1.1 million water samples from the global oceans collected on 840 cruises. The data for the 12 GLODAP core variables (salinity, oxygen, nitrate, silicate, phosphate, dissolved inorganic carbon, total alkalinity, pH, CFC-11, CFC-12, CFC-113, and CCl4) have undergone extensive quality control, especially systematic evaluation of bias. The data are available in two formats: (i) as submitted by the data originator but updated to WOCE exchange format and (ii) as a merged data product with adjustments applied to minimize bias. These adjustments were derived by comparing the data from the 116 new cruises with the data from the 724 quality-controlled cruises of the GLODAPv2 data product. They correct for errors related to measurement, calibration, and data handling practices, taking into account any known or likely time trends or variations. The compiled and adjusted data product is believed to be consistent to better than 0.005 in salinity, 1 % in oxygen, 2 % in nitrate, 2 % in silicate, 2 % in phosphate, 4 µmol kg−1 in dissolved inorganic carbon, 4 µmol kg−1 in total alkalinity, 0.01–0.02 in pH, and 5 % in the halogenated transient tracers. The compilation also includes data for several other variables, such as isotopic tracers. These were not subjected to bias comparison or adjustments. The original data, their documentation and DOI codes are available in the Ocean Carbon Data System of NOAA NCEI (https://www.nodc.noaa.gov/ocads/oceans/GLODAPv2_2019/, last access: 17 September 2019). This site also provides access to the merged data product, which is provided as a single global file and as four regional ones – the Arctic, Atlantic, Indian, and Pacific oceans – under https://doi.org/10.25921/xnme-wr20 (Olsen et al., 2019). The product files also include significant ancillary and approximated data. These were obtained by interpolation of, or calculation from, measured data. This paper documents the GLODAPv2.2019 methods and provides a broad overview of the secondary quality control procedures and results.
    Repository Name: EPIC Alfred Wegener Institut
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  • 10
    Publication Date: 2019-10-24
    Description: We present a Lagrangian convective transport scheme developed for global chemistry and transport models, which considers the variable residence time that an air parcel spends in convection. This is particularly important for accurately simulating the tropospheric chemistry of short-lived species, e.g., for determining the time available for heterogeneous chemical processes on the surface of cloud droplets. In current Lagrangian convective transport schemes air parcels are stochastically redistributed within a fixed time step according to estimated probabilities for convective entrainment as well as the altitude of detrainment. We introduce a new scheme that extends this approach by modeling the variable time that an air parcel spends in convection by estimating vertical updraft velocities. Vertical updraft velocities are obtained by combining convective mass fluxes from meteorological analysis data with a parameterization of convective area fraction profiles. We implement two different parameterizations: a parameterization using an observed constant convective area fraction profile and a parameterization that uses randomly drawn profiles to allow for variability. Our scheme is driven by convective mass fluxes and detrainment rates that originate from an external convective parameterization, which can be obtained from meteorological analysis data or from general circulation models. We study the effect of allowing for a variable time that an air parcel spends in convection by performing simulations in which our scheme is implemented into the trajectory module of the ATLAS chemistry and transport model and is driven by the ECMWF ERA-Interim reanalysis data. In particular, we show that the redistribution of air parcels in our scheme conserves the vertical mass distribution and that the scheme is able to reproduce the convective mass fluxes and detrainment rates of ERA-Interim. We further show that the estimated vertical updraft velocities of our scheme are able to reproduce wind profiler measurements performed in Darwin, Australia, for velocities larger than 0.6 m s−1. SO2 is used as an example to show that there is a significant effect on species mixing ratios when modeling the time spent in convective updrafts compared to a redistribution of air parcels in a fixed time step. Furthermore, we perform long-time global trajectory simulations of radon-222 and compare with aircraft measurements of radon activity.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
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  • 11
    Publication Date: 2019-08-15
    Description: Many applications of atmospheric composition and climate data involve the comparison or combination of vertically resolved atmospheric state variables. Calculating differences and combining data require harmonization of data representations in terms of physical quantities and vertical sampling at least. If one or both datasets result from a retrieval process, knowledge of prior information and averaging kernel matrices in principle allows retrieval differences to be accounted for as well. Spatiotemporal mismatch of the sensed air masses and its contribution to the data discrepancies can be estimated with chemistry transport modeling support. In this work an overview of harmonization or matching operations for atmospheric profile observations is provided. The effect of these manipulations on the information content of the original data and on the uncertainty budget of data comparisons is examined and discussed.
    Print ISSN: 1867-1381
    Electronic ISSN: 1867-8548
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 12
    Publication Date: 2019-07-15
    Description: We study forty-one days with daily median surface accumulation mode aerosol particle concentrations below 50 cm−3 (ultra-clean conditions) observed at Ascension Island (7.9° S, 14.4° W) between June 2016 and October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Interestingly, these days occur during a period of great relevance for aerosol-cloud-radiation interactions, the southeast Atlantic (SEATL) biomass-burning season (approximately June–October). That means that these critical months can feature both the highest surface aerosol numbers, from smoke intrusion into the marine boundary layer, as well as the lowest. While carbon monoxide and refractory black carbon concentrations on ultra-clean days do not approach those on days with heavy smoke, they also frequently exceed background concentrations calculated in the non-burning season from December 2016–April 2017. This is evidence that even what become ultra-clean boundary layers can make contact with and entrain from an overlying SEATL smoke layer before undergoing a process of rapid aerosol removal. Because many ultra-clean and polluted boundary layers observed at Ascension Island follow similar isobaric back-trajectories, the variability in this entrainment is likely closely tied to the variability in the overlying smoke rather than large-scale horizontal circulation through the boundary layer. Finally, surface drizzle rates, frequencies and accumulation – as well as retrievals of liquid water path – all consistently tend toward higher values on ultra-clean days. This implicates enhanced coalescence scavenging in low clouds as the key driver of ultra-clean events in the southeast Atlantic marine boundary layer. These enhancements occur against and are likely mediated by the backdrop of a seasonal increase in daily mean cloud fraction and daily median liquid water path over ASI, peaking in September and October in both LASIC years. Therefore the seasonality in ultra-clean day occurrence seems directly linked to the seasonality in SEATL cloud properties. These results highlight the importance of two-way aerosol-cloud interactions in the region.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 13
    Publication Date: 2019-07-15
    Description: Over the last four decades, Asian countries have undergone significant economic development leading to rapid urbanization and industrialization in the region. Consequently, fossil fuel consumption has risen dramatically worsening the air quality in Asia. Fossil fuel combustion emits particulate matter containing toxic metals that can adversely affect living organisms, including humans. Thus, it is imperative to investigate the temporal and spatial extent of metal pollution in Asia. Recently, we reported a continuous and high-resolution 1650–1991 ice core record from the Guliya ice cap in northwestern Tibet, China showing a contamination of Cd, Pb and Zn during the 20th century. Here, we present a new continuous and high-resolution ice core record of trace metals from the Guliya ice cap that comprises the years between 1971 and 2015, extending the 1650–1991 ice core record into the 21st century. Non-crustal Cd, Pb, Zn and Ni enrichments increased since the 1990s reaching a maximum in 2008. The enrichments of Cd, Pb, Zn, and Ni increased by ~ 75 %, 35 %, 30 %, and 10 %, respectively during the 2000–2015 period relative to 1971–1990. Our analysis suggests that emissions from Pakistan's fossil fuel combustion (by road transportation and the manufacturing and construction industries) became the dominant source of Cd, Pb, Zn, and Ni deposited on Guliya between 1995 and 2015. However, it is possible that emissions from Central Asia, Afghanistan, India, Nepal, and the Xinjiang province in China have also impacted Guliya during the 21st century. The enrichments of Cd, Zn, and Ni declined after 2008 likely due to a coal consumption decrease in Pakistan at that time. This new record demonstrates that the current emissions in Asia are impacting remote high-altitude glaciers in the region and that mitigation policies and technologies should be enforced to improve the air quality as economic development continues in most Asian countries.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 14
    Publication Date: 2019-07-11
    Description: We investigate the possibility that the refractory, infrared-light-absorbing carbon particulate material known as tar balls or tar brown carbon (tar brC) generates a unique signal in the scattering and incandescent detectors of the single particle soot photometer (SP2). As recent studies have defined tar brC in different ways, we begin by reviewing the literature and proposing a material-based definition of tar. We then show that tar brC results in unique SP2 signals due to a combination of complete or partial evaporation, with no or very little incandescence. Approximately 70 % of tar particles incandesced. At the time of incandescence the ratio of light scattering to incandescence from these particles was up to twofold greater than from soot black carbon (BC). In our sample, where the mass of tar was threefold greater than the mass of soot, this led to a bias of
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 15
    Publication Date: 2019-07-12
    Description: Changes in atmospheric methane abundance have implications for both chemistry and climate as methane is both a strong greenhouse gas and an important precursor for tropospheric ozone. A better understanding of the drivers of trends and variability in methane abundance over the recent past is therefore critical for building confidence in projections of future methane levels. In this work, the representation of methane in the atmospheric chemistry model AM4.1 is improved by optimizing total methane emissions (to an annual mean of 576 ± 32 Tg yr−1) to match surface observations over 1980–2017. The simulations with optimized global emissions are in general able to capture the observed global trend, variability, seasonal cycle, and latitudinal gradient of methane. Simulations with different emission adjustments suggest that increases in methane sources (mainly from energy and waste sectors) balanced by increases in methane sinks (mainly due to increases in OH levels) lead to methane stabilization (with an imbalance of 5 Tg yr−1) during 1999–2006, and that increases in methane sources combined with little change in sinks (despite small decreases in OH levels) during 2007–2012 lead to renewed methane growth (with an imbalance of 14 Tg yr−1 for 2007–2017). Compared to 1999–2006, both methane emissions and sinks are greater (by 31 Tg yr−1 and 22 Tg yr−1, respectively) during 2007–2017. Our results also indicate that the energy sector is more likely a major contributor to the methane renewed growth after 2006 than wetland, as increases in wetland emissions alone are not able to explain the renewed methane growth with constant anthropogenic emissions. In addition, a significant increase in wetland emissions would be required starting in 2006, if anthropogenic emissions declined, for wetland emissions to drive renewed growth in methane, which is a less likely scenario. Simulations with varying OH levels indicate that 1 % change in OH levels could lead to an annual mean of ~ 4 Tg yr−1 difference in the optimized emissions and 0.08 year difference in the estimated tropospheric methane lifetime. Continued increases in methane emissions along with decreases in tropospheric OH concentrations during 2008–2015 prolong methane lifetime and therefore amplify the response of methane concentrations to emission changes. Uncertainties still exist in the partitioning of emissions among individual sources and regions.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 16
    Publication Date: 2019-07-11
    Description: In the framework of the MarParCloud (Marine biological production, organic aerosol particles and marine clouds: a Process Chain) project, measurements were carried out on the islands of Cape Verde, to investigate the abundance, properties, and sources of aerosol particles in general and cloud condensation nuclei (CCN) in particular, both close to sea and cloud level heights. A thorough comparison of particle number concentration (PNC), particle number size distribution (PNSD) and CCN number concentration (NCCN) at the Cape Verde Atmospheric Observatory (CVAO, sea level station) and Monte Verde (MV, cloud level station) reveals that during times without clouds the aerosol at CVAO and MV are similar and the boundary layer is generally well mixed. Therefore, data obtained at CVAO can be used to describe the aerosol particles at cloud level. Cloud events were observed at MV during roughly 58 % of the time and during these, a large fraction of particles were activated to cloud droplets. A trimodal parameterization method was deployed to characterize PNC at CVAO. Based on number concentrations in different aerosol modes, four well separable types of PNSDs were found, which were named the marine type, mixture type, dust type1 and dust type2. Aerosol particles differ depending on their origins. When the air masses came from the Atlantic Ocean, sea spray can be assumed to be one source for particles, besides for new particle formation. For these air masses, PNSDs featured the lowest number concentration in Aitken, accumulation and coarse mode. Particle number concentrations for the sea spray aerosol (SSA, i.e., the coarse mode for these air masses) accounted for about 3.7 % of NCCN,0.30 % (CCN number concentration at 0.30 % supersaturation) and about 1.1 % to 4.4 % of Ntotal (total particle number concentration). When the air masses came from the Saharan desert, we observed enhanced Aitken, accumulation and coarse mode particle number concentrations and overall increased NCCN. NCCN,0.30 % during the strongest observed dust periods is about 2.5 times higher than that during marine periods. However, the particle hygroscopicity parameter κ for these two most different periods shows no significant difference and is generally similar, independent of air mass. Overall, κ averaged 0.28, suggesting the presence of organic material in particles. This is consistent with previous model work and field measurement. There is a slight increase of κ with increasing particle size, indicating the addition of soluble, likely inorganic material during cloud processing.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 17
    Publication Date: 2019-06-26
    Description: Recent studies have recognized highly oxygenated organic molecules (HOM) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOC) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOM upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOC, and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOM to NOx and seed aerosol. Using a nitrate-based chemical ionization mass spectrometer (CI-APi-TOF), we observed the formation of HOM in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOM formed through multiple OH-oxidation steps. The composition of the identified HOM in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOM in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis of multi-generation HOM. Based on our results that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration, we conclude that atmospheric models should account for such dependency and the chemical regime when implementing the quantitative results of laboratory studies. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 18
    Publication Date: 2019-06-27
    Description: The largest contributors to the uncertainty in assessing the anthropogenic contribution in radiative forcing are the direct and indirect effects of aerosol particles on the Earth's radiative budget. Soot particles are of special interest since their properties can change significantly due to aging processes once they are emitted to the atmosphere. Probably the largest obstacle for the investigation of these processes in the laboratory is the long atmospheric lifetime of one week, demanding tailored experiments that cover this time span. This work presents results on the ability of two types of soot to act as cloud condensation nuclei (CCN) after exposure to atmospherically relevant levels of ozone and humidity. Aging times of up to 12 h were achieved by successful application of the continuous-flow stirred tank reactor (CSTR) concept while allowing for size-selection of particles prior to the aging step. 100 nm particles rich in organic carbon (OC) that were initially CCN-inactive showed significant CCN-activity at supersaturations (SS) down to 0.3 % after 10 h of exposure to 200 ppb of ozone. While this process was not affected by different levels of relative humidity in the range 5–75 %, a high sensitivity towards the ambient/reaction temperature was observed. Soot particles with a lower OC-content demanded an approximately four-fold longer aging duration to show CCN-activity for the same SS. Prior to the slow change in the CCN-activity, a rapid increase in the particle diameter was detected which occurred within several minutes. This study highlights the applicability of the CSTR-approach for the simulation of atmospheric aging processes, as aging durations beyond 12 h can be achieved in comparably small aerosol chamber volumes (
    Electronic ISSN: 1680-7375
    Topics: Geosciences
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  • 19
    Publication Date: 2019-06-24
    Description: Refractory Black Carbon (rBC) in the atmosphere is known for its significant impact on the climate system in the atmosphere. The relationship between the microphysical and optical properties of rBC remain uncertain and are largely influenced by the size, coating thickness and mixing state of particles. This study presents a coupling of a centrifugal particle mass analyser (CPMA) and a single particle soot photometer (SP2) for the morphology-independent quantification of the mixing state of rBC-containing particles, used in the urban site of Beijing as part of the Air Pollution and Human Health-Beijing (APHH-Beijing) project during winter (10th Nov–10th Dec) and summer (18th May–25th June). An inversion method is applied to the measurements to present a two-variable distribution of both rBC core mass and total mass of rBC-containing particles and present the mass-resolved mixing state of rBC-containing particles. The mass ratio between non-rBC coating and rBC core (MR) is calculated to determine the coating thickness of the rBC-containing particles. The bulk MR was found to vary between 2–12 in winter and between 2–3 in summer. This mass-resolved mixing state is used to derive the mixing state index (χ) for the rBC-containing particles. χ quantifies whether the coating is evenly distributed across the rBC-containing particle population and is used to determine the degree of internal and external mixture of rBC-containing particles. The rBC-containing particles in Beijing were found to be 55%–70 % internally mixed in winter depending on the dominant air masses. χ of rBC-containing particles was highly positively associated with increased bulk MR, rBC mass loading or pollution level in winter, whereas χ of rBC-containing particles in summer varied significantly (ranging 60 %–75 %) within the narrowly-distributed bulk MR and was found to be independent of air mass sources. This concludes that the bulk MR may only act as a predictor of mixing state in winter, and χ is better to quantify the mixing state of rBC-containing particles. The same level of bulk MR corresponded with a higher χ in summer than in winter and this tended to suggest a limited formation of coatings on rBC largely depended on primary sources. However, with the higher Non-refractory PM1 (NR-PM1) concentration in winter, the coagulation process may still lead relative thick coatings. In summer the higher secondary compounds made the rBC-containing particles more homogeneous. But due to the higher temperatures and limited pollution level, the coating thickness in summer is limited. The mixing state of rBC-containing particles should also depend on the coating formation mechanism, both primary source influence and secondary coating formation mechanism should be considered in interpreting the rBC-containing particles mixing state in the atmosphere. This particle morphology-independent and mass-based data format as introduced in this study could be conviently applied in particle-resolved or other process models to investigate atmospheric rBC aging and mixing state properties.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
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  • 20
    Publication Date: 2019-06-21
    Description: This study presents a statistical analysis of the properties of ice hydrometeors in tropical mesoscale convective systems observed during four different aircraft campaigns. Among the instruments on board the aircraft, we focus on the synergy of a 94 GHz cloud radar and 2 optical array probes (OAP; measuring hydrometeor sizes from 10 µm to about 1 cm). For two campaigns, an accurate simultaneous measurement of the ice water content is available, while for the two others, ice water content is retrieved from the synergy of the radar reflectivity measurements and hydrometeor size and morphological retrievals from OAP probes. The statistics of ice hydrometeor properties is calculated as a function of radar reflectivity factor measurement percentiles and temperature. Hence, MCS microphysical properties (ice water content, visible extinction, mass-size relationship coefficients, total concentrations and second and third moment of hydrometeors size distribution) are sorted in temperature (thus altitude) zones, and subsequently each individual campaign is analysed with respect to median microphysical properties of the global dataset (merging all 4 campaign datasets). The study demonstrates that ice water content, visible extinction, total crystal concentration, and second and third moments of hydrometeors size distributions are similar in all 4 type of MCS for IWC larger than 0.1 g m−3. Finally, two parameterizations are developed for deep convective systems. The first one concerns the calculation of the visible extinction as a function of temperature and ice water content. The second one concerns the calculation of hydrometeor size distributions as a function of ice water content and temperature that can be used in numerical weather prediction.
    Electronic ISSN: 1680-7375
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  • 21
    Publication Date: 2019-06-21
    Description: Oxidation of organic compounds in the atmosphere produces an immensely complex mixture of product species, posing a challenge both for their measurement in laboratory studies and their inclusion in air quality and climate models. Mass spectrometry techniques can measure thousands of these species, giving insight into these chemical processes, but the data sets themselves are highly complex. Data reduction techniques that group compounds in a chemically and kinetically meaningful way provide a route to simplify the chemistry of these systems, but have not been systematically investigated. Here we evaluate three approaches to reducing the dimensionality of oxidation systems measured in an environmental chamber: positive matrix factorization (PMF), hierarchical clustering analysis (HCA), and a parameterization to describe kinetics in terms of multigenerational chemistry (gamma kinetics parameterization, GKP). The evaluation is implemented by means of two data sets: synthetic data consisting of a three-generation oxidation system with known rate constants, generation numbers, and chemical pathways; and the measured products of OH-initiated oxidation of a substituted aromatic compound in a chamber experiment. We find that PMF accounts for changes in the average composition of all products during specific periods of time, but does not sort compounds into generations or by another reproducible chemical process. HCA, on the other hand, can identify major groups of ions and patterns of behavior, and maintains bulk chemical properties like carbon oxidation state that can be useful for modeling. The continuum of kinetic behavior observed in a typical chamber experiment can be parameterized by fitting species' time traces to the GKP, which approximates the chemistry as a linear, first-order kinetic system. Fitted parameters for each species are the number of reaction steps with OH needed to produce the species (the generation) and an effective kinetic rate constant that describes the formation and loss rates of the species. The thousands of species detected in a typical laboratory chamber experiment can be organized into a much smaller number (10–30) of groups, each of which has characteristic chemical composition and kinetic behavior. This quantitative relationship between chemical and kinetic characteristics, and the significant reduction in the complexity of the system, provide an approach to understanding broad patterns of behavior in oxidation systems and could be exploited for mechanism development and atmospheric chemistry modeling.
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  • 22
    Publication Date: 2019-06-19
    Description: Sunlit snow is highly photochemically active and plays an important role in the exchange of gas-phase species between the cryosphere to the atmosphere. Here, we investigate the behaviour of two selected species in surface snow: mercury (Hg) and iodine (I). Hg can deposit year-round and accumulate in the snowpack. However, photo-induced re-emission of gas phase Hg from the surface has been widely reported. Iodine is active in atmosphere new particle formation, especially in the marine boundary layer, and in the destruction of atmospheric ozone. It can also undergo photochemical re-emission. Although previous studies indicate possible post-depositional processes, little is known about the diurnal behaviour of these two species and their interaction in surface snow. The mechanisms are still poorly constrained and no field experiments have been performed in different seasons to investigate the magnitude of re-emission processes. Three high temporal resolution (hourly samples) 3 days long sampling campaign were carried out near Ny-Ålesund (Svalbard) to study the behaviour of mercury and iodine in surface snow under different sunlight and environmental conditions (24 h-darkness, 24 h-sunlight and day/night cycles). Our results indicate a clearly different behaviour of Hg and I in surface snow during the different campaign. The day/night experiments demonstrate the existence of a diurnal cycle in surface snow for Hg and iodine, indicating that these species are indeed influenced by the daily solar radiation cycle. Differently bromine did not show any diurnal cycle. The diurnal cycle disappeared also for Hg and iodine during the 24 h-sunlight period and during 24 h-darkness experiments supporting the idea of the occurrence (absence) of a continuous recycling/exchange at the snow-air interface. These results demonstrate that this surface snow recycling is seasonally dependent, through sunlight. They also highlight the non-negligible role that snowpack emissions have on ambient air concentrations and potentially on iodine-induced atmospheric nucleation processes.
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  • 23
    Publication Date: 2019-06-20
    Description: The global response of air temperature at 2 metre above the surface to the eruptions of Mount Agung in March 1963, El Chichón in April 1982, and Mount Pinatubo in June 1991 is investigated using 11 global atmospheric reanalysis data sets (JRA-55, JRA-25, MERRA-2, MERRA, ERA-Interim, ERA-40, CFSR, NCEP-NCAR R-1, 20CR version 2c, ERA-20C, and CERA-20C). Multiple linear regression (MLR) is applied to the monthly mean time series of temperature for two periods, 1980–2010 (for 10 reanalyses) and 1958–2001 (for six reanalyses), by considering explanatory factors of seasonal harmonics, linear trends, Quasi-Biennial Oscillation (QBO), solar cycle, tropical sea surface temperature (SST) variations in the Pacific, Indian, and Atlantic Oceans, and Arctic SST variations. Empirical orthogonal function (EOF) analysis is applied to these climatic indices to obtain a set of orthogonal indices to be used for the MLR. The residuals of the MLR are used to define the volcanic signals for the three eruptions separately. First, latitudinally averaged time series of the residuals are investigated and compared with the results from previous studies. Then, the geographical distribution of the response during the peak cooling period after each eruption is investigated. In general, different reanalyses show similar geographical patterns of the response, but with the largest differences in the polar regions. The Pinatubo response shows largest average cooling in the 60° N–60° S region among the three eruptions, with a peak cooling of 0.10–0.15 K. The El Chichón response shows slightly larger cooling in the NH than in the Southern Hemisphere (SH), while the Agung response shows larger cooling in the SH. These hemispheric differences are consistent with the distribution of stratospheric aerosol optical depth after these eruptions; however, the peak cooling after these two eruptions is comparable in magnitude to unexplained cooling events in other periods without volcanic influence. Other methods in which the MLR model is used with different sets of indices are also tested, and it is found that careful treatment of tropical SST variability is necessary to evaluate the surface response to volcanic eruptions in observations and reanalyses.
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  • 24
    Publication Date: 2019-06-14
    Description: The uptake of hydrochloric acid (HCl), ethanol (C2H5OH), 1-butanol (1-C4H9OH), formic acid HC(O)OH and tri-fluoro acetic (CF3C(O)OH) acid to growing ice surfaces was investigated at temperatures between 194 and 228 K. HCl displayed extensive, continuous uptake during ice growth, which was strongly dependent on the ice growth velocity, the temperature of the ice surface and the gas phase concentration of HCl. Tri-fluoro acetic acid was also observed to be trapped in growing ice, albeit approximately an order of magnitude less efficiently than HCl, whereas the adsorption and desorption kinetics of ethanol, 1-butanol, formic acid on ice was not measurably different to that for non-growing ice, even at very large ice-growth rates. We present a parameterisation of the uptake coefficient for HCl on growing ice films (gamma_trap) and compare the results to an existing framework that describes the non-equilibrium trapping of trace gases on ice. The trapping of HCl in growing ice crystals in the atmosphere is assessed and compared to the gas- and ice-phase partitioning resulting from equilibrium surface adsorption and solubility.
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  • 25
    Publication Date: 2019-06-17
    Description: Peat fuels representing four biomes of boreal (western Russia and Siberia), temperate (northern Alaska, U.S.A.), subtropical (northern and southern Florida, U.S.A), and tropical (Borneo, Malaysia) regions were burned in a laboratory chamber to determine gas and particle emission factors (EFs). Tests with 25 % fuel moisture were conducted with predominant smoldering combustion conditions (average modified combustion efficiency [MCE] = 0.82 ± 0.08). Average fuel-based EFCO2 (carbon dioxide) are highest (1400 ± 38 g kg−1) and lowest (1073 ± 63 g kg−1) for the Alaskan and Russian peats, respectively. EFCO (carbon monoxide) and EFCH4 (methane) are ~12 %‒15 % and ~0.3 %‒0.9  % of EFCO2, in the range of 157‒171 g kg−1 and 3‒10 g kg−1, respectively. EFs for nitrogen species are at the same magnitude of EFCH4, with an average of 5.6 ± 4.8 and 4.7 ± 3.1 g kg−1 for EFNH3 (ammonia) and EFHCN (hydrogen cyanide); 1.9 ± 1.1 g kg−1 for EFNOx (nitrogen oxides); as well as 2.4 ± 1.4 and 2.0 ± 0.7 g kg−1 for EFNOy (reactive nitrogen) and EFN2O (nitrous oxide). An oxidation flow reactor (OFR) was used to simulate atmospheric aging times of ~2 and ~7 days to compare fresh (upstream) and aged (downstream) emissions. Filter-based EFPM2.5 varied by 〉4-fold (14‒61 g kg−1) without appreciable changes between fresh and aged emissions. The majority of EFPM2.5 consists of EFOC (organic carbon), with EFOC/EFPM2.5 ratios in the range of 52 %‒98 % for fresh emissions, and ~15 % degradation after aging. Reductions of EFOC (~7‒9 g kg−1) after aging are most apparent for boreal peats with the largest degradation in organic carbon that evolves at 〈140 °C, indicating the loss of high vapor pressure semi-volatile organic compounds upon aging. The highest EFLevoglucosan is found for Russian peat (~16 g kg−1), with ~35 %‒50 % degradation after aging. EFs for water-soluble OC (EFWSOC) accounts for ~20 %‒62 % of fresh EFOC. The majority (〉95 %) of the total emitted carbon is in the gas phase with 54 %‒75 % CO2, followed by 8 %‒30 % CO. Nitrogen in the measured species explains 24 %‒52 % of the consumed fuel nitrogen with an average of 35 ± 11 %, consistent with past studies that report ~one- to two-thirds of the fuel nitrogen measured in biomass smoke. The majority (〉99 %) of the total emitted nitrogen is in the gas phase, with an average of 16.7 % fuel N emitted as NH3 and 9.5 % of fuel N emitted as HCN. N2O and NOy constituted 5.7 % and 2.9 % of consumed fuel N. EFs from this study can be used to refine current emissions inventories.
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  • 26
    Publication Date: 2019-06-12
    Description: Shipborne measurements of nitryl chloride (ClNO2), hydrogen chloride (HCl) and sulphur dioxide (SO2) were made during the AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign in summer 2017. The dataset includes measurements over the Mediterranean Sea, the Suez Canal, the Red Sea, the Gulf of Aden, the Arabian Sea, the Gulf of Oman and the Arabian Gulf (also known as Persian Gulf) with observed ClNO2 mixing ratios ranging from the limit of detection to ≈ 600 pptv. We examined the regional variability in the generation of ClNO2 via the uptake of dinitrogen pentoxide (N2O5) to Cl-containing aerosol and its importance for Cl-atom generation in a marine boundary layer under the (variable) influence of emissions from shipping and oil industry. The yield of ClNO2 formation per NO3 radical generated was generally low (median of ≈ 1–5 % depending on the region), mainly as a result of gas-phase loss of NO3 dominating over heterogeneous loss of N2O5, the latter being disfavoured by the high temperatures found throughout the campaign. The contributions of ClNO2 photolysis and OH-induced HCl oxidation to Cl-radical formation were derived and their relative contributions over the diel cycle compared. The results indicate that over the northern Red Sea, the Gulf of Suez and the Gulf of Oman the formation of Cl-atoms will enhance the oxidation rates of some VOCs, especially in the early morning.
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  • 27
    Publication Date: 2019-06-07
    Description: The physicochemical properties of aerosols and their impacts on cloud microphysical properties are examined using data collected from the Department of Energy Atmospheric Radiation Measurement (ARM) facility over the Southern Great Plains region of the United States (ARM-SGP). A total of 16 low-level stratus cloud cases under daytime coupled boundary layer conditions are selected. The aerosol-cloud interaction index (ACIr) is used to quantify the aerosol impacts with respect to cloud-droplet effective radius. The mean value of ACIr calculated from all selected samples is 0.145 ± 0.05 and ranges from 0.09 to 0.24 at a range of cloud liquid water paths (LWP = 20–300 g m−2). The magnitude of ACIr decreases with increasing LWP which suggests a cloud microphysical response to diminished aerosol loading presumably due to enhanced collision-coalescence processes and enlarged particle size. In the presence of weak light-absorbing aerosols, the low-level clouds feature a higher number concentration of cloud condensation nuclei (NCCN) and smaller effective radii (re) while the opposite is true for strong light-absorbing aerosols. Furthermore, the mean activation ratio of aerosols to CCN (NCCN / Na) for weakly (strongly) absorbing aerosols is 0.54 (0.45), owing to the different hygroscopic abilities associated with the dominant aerosol species. In terms of the sensitivity of cloud droplet number concentration (Nd) to aerosol loading, the conversion ratio of Nd / NCCN for weakly (strongly) absorptive aerosols is 0.68 (0.54). Consequently, we expect larger shortwave radiative cooling effect from clouds influenced by weakly absorbing aerosols than strongly absorbing aerosols.
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  • 28
    Publication Date: 2019-06-11
    Description: Air pollution in West Africa is far to be well characterized. It was the rationale of the Air Pollution and Health work package in the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) program. Intensive measurement campaigns were running in two West African capitals (Abidjan in Côte d’Ivoire and Cotonou in Benin), to examine size distribution of the particulate matter (PM) concentrations and their chemical composition (Elemental Carbon (EC), Organic Carbon (OC), Water-soluble organic carbon (WSOC), Water-soluble inorganic ions (WSI) and trace metals). This work aims to characterize PM from different sites in Abidjan, the economic capital of Cote d’Ivoire, typical of Domestic Fire (ADF), Traffic (AT) and Waste Burning (AWB) and Cotonou, the capital of Benin, representative of Traffic (CT). These selected sites, impacted by a large amount of pollution levels, are representative of the main combustion sources prevailing in South West Africa during dry and wet seasons. To address this concern, intensive campaigns in Abidjan and Cotonou have been conducted in July (2015 and 2016) and January (2016 and 2017). Results show a well-marked seasonality, inter-annual and spatial variabilities and the PM levels at the studied areas are generally higher than the WHO guidelines. The average mass concentrations in the wet season were 90.3, 104.1, 69.1 and 517.3 μg m−3 at AT, CT, AWB and ADF sites, respectively. The largest value at ADF site is due to the contribution of smoking and roasting activities. By contrast in the dry season, the concentrations increase to 141.3, 269.7 and 175.3 μg m−3 at AT, CT and AWB site, respectively whereas at ADF site concentration decreases to 375.7 μg m−3. The chemical aerosol mass closure shows that dust contributed for 25–65 % at the both traffic and AWB sites, and 10–30 % at ADF with a clear seasonal cycle. A large variability of POM is observed with contribution range of 37–68 % at ADF, 20–42 % at AT, 10–34 % at AWB and 15–22 % at CT. The contribution of WSI to bulk PM (lower than 20 %) is 2–3 times larger in wet season than dry season, except at ADF site where no season variation is observed. The most dominant species in WSI fraction at ADF are chloride (18–36 % of the total ions), potassium (8–22 %) and calcium (13–25 %), while at the rest of the sites, nitrates (21–36 %), chlorides (6–30 %) and sulfates (9–20 %) are higher. At all sites, the proportion of EC is twice higher in dry season than in wet season. Carbonaceous aerosol (sum of EC and POM) and dust particles are the two major contributors to the different particle fractions with carbonaceous aerosol predominant at Abidjan and dust at Cotonou. The highest carbonaceous aerosol contribution is obtained at ADF (up to 75 % of total PM), while at the other sites its contribution ranges between 18 and 35 %. WSOC levels are higher at the traffic sites during the dry season, while during the wet season they are maximum at ADF and AWB sites. Element trace characterization is also determined, showing predominance of Al, Na and Ca followed by Fe, K and Mg. Our study highlights the contribution of different traffic emissions in two major West African cities in atmospheric aerosol composition but also the one of domestic fire and waste combustion sources. It constitutes an original database to characterize urban air pollution for specific African combustion sources.
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  • 29
    Publication Date: 2019-06-07
    Description: The ozone variation characteristics and the impact of synoptic and local meteorological factors in North China were analysed quantitively during the warm season from 2013 to 2017 based on multi-city, in-situ ozone and meteorological data, as well as meteorological reanalysis. The domain-averaged maximum daily 8-h running average O3 (MDA8 O3) concentration was 122 ± 11 μg m−3 with an increase rate of 7.88 μg m−3 year−1, and the three most highly-polluted months were June (149 μg m−3), May (138 μg m−3) and July (132 μg m−3), which was closely related to synoptic circulation variations. Twenty-six synoptic circulation types (merged into 5 weather categories) were objectively identified using the Lamb-Jenkinson method. The highly-polluted weather categories included S-W-N directions, LP (low-pressure related circulation patterns) and C (cyclone type), and corresponding domain-averaged MDA8 O3 concentration were 122, 126 and 128 μg m−3, respectively. Based on the frequency and intensity changes of synoptic circulations, 39.2 % of the inter-annual domain-averaged O3 increase from 2013 to 2017 was attributed to synoptic changes, and intensity of synoptic circulations was the dominant factor. Using synoptic classification and local meteorological factors, the segmented synoptic-regression approach was established to evaluate and forecast daily ozone variations on an urban scale. The results showed that this method is practicable in most cities, and that the dominant factors are the maximum temperature, southerly winds, relative humidity in the previous and in the same day, and total cloud cover. Overall, 43–64 % of the day-to-day variability of MDA8 O3 concentrations was due to local meteorological variations in most cities over North China, except for QHD~32 % and ZZ~25 %. Our quantitative exploration on synoptic and local meteorological factors influencing both on inter-annual and day-to-day ozone variations will provide the scientific basis for evaluating emission reduction measures, since the national and local governments have implemented a series of measures to mitigate air pollution in North China in these five years.
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  • 30
    Publication Date: 2019-06-07
    Description: Nocturnal low-level stratus clouds (LLC) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLC in the state–of–the–art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-Aerosol-Cloud-Chemistry Interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLC during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to reduced contribution from horizontal cold-air advection. This in turn results in a weaker cooling and thus saturation is not reached. Such deviation in the dynamics of NLLJ is related to the arrival of cold air mass propagating northwards from the coast called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from e.g. central or south Africa, reduce the background moisture over the large parts of SWA. Based on the backward trajectories analysis, another possible reason for clear nights is descending of air originating from drier levels above the marine boundary layer.
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  • 31
    Publication Date: 2019-06-06
    Description: The Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE) was devoted to the investigation of Saharan dust properties over the Caribbean. The campaign took place in June–July 2013. A wide set of ground-based and airborne aerosol instrumentation was deployed at Barbados island for a comprehensive experiment. Several sun photometers performed measurements during this campaign: two AERONET Cimel sun photometers and the Sun and Sky Automatic Radiometer (SSARA). The sun photometers were co-located with the ground-based multi-wavelength lidars BERTHA and POLIS. Aerosol properties derived from direct sun and sky radiance observations are analyzed, and a comparison with the co-located lidar and in-situ data is provided. The time series of aerosol optical depth allows identifying successive dust events with short periods in between in which the marine background conditions were observed. Moderate aerosol optical depth in the range 0.3 to 0.6 was found during the dust periods. The sun photometer infrared channel at 1640 nm wavelength was used in the retrieval to investigate possible improvements and expected larger sensitivity to coarse particles. The comparison between column (AOD) and surface (dust concentration) data demonstrates the connection between the Saharan Air Layer and the boundary layer in the Caribbean region, as it is shown by the synchronized detection of the successive dust events in both data sets. However the comparison of size distributions derived from sun photometer data and in-situ observations reveal the difficulties to carry out a column closure study.
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  • 32
    Publication Date: 2019-06-14
    Description: Using a transient simulation for the period 1960–2100 with the state-of-the-art ECHAM5/MESSy Atmospheric Chemistry (EMAC) global model and a tropopause fold identification algorithm, we explore the future projected changes in tropopause folds, Stratosphere-to-Troposphere Transport (STT) of ozone and tropospheric ozone under the RCP6.0 scenario. Statistically significant changes in tropopause fold frequencies are identified in both Hemispheres, occasionally exceeding 3 %, which are associated with the projected changes in the position and intensity of the subtropical jet streams. A strengthening of ozone STT is projected for future at both Hemispheres, with an induced increase of transported stratospheric ozone tracer throughout the whole troposphere, reaching up to 10 nmol/mol in the upper troposphere, 8 nmol/mol in the middle troposphere and 3 nmol/mol near the surface. Notably, the regions exhibiting the maxima changes of ozone STT at 400 hPa, coincide with that of the highest fold frequencies, highlighting the role of tropopause folding mechanism in STT process under a changing climate. For both the eastern Mediterranean and Middle East (EMME), and the Afghanistan (AFG) regions, which are known as hotspots of fold activity and ozone STT during the summer period, the year-to-year variability of middle tropospheric ozone with stratospheric origin is largely explained by the short-term variations of ozone at 150 hPa and tropopause folds frequency. Finally, ozone in the lower troposphere is projected to decrease under the RCP6.0 scenario during MAM (March, April and May) and JJA (June, July and August) at the Northern Hemisphere, and during DJF (December, January and February) at the Southern Hemisphere, due to the decline of ozone precursors emissions, while in the rest of the troposphere ozone shows a remarkable increase owing to the STT strengthening.
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  • 33
    Publication Date: 2019-06-12
    Description: The long-term temporal-spatial variations of aerosol optical properties in Tibetan Plateau (TP) and the potential long-range transport from surrounding areas to TP were analyzed in this work, by using multiple years of sunphotometer measurements (CE318) at five stations in TP, satellite aerosol productions from Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), back-trajectory analysis from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) and model simulation of the Goddard Earth Observing System (GEOS)-Chem chemistry transport model. The results from ground-based observations show that the annual aerosol optical depth (AOD) at most TP sites increased in the past decades with trends of 0.001 ± 0.003/year at Lhasa, 0.013 ± 0.003/year at Mt_WLG, 0.002 ± 0.002/year at NAM_CO, and 0.000 ± 0.002/year at QOMS_CAS. The increasing trend is also found for the aerosol Extinction Ångstrom exponent (EAE) at most sites, except for Mt_WLG sites with an obvious decreasing trend. Spatially, the AOD observed from MODIS shows negative trends in the northwest edge closed to the Taklimakan Desert and east of the Qaidam Basin and slightly positive trends in most of the other area of TP. Different aerosol types and sources contribute to the polluted day (with CE318 AOD at 440 nm 〉 0.4) in the five sites of TP: dust dominant in Lhasa, Mt_WLG and Muztagh with sources from the Taklimakan Desert but fine aerosol pollution dominant at NAM_CO and QOMS_CAS with the transport from South Asia. A case of aerosol pollution at Lhasa, NAM_CO and QOMS_CAS during 28 April–3 May 2016 reveals that the smoke aerosols in South Asia were lifted up to 10 km and transported to TP, while the dust from Taklimakan Desert could climb the north slope of TP and then be transported to center TP. The long-range transport thereby seriously impact aerosol loading over the TP.
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  • 34
    Publication Date: 2019-06-11
    Description: Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical non-interacting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We have studied the statistics of collisions of sulfuric acid molecules in vacuum by atomistic molecular dynamics simulations. We have found that the effective collision cross section of the H2SO4 molecule, as described by an OPLS-All Atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor 2.2, compared to kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, fitted to the attractive part of the atomistic intermolecular potential of mean force.
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  • 35
    Publication Date: 2019-06-11
    Description: A demonstration study to examine the feasibility to retrieve dust radiative effects based on combined satellite data from MODIS (Moderate Resolution Imaging Spectroradiometer), CERES (Clouds and the Earth’s Radiant Energy System) and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) li-dar vertical profiles along their orbit is presented. The radiative transfer model GAME (Global Atmos-pheric Model) is used to estimate the shortwave and longwave dust radiative effects below the CALIP-SO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite) orbit assuming an aerosol parameterization based on CALIOP vertical distribution at a horizontal resolution of 5 km and additional AERONET (Aerosol Robotic Network) data. Two study cases are analysed; a strong long-range transport mineral dust event (AOD = 0.52) originated in the Sahara Desert and reaching the United Kingdom and a weaker event (AOD = 0.16) affecting Eastern Europe. The obtained radiative fluxes are first validated in terms of radiative forcing efficiency at a single point with space-time co-located lidar ground-based measurements from EARLINET (European Aerosol Research Lidar Network) stations below the orbit. The methodology is then applied to the full orbit. The obtained results indicate that the radiative effects show a strong dependence on the aerosol load, highlighting the need of accurate AOD measurements for forcing studies, and on the surface albedo. The calculated dust radiative effects and heating rates below the orbits are in good agreement with previous studies of mineral dust, with the forcing efficiency obtained at the surface ranging between −80.3 and −63.0 W m−2 for the weaker event and −119.1 and −79.3 W m−2 for the strong one. Results thus demonstrate the validity of the presented method to retrieve 2-D accurate radiative properties with large spatial and temporal coverage.
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  • 36
    Publication Date: 2019-06-11
    Description: We combine available observational data sets with Lagrangian atmospheric modelling in order to analyze the spatial and temporal variability of the CHBr3 injection into the stratosphere. Regional maxima with mixing ratios of up to 0.4–0.5 ppt at 17 km altitude are diagnosed to be over Central America (1) and over the Maritime Continent/West Pacific (2), both of which are confirmed by high-altitude aircraft campaigns. The CHBr3 maximum over Central America is caused by the co-occurrence of convectively-driven short transport time scales and strong regional sources, which in conjunction drive the seasonality of CHBr3 injection. Model results at a daily resolution reveal isolated, exceptionally high CHBr3 values in this region which are confirmed by measurements during the ACCENT campaign and do not occur in spatially or temporally averaged model fields. CHBr3 injection over the West Pacific is centered south of the equator due to strong oceanic sources underneath prescribed by the here applied bottom-up emission inventory. The globally strongest stratospheric CHBr3 injection of up to 0.6 ppt is diagnosed to occur over the region of India, Bay of Bengal and Arabian Sea (3), however, no data from aircraft campaigns are available to confirm this finding. Interannual variability of stratospheric CHBr3 injection of 10–20 % is to a large part driven by the variability of coupled ocean-atmosphere circulation systems. Long-term changes, on the other hand, correlate with the regional SST trends resulting in positive trends of stratospheric CHBr3 injection over the West Pacific and Asian monsoon region and negative trends over the East Pacific. For the tropical mean, these opposite regional trends balance each other out resulting in a relatively weak positive trend of 0.017 ± 0.012 ppt Br/dec for 1979–2013, corresponding 3 % Br/dec. The overall contribution of CHBr3 together with CH2Br2 to the stratospheric halogen loading accounts for 4.7 ppt Br, in good agreement with existing studies, with 50 %/50 % being injected in form of source and product gases, respectively.
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  • 37
    Publication Date: 2019-06-11
    Description: Dry deposition is the second largest sink of tropospheric ozone. Increasing evidence has shown that ozone dry deposition actively links meteorology and hydrology with ozone air quality. However, there is little systematic investigation on the performance of different ozone dry deposition parameterizations at the global scale, and how parameterization choice can impact surface ozone simulations. Here we present the results of the first global, multi-decade modelling and evaluation of ozone dry deposition velocity (vd) using multiple ozone dry deposition parameterizations. We use consistent assimilated meteorology and satellite-derived leaf area index (LAI) to simulate vd over 1982–2011 driven by four sets of ozone dry deposition parametrization that are representative of the current approaches of global ozone dry deposition modelling, such that the differences in simulated vd are entirely due to differences in deposition model structures. In addition, we use the surface ozone sensitivity to vd predicted by a chemical transport model to estimate the impact of mean and variability of ozone dry deposition velocity on surface ozone. Our estimated vd from four different parameterizations are evaluated against field observations, and while performance varies considerably by land cover types, our results suggest that none of the parameterizations are universally better than the others. Discrepancy in simulated mean vd among the parameterizations is estimated to cause 2 to 5 ppbv of discrepancy in surface ozone in the Northern Hemisphere (NH) and up to 8 ppbv in tropical rainforest in July, and up to 8 ppbv in tropical rainforests and seasonally dry tropical forests in Indochina in December. Parameterization-specific biases based on individual land cover type and hydroclimate are found to be the two main drivers of such discrepancies. We find statistically significant trends in the multiannual time series of simulated July daytime vd in all parameterizations, driven by warming and drying (southern Amazonia, southern African savannah and Mongolia) or greening (high latitudes). The trends in July daytime vd is estimated to be 1 % yr−1 and leads to up to 3 ppbv of surface ozone changes over 1982–2011. The interannual coefficient of variation (CV) of July daytime mean vd in NH is found to be 5 %–15 %, with spatial distribution that varies with the dry deposition parameterization. Our sensitivity simulations suggest this can contribute between 0.5 to 2 ppbv to interannual variability (IAV) in surface ozone, but all models tend to underestimate interannual CV when compared to long-term ozone flux observations. We also find that IAV in some dry deposition parameterizations are more sensitive to LAI while others are more sensitive to climate. Comparisons with other published estimates of the IAV of background ozone confirm that ozone dry deposition can be an important part of natural surface ozone variability. Our results demonstrate the importance of ozone dry deposition parameterization choice on surface ozone modelling, and the impact of IAV of vd on surface ozone, thus making a strong case for further measurement, evaluation and model-data integration of ozone dry deposition on different spatiotemporal scales.
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  • 38
    Publication Date: 2019-06-12
    Description: We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier-Transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0° S, 39.6° E) from March to December 2007 and September to November 2011. This was the first continuous measurements of chlorine species throughout the ozone hole period from the ground in Antarctica. We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura/MLS (Microwave Limb Sounder) satellite sensor, and ClONO2 data taken with the Envisat/MIPAS (The Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. HCl and ClONO2 decrease occurred at both 18 and 22 km, and soon ClONO2 was almost depleted in early winter. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO enhanced period, negative correlation between ClO and ClONO2 was observed in the time-series of the data at Syowa Station. This negative correlation was associated with the distance between Syowa Station and the inner edge of the polar vortex. We used MIROC3.2 Chemistry-Climate Model (CCM) results to see the comprehensive behavior of chlorine and related species inside the polar vortex and the edge region in more detail. From CCM model results, rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase of ClO when sunlight became available, and conversion of ClO into HCl, was successfully reproduced. HCl decrease in the winter polar vortex core continued to occur due to the transport of ClONO2 from the subpolar region to higher latitudes, providing a flux of ClONO2 from more sunlit latitudes into the polar vortex. Temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistry related to Polar Stratospheric Cloud (PSC) occurrence deep inside the polar vortex, and transport of an NOx-rich airmass from lower latitudinal polar vortex boundary region which can produce additional ClONO2 by reaction of ClO with NO2. The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO2) were found to be highly dependent on the availability of ambient O3. At an altitude where most ozone was depleted in Antarctica (18 km), most ClO was converted to HCl. However, at an altitude where there were some O3 available (22 km), additional increase of ClONO2 from initial value can occur, similar to the case in the Arctic.
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  • 39
    Publication Date: 2019-06-07
    Description: In vegetation canopies with complex architectures, diffuse solar radiation can enhance carbon assimilation through photosynthesis because isotropic light is able to reach deeper layers of the canopy. Although this effect has been studied in the past decade, the mechanisms and impacts of this enhancement over South America remain poorly understood. Over the Amazon deforestation arch large amounts of aerosols are released into the atmosphere due to biomass burning, which provides an ideal scenario for further investigation of this phenomenon in the presence of canopies with complex architecture. In this paper, the relation of aerosol optical depth and surface fluxes of mass and energy are evaluated over three study sites with Artificial Neural Networks and radiative transfer modeling. Results indicate a significant effect of the aerosol on flux of carbon dioxide between the vegetation and the atmosphere, as well as on energy exchange, including that surface fluxes are sensitive to second order radiative impacts of aerosols on temperature, humidity, and friction velocity. CO2 exchanges increased in the presence of aerosol in up to 55 % in sites with complex canopy architecture. A decrease of approximately 12 % was observed for a site with shorter vegetation. Energy fluxes were negatively impacted by aerosols over all study sites.
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  • 40
    Publication Date: 2019-06-06
    Description: Biogenic volatile organic compounds exert a strong influence on regional air quality and climate through their roles in the chemical formation of ozone and fine mode aerosol. Dimethyl sulphide (DMS), in particular, can also impact cloud formation and the radiative budget as it produces sulfate aerosols upon atmospheric oxidation. Recent studies have reported DMS emissions from terrestrial sources , however their magnitudes have been too low to account for the observed ecosystem scale DMS fluxes. Big-leaf Mahogany (Swietenia macrophylla) is an agro-forestry and natural forest tree known for it good quality timber and listed under the Convention on International Trade in Endangered Species (CITES). It is widely grown in several South American, Central American, North American and Asian atmospheric environments (〉 2.4 million km2 collectively). Here, we investigated emissions of monoterpenes, isoprene and DMS as well as seasonal carbon assimilation from four big-leaf Mahogany trees in their natural outdoor environment using a dynamic branch cuvette system, high sensitivity proton transfer reaction mass spectrometer and cavity ring down spectrometer. The emissions were characterized in terms of environmental response functions such as temperature, radiation and physiological growth phases including leaf area over the course of four seasons (summer, monsoon, post-monsoon, winter) in 2018–19. We discovered remarkably high emissions of DMS (average in post monsoon: ~ 19 ng/g leaf dry weight/hr) relative to previous known tree DMS emissions, high monoterpenes (average in monsoon: ~ 15 µg/g leaf dry weight/hr which are comparable to oak trees) and low emissions of isoprene. Distinct linear relationships existed in the emissions of all three BVOCs with higher emissions during the reproductive phase (monsoon and post-monsoon seasons) and lower emissions in the vegetative phase (summer and winter seasons) for the same amount of cumulative assimilated carbon. Temperature and PAR dependency of the BVOC emissions enabled formulation of a new parametrization for use in global BVOC emission models. Finally, using the measured seasonal fluxes, we provide the first estimates for the global emissions from Mahogany trees which amount to circa 210–320 Gg yr−1 for monoterpenes, 370–550 Mg yr−1 for DMS and 1700–2600 Mg yr−1 for isoprene. Finally, through the results obtained in this study, we have been able to discover and identify Mahogany as one of the missing natural sources of ambient DMS over the Amazon rainforest as well. These new emission findings, seasonal patterns, and estimates will be useful for initiating new studies to further improve the global BVOC terrestrial budget.
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  • 41
    Publication Date: 2019-06-04
    Description: The photooxidation of the most abundant monoterpene α-pinene, by the hydroxyl radical (OH) was investigated at atmospheric concentrations in the atmospheric simulation chamber SAPHIR. Concentrations of nitric oxide (NO) were below 120 pptv. Yields of organic oxidation products are determined from measured time series giving values of 0.11 ± 0.05, 0.19 ± 0.06, and 0.05 ± 0.03 for formaldehyde, acetone, and pinonaldehyde, respectively. The pinonaldehyde yield is at the low side of yields measured in previous laboratory studies, ranging from 0.06 to 0.87. These studies were mostly performed at reactant concentrations much higher than observed in the atmosphere. Time series of measured radical and trace gas concentrations are compared to results from model calculations applying the Master Chemical Mechanism (MCM) 3.3.1. The model predicts pinonaldehyde mixing ratios that are at least a factor of 4 higher than measured values. At the same time, modelled hydroxyl and hydroperoxy (HO2) radical concentrations are approximately 25 % lower than measured values. Vereecken et al. (2007) suggested a shift of the initial organic peroxy radical (RO2) distribution towards RO2 species that do not yield pinonaldehyde, but produce other organic products. Implementing these modifications reduces the model-measurement gap of pinonaldehyde by 20 % and also improves the agreement in modelled and measured radical concentrations by 10 %. However, the chemical oxidation mechanism needs further adjustment to explain observed radical and pinonaldehyde concentrations. This could be achieved by adjusting the initial RO2 distribution, but could also be done by implementing alternative reaction channels of RO2 species that currently lead to the formation of pinonaldehyde in the model.
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  • 42
    Publication Date: 2019-06-03
    Description: The legal commercialization of cannabis for recreational and medical use has effectively created a new and almost unregulated cultivation industry. In 2018, within the Denver County limits, there were more than 600 registered cannabis cultivation facilities (CCFs) for recreational and medical use, mostly housed in commercial warehouses. Measurements have found concentrations of highly reactive terpenes from the headspace above cannabis plants that, when released in the atmosphere, could impact air quality. Here we developed the first emission inventory for cannabis emissions of terpenes. The range of possible emissions from these facilities was 66–657 metric tons/year of terpenes across the state of Colorado; half of the emissions are from Denver County. Our estimates are based on the best available information and highlight the critical data gaps needed to reduce uncertainties. These realizations of inventories were then used with a regulatory air quality model, developed by the State of Colorado to predict regional ozone impacts. It was found that most of the predicted changes occur in the vicinity of CCFs concentrated in Denver. An increase of 362 metric tons/year of terpene emissions in Denver County resulted in increases of up to 0.34 ppb in hourly ozone concentrations during the morning and 0.67 ppb at night. Model predictions indicate that in Denver County every 1,000 metric tons/year increase of terpenes results in 1 ppb increase in daytime hourly ozone concentrations and a maximum daily 8-hour average (MDA8) increase of 0.3 ppb. The emission inventories developed here are highly uncertain, but highlight the need for more detailed cannabis and CCFs data to fully understand the possible impacts of this new industry on regional air quality.
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  • 43
    Publication Date: 2019-06-03
    Description: This study reports on the glaciochemistry of a deep ice core (182 m long) drilled in 2009 at Mount Elbrus (43°21′ N, 42°26′ E; 5115 m above sea level) in the Caucasus, Russia. Radiocarbon dating of the particulate organic carbon fraction in the ice suggests a basal ice age of ~ 1670 ± 400 cal yr BP. Based on chemical stratigraphy, the upper 168.6 m of the core were dated by counting annual layers. The seasonally resolved chemical records cover the years 1774–2009 (Common Era), thus, being useful to reconstruct many aspects of atmospheric pollution in central Europe from pre-industrial times to present-day. After having examined the extent to which the arrival of large dust plumes originating from Sahara and Middle East modifies the chemical composition of the Elbrus (ELB) snow and ice layers, we focus on the sulfur pollution. The ELB sulfate levels indicate a four- and six-fold increase from 1774–1900 to 1980–1995 in winter and summer, respectively. Remaining close to 116 ± 28 ppb during the nineteen century, the summer sulfate levels started to rise at a mean rate of ~ 6 ppb per year from 1920 to 1950. The summer sulfate increase accelerated between 1950 and 1975 (11 ppb per year), levels reaching a maximum between 1980 and 1990 (730 ± 152 ppb) and subsequently decreasing to 630 ± 130 ppb at the beginning of the twenty first century. Long-term sulfate trends observed in the ELB ice cores are compared with those previously obtained in Alpine ice, the most important difference consists in a more pronounced decrease of the sulfur pollution over the three last decades in western than central Europe.
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  • 44
    Publication Date: 2019-05-29
    Description: Climate change model evaluations need a better understanding of the atmo- spheric aerosols' optical properties and with this of the refractive index (RI) of atmospheric aerosols as well. Due to the remoteness of Antarctica only a very few data on the refractive index exists from there. In this paper we calculate the real refractive index of atmospheric aerosols from number size distribution measurements at a coastal Antarctic measurement site. In our calculations we used the overlapping size range (120–340 nm) of a scanning mobility sizer (SMPS), which sizes the particles by their electrical mobility, and a laser aerosol spectrometer (LAS), which sizes the particles by their optical scattering signal. Based on almost a complete year of measurement and 2439 measurement points, the average effective refractive index (RIeff) turned out to be 1.44. This is in a good agreement with the RI value of 1.47 which we derived from the chemical composition filter measurements. At our measurement site the aerosol has a very characteristic seasonal pattern in both number concentration and chemical composition. Despite this, we could not identify any significant sea- sonal variability in the effective refractive index, the monthly averages remain within the range of 1.40–1.50. Two austral winter months June and September has a slightly higher average values (1.50 and 1.47). We could not identify any in uence of the occurring wind direction on the retrieved RIeff either. For the few examples of north winds coming from the Neumayer station (occurs very rarely, this is the reason why the measurement site was built to the south), we don't see different values than for the other wind directions. During an artificial, high contamination episode, when diesel engines were operated right next to the measurement site, we had an hour of constant conditions such that one RI fit was possible. This fit resulted in an unusual high RI of 1.59, which is most probably due to the high black carbon content of the diesel engine emission. Therefore, we also assume that even during northerly wind directions we did not have significant in uence from the Neumayer station. During a shorter period between 2017 December and 2018 January we used the time averaged LAS and SMPS number size distributions to get some in- formation on the size dependency of the refractive index. The RI was fit in 5 different particle size ranges, and we have found a slight decrease of the re- fractive index with the particle size from 1.47 in the 116–168 nm to 1.37 in the 346–478 nm range.
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  • 45
    Publication Date: 2019-06-03
    Description: A long-live gravity wave (GW) in atmospheric boundary layer (ABL) during a field experiment in Anqing, China (116°58′ E, 30°37′ N) is analysed. Persistent GWs over 10 hours with periods ranging from 10 to 30 min in the ABL within 2 km height are detected by a coherent Doppler lidar from 4 to 5 in September 2018. The amplitudes of the vertical wind due to these GWs are about 0.15~0.2 m s−1. The lifetime of the GWs is more than 20 wave cycles. There is no apparent phase progression with altitude. The vertical and zonal perturbations of the GWs are apparent quadrature with vertical perturbations generally leading ahead of zonal ones. Based on experiments and simplified 2-Dimensional Computational Fluid Dynamics (CFD) numerical simulations, a reasonable generation mechanism of this persistent wave is proposed. A westerly low-level jet of ~ 5 m s−1 exists at the altitude of 1~2 km in the ABL. The wind shear around the low-level jet lead to the wave generation in the condition of light horizontal wind. Furthermore, a combination of thermal and Doppler ducts occurs in the ABL. Thus, the ducted wave motions are trapped in the ABL with long lifetime.
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  • 46
    Publication Date: 2019-05-20
    Description: Aerosols in the mid- and upper-troposphere have a long enough lifetime for trans-Pacific transport from East Asia to North America to influence air quality in the West Coast of the United States (US). Here, we conduct quasi-global simulations (180° W–180° E and 70° S–75° N) from 2010 to 2014 using an updated version of WRF-Chem (Weather Research and Forecasting model fully coupled with chemistry) to analyze the spatiotemporal characteristics and source contributions of trans-Pacific aerosol transport. We find that trans-Pacific total aerosols have a maximum mass concentration (about 15 μg m−3) in the boreal spring with a peak between 3 and 4 km above the surface around 40° N. Sea-salt and dust dominate the total aerosol mass concentration below 1 km and above 4 km, respectively. About 80.8 Tg of total aerosols (48.7 Tg of dust) are exported annually from East Asia, of which 26.7 Tg of aerosols (13.4 Tg of dust) reach the West Coast of the US. Dust contributions from four desert regions in the Northern Hemisphere are analyzed using a tracer-tagging technique. About 4.9, 3.9, and 4.5 Tg year−1 of dust aerosol emitted from North Africa, Middle East and Central Asia, and East Asia, respectively, can be transported to the West Coast of the US. The trans-Pacific aerosols dominate the column-integrated aerosol mass (~ 65.5 %) and number concentration (~ 80 %) over the western North America. Radiation budget analysis shows that the inflow aerosols could contribute about 86.4 % (−2.91  W m−2) at the surface, 85.5 % (+1.36 W m−2) in the atmosphere and 87.1 % (−1.55 W m−2) at the top of atmosphere to total aerosol radiative effect over western North America. However, near the surface in the central and eastern North America, aerosols are mainly derived from local emissions and the radiative effect of imported aerosols decreases rapidly. This study motivates further investigations of the potential impacts of trans-Pacific aerosols from East Asia on regional air quality and hydrological cycle in North America.
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  • 47
    Publication Date: 2019-05-16
    Description: Data assimilation algorithms rely on a basic assumption of an unbiased observation error. However, the presence of inconsistent measurements with nontrivial biases or inseparable baselines is unavoidable in practice. Assimilation analysis might diverge from reality, since the data assimilation itself cannot distinguish whether the differences between model simulations and observations are due to the biased observations or model deficiencies. Unfortunately, modeling of observation biases or baselines which show strong spatiotemporal variability is a challenging task. In this study, we report how data-driven machine learning can be used to perform observation bias correction for data assimilation through a real application, which is the dust emission inversion using PM10 observations. PM10 observations are considered as unbiased, however, a bias correction is necessary if they are used as a proxy for dust during dust storms since they actually represent a sum of dust particles and non-dust aerosols. Two observation bias correction methods have been designed in order to use PM10 measurements as proxy for the dust storm loads under severe dust conditions. The first one is the conventional chemical transport (CTM) model that simulates life cycles of non-dust aerosols. The other one is the machine learning model that describes the relations between the regular PM10 and other air quality measurement. The latter is trained by learning two-year's historical samples. The machine learning based non-dust model is shown to be in better agreements with observations compared to the CTM. The dust emission inversion tests have been performed, either through assimilating the raw measurements, or the bias-corrected dust observations using either the CTM or machine learning model. The emission field, surface dust concentration and forecast skill are evaluated. The worst case is when we directly assimilate the original observations. The forecasts driven by the posterior emission in this case even results in larger errors than the reference prediction. This shows the necessities of bias correction in data assimilation. The best results are obtained when using a machine learning model for bias correction, with the existing measurements used more precisely and the resulting forecasts close to reality.
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  • 48
    Publication Date: 2019-05-15
    Description: Spatial differences in the monthly baseline CO2 since 1992 from Mauna Loa, (mlo, 19.5° N, 155.6° W, 3379 m), Cape Grim (cgo, 40.7° S, 144.7° E, 94 m) and South Pole (spo, 90° S, 2810 m), are examined for consistency between four monitoring networks. For each site pair, a composite based on the average of NOAA, CSIRO and two independent SIO analysis methods is presented. Averages of the monthly standard deviations are 0.25, 0.23 and 0.16 ppm for mlo-cgo, mlo-spo and cgo-spo respectively. This high degree of consistency and near-monthly temporal differentiation (compared to CO2 growth rates) provides an opportunity to use the composite differences for verification of global carbon cycle model simulations. Interhemispheric CO2 variation is predominantly imparted by the mlo data. The peaks and dips of the seasonal variation in interhemispheric difference act largely independently. The peaks mainly occur in May, near the peak of Northern Hemisphere terrestrial respiration. Boreal spring is when interhemispheric exchange via eddy processes dominates, with increasing contributions from mean transport into boreal summer. The dips occur in September, when the CO2 partial pressure difference is near zero, just after the peak in the mean interhemispheric exchange via the Hadley circulation. Surface-air terrestrial flux anomalies would need to be up to an order of magnitude larger than found in order to explain the peak and dip CO2 variations (large enough to significantly influence short-term northern hemisphere growth rate variations). Recent features in the composite records, inconsistent in timing and amplitude with air-surface fluxes, are largely consistent with interhemispheric transport variations. These include the remarkable stability in annual CO2 inter-hemispheric difference in the 5-year relatively ENSO-quiet period 2010–2014, and the 2017 recovery in the CO2 interhemispheric gradient from the unprecedented ENSO event in 2015–16.
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  • 49
    Publication Date: 2019-05-16
    Description: An organic aerosol particle has a lifetime of approximately one week in the atmosphere during which it will be exposed to sunlight. Yet, the effect of photochemistry on the propensity of organic matter to participate in the initial cloud-forming steps is difficult to predict. In this study, we quantify on a molecular scale the effect of photochemical exposure of naturally occurring dissolved organic matter (DOM) and of a fulvic acid standard on its ability to form mixed-phase clouds, by acting as cloud condensation nuclei (CCN) and by acting as ice nucleating particles (INPs). We find that photochemical processing, equivalent to 4.6 days in the atmosphere, of DOM increases its ability to form cloud droplets by up to a factor of 2.5 but decreases its ability to form ice crystals at a loss rate of −0.04°CT50 h−1 of sunlight at ground level. In other words, the ice nucleation activity of photooxidized DOM can require up to 4 degrees colder temperatures for 50 % of the droplets to activate as ice crystals under immersion freezing conditions. This temperature change could impact the ratio of ice to water droplets within a mixed phase cloud by delaying the onset of glaciation and by increasing the supercooled liquid fraction of the cloud, thereby modifying the radiative properties and the lifetime of the cloud. Concurrently, a photomineralization mechanism was quantified by monitoring the loss of organic carbon and the simultaneous production of organic acids, such as formic, acetic, oxalic and pyruvic acids, CO and CO2. This mechanism explains and predicts the observed increase in CCN and decrease in INP efficiencies. Indeed, we show that photochemical processing can be a dominant atmospheric aging process, impacting CCN and INP efficiencies and concentrations. Photomineralization can thus alter the aerosol-cloud radiative effects of organic matter by modifying the supercooled liquid water-to-ice crystal ratio in mixed-phase clouds with implications for cloud lifetime, precipitation patterns and the hydrological cycle.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
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  • 50
    Publication Date: 2019-08-13
    Description: The viscosity of primary and secondary organic aerosol (SOA) has important implications for the processing of aqueous organic aerosol phases in the atmosphere, their involvement in climate forcing, and transboundary pollution. Here we introduce a new thermodynamics-based group-contribution model, which is capable of accurately predicting the dynamic viscosity of a mixture over several orders of magnitude (~ 10−3 to 〉 1012 Pa s) as a function of temperature and mixture composition, accounting for the effect of relative humidity on aerosol water content. The mixture viscosity modelling framework builds on the thermodynamic activity coefficient model AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) for predictions of liquid mixture non-ideality, including liquid–liquid phase separation, and the calorimetric glass transition temperature model by DeRieux et al. (2018) for pure-component viscosity values of organic components. Comparing this new model with simplified modelling approaches reveals that the group-contribution method is the most accurate in predicting mixture viscosity, although accurate pure-component viscosity predictions (and associated experimental data) are key and one of the main sources of uncertainties in current models, including the model presented here. Nonetheless, we find excellent agreement between the viscosity predictions and measurements for systems in which mixture constituents have a molar mass below 350 g mol−1. As such, we demonstrate the validity of the model in quantifying mixture viscosity for aqueous binary mixtures (glycerol, citric acid, sucrose, and trehalose), aqueous multicomponent mixtures (citric acid + sucrose and a mixture of nine dicarboxylic acids), and aqueous SOA surrogate mixtures derived from the oxidation of α-pinene, toluene, or isoprene. We also use the model to assess the expected change in SOA particle viscosity during idealized adiabatic air parcel transport from the surface to higher altitudes within the troposphere. This work demonstrates the capability and flexibility of our model in predicting the viscosity for organic mixtures of varying degrees of complexity and its applicability for modelling SOA viscosity over a wide range of temperatures and relative humidities.
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  • 51
    Publication Date: 2019-08-12
    Description: Continuous efforts have been made to monitor atmospheric CO2 as it is one of the most influential greenhouse gases in Earth's atmosphere. Inverse modeling, which is one of the methods to carry out such monitoring, derives estimated CO2 mole fractions in the air from calculated surface carbon fluxes using model and observed CO2 mole fraction data. Although observation data is crucial for successful modeling, comparatively fewer in-situ observation sites are located in Asia compared to Europe or North America. Based on the importance of the terrestrial ecosystem of Asia for global carbon exchanges, more observation stations and an effective observation network design are required. In this paper, several observation network experiments were conducted to optimize the surface carbon flux of Asia using CarbonTracker and observation system simulation experiments (OSSE). The impacts of the redistribution of and additions to the existing observation network of Asia were evaluated using hypothetical in-situ observation sites. In the case of the addition experiments, 10 observation stations, which is a practical number for real implementation, were added through three strategies: random addition, the influence matrix (i.e., self-sensitivity), and ecoregion information within the model. The simulated surface carbon flux in Asia in summer can be improved by redistributing the existing observation network. The addition experiments revealed that considering both the distribution of normalized self-sensitivity and ecoregion information can yield better simulated surface carbon fluxes compared to random addition, regardless of the season. This study provides a diagnosis of the existing observation network and useful information for future observation network design in Asia to estimate the surface carbon flux, and also suggests the use of an influence matrix for designing carbon observation networks. Unlike other previous observation network studies with many numerical experiments for optimization, comparatively fewer experiments were required in this study. Thus, the methodology used in this study may be used for designing observation networks for monitoring greenhouse gases at both continental and global scales.
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  • 52
    Publication Date: 2019-05-15
    Description: Atmospheric gravity waves play a key role in the transfer of energy and momentum between layers of the Earth's atmosphere. However, nearly all Global Circulation Models (GCMs) seriously under-represent the momentum fluxes of gravity waves at latitudes near 60° S. This can result in modelled winter stratospheres that are unrealistically cold – a significant bias known as the "cold-pole problem". There is thus a need for measurements of gravity-wave fluxes near 60S to test and constrain GCMs. Such measurements are notoriously difficult, because they require 3-D observations of wave properties if the fluxes are to be estimated without using significant limiting assumptions. Here we use 3-D satellite measurements of stratospheric gravity waves from NASA's AIRS/Aqua instrument. We present the first extended application of a 3-D Stockwell transform (3DST) method to determine localised gravity-wave amplitudes, wavelengths and directions of propagation around the entire region of the Southern Ocean near 60° S during austral winter 2010. We first validate our method using a synthetic wave field and two case studies of real gravity waves over the Southern Andes and the island of South Georgia. A new technique to overcome wave amplitude attenuation problems in previous methods is also presented. We then characterise large-scale gravity-wave occurrence frequencies, directional momentum fluxes and short-timescale intermittency over the entire Southern Ocean. Our results show that highest wave-occurrence frequencies, amplitudes and momentum fluxes are observed in the stratosphere over the mountains of the Southern Andes and Antarctic Peninsula. However, we find that around 60–80 % of total zonal-mean momentum flux is located over the open Southern Ocean during June–August, where a large "belt" of increased wave-occurrence frequencies, amplitudes and fluxes is observed. Our results also suggest significant short-timescale variability of fluxes from both orographic and non-orographic sources in the region. A particularly striking result is a widespread convergence of gravity-wave momentum fluxes towards latitudes around 60° S from the north and south. We propose that this convergence, which is observed at nearly all longitudes during winter, accounts for a significant part of the under-represented flux in GCMs at these latitudes.
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  • 53
    Publication Date: 2019-05-10
    Description: Emissions of biogenic volatile organic compounds (BVOCs) play important roles in ecophysiology and atmospheric chemistry at large spatial and temporal scales. Tropical regions are a main global source of BVOCs and magnitude and chemical compositions are highly variable. This study is based on the measurements of monoterpenes using proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) at a semi-arid site in western India during the winter-to-summer transition. Mixing ratio of monoterpenes showed strong diurnal variation with elevated values from evening till midnight and lowest in the afternoon. The daily data does not show clear trends with monthly means of ~ 0.35 ppbv during each month. Exceptionally high levels of 3–6 ppbv were measured during the sporadic biomass burning and bonfire event during Holi festival. The daytime data of monoterpenes do not clearly reflect the impact of biogenic emission due to the competing influences of mixing and OH-reaction loss. In the afternoon, the monoterpenes/benzene ratio of 0.43 ppbv ppbv−1 in second half of March was ~ 3 times higher than that in first half of February. It showed strong response with temperature as it increased from 0.27 ppbv ppbv−1 ( 30 °C). The dependence with wind speed is represented by exponential decay but rate of decline in February was ~ 2 times greater than that in March. The ratios of monoterpenes/isoprene in the night were significant higher than those during the day indicating light independent but temperature dependent emissions of monoterpenes. The nighttime MTs/isoprene ratio increased from 0.25 ppbv ppbv−1 in the first half of February to 0.43 ppbv ppbv−1 in the second half of March. Overall, the ratios of monoterpenes/isoprene agree with the values reported for a topical forest region in SE Asia. The estimated contribution from local biogenic sources to ambient monoterpenes increased from 31 % in first half of February to 67 % in second half of March. This trend suggests the increasing biogenic contribution from February to March. The NW winds and higher ambient temperatures in March favored the local emissions and regional transport of BVOCs.
    Electronic ISSN: 1680-7375
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  • 54
    Publication Date: 2019-08-09
    Description: Southern West Africa (SWA) is an African pollution hotspot but a relatively poorly sampled region of the world. We present an overview of in-situ aerosol optical measurements collected over SWA in June and July 2016 as part as the DACCIWA (Dynamics–Aerosol–Chemistry–Clouds Interactions in West Africa) airborne campaign. The aircraft sampled a wide range of air masses, including anthropogenic pollution plumes emitted from the coastal cities, long-range transported biomass burning plumes from Central and Southern Africa and dust plumes from the Sahara and Sahel region, as well as mixtures of these plumes. The specific objective of this work is to characterize the regional variability of the vertical distribution of aerosol particles and their spectral optical properties (single scattering albedo: SSA, asymmetry parameter, extinction mass efficiency, scattering Ångström exponent and absorption Ångström exponent: AAE). First findings indicate that aerosol optical properties in the planetary boundary layer were dominated by a widespread and persistent biomass burning loading from the Southern Hemisphere. Despite a strong increase of aerosol number concentration in air masses downwind of urban conglomerations, spectral SSA were comparable to the background and showed signatures of the absorption characteristics of biomass burning aerosols. In the free troposphere, moderately to strongly absorbing aerosol layers, dominated by either dust or biomass burning particles, occurred occasionally. In aerosol layers dominated by mineral dust particles, SSA varied from 0.81 to 0.92 at 550 nm depending on the variable proportion of anthropogenic pollution particles externally mixed with the dust. Biomass burning aerosol particles were significantly more light absorbing than those previously measured in other areas (e.g. Amazonia, North America) with SSA ranging from 0.71 to 0.77 at 550 nm. The variability of SSA was mainly controlled by variations in aerosol composition rather than in aerosol size distribution. Correspondingly, values of AAE ranged from 0.9 to 1.1, suggesting that lens-coated black carbon particles were the dominant absorber in the visible range for these biomass burning aerosols. Comparison with literature shows a consistent picture of increasing absorption enhancement of biomass burning aerosol from emission to remote location and underscores that the evolution of SSA occurred a long time after emission. The results presented here build a fundamental basis of knowledge about the aerosol optical properties observed over SWA during the monsoon season and can be used in climate modelling studies and satellite retrievals. In particular and regarding the very high absorbing properties of biomass burning aerosols over SWA, our findings suggest that considering the effect of internal mixing on absorption properties of black carbon particles in climate models should help better assessing the direct and semi-direct radiative effects of biomass burning particles.
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  • 55
    Publication Date: 2019-08-08
    Description: Urban air quality is one of the most prominent environmental concerns for a modern city dweller. Accurate monitoring of air quality is difficult due to intrinsic urban landscape heterogeneity and superposition of multiple polluting sources. Existing approaches often do not provide the necessary spatial details and peak concentrations of pollutants, especially at larger distances from measuring stations. A more advanced approach is needed. This study presents a very high-resolution air quality assessment with the large-eddy simulation model PALM. This fully three-dimensional primitive-equation hydro-dynamical model resolves both structural details of the complex urban surface and turbulent eddies larger than 10 m in size. We ran a set of 9 meteorological scenarios in order to assess the dispersion of pollutants in Bergen, a middle-sized Norwegian city embedded in a coastal valley. This set of scenarios represents typically observed conditions with high air pollution from nitrogen dioxide (NO2) and particulate matter (PM2.5). The modelling methodology helped to identify pathways and patterns of air pollution caused by the three main local air pollution sources in the city. These are road vehicle traffic, domestic house heating with wood-burning fireplaces and ships docked in the harbour area next to the city centre. The study produced vulnerability maps, highlighting the most impacted districts for each scenario.
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  • 56
    Publication Date: 2019-08-09
    Description: Between June and September large amounts of biomass burning aerosol are released into the atmosphere from agricultural fires in Central and southern Africa. Recent studies have suggested that this plume is carried westward over the Atlantic Ocean at altitudes between 2 and 4 km and then northward with the monsoon flow at low levels to increase the atmospheric aerosol load over coastal cities in southern West Africa (SWA), thereby exacerbating air pollution problems. However, the processes by which these fire emissions are transported into the planetary boundary layer are still unclear. One potential factor is the large-scale subsidence related to the southern branch of the monsoon Hadley cell over the tropical Atlantic. Here we use convection-permitting model simulations with COSMO-ART to investigate for the first time to what extent mixing related to cloud venting contributes to the downward transport of the biomass burning plume. Based on a monthly climatology, model simulations compare satisfactory with wind fields from reanalysis data, cloud observations, and satellite retrieved CO mixing ratio. For a case study on 02 July 2016, modelled clouds and rainfall show overall good agreement with Spinning Enhanced Visible and InfraRed Imager (SEVIRI) cloud products and Global Precipitation Measurement Integrated Multi-satellite Retrievals (GPM-IMERG) rainfall estimates. However, there is a tendency for the model to produce too much clouds and rainfall over the Gulf of Guinea. Looking into the CO dispersion, used as an indicator for the biomass burning plume, we identified individual mixing events south of the coast of Côte d’Ivoire due to midlevel convective clouds injecting parts of the biomass burning plume into the boundary layer. This cloud venting is modulated by the underlying sea surface temperatures. Idealized tracer experiments suggest that about 20 % of the CO mass from the 2–4 km layer are mixed below 1km within two days over the Gulf of Guinea. There is even stronger vertical mixing when the biomass burning plume reaches land due to daytime heating. In that case, the long-range transported biomass burning plume is mixed with local anthropogenic emissions. Future work should provide more robust statistics on the cloud venting effect over the Gulf of Guinea and include aspects of aerosol deposition.
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  • 57
    Publication Date: 2019-08-09
    Description: In order to better understand the molecular composition and sources of organic aerosols in Tianjin, a coastal megacity in North China, ambient fine aerosol (PM2.5) samples were collected on a day/night basis during November–December 2016 and May–June 2017. Organic molecular compositions in PM2.5, including aliphatic lipids (n-alkanes, fatty acids and fatty alcohols), sugar compounds and photooxidation products from isoprene, monoterpene, β-caryophyllene, naphthalene and toluene, were analysed using gas chromatography-mass spectrometry. Fatty acids, fatty alcohols and saccharides were identified as the most abundant organic compound classes among all the tracers during both seasons. High concentrations of most organics at night in winter may be attributed to intensive residential activities such as house heating and the low boundary layer height. Based on the tracer methods, the contributions of the sum of primary and secondary organic carbon (POC and SOC) to aerosol organic carbon (OC) were 24.8 % (daytime) versus 27.6 % (nighttime) in winter and 38.9 % (daytime) versus 32.5 % (nighttime) in summer. In detail, POC derived from fungal spores, plant debris, and biomass burning accounted for 2.78–31.6 % (12.4 %) of OC in the daytime versus 4.72–45.9 % (16.3 %) at night in winter, and 1.28–9.89 % (5.24 %) versus 2.08–47.2 % (10.6 %) in summer. Biomass burning derived OC was the predominant source of POC in this study, especially at night (16.0 ± 6.88 % in winter and 9.62 ± 8.73 % in summer). Biogenic SOC from isoprene, α/β-pinene and β-caryophyllene exhibited obvious seasonal and diurnal variations, contributing 2.23 ± 1.27 % (2.30 ± 1.35 % in the daytime and 2.18 ± 1.19 % at night) and 8.60 ± 4.02 % (8.98 ± 3.67 % and 8.21 ± 4.39 %) to OC in winter and summer, respectively. Isoprene and α/β-pinene SOC were obviously elevated in summer, especially in the daytime, mainly due to strong photooxidation. Anthropogenic SOC from toluene and naphthalene oxidation contributed higher to OC in summer (21.0 ± 18.5 %) than in winter (9.58 ± 3.68 %). In summer, toluene SOC was the dominant contributor to aerosol OC, and biomass burning OC also accounted for a large portion to OC, especially in the nighttime, which indicate that land/sea breezes also play an important role in aerosol chemistry at the coastal city of Tianjin in North China.
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  • 58
    Publication Date: 2019-08-09
    Description: The atmospheric fate of a series of Methyl Saturated Alcohols (MSA) has been evaluated through the kinetic and reaction product studies with the main atmospheric oxidants. Rate coefficients (in cm3 molecule−1 s−1 unit) measured at ~ 298 K and atmospheric pressure (~ 740 Torr) were as follows: (3.71 ± 0.53) × 10−10, (1.91 ± 0.65) × 10−11 and (2.92 ± 1.38) × 10−15 for reaction of E-4-methyl-cyclohexanol with Cl, OH and NO3, respectively. (2.70 ± 0.55) × 10−10 and (5.57 ± 0.66) × 10−12 for reaction of 3,3-dimethyl-1-butanol with Cl and OH radical respectively and (1.21 ± 0.37) × 10−10 and (10.51 ± 0.81) × 10−12 for reaction of 3,3-dimethyl-2-butanol with Cl and OH radical respectively. The main detected products were 4-methylcyclohexanone, 3,3-dimethylbutanal and 3,3-dimethyl-2-butanone for the reactions of E-4-methyl-cyclohexanol, 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol respectively with the three oxidants. A tentative estimation of yields have been done obtaining the following ranges (25–60) % for 4-methylcyclohexanone, (40–60) % for 3,3-dimethylbutanal and (40–80) % for 3,3-dimethyl-2-butanone. Other products as HCOH, 2,2-dimethylpropanal and acetone have been identified in the reaction of 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. The yields of these products indicate a hydrogen abstraction mechanism at different sites of the alkyl chain in the case of Cl reaction and a predominant site in the case of OH and NO3 reactions, supported by SAR methods prediction. Tropospheric lifetimes (τ) of these MSA have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 days for OH reactions, 8–13 days for NO3 radical reactions and 1–3 months for Cl atoms. In coastal areas the lifetime due to the reaction with Cl decreases to hours. The global tropospheric lifetimes calculated, and the polyfunctional compounds detected as reaction products in this work, imply that the Methyl Saturated Alcohols could contribute to ozone and nitrated compound formation at local, but also regional and even to global scale. Therefore, the use of large saturated alcohols as additives in biofuels must be taken with caution.
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  • 59
    Publication Date: 2019-08-08
    Description: Biomass burning is a large source of uncontrolled air pollutants, including particulate matter (i.e. PM2.5), black carbon (BC), volatile organic compounds (VOCs), and carbon monoxide (CO), which have significant effects on air quality, human health, and climate. Measurements of PM2.5, BC, and CO made at the Yale Coastal Field Station in Guilford, CT and five other sites in the metropolitan New York City (NYC) area indicate long-distance transport of pollutants from wildfires and other biomass burning to surface-level sites in the region. Here, we examine two such events occurring on August 16th–17th and 27th–29th, 2018. In addition to regionally-consistent enhancements in the surface concentrations of gases and particulates associated with biomass burning, satellite imagery confirms the presence of smoke plumes in the NYC-Connecticut region during these events. Backward-trajectory modeling indicates that air masses arriving in coastal Connecticut on August 16th–17th passed over the west coast of Canada, near multiple large wildfires. In contrast, air parcels arriving on August 27th–29th passed over active fires in the southeastern United States. The results of this study demonstrate that biomass burning events throughout the U.S. and Canada (more than 4000 km away), which are increasing in frequency, impact surface-level air quality beyond regional scales, including in NYC and the northeastern U.S.
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  • 60
    Publication Date: 2019-05-17
    Description: Improving the understanding of the health and climate impacts of PM1 remains challenging and is restricted by the limitations of current measurement techniques. Detailed investigation of secondary organic aerosol (SOA), which is typically the dominating fraction of the organic aerosol (OA), requires instrumentation capable of real-time, in situ measurements of molecular composition. In this study, we present the first ambient measurements by a novel extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF). The EESI-TOF was deployed along with a high resolution time of flight aerosol mass spectrometer (HR-ToF-AMS) during summer 2016 at an urban location (Zurich, Switzerland). Positive matrix factorization (PMF), implemented within the Multilinear Engine (ME-2), was applied to the data from both instruments to quantify the primary and secondary contributions to OA. From the EESI-TOF analysis, a 6-factor solution was selected as the most representative and interpretable solution for the investigated dataset, including two primary and four secondary factors. The primary factors are dominated by cooking and cigarette smoke signatures while the secondary factors are discriminated according to their daytime (two factors) and nighttime (two factors) chemistry. All four factors showed strong influence by biogenic emissions but exhibited significant day/night differences. Factors dominating during daytime showed predominantly ions characteristic of monoterpene and sesquiterpene oxidation while the nighttime factors included less oxygenated terpene oxidation products, as well as organonitrates which were likely derived from NO3 radical oxidation of monoterpenes. Overall, the signal measured by the EESI-TOF and AMS showed a good correlation. Further, the two instruments were in excellent agreement in terms of both the mass contribution apportioned to the sum of POA and SOA factors and the total SOA signal. However, while the OOA factors separated by AMS analysis exhibited a flat diurnal pattern, the EESI-TOF factors illustrated significant chemical variation throughout the day. The captured variability, inaccessible from AMS PMF analysis, was shown to be consistent with the variations in the physiochemical processes influencing chemical composition and SOA formation. The improved source separation and interpretability of EESI-TOF results suggest it to be a promising approach to source apportionment and atmospheric composition research.
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  • 61
    Publication Date: 2019-05-20
    Description: This study investigates the impact of island diameter size and orographic height on precipitation, convective organization and aerosol transport on and around tropical islands. Twenty-four model simulations are set up and run, in which the island diameter (50 km, 100 km, and 200 km) and peak orographic height (flat, 500 m, 1 km, and 2 km) are independently and simultaneously varied under weak and strong zonal wind regimes. In these simulations, unlike many of the previous island flow investigations, the full three-dimensional flow over and around islands is resolved. Analysis of these numerical experiments demonstrates that island orographic height is a stronger control than island size on precipitation, convective organization, and aerosol redistribution. The wind regime is found to modulate these results. Under the strong zonal wind regime, increasing orographic height induces changes to the flow around the island, leading to lee-vortex formation, reverse flow, and the earlier development of deep convection than in the flat simulation. In the weaker zonal wind experiment, increasing orographic height enhances the role of radiational heating, leading to enhanced upslope flow and stronger convergence, which produces earlier deep convection than in the flat simulation. The timing and location of the sea breeze/mountain breeze convergence determines the location of the initial vertical aerosol mixing with cloud formation and entrainment, whereas the timing of deep convection formation dictates how quickly the aerosols are mixed out of the boundary layer. Finally, it is demonstrated that cold pools play an important role in the propagation of convection towards the shore in the simulations with orography.
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  • 62
    Publication Date: 2019-08-07
    Description: We introduce a Multi-mOdel Multi-cOnstituent Chemical data assimilation (MOMO-Chem) framework that directly accounts for model error in transport and chemistry by integrating a portfolio of forward chemical transport models (GEOS-Chem, AGCM-CHASER, MIROC-Chem, MIROC-Chem-H) into a state-of-the-art ensemble Kalman filter data assimilation system that simultaneously optimizes both concentrations and emissions of multiple species through ingestion of a suite of measurements (ozone, NO2, CO, HNO3) from multiple satellite sensors. In spite of substantial model differences, the observational density and accuracy was sufficient for the assimilation to reduce the multi-model spread by 20–85 % for ozone, and annual mean bias by 39–97 % for ozone in the middle troposphere, while simultaneously reducing the tropospheric NO2 column biases by more than 40 %, and the negative biases of surface CO in the Northern Hemisphere by 41–94 %. For tropospheric mean OH, the multi-model mean meridional hemispheric gradient was reduced from 1.32 ± 0.03 to 1.19 ± 0.03, while the multi-model spread was reduced by 24–58 % over polluted areas. These improvements extended to emissions where uncertainty ranges in the a posteriori emissions due to model errors were quantified in 4–31 % for NOx and 13–35 % for CO regional emissions. Harnessing assimilation increments in both NOx and ozone, we show that the sensitivity of ozone and NO2 surface concentrations to NOx emissions varied by a factor of 2 for end-member models revealing fundamental differences in the representation of fast chemical and dynamical processes. Consequently, diagnostic information readily available from MOMO-Chem has the potential to improve chemical predictions through relationships such as emergent constraints.
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  • 63
    Publication Date: 2019-08-06
    Description: In this study an optical parameter derived from lidar measurements is found to be relevant to monitor the evolution of near-surface particulate concentrations. This highlights the opportunities offered by future spaceborne lidar missions in air quality assessment on a global scale. This work is carried out following a dedicated field campaign in the Paris area (France) during winter 2016–2017, from 1st November to 31st January. Two of the most intense winter aerosol pollution events occurring over the last decade were sampled using a ground-based N2-Raman. The lidar operated continuously at the wavelength of 355 nm, favourable to the measurement of submicron aerosols mainly linked to traffic emissions. The data analysis uses the synergy between ground-based and spaceborne lidar observations, and data from the air quality monitoring network Airparif. The first severe aerosol pollution event occurred on 1st December 2016; it concerned a circular area of 250 km in diameter around Paris with maximum PM10 (PMx is the mass concentration of particles with an aerodynamic diameter smaller than x µm) values of 121 ± 63 µg m-3. The second event took place from 21st to 22nd January which covered all of Western Europe, with maxima of PM10 (156 ± 33 µg m-3) and aerosol extinction coefficient (AEC) between 0.6 and 1 km-1, within the winter atmospheric boundary layer. These two major aerosol pollution events share very low boundary layer height, down to 300 m above ground level. However, they did not take place in the same weather condition; moreover, they are associated with significantly different lidar ratios: 72 ± 15 sr and 56 ± 15 sr, respectively in December and January. Such results are consistent with available spaceborne lidar data (70 ± 25 sr) and values found in the literature. During these two events, the continuous temporal evolution of the aerosol extinction coefficient allows us to investigate the representativeness of optical parameters found in the planetary boundary layer to assess surface aerosol concentration. No one-to-one relationship between the aerosol optical thickness (AOT) and PM2.5 values stands out within our study. In contrast, the maximum lidar-derived aerosol extinction coefficient found within the planetary boundary layer is identified as a consistent variable to assess the evolution of ground aerosol concentration.
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  • 64
    Publication Date: 2019-08-06
    Description: Organic nitrate esters are key products of terpene oxidation in the atmosphere. We report here the preparation and purification of nine nitrate esters derived from (+)-(3)-carene, limonene, α-pinene, β-pinene and perillic alcohol. The availability of these compounds will enable detailed investigations into the structure reactivity relationships of aerosol formation and processing and will allow individual investigations into aqueous phase reactions of organic nitrate esters.
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  • 65
    Publication Date: 2019-08-07
    Description: Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). A series of toluene and 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevant polluted conditions (NOx ~ 10 ppb). An extensive suite of instrumentation including two Proton-Transfer Reaction Mass-Spectrometers (PTR-MS) and two Chemical Ionization Mass-Spectrometers (NH4+ CIMS and I- CIMS) allowed for quantification of reactive carbon in multiple generations of oxidation. Hydroxyl radical (OH)-initiated oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring scission products such as epoxides, and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to formation of compounds with high oxygen content (O:C ratio up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both autooxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present speciated elemental composition of SOA for both precursors and confirm that highly oxygenated products make up a significant fraction of SOA. Ring scission products are also detected in both the gas and particle phases, and their yields and speciation overall agree with the kinetic model prediction.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 66
    Publication Date: 2019-08-07
    Description: Mineral dust plays an important role in the climate of the Tibetan Plateau (TP) by modifying the radiation budget, cloud macro- and microphysics, precipitation, and snow albedo. Meanwhile, the TP with the highest topography in the word can affect intercontinental transport of dust plumes and induce typical distribution characteristics of dust at different altitudes. In this study, we conduct a quasi-global simulation to investigate the characteristics of dust source contribution and transport over the TP at different altitude by using a fully coupled meteorology–chemistry model (WRF–Chem) with a tracer-tagging technique. Generally, the simulation reasonably captures the spatial distribution of satellite retrieved dust aerosol optical depth (AOD) at different altitudes. Model results show that dust particles are emitted into atmosphere through updrafts over major desert regions, and then transported to the TP. The East Asian dust is transported and lifted up to the TP, contributing a mass loading of 50 mg/m2 at 3 km height and 5 mg/m2 at 12 km height over the northern slop of the TP. Dust from North Africa and Middle East are concentrated over both northern and southern slopes below 6 km, where mass loadings range from 10 to 100 mg/m2 and 1 to 10 mg/m2 below 3 km and above 9 km, respectively. As the dust is transported to the north and over the TP, mass loadings are 5–10 mg/m2 above 6 km. The imported dust mass flux from East Asia to the TP is 7.9 Tg/year mostly occuring at the heights of 3–6 km. The North African and Middle East dust particles are transported eastward following the westerly jet, and then imported into the TP at West side with the dust mass flux of 7.8 and 26.6 Tg/year, respectively. The maximum mass flux of the North African dust mainly occurs in 0–3 km (3.9 Tg/year), while the Middle East within 6–9 km (12.3 Tg/year). The dust outflow occurs at East side (−17.89 Tg/year) and South side (−11.22 Tg/year) of the TP with a peak value (8.7 Tg/year) in 6–9 km. Moreover, the dust mass is within the size range of 1.25~5.0 μm and dust number is concentrated in the size range of 0.156~1.25 μm. Compared with other aerosols, the dust contributes more than 50 % to the total AOD over the TP. The direct radiative forcing induced by the dust is −1.28 W/m2 at the top of the atmosphere (cooling), 0.41 W/m2 in the atmosphere (warming) and −1.68 W/m2 at the surface (cooling). Our quantitative analyses of the dust contribution from different source regions and the associated radiative forcing can help better understand the role of dust on the climate over the TP and surrounding regions.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 67
    Publication Date: 2019-05-13
    Description: Despite the increased awareness of heterogeneous reaction on mineral dust, the knowledge of how the intensity of solar irradiation influences the photochemistry activity remains a crucially important part in atmospheric research. Relevant studies have not seriously discussed the photochemistry under weak sunlight during haze, and thus ignored some underlying pollution and toxicity. Here, we investigated the heterogeneous formation of nitrate and nitrite under various illumination conditions by laboratory experiments and field observations. Observed by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), water-solvated nitrate was the main surface product, followed by other species varying with illumination condition. The growth of nitrate formation rate tends to be slow after the initial fast with increasing light intensity. For example, the geometric uptake coefficient (γgeo) under 30.5 mW/cm2 (5.72 × 10−6) has exceeded the 50 % of that under 160 mW/cm2 (1.13 × 10−5). This case can be explained by the excess NO2 adsorption under weak illumination while the excess photoinduced active species under strong irradiation. Being negatively associated with nitrate (R2 = 0.748, P 
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 68
    Publication Date: 2019-05-13
    Description: The radical terminating, termolecular reaction between OH and NO2 exerts great influence on the NOy / NOx ratio and O3 formation in the atmosphere. Evaluation panels (IUPAC and NASA) recommend rate coefficients for this reaction that disagree by as much as a factor 1.6 at low temperature and pressure. In this work, the title reaction was studied by pulsed laser photolysis-laser induced fluorescence over the pressure range 16–1200 mbar and temperature 217–333 K in N2 bath-gas, with experiments at 295 K (67–333 mbar) for O2. In-situ measurement of NO2 using two optical-absorption set-ups enabled generation of highly precise, accurate rate coefficients in the fall-off pressure range, appropriate for atmospheric conditions. We found, in agreement with previous work, that O2 bath-gas has a lower collision efficiency than N2 with a relative collision efficiency to N2 of 0.74. Using the widely used Troe-type formulation for termolecular reactions we present a new set of parameters with k0(N2) = 2.6 × 10−30 cm6 molecule−2 s−1, k0(O2) = 2.0 × 10−30 cm6 molecule−2 s−1, m = 3.6, k∞ = 6.3 × 10−11 cm3 molecule−1 s−1, Fc = 0.39 and compare our results to previous studies in N2 and O2 bath-gases.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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