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Savarino J, Kaiser J, Morin S, Sigman D M, Thiemens M H, |
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Title |
Nitrogen and oxygen isotopic constraints on the origin of atmospheric nitrate in coastal Antarctica
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2007 |
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Atmospheric Chemistry and Physics |
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7 |
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8 |
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1925 -1945 |
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Throughout the year 2001, aerosol samples were collected continuously for 10 to 15 days at the French Antarctic Station Dumont d'Urville DDU) (66 degrees 40' S, 140 degrees 01' E, 40m above mean sea level). The nitrogen and oxygen isotopic ratios of particulate nitrate at DDU exhibit seasonal variations that are among the most extreme observed for nitrate on Earth. In association with concentration measurements, the isotope ratios delineate four distinct periods, broadly consistent with previous studies on Antarctic coastal areas. During austral autumn and early winter March to mid-July), nitrate concentrations attain a minimum between 10 and 30 ng m(-3) referred to as Period 2). Two local maxima in August (55 ng m(-3)) and November/December (165 ng m(-3)) are used to assign Period 3 (mid-July to September) and Period 4 October to December). Period 1 January to March) is a transition period between the maximum concentration of Period 4 and the background concentration of Period 2. These seasonal changes are reflected in changes of the nitrogen and oxygen isotope ratios. During Period 2, which is characterized by background concentrations, the isotope ratios are in the range of previous measurements at midlatitudes: delta O-18(vsmow)=(77.2 8.6)parts per thousand; Delta O-17=(29.8 4.4)parts per thousand; delta N-15(air)=(- 4.4 5.4)parts per thousand mean one standard deviation). Period 3 is accompanied by a significant increase of the oxygen isotope ratios and a small increase of the nitrogen isotope ratio to delta O-18(vsmow)=( 98.8 13.9)parts per thousand; Delta O-17=(38.8 4.7)parts per thousand and delta N-15(air)=(4.3 8.20 parts per thousand). Period 4 is characterized by a minimum N-15/N-14 ratio, only matched by one prior study of Antarctic aerosols, and oxygen isotope ratios similar to Period 2: delta O-18(vsmow)=(77.2 7.7)parts per thousand; Delta O-17=(31.1 3.2)parts per thousand; delta N-15(air)=(- 32.7 8.4)parts per thousand. Finally, during Period 1, isotope ratios reach minimum values for oxygen and intermediate values for nitrogen: delta O-18(vsmow)= 63.2 2.5 parts per thousand; Delta O-17= 24.0 1.1 parts per thousand; delta N-15(air)=- 17.9 4.0 parts per thousand). Based on the measured isotopic composition, known atmospheric transport patterns and the current understanding of kinetics and isotope effects of relevant atmospheric chemical processes, we suggest that elevated tropospheric nitrate levels during Period 3 are most likely the result of nitrate sedimentation from polar stratospheric clouds (PSCs), whereas elevated nitrate levels during Period 4 are likely to result from snow re-emission of nitrogen oxide species. We are unable to attribute the source of the nitrate during periods 1 and 2 to local production or long-range transport, but note that the oxygen isotopic composition is in agreement with day and night time nitrate chemistry driven by the diurnal solar cycle. A precise quantification is difficult, due to our insufficient knowledge of isotope fractionation during the reactions leading to nitrate formation, among other reasons.
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1580 |
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Philippe Ricaud, Massimo Del Guasta, Angelo Lupi, Romain Roehrig, Eric Bazile, Pierre Durand, Jean-Luc Attié, Alessia Nicosia, Paolo Grigioni |
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Title |
Supercooled liquid water clouds observed over Dome C, Antarctica: temperature sensitivity and cloud radiative forcing |
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Year |
2024 |
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Atmospheric Chemistry and Physics |
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24 |
Issue |
1 |
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613-630 |
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Clouds affect the Earth climate with an impact that depends on the cloud nature (solid and/or liquid water). Although the Antarctic climate is changing rapidly, cloud observations are sparse over Antarctica due to few ground stations and satellite observations. The Concordia station is located on the eastern Antarctic Plateau (75∘ S, 123∘ E; 3233 m above mean sea level), one of the driest and coldest places on Earth. We used observations of clouds, temperature, liquid water, and surface irradiance performed at Concordia during four austral summers (December 2018–2021) to analyse the link between liquid water and temperature and its impact on surface irradiance in the presence of supercooled liquid water (liquid water for temperature less than 0 ∘C) clouds (SLWCs). Our analysis shows that, within SLWCs, temperature logarithmically increases from −36.0 to −16.0 ∘C when liquid water path increases from 1.0 to 14.0 g m−2. The SLWC radiative forcing is positive and logarithmically increases from 0.0 to 70.0 W m−2 when liquid water path increases from 1.2 to 3.5 g m−2. This is mainly due to the downward longwave component that logarithmically increases from 0 to 90 W m−2 when liquid water path increases from 1.0 to 3.5 g m−2. The attenuation of shortwave incoming irradiance (that can reach more than 100 W m−2) is almost compensated for by the upward shortwave irradiance because of high values of surface albedo. Based on our study, we can extrapolate that, over the Antarctic continent, SLWCs have a maximum radiative forcing that is rather weak over the eastern Antarctic Plateau (0 to 7 W m−2) but 3 to 5 times larger over West Antarctica (0 to 40 W m−2), maximizing in summer and over the Antarctic Peninsula. |
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8780 |
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Cynthia H. Whaley, Kathy S. Law, Jens Liengaard Hjorth, Henrik Skov, Stephen R. Arnold, Joakim Langner, Jakob Boyd Pernov, Garance Bergeron, Ilann Bourgeois, Jesper H. Christensen, Rong-You Chien, Makoto Deushi, Xinyi Dong, Peter Effertz, Gregory Faluvegi, Mark Flanner, Joshua S. Fu, Michael Gauss, Greg Huey, Ulas Im, Rigel Kivi, Louis Marelle, Tatsuo Onishi, Naga Oshima, Irina Petropavlovskikh, Jeff Peischl, David A. Plummer, Luca Pozzoli, Jean-Christophe Raut, Tom Ryerson, Ragnhild Skeie, Sverre Solberg, Manu A. Thomas, Chelsea Thompson, Kostas Tsigaridis, Svetlana Tsyro, Steven T. Turnock, Knut von Salzen, David W. Tarasick |
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Title |
Arctic tropospheric ozone: assessment of current knowledge and model performance |
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Year |
2023 |
Publication |
Atmospheric Chemistry and Physics |
Abbreviated Journal |
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Volume |
23 |
Issue |
1 |
Pages |
637-661 |
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As the third most important greenhouse gas (GHG) after carbon dioxide (CO2) and methane (CH4), tropospheric ozone (O3) is also an air pollutant causing damage to human health and ecosystems. This study brings together recent research on observations and modeling of tropospheric O3 in the Arctic, a rapidly warming and sensitive environment. At different locations in the Arctic, the observed surface O3 seasonal cycles are quite different. Coastal Arctic locations, for example, have a minimum in the springtime due to O3 depletion events resulting from surface bromine chemistry. In contrast, other Arctic locations have a maximum in the spring. The 12 state-of-the-art models used in this study lack the surface halogen chemistry needed to simulate coastal Arctic surface O3 depletion in the springtime; however, the multi-model median (MMM) has accurate seasonal cycles at non-coastal Arctic locations. There is a large amount of variability among models, which has been previously reported, and we show that there continues to be no convergence among models or improved accuracy in simulating tropospheric O3 and its precursor species. The MMM underestimates Arctic surface O3 by 5 % to 15 % depending on the location. The vertical distribution of tropospheric O3 is studied from recent ozonesonde measurements and the models. The models are highly variable, simulating free-tropospheric O3 within a range of ±50 % depending on the model and the altitude. The MMM performs best, within ±8 % for most locations and seasons. However, nearly all models overestimate O3 near the tropopause (∼300 hPa or ∼8 km), likely due to ongoing issues with underestimating the altitude of the tropopause and excessive downward transport of stratospheric O3 at high latitudes. For example, the MMM is biased high by about 20 % at Eureka. Observed and simulated O3 precursors (CO, NOx, and reservoir PAN) are evaluated throughout the troposphere. Models underestimate wintertime CO everywhere, likely due to a combination of underestimating CO emissions and possibly overestimating OH. Throughout the vertical profile (compared to aircraft measurements), the MMM underestimates both CO and NOx but overestimates PAN. Perhaps as a result of competing deficiencies, the MMM O3 matches the observed O3 reasonably well. Our findings suggest that despite model updates over the last decade, model results are as highly variable as ever and have not increased in accuracy for representing Arctic tropospheric O3. |
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1215 |
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yes |
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8507 |
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Eleftherios Ioannidis, Kathy S. Law, Jean-Christophe Raut, Louis Marelle, Tatsuo Onishi, Rachel M. Kirpes, Lucia M. Upchurch, Thomas Tuch, Alfred Wiedensohler, Andreas Massling, Henrik Skov, Patricia K. Quinn, Kerri A. Pratt |
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Title |
Modelling wintertime sea-spray aerosols under Arctic haze conditions |
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Journal |
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Year |
2023 |
Publication |
Atmospheric chemistry and physics |
Abbreviated Journal |
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Volume |
23 |
Issue |
10 |
Pages |
5641-5678 |
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Abstract |
Anthropogenic and natural emissions contribute to enhanced concentrations of aerosols in the Arctic winter and early spring, with most attention being paid to anthropogenic aerosols that contribute to so-called Arctic haze. Less-well-studied wintertime sea-spray aerosols (SSAs) under Arctic haze conditions are the focus of this study, since they can make an important contribution to wintertime Arctic aerosol abundances. Analysis of field campaign data shows evidence for enhanced local sources of SSAs, including marine organics at Utqiaġvik (formerly known as Barrow) in northern Alaska, United States, during winter 2014. Models tend to underestimate sub-micron SSAs and overestimate super-micron SSAs in the Arctic during winter, including the base version of the Weather Research Forecast coupled with Chemistry (WRF-Chem) model used here, which includes a widely used SSA source function based on Gong et al. (1997). Quasi-hemispheric simulations for winter 2014 including updated wind speed and sea-surface temperature (SST) SSA emission dependencies and sources of marine sea-salt organics and sea-salt sulfate lead to significantly improved model performance compared to observations at remote Arctic sites, notably for coarse-mode sodium and chloride, which are reduced. The improved model also simulates more realistic contributions of SSAs to inorganic aerosols at different sites, ranging from 20 %–93 % in the observations. Two-thirds of the improved model performance is from the inclusion of the dependence on SSTs. The simulation of nitrate aerosols is also improved due to less heterogeneous uptake of nitric acid on SSAs in the coarse mode and related increases in fine-mode nitrate. This highlights the importance of interactions between natural SSAs and inorganic anthropogenic aerosols that contribute to Arctic haze. Simulation of organic aerosols and the fraction of sea-salt sulfate are also improved compared to observations. However, the model underestimates episodes with elevated observed concentrations of SSA components and sub-micron non-sea-salt sulfate at some Arctic sites, notably at Utqiaġvik. Possible reasons are explored in higher-resolution runs over northern Alaska for periods corresponding to the Utqiaġvik field campaign in January and February 2014. The addition of a local source of sea-salt marine organics, based on the campaign data, increases modelled organic aerosols over northern Alaska. However, comparison with previous available data suggests that local natural sources from open leads, as well as local anthropogenic sources, are underestimated in the model. Missing local anthropogenic sources may also explain the low modelled (sub-micron) non-sea-salt sulfate at Utqiaġvik. The introduction of a higher wind speed dependence for sub-micron SSA emissions, also based on Arctic data, reduces biases in modelled sub-micron SSAs, while sea-ice fractions, including open leads, are shown to be an important factor controlling modelled super-micron, rather than sub-micron, SSAs over the north coast of Alaska. The regional results presented here show that modelled SSAs are more sensitive to wind speed dependence but that realistic modelling of sea-ice distributions is needed for the simulation of local SSAs, including marine organics. This study supports findings from the Utqiaġvik field campaign that open leads are the primary source of fresh and aged SSAs, including marine organic aerosols, during wintertime at Utqiaġvik; these findings do not suggest an influence from blowing snow and frost flowers. To improve model simulations of Arctic wintertime aerosols, new field data on processes that influence wintertime SSA production, in particular for fine-mode aerosols, are needed as is improved understanding about possible local anthropogenic sources. |
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8504 |
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Florent Tencé, Julien Jumelet, Marie Bouillon, David Cugnet, Slimane Bekki, Sarah Safieddine, Philippe Keckhut, Alain Sarkissian |
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14 years of lidar measurements of polar stratospheric clouds at the French Antarctic station Dumont d'Urville |
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Year |
2023 |
Publication |
Atmospheric chemistry and physics |
Abbreviated Journal |
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Volume |
23 |
Issue |
1 |
Pages |
431-451 |
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Polar stratospheric clouds (PSCs) play a critical role in the stratospheric ozone depletion processes. The last 30 years have seen significant improvements in our understanding of the PSC processes but PSC parametrization in global models still remains a challenge due to the necessary trade-off between the complexity of PSC microphysics and model parametrization constraints. The French Antarctic station Dumont d'Urville (DDU, 66.6∘ S, 140.0∘ E) has one of the few high latitude ground-based lidars in the Southern Hemisphere that has been monitoring PSCs for decades. This study focuses on the PSC data record during the 2007–2020 period. First, the DDU lidar record is analysed through three established classification schemes that prove to be mutually consistent: the PSC population observed above DDU is estimated to be of 30 % supercooled ternary solutions, more than 60 % nitric acid trihydrate mixtures and less than 10 % of water–ice dominated PSC. The Cloud–Aerosol Lidar with Orthogonal Polarization PSC detection around the station are compared to DDU PSC datasets and show a good agreement despite more water–ice PSC detection. Detailed 2015 lidar measurements are presented to highlight interesting features of PSC fields above DDU. Then, combining a temperature proxy to lidar measurements, we build a trend of PSC days per year at DDU from ERA5 (the fifth generation of European ReAnalysis) and NCEP (National Centers for Environment Protection reanalysis) reanalyses fitted on lidar measurements operated at the station. This significant 14-year trend of −4.6 PSC days per decade is consistent with recent temperature satellite measurements at high latitudes. Specific DDU lidar measurements are presented to highlight fine PSC features that are often sub-scale to global models and spaceborne measurements. |
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209 |
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8491 |
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Cynthia H. Whaley, Rashed Mahmood, Knut von Salzen, Barbara Winter, Sabine Eckhardt, Stephen Arnold, Stephen Beagley, Silvia Becagli, Rong-You Chien, Jesper Christensen, Sujay Manish Damani, Xinyi Dong, Konstantinos Eleftheriadis, Nikolaos Evangeliou, Gregory Faluvegi, Mark Flanner, Joshua S. Fu, Michael Gauss, Fabio Giardi, Wanmin Gong, Jens Liengaard Hjorth, Lin Huang, Ulas Im, Yugo Kanaya, Srinath Krishnan, Zbigniew Klimont, Thomas Kühn, Joakim Langner, Kathy S. Law, Louis Marelle, Andreas Massling, Dirk Olivié, Tatsuo Onishi, Naga Oshima, Yiran Peng, David A. Plummer, Olga Popovicheva, Luca Pozzoli, Jean-Christophe Raut, Maria Sand, Laura N. Saunders, Julia Schmale, Sangeeta Sharma, Ragnhild Bieltvedt Skeie, Henrik Skov, Fumikazu Taketani, Manu A. Thomas, Rita Traversi, Kostas Tsigaridis, Svetlana Tsyro, Steven Turnock, Vito Vitale, Kaley A. Walker, Minqi Wang, Duncan Watson-Parris, Tahya Weiss-Gibbons |
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Model evaluation of short-lived climate forcers for the Arctic Monitoring and Assessment Programme: a multi-species, multi-model study |
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2022 |
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Atmospheric Chemistry and Physics |
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Volume |
22 |
Issue |
9 |
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5775-5828 |
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While carbon dioxide is the main cause for global warming, modeling short-lived climate forcers (SLCFs) such as methane, ozone, and particles in the Arctic allows us to simulate near-term climate and health impacts for a sensitive, pristine region that is warming at 3 times the global rate. Atmospheric modeling is critical for understanding the long-range transport of pollutants to the Arctic, as well as the abundance and distribution of SLCFs throughout the Arctic atmosphere. Modeling is also used as a tool to determine SLCF impacts on climate and health in the present and in future emissions scenarios. In this study, we evaluate 18 state-of-the-art atmospheric and Earth system models by assessing their representation of Arctic and Northern Hemisphere atmospheric SLCF distributions, considering a wide range of different chemical species (methane, tropospheric ozone and its precursors, black carbon, sulfate, organic aerosol, and particulate matter) and multiple observational datasets. Model simulations over 4 years (2008–2009 and 2014–2015) conducted for the 2022 Arctic Monitoring and Assessment Programme (AMAP) SLCF assessment report are thoroughly evaluated against satellite, ground, ship, and aircraft-based observations. The annual means, seasonal cycles, and 3-D distributions of SLCFs were evaluated using several metrics, such as absolute and percent model biases and correlation coefficients. The results show a large range in model performance, with no one particular model or model type performing well for all regions and all SLCF species. The multi-model mean (mmm) was able to represent the general features of SLCFs in the Arctic and had the best overall performance. For the SLCFs with the greatest radiative impact (CH4, O3, BC, and SO42-), the mmm was within ±25 % of the measurements across the Northern Hemisphere. Therefore, we recommend a multi-model ensemble be used for simulating climate and health impacts of SLCFs. Of the SLCFs in our study, model biases were smallest for CH4 and greatest for OA. For most SLCFs, model biases skewed from positive to negative with increasing latitude. Our analysis suggests that vertical mixing, long-range transport, deposition, and wildfires remain highly uncertain processes. These processes need better representation within atmospheric models to improve their simulation of SLCFs in the Arctic environment. As model development proceeds in these areas, we highly recommend that the vertical and 3-D distribution of SLCFs be evaluated, as that information is critical to improving the uncertain processes in models. |
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1680-7316 |
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8506 |
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Pete D. Akers, Joël Savarino, Nicolas Caillon, Olivier Magand, Emmanuel Le Meur |
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Title |
Photolytic modification of seasonal nitrate isotope cycles in East Antarctica |
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2022 |
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Atmospheric Chemistry and Physics |
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22 |
Issue |
24 |
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15637-15657 |
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Nitrate in Antarctic snow has seasonal cycles in nitrogen and oxygen isotopic ratios that reflect its sources and atmospheric formation processes, and as a result, nitrate archived in Antarctic ice should have great potential to record atmospheric chemistry changes over thousands of years. However, sunlight that strikes the snow surface results in photolytic nitrate loss and isotopic fractionation that can completely obscure the nitrate's original isotopic values. To gain insight into how photolysis overwrites the seasonal atmospheric cycles, we collected 244 snow samples along an 850 km transect of East Antarctica during the 2013–2014 CHICTABA traverse. The CHICTABA route's limited elevation change, consistent distance between the coast and the high interior plateau, and intermediate accumulation rates offered a gentle environmental gradient ideal for studying the competing pre- and post-depositional influences on archived nitrate isotopes. We find that nitrate isotopes in snow along the transect are indeed notably modified by photolysis after deposition, and drier sites have more intense photolytic impacts. Still, an imprint of the original seasonal cycles of atmospheric nitrate isotopes is present in the top 1–2 m of the snowpack and likely preserved through archiving in glacial ice at these sites. Despite this preservation, reconstructing past atmospheric values from archived nitrate in similar transitional regions will remain a difficult challenge without having an independent proxy for photolytic loss to correct for post-depositional isotopic changes. Nevertheless, nitrate isotopes should function as a proxy for snow accumulation rate in such regions if multiple years of deposition are aggregated to remove the seasonal cycles, and this application can prove highly valuable in its own right. |
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8583 |
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Yanzhi Cao, Zhuang Jiang, Becky Alexander, Jihong Cole-Dai, Joel Savarino, Joseph Erbland, Lei Geng |
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On the potential fingerprint of the Antarctic ozone hole in ice-core nitrate isotopes: a case study based on a South Pole ice core |
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2022 |
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Atmospheric chemistry and physics |
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22 |
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20 |
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13407-13422 |
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Column ozone variability has important implications for surface photochemistry and the climate. Ice-core nitrate isotopes are suspected to be influenced by column ozone variability and δ15N(NO) has been sought to serve as a proxy of column ozone variability. In this study, we examined the ability of ice-core nitrate isotopes to reflect column ozone variability by measuring δ15N(NO) and Δ17O(NO) in a shallow ice core drilled at the South Pole. The ice core covers the period 1944–2005, and during this period δ15N(NO) showed large annual variability ((59.2 ± 29.3) ‰ ), but with no apparent response to the Antarctic ozone hole. Utilizing a snow photochemical model, we estimated 6.9 ‰ additional enrichments in δ15N(NO) could be caused by the development of the ozone hole. Nevertheless, this enrichment is small and masked by the effects of the snow accumulation rate at the South Pole over the same period of the ozone hole. The Δ17O(NO) record has displayed a decreasing trend by ∼ 3.4 ‰ since 1976. This magnitude of change cannot be caused by enhanced post-depositional processing related to the ozone hole. Instead, the Δ17O(NO) decrease was more likely due to the proposed decreases in the O3 HOx ratio in the extratropical Southern Hemisphere. Our results suggest ice-core δ15N(NO) is more sensitive to snow accumulation rate than to column ozone, but at sites with a relatively constant snow accumulation rate, information of column ozone variability embedded in δ15N(NO) should be retrievable. |
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8455 |
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Étienne Vignon, Lea Raillard, Christophe Genthon, Massimo Del Guasta, Andrew J. Heymsfield, Jean-Baptiste Madeleine, Alexis Berne |
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Ice fog observed at cirrus temperatures at Dome C, Antarctic Plateau |
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2022 |
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Atmospheric Chemistry and Physics |
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22 |
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19 |
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12857-12872 |
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As the near-surface atmosphere over the Antarctic Plateau is cold and pristine, its physico-chemical conditions resemble to a certain extent those of the high troposphere where cirrus clouds form. In this paper, we carry out an observational analysis of two shallow fog clouds forming in situ at cirrus temperatures – that is, temperatures lower than 235 K – at Dome C, inner Antarctic Plateau. The combination of lidar profiles with temperature and humidity measurements from advanced thermo-hygrometers along a 45 m mast makes it possible to characterise the formation and development of the fog. High supersaturations with respect to ice are observed before the initiation of fog, and the values attained suggest that the nucleation process at play is the homogeneous freezing of solution aerosol droplets. This is the first time that in situ observations show that this nucleation pathway can be at the origin of an ice fog. Once nucleation occurs, the relative humidity gradually decreases down to subsaturated values with respect to ice in a few hours, owing to vapour deposition onto ice crystals and turbulent mixing. The development of fog is tightly coupled with the dynamics of the boundary layer which, in the first study case, experiences a weak diurnal cycle, while in the second case, it transits from a very stable to a weakly stable dynamical regime. Overall, this paper highlights the potential of the site of Dome C for carrying out observational studies of very cold cloud microphysical processes in natural conditions and using in situ ground-based instruments. |
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1013 |
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8769 |
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Albane Barbero, Roberto Grilli, Markus M. Frey, Camille Blouzon, Detlev Helmig, Nicolas Caillon, Joël Savarino |
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Summer variability of the atmospheric NO2 : NO ratio at Dome C on the East Antarctic Plateau |
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2022 |
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Atmospheric Chemistry and Physics |
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22 |
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18 |
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12025-12054 |
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Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the lower atmosphere of the continental plateau and at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 × 106 cm−3. Such high reactivity in the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NOx ≡ NO + NO2) produced during photo-denitrification of the snowpack, but its underlying mechanisms are not yet fully understood, as some of the chemical species involved (NO2, in particular) have not yet been measured directly and accurately. To overcome this crucial lack of information, newly developed optical instruments based on absorption spectroscopy (incoherent broadband cavity-enhanced absorption spectroscopy, IBBCEAS) were deployed for the first time at Dome C (−75.10 lat., 123.33 long., 3233 m a.s.l.) during the 2019–2020 summer campaign to investigate snow–air–radiation interaction. These instruments directly measure NO2 with a detection limit of 30 pptv (parts per trillion by volume or 10−12 mol mol−1) (3σ). We performed two sets of measurements in December 2019 (4 to 9) and January 2020 (16 to 25) to capture the early and late photolytic season, respectively. Late in the season, the daily averaged NO2:NO ratio of 0.4 ± 0.4 matches that expected for photochemical equilibrium through Leighton's extended relationship involving ROx (0.6 ± 0.3). In December, however, we observed a daily averaged NO2:NO ratio of 1.3 ± 1.1, which is approximately twice the daily ratio of 0.7 ± 0.4 calculated for the Leighton equilibrium. This suggests that more NO2 is produced from the snowpack early in the photolytic season (4 to 9 December), possibly due to stronger UV irradiance caused by a smaller solar zenith angle near the solstice. Such a high sensitivity of the NO2:NO ratio to the sun's position is of importance for consideration in atmospheric chemistry models. |
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1177 |
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8591 |
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