Publications

Abstract: During The East Antarctic International Ice Sheet Traverse (Eaiist, December 2019), In An Unexplored Part Of The East Antarctic Plateau, Snow Samples Were Collected To Expand Our Knowledge Of The Latitudinal Variability Of Iodine, Bromine And Sodium As Well As Their Relation In Connection With Emission Processes And Photochemical Activation In This Unexplored Area. A Total Of 32 Surface (0-5 Cm) And 32 Bulk (Average Of 1 M Depth) Samples Were Taken And Analysed By Inductively Coupled Plasma Mass Spectrometry (Icp-Ms). Our Results Show That There Is No Relevant Latitudinal Trend For Bromine And Sodium. For Bromine They Also Show That It Has No Sig-Nificant Post-Depositional Mechanisms While Its Inland Surface Snow Concentration Is Influenced By Spring Coastal Bromine Explosions. Iodine Concentrations Are Several Orders Of Magnitude Lower Than Bromine And Sodium And They Show A Decreasing Trend In The Surface Samples Concentration Moving Southward. This Suggests That Other Processes Affect Its Accumulation In Surface Snow, Probably Related To The Radial Reduction In The Ozone Layer Moving Towards Central Antarctica. Even Though All Iodine, Bromine And Sodium Present Similar Long-Range Transport From The Dominant Coastal Antarctic Sources, The Annual Seasonal Cycle Of The Ozone Hole Over Antarctica Increases The Amount Of Uv Radiation (In The 280-320 Nm Range) Reaching The Surface, Thereby Affecting The Surface Snow Photoactivation Of Iodine. A Comparison Between The Bulk And Surface Samples Supports The Conclusion That Iodine Undergoes Spring And Summer Snow Recycling That Increases Its Atmospheric Lifetime, While It Tends To Accumulate During The Winter Months When Photochemistry Ceases.

Abstract: Ammonia (Nh3) Participates In The Nucleation And Growth Of Aerosols And Thus Plays A Major Role In Atmospheric Transparency, Pollution, Health, And Climate-Related Issues. Understanding Its Emission Sources Through Nitrogen Stable Isotopes Is Therefore A Major Focus Of Current Work To Mitigate The Adverse Effects Of Aerosol Formation. Since Ice Cores Can Preserve The Past Chemical Composition Of The Atmosphere For Centuries, They Are A Top Tool Of Choice For Understanding Past Nh3 Emissions Through Ammonium (Nh4+), The Form Of Nh3 Archived In Ice. However, The Remote Or High-Altitude Sites Where Glaciers And Ice Sheets Are Typically Localized Have Relatively Low Fluxes Of Atmospheric Nh4+ Deposition, Which Makes Ice Core Samples Very Sensitive To Laboratory Nh3 Contamination. As A Result, Accurate Techniques For Identifying And Tracking Nh3 Emissions Through Concentration And Isotopic Measurements Are Highly Sought To Constrain Uncertainties In Nh3 Emission Inventories And Atmospheric Reactivity Unknowns. Here, We Describe A Solid-Phase Extraction Method For Nh4+ Samples Of Low Concentration That Limits External Contamination And Produces Precise Isotopic Results. By Limiting Nh3Atm Exposure With A Scavenging Fume Hood And Concentrating The Targeted Nh4+ Through Ion Exchange Resin, We Successfully Achieve Isotopic Analysis Of 50 Nmol Nh4+ Samples With A 0.6 Parts Per Thousand Standard Deviation. This Extraction Method Is Applied To An Alpine Glacier Ice Core From Col Du Dome, Mont Blanc, Where We Successfully Demonstrate The Analytical Approach Through The Analysis Of Two Replicate 8 M Water Equivalent Ice Cores Representing 4 Years Of Accumulation With A Reproducibility Of +/- 2.1 Parts Per Thousand. Applying This Methodology To Other Ice Cores In Alpine And Polar Environments Will Open New Opportunities For Understanding Past Changes In Nh3 Emissions And Atmospheric Chemistry.

Abstract: A hydrogeochemical and stable isotopic (delta(15) N-NO3 and delta O-18(NO3)) multitracer approach was combined with previous geological and hydrogeological knowledge in a groundwater-dominated basin, located within the semiarid region of the Bolivian Altiplano (SE of Lake Titicaca). Major natural processes and anthropogenic impacts controlling water chemistry and isotopic compositions of groundwater were identified and corresponding aquifer impacted zones determined. The main natural processes are, by following water flowlines, (1) silicate weathering in the piedmont subsystem (similar to 4,600-3,910 m asl, Ca(Mg)HCO3 facies), (2) Na-Ca exchange within glacial-fluvial deposits overlying paleolacustrine deposits (similar to 3,910 to 3,860 m asl, Na-HCO3 facies), and (3) evaporite dissolution in the confined zone of the lacustrine plain (similar to 3,860-3,810 m asl, Na-Cl-SO4 facies). The highest contributions of anthropogenic nitrate in groundwater have been observed at 3,960-3,860 m asl in the piedmont subsystem and were isotopically associated with leaching from areas influenced by manure piles, synthetic N fertilizers, and sewage collector pipes. In this subsystem, natural water-rock interactions could be deciphered with minimal anthropogenic impact, allowing nitrate sources to be clearly identified. Denitrification, occurring in the topographic lows of the piedmont subsystem, was identified as the main natural attenuation process. The multitracer approach provided a consistent understanding of the major processes that take place along the groundwater flow system and confirmed the significant role of anthropogenic nitrate. This aquifer system thus represents an ideal model of the region's hydrochemical evolution along the gravity-driven flow caused by natural water-rock interaction processes and the influence of anthropogenic contamination.

Abstract: 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 Delta N-15(No3-) 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 Delta N-15(No3-) And Delta O-17(No3-) In A Shallow Ice Core Drilled At The South Pole. The Ice Core Covers The Period 1944-2005, And During This Period Delta N-15(No3-) Showed Large Annual Variability ((59.2 +/- 29.3)Parts Per Thousand), But With No Apparent Response To The Antarctic Ozone Hole. Utilizing A Snow Photochemical Model, We Estimated 6.9 Parts Per Thousand Additional Enrichments In Delta N-15(No3-) 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 Delta O-17(No3-) Record Has Displayed A Decreasing Trend By Similar To 3.4 Parts Per Thousand Since 1976. This Magnitude Of Change Cannot Be Caused By Enhanced Post-Depositional Processing Related To The Ozone Hole. Instead, The Delta O-17(No3-) Decrease Was More Likely Due To The Proposed Decreases In The O-3 / Hox Ratio In The Extratropical Southern Hemisphere. Our Results Suggest Ice-Core Delta N-15(No3-) 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 Delta N-15(No3-) Should Be Retrievable.

Abstract: Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the C-14 fraction modern (f(M)) and delta C-13 of total carbon (TC) and delta N-15 of NH4+ in the PM2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean delta C-13 values were similar to -25.1% +/- 2.0% in warm and -24.2%+/- 0.82% in cold seasons. Mean delta N-15 values were higher in warm (16.4%+/- 2.8%) than in cold seasons (4.0%+/- 6.1%), highlighting the temperature effects on atmospheric NH3 levels and phase- equilibrium isotopic exchange during the conversion of NH3 to NH4+. While 37% +/- 10% of TC was apportioned to fossilfuel sources on the basis of f(M) values, delta N-15 indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH3 slip) to NH3: 60% +/- 26% in warm season and 66% +/- 22% in cold season, based on a Bayesian isotope-mixing model. The collective evidence of multiple isotope analysis reasonably supports the major contribution of fossil-fuel-combustion sources to NH4+, in conjunction with TC, and an increased contribution from vehicle emissions during the severe PM2.5 pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM2.5 in the urban atmosphere.

Abstract: Satellite observations of snow-covered regions in the microwave range have the potential to retrieve essential climate variables such as snow height. This requires a precise understanding of how microwave scattering is linked to snow microstructural properties (density, grain size, grain shape and arrangement). This link has so far relied on empirical adjustments of the theories, precluding the development of robust retrieval algorithms. Here we solve this problem by introducing a new microstructural parameter able to consistently predict scattering. This “microwave grain size” is demonstrated to be proportional to the measurable optical grain size and to a new factor describing the chord length dispersion in the microstructure, a geometrical property known as polydispersity. By assuming that the polydispersity depends on the snow grain type only, we retrieve its value for rounded and faceted grains by optimization of microwave satellite observations in 18 Antarctic sites, and for depth hoar in 86 Canadian sites using ground-based observations. The value for the convex grains (0.6) compares favorably to the polydispersity calculated from 3D micro-computed tomography images for alpine grains, while values for depth hoar show wider variations (1.2-1.9) and are larger in Canada than in the Alps. Nevertheless, using one value for each grain type, the microwave observations in Antarctica and in Canada can be simulated from in-situ measurements with good accuracy with a fully physical model. These findings improve snow scattering modeling, enabling future more accurate uses of satellite observations in snow hydrological and meteorological applications.

Abstract: To fully decipher the role of nitrate photolysis on the atmospheric oxidative capacity in snow-covered regions, NOx flux must be determined with more precision than existing estimates. Here, we introduce a method based on dynamic flux chamber measurements for evaluating the NOx production by photolysis of snowpack nitrate in Antarctica. Flux chamber experiments were conducted for the first time in Antarctica, at the French-Italian station Concordia, Dome C (75 degrees 06'S, 123 degrees 20'E, 3233 m a.s.l) during the 2019-2020 summer campaign. Measurements were gathered with several snow samples of different ages ranging from newly formed drifted snow to 6-year-old firn. Contrary to existing literature expectations, the daily average photolysis rate coefficient, JNO3 over bar , did not significantly vary between differently aged snow samples, suggesting that the photolabile nitrate in snow behaves as a single-family source with common photochemical properties, where a JNO3 over bar = (2.37 +/- 0.35) x 10(-8) s(-1) (1 sigma) has been calculated from December 10(th) 2019 to January 7(th) 2020. At Dome C summer daily average NOx flux, FNOx, based on measured NOx production rates was estimated to be (4.3 +/- 1.2) x 10(8) molecules cm(-2) s(-1), which is 1.5-7 times less than the net NOx flux observed previously above snow at Dome C using the gradient flux method. Using these results, we extrapolated an annual continental snow sourced NOx budget of 0.017 +/- 0.003 Tg center dot N y(-1), similar to 2 times the nitrogen budget, (N-budget), of the stratospheric denitrification previously estimated for Antarctica. These quantifications of nitrate photolysis using flux chamber experiments provide a road-map toward a new parameterization of the sigma NO3-(lambda,T)phi(T,pH) product that can improve future global and regional models of atmospheric chemistry.

Abstract: The CARA program has been running since 2008 by the French reference laboratory for air quality monitoring (LCSQA) and the regional monitoring networks, to gain better knowledge-at a national level-on particulate matter (PM) chemistry and its diverse origins in urban environments. It results in strong collaborations with international-level academic partners for state-of-the-art, straightforward, and robust results and methodologies within operational air quality stakeholders (and subsequently, decision makers). Here, we illustrate some of the main outputs obtained over the last decade, thanks to this program, regarding methodological aspects (both in terms of measurement techniques and data treatment procedures) as well as acquired knowledge on the predominant PM sources. Offline and online methods are used following well-suited quality assurance and quality control procedures, notably including inter-laboratory comparison exercises. Source apportionment studies are conducted using various receptor modeling approaches. Overall, the results presented herewith underline the major influences of residential wood burning (during the cold period) and road transport emissions (exhaust and non-exhaust ones, all throughout the year), as well as substantial contributions of mineral dust and primary biogenic particles (mostly during the warm period). Long-range transport phenomena, e.g., advection of secondary inorganic aerosols from the European continental sector and of Saharan dust into the French West Indies, are also discussed in this paper. Finally, we briefly address the use of stable isotope measurements (delta N-15) and of various organic molecular markers for a better understanding of the origins of ammonium and of the different organic aerosol fractions, respectively.

Abstract: O-17-excess (Delta O-17 = delta O-17 – 0.52 x delta O-18) of sulfate trapped in Antarctic ice cores has been proposed as a potential tool for assessing past oxidant chemistry, while insufficient understanding of atmospheric sulfate formation around Antarctica hampers its interpretation. To probe influences of regional specific chemistry, we compared year-round observations of Delta O-17 of non-sea-salt sulfate in aerosols (Delta O-17(SO42-)(nss)) at Dome C and Dumont d'Urville, inland and coastal sites in East Antarctica, throughout the year 2011. Although Delta O-17(SO42-)(nss) at both sites showed consistent seasonality with summer minima (similar to 1.0 parts per thousand) and winter maxima (similar to 2.5 parts per thousand) owing to sunlight-driven changes in the relative importance of O-3 oxidation to OH and H2O2 oxidation, significant intersite differences were observed in austral spring-summer and autumn. The cooccurrence of higher Delta O-17(SO42-)(nss) at inland (2.0 parts per thousand +/- 0.1 parts per thousand) than the coastal site (1.2 parts per thousand +/- 0.1 parts per thousand) and chemical destruction of methanesulfonate (MS-) in aerosols at inland during spring-summer (October-December), combined with the first estimated Delta O-17(MS-) of similar to 16 parts per thousand, implies that MS- destruction produces sulfate with high Delta O-17(SO42-)(nss) of similar to 12 parts per thousand. If contributing to the known postdepositional decrease of MS- in snow, this process should also cause a significant postdepositional increase in Delta O-17(SO42-)(nss) over 1 parts per thousand, that can reconcile the discrepancy between Delta O-17(SO42-)(nss) in the atmosphere and ice. The higher Delta O-17(SO42-)(nss) at the coastal site than inland during autumn (March-May) may be associated with oxidation process involving reactive bromine and/or sea-salt particles around the coastal region.

Abstract: The chemistry of reactive gases inside the snowpack and in the lower atmosphere was investigated at Concordia Station (Dome C), Antarctica, from December 2012 to January 2014. Measured species included ozone, nitrogen oxides, gaseous elemental mercury (GEM), and formaldehyde, for study of photochemical reactions, surface exchange, and the seasonal cycles and atmospheric chemistry of these gases. The experiment was installed approximate to 1 km from the station main infrastructure inside the station clean air sector and within the station electrical power grid boundary. Ambient air was sampled continuously from inlets mounted above the surface on a 10m meteorological tower. In addition, snowpack air was collected at 30 cm intervals to 1.2m depth from two manifolds that had both above- and below-surface sampling inlets. Despite being in the clean air sector, over the course of the 1.2-year study, we observed on the order of 50 occasions when exhaust plumes from the camp, most notably from the power generation system, were transported to the study site. Continuous monitoring of nitrogen oxides (NOx) provided a measurement of a chemical tracer for exhaust plumes. Highly elevated levels of NOx (up to 1000 x background) and lowered ozone (down to approximate to 50 %), most likely from reaction of ozone with nitric oxide, were measured in air from above and within the snowpack. Within 5-15 min from observing elevated pollutant levels above the snow, rapidly increasing and long-lasting concentration enhancements were measured in snowpack air. While pollution events typically lasted only a few minutes to an hour above the snow surface, elevated NOx levels were observed in the snowpack lasting from a few days to approximate to 1 week. GEM and formaldehyde measurements were less sensitive and covered a shorter measurement period; neither of these species' data showed noticeable concentration changes during these events that were above the normal variability seen in the data. Nonetheless, the clarity of the NOx and ozone observations adds important new insight into the discussion of if and how snow photochemical experiments within reach of the power grid of polar research sites are possibly compromised by the snowpack being chemically influenced (contaminated) by gaseous and particulate emissions from the research camp activities. This question is critical for evaluating if snowpack trace chemical measurements from within the camp boundaries are representative for the vast polar ice sheets.

Abstract: The nitrogen stable isotopic composition in nitrate (delta N-15-NO3-) measured in ice cores from low-snow-accumulation regions in East Antarctica has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO) due to the sensitivity of nitrate (NO3-) photolysis to UV radiation. However, understanding the transfer of reactive nitrogen at the air-snow interface in polar regions is paramount for the interpretation of ice core records of delta N-15-NO3- and NO3- mass concentrations. As NO 3 undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of delta N-15-NO3- and air-snow transfer modelling are necessary to understand and quantify the complex photochemical processes at play. As part of the Isotopic Constraints on Past Ozone Layer Thickness in Polar Ice (ISOL-ICE) project, we report new measurements of NO3- mass concentration and delta N-15-NO3- in the atmosphere, skin layer (operationally defined as the top 5 mm of the snowpack), and snow pit depth profiles at Kohnen Station, Dronning Maud Land (DML), Antarctica. We compare the results to previous studies and new data, presented here, from Dome C on the East Antarctic Plateau. Additionally, we apply the conceptual 1D model of TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of NO3- recycling on delta N-15-NO3- and NO3- mass concentrations archived in snow and firn. We find clear evidence of NO3- photolysis at DML and confirmation of previous theoretical, field, and laboratory studies that UV photolysis is driving NO3- recycling and redistribution at DML. Firstly, strong denitrification of the snowpack is observed through the delta N-15-NO3- signature, which evolves from the enriched snowpack (-3 parts per thousand to 100 parts per thousand), to the skin layer (-20 parts per thousand to 3 parts per thousand), to the depleted atmosphere (-50% to -2 parts per thousand), corresponding to mass loss of NO3- from the snowpack. Based on the TRANSITS model, we find that NO3- is recycled two times, on average, before it is archived in the snowpack below 15 cm and within 0.75 years (i.e. below the photic zone). Mean annual archived delta N-15-NO3- and NO3- mass concentration values are 50 parts per thousand and 60 ng g(-1), respectively, at the DML site. We report an e-folding depth (light attenuation) of 2-5 cm for the DML site, which is considerably lower than Dome C. A reduced photolytic loss of NO3- at DML results in less enrichment of delta N-15-NO3- than at Dome C mainly due to the shallower e-folding depth but also due to the higher snow accumulation rate based on TRANSITS-modelled sensitivities. Even at a relatively low snow accumulation rate of 6 cm yr(-1) (water equivalent; w.e. ), the snow accumulation rate at DML is great enough to preserve the seasonal cycle of NO3- mass concentration and delta N-15-NO3-, in contrast to Dome C where the depth profiles are smoothed due to longer exposure of surface snow layers to incoming UV radiation before burial. TRANSITS sensitivity analysis of delta N-15-NO3- at DML highlights that the dominant factors controlling the archived delta N-15-NO3- signature are the e-folding depth and snow accumulation rate, with a smaller role from changes in the snowfall timing and TCO. Mean TRANSITS model sensitivities of archived delta N-15-NO3- at the DML site are 100% for an e-folding depth change of 8 cm, 110% for an annual snow accumulation rate change of 8.5 cm yr(-1) w.e., 10% for a change in the dominant snow deposition season between winter and summer, and 10% for a TCO change of 100DU (Dobson units). Here we set the framework for the interpretation of a 1000-year ice core record of delta N-15-NO3- from DML. Ice core delta N-15-NO3- records at DML will be less sensitive to changes in UV than at Dome C; however the higher snow accumulation rate and more accurate dating at DML allows for higher-resolution delta N-15-NO3- records.

Abstract: High quality records of stratospheric volcanic eruptions, required to model past climate variability, have been constructed by identifying synchronous (bipolar) volcanic sulfate horizons in Greenland and Antarctic ice cores. Here we present a new 2600-year chronology of stratospheric volcanic events using an independent approach that relies on isotopic signatures (Delta S-33 and in some cases Delta O-17) of ice core sulfate from five closely-located ice cores from Dome C, Antarctica. The Dome C stratospheric reconstruction provides independent validation of prior reconstructions. The isotopic approach documents several high-latitude stratospheric events that are not bipolar, but climatically-relevant, and diverges deeper in the record revealing tropospheric signals for some previously assigned bipolar events. Our record also displays a collapse of the Delta O-17 anomaly of sulfate for the largest volcanic eruptions, showing a further change in atmospheric chemistry induced by large emissions. Thus, the refinement added by considering both isotopic and bipolar correlation methods provides additional levels of insight for climate-volcano connections and improves ice core volcanic reconstructions.

Abstract: Despite widespread applications of sulfur isotope mass-independent fractionation (MIF) signals for probing terrestrial and extra-terrestrial environments, there has been no international sulfur isotope reference material available for normalization of Delta S-33 and Delta S-36 data. International reference materials to anchor isotope values are useful for interlaboratory data comparisons and are needed to evaluate, e.g., whether issues exist associated with blanks and mass spectrometry when using different analytical approaches. We synthesized two sodium sulfate samples enriched in S-33 with different magnitudes, and termed them S-MIF-1 and S-MIF-2, respectively. The sulfur isotopic compositions of these two samples were measured in five different laboratories using two distinct techniques to place them on the V-CDT scale for delta S-34 and a provisional V-CDT scale for Delta S-33 and Delta S-36. We obtained average delta S-34 values of S-MIF-1 = 10.26 +/- 0.22 parts per thousand and S-MIF-2 = 21.53 +/- 0.26 parts per thousand (1 sigma, versus V-CDT). The average Delta S-33 and Delta S-36 values of S-MIF-1 were determined to be 9.54 +/- 0.09 parts per thousand and -0.11 +/- 0.25 parts per thousand, respectively, while the average Delta S-33 and Delta S-36 values of S-MIF-2 are 11.39 +/- 0.08 parts per thousand and -0.33 +/- 0.13 parts per thousand (1 sigma, versus V-CDT). The lack of variation among the interlaboratory isotopic values suggests sufficient homogeneity of S-MIF-1 and S-MIF-2, especially for Delta S-33. Although additional measurements may be needed to ensure the accuracy of the isotopic compositions of S-MIF-1 and S-MIF-2, they can serve as working standards for routine Delta S-33 analysis to improve data consistency, and have the potential to serve as secondary sulfur isotope reference materials to address issues such as scale contraction/expansion and for normalization and reporting of Delta S-33 and Delta S-36 between laboratories. For the same reasons as listed for sulfur isotopes, the same standards were also artificially enriched in O-17. The calibration is still in progress but first estimations gave Delta O-17 = 3.3 +/- 0.3 parts per thousand with unassigned delta O-18.

Abstract: Atmospheric nitrate (NO3- = particulate NO3- + gas-phase nitric acid [HNO3]) and sulfate (SO42-) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO3- and total suspended particulate sulfate (SO4(TSP)2-) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO3- was largely affected by snowpack emissions in which [NO3-] and delta N-15(NO3-) were highest (49.3 +/- 21.4 ng/m(3), n = 8) and lowest (-47.0 +/- 11.7 parts per thousand, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Delta O-17(NO3-) reflected tropospheric chemistry year-round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HOx- to O-3-dominated oxidation chemistry. SO4(TSP)2- concentrations were highest during austral summer and fall (86.7 +/- 73.7 ng/m(3), n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as delta S-34(SO42-)((TSP)) values (18.5 +/- 1.0 parts per thousand, n = 10) were similar to literature delta S-34(DMS) values. The seasonal cycle of Delta O-17(SO42-)((TSP)) exhibited minima during austral summer (0.9 +/- 0.1 parts per thousand, n = 5) and maxima during austral fall (1.3 +/- 0.3 parts per thousand, n = 6) and austral spring (1.6 +/- 0.1 parts per thousand, n = 5), indicating a shift from HOx- to O-3-dominated chemistry in the atmospheric derived SO42- component. Overall, the budgets of NO3- and SO4(TSP)2- at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

Abstract: Nitrogen (N) emissions associated with urbanization exacerbate the atmospheric N influx to remote ecosystems – like mountains -, leading to well-documented detrimental effects on ecosystems (e.g., soil acidification, pollution of freshwaters). Here, the importance and fate of N deposition in a watershed was evaluated along a montane to urban gradient, using a multi-isotopic tracers approach (Delta O-17, delta N-15, delta O-18 of nitrate, delta H-2 and delta O-18 of water). In this setting, the montane streams had higher proportions of atmospheric nitrate compared to urban streams, and exported more atmospheric nitrate on a yearly basis (0.35 vs 0.10 kg-N ha(-1) yr(-1)). In urban areas, nitrate exports were driven by groundwater, whereas in the catchment head nitrate exports were dominated by surface runoff. The main sources of nitrate to the montane streams were microbial nitrification and atmospheric deposition, whereas microbial nitrification and sewage leakage contributed most to urban streams. Based on the measurement of delta N-15 and delta O-18-NO3-, biological processes such as denitrification or N assimilation were not predominant in any streams in this study. The observed low delta N-15 and delta O-18 range of terrestrial nitrate (i.e., nitrate not coming from atmospheric deposition) in surface water compared to literature suggests that atmospheric deposition may be underestimated as a direct source of N. (c) 2018 Elsevier B.V. All rights reserved.

Abstract: This work presents measurements of time-resolved mass-independently fractionated sulfate of volcanic origin from Antarctic ice core records that cover the last 2,600years. These measurements are used to evaluate the time dependence of the deposited isotopic signal and to extract the isotopic characteristics of the reactions yielding sulfate from stratospheric volcanic eruptions in the modern atmosphere. Time evolution of the signal in snow (years) with respect to the fast SO2 oxidation in the stratosphere suggests that photochemically produced condensed phase is rapidly and continuously separated from the gas phase and preserved during transportation and deposition on the polar ice cap. On some eruptions, a nonzero isotopic mass balance highlights that a part of the signal can be lost during transport and/or deposition. The large number of volcanic events studied allows the S-33 versus S-36 and S-34 versus S-33 slopes to be constrained at -1.56 (1 sigma=0.25) and 0.09 (1 sigma=0.02), respectively. The S-33 versus S-36 slope refines a prior determinations of S-36/S-33=-4 and overlaps the range observed for sulfur seen in early Earth samples (Archean). In recent volcanogenic sulfate, the S-33 versus S-34 differs, however, from the Archean record. The similitude for S-36/S-33 and the difference for S-33/S-34 suggest similar mass-independently fractionated sulfate processes to the Archean atmosphere. Using a simple model, we highlight that a combination of several mechanisms is needed to reproduce the observed isotopic trends and suggest a greater contribution from mass-dependent oxidation by OH in the modern atmosphere. Plain Language Summary Large volcanic eruptions inject sulfurous gases in the stratosphere, where they rapidly form sulfuric acid aerosols. These aerosols can reside in the stratosphere for years, cover the entire globe, and profoundly modify the climate by scattering and absorbing solar radiation. Sulfuric acid aerosols formed by this process acquire an isotopic anomaly that traces these processes and allows identification of these eruptions in ice core records, providing a means to distinguish between high and low climatic impact eruptions in ice core volcanic deposits. This study provides a characterization of this time-dependent isotopic signature that is used to constrain its origin and to understand the processes underlying its production and evolution.

Abstract: Distinct diurnal and seasonal variations of mercury (Hg) have been observed in near-surface air at Concordia Station on the East Antarctic Plateau, but the processes controlling these characteristics are not well understood. Here, we use a box model to interpret the Hg-0 (gaseous elemental mercury) measurements in thes year 2013. The model includes atmospheric Hg-0 oxidation (by OH, O-3, or bromine), surface snow Hg-II (oxidized mercury) reduction, and air-snow exchange, and is driven by meteorological fields from a regional climate model. The simulations suggest that a photochemically driven mercury diurnal cycle occurs at the air-snow interface in austral summer. The fast oxidation of Hg-0 in summer may be provided by a two-step bromine-initiated scheme, which is favored by low temperature and high nitrogen oxides at Concordia. The summertime diurnal variations of Hg-0 (peaking during daytime) may be confined within several tens of meters above the snow surface and affected by changing mixed layer depths. Snow re-emission of Hg-0 is mainly driven by photoreduction of snow HgII in summer. Intermittent warming events and a hypothesized reduction of Hg-II occurring in snow in the dark may be important processes controlling the mercury variations in the non-summer period, although their relative importance is uncertain. The Br-initiated oxidation of Hg-0 is expected to be slower at Summit Station in Greenland than at Concordia (due to their difference in temperature and levels of nitrogen oxides and ozone), which may contribute to the observed differences in the summertime diurnal variations of Hg-0 between these two polar inland stations.

Abstract: Under the framework of the GMOS project (Global Mercury Observation System) atmospheric mercury monitoring has been implemented at Concordia Station on the high-altitude Antarctic plateau (75 degrees 06'S, 123 degrees 20'E, 3220m above sea level). We report here the first year-round measurements of gaseous elemental mercury (Hg(0)) in the atmosphere and in snowpack interstitial air on the East Antarctic ice sheet. This unique data set shows evidence of an intense oxidation of atmospheric Hg(0) in summer (24-hour daylight) due to the high oxidative capacity of the Antarctic plateau atmosphere in this period of the year. Summertime Hg(0) concentrations exhibited a pronounced daily cycle in ambient air with maximal concentrations around midday. Photochemical reactions and chemical exchange at the air-snow interface were prominent, highlighting the role of the snowpack on the atmospheric mercury cycle. Our observations reveal a 20 to 30% decrease of atmospheric Hg(0) concentrations from May to mid-August (winter, 24 h darkness). This phenomenon has not been reported elsewhere and possibly results from the dry deposition of Hg(0) onto the snowpack. We also reveal the occurrence of multi-day to weeklong atmospheric Hg(0) depletion events in summer, not associated with depletions of ozone, and likely due to a stagnation of air masses above the plateau triggering an accumulation of oxidants within the shallow boundary layer. Our observations suggest that the inland atmospheric reservoir is depleted in Hg(0) in summer. Due to katabatic winds flowing out from the Antarctic plateau down the steep vertical drops along the coast and according to observations at coastal Antarctic stations, the striking reactivity observed on the plateau most likely influences the cycle of atmospheric mercury on a continental scale.

Abstract: Current volcanic reconstructions based on ice core analysis have significantly improved over the past few decades by incorporating multiple-core analyses with a high temporal resolution from different parts of the polar regions into a composite common volcanic eruption record. Regional patterns of volcanic deposition are based on composite records, built from cores taken at both poles. However, in many cases only a single record at a given site is used for these reconstructions. This assumes that transport and regional meteorological patterns are the only source of the dispersion of the volcanic products. Here we evaluate the local-scale variability of a sulfate profile in a low-accumulation site (Dome C, Antarctica), in order to assess the representativeness of one core for such a reconstruction. We evaluate the variability with depth, statistical occurrence, and sulfate flux deposition variability of volcanic eruptions detected in five ice cores, drilled 1m apart from each other. Local-scale variability, essentially attributed to snow drift and surface roughness at Dome C, can lead to a non-exhaustive record of volcanic events when a single core is used as the site reference, with a bulk probability of 30% of missing volcanic events and close to 65% uncertainty on one volcanic flux measurement (based on the standard deviation obtained from a five-core comparison). Averaging n records reduces the uncertainty of the deposited flux mean significantly (by a factor 1/root n); in the case of five cores, the uncertainty of the mean flux can therefore be reduced to 29 %.

Abstract: Surface ozone has been measured since 2004 at the coastal East Antarctic site of Dumont d'Urville (DDU), and since 2007 at the Concordia station located on the high East Antarctic plateau. This paper discusses long-term changes, seasonal and diurnal cycles, as well as inter-annual summer variability observed at these two East Antarctic sites. At Concordia, near-surface ozone data were complemented by balloon soundings and compared to similar measurements done at the South Pole. The DDU record is compared to those obtained at the coastal site of Syowa, also located in East Antarctica, as well as the coastal sites of Neumayer and Halley, both located on the coast of the Weddell Sea in West Antarctica. Surface ozone mixing ratios exhibit very similar seasonal cycles at Concordia and the South Pole. However, in summer the diurnal cycle of ozone is different at the two sites with a drop of ozone in the afternoon at Concordia but not at the South Pole. The vertical distribution of ozone above the snow surface also differs. When present, the ozone-rich layer located near the ground is better mixed and deeper at Concordia (up to 400aEuro-m) than at the South Pole during sunlight hours. These differences are related to different solar radiation and wind regimes encountered at these two inland sites. DDU appears to be the coastal site where the impact of the late winter/spring bromine chemistry is the weakest, but where the impact of elevated ozone levels caused by NOx snow emissions from the high Antarctic plateau is the highest. The highest impact of the bromine chemistry is seen at Halley and Neumayer, and to a lesser extent at Syowa. These three sites are only weakly impacted by the NOx chemistry and the net ozone production occurring on the high Antarctic plateau. The differences in late winter/spring are attributed to the abundance of sea ice offshore from the sites, whereas those in summer are related to the topography of East Antarctica that promotes the katabatic flow bringing oxidant-rich inland air masses to the site. There appears to be a decreasing change in summer surface ozone at the two East Antarctic sites of Concordia and DDU over the most recent period (2004-2014 and 2007-2014). Further research, including continued monitoring, is needed at these two sites to better separate the effect of synoptic transport from possible change of NOx snow emissions in response to recovery of the stratospheric ozone layer leading to penetration of more UV radiation to the surface.

Abstract: The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture. However, recent analyses of snow from shallow pits indicate that the climatic signal can become erased in very low accumulation regions, due to local processes of snow reworking. The signal-to-noise ratio decreases and the climatic signal can then only be retrieved using stacks of several snow pits. Obviously, the signal is not completely lost at this stage, otherwise it would be impossible to extract valuable climate information from ice cores as has been done, for instance, for the last glaciation. To better understand how the climatic signal is passed from the precipitation to the snow, we present here results from varied snow samples from East Antarctica. First, we look at the relationship between isotopes and temperature from a geographical point of view, using results from three traverses across Antarctica, to see how the relationship is built up through the distillation process. We also take advantage of these measures to see how second-order parameters (d-excess and O-17-excess) are related to delta O-18 and how they are controlled. d-excess increases in the interior of the continent (i.e., when delta O-18 decreases), due to the distillation process, whereas O-17-excess decreases in remote areas, due to kinetic fractionation at low temperature. In both cases, these changes are associated with the loss of original information regarding the source. Then, we look at the same relationships in precipitation samples collected over 1 year at Dome C and Vostok, as well as in surface snow at Dome C. We note that the slope of the delta O-18 vs. temperature (T) relationship decreases in these samples compared to those from the traverses, and thus caution is advocated when using spatial slopes for past climate reconstruction. The second-order parameters behave in the same way in the precipitation as in the surface snow from traverses, indicating that similar processes are active and that their interpretation in terms of source climatic parameters is strongly complicated by local temperature effects in East Antarctica. Finally we check if the same relationships between delta O-18 and second-order parameters are also found in the snow from four snow pits. While the d-excess remains opposed to delta O-18 in most snow pits, the O-17-excess is no longer positively correlated to delta O-18 and even shows anti-correlation to delta O-18 at Vostok. This may be due to a stratospheric influence at this site and/or to post-deposition processes.

Abstract: Stable isotope ratios of nitrate preserved in deep ice cores are expected to provide unique and valuable information regarding paleoatmospheric processes. However, due to the post-depositional loss of nitrate in snow, this information may be erased or significantly modified by physical or photochemical processes before preservation in ice. We investigated the role of solar UV photolysis in the post-depositional modification of nitrate mass and stable isotope ratios at Dome C, Antarctica, during the austral summer of 2011/2012. Two 30 cm snow pits were filled with homogenized drifted snow from the vicinity of the base. One of these pits was covered with a plexiglass plate that transmits solar UV radiation, while the other was covered with a different plexiglass plate having a low UV transmittance. Samples were then collected from each pit at a 2-5 cm depth resolution and a 10-day frequency. At the end of the season, a comparable nitrate mass loss was observed in both pits for the top-level samples (0-7 cm) attributed to mixing with the surrounding snow. After excluding samples impacted by the mixing process, we derived an average apparent nitrogen isotopic fractionation ((15)epsilon(app)) of -67.8 +/- 12% for the snow nitrate exposed to solar UV using the nitrate stable isotope ratios and concentration measurements. For the control samples in which solar UV was blocked, an apparent average (15)epsilon(app) value of -12.0 +/- 1.7% was derived. This difference strongly suggests that solar UV photolysis plays a dominant role in driving the isotopic fractionation of nitrate in snow. We have estimated a purely photolytic nitrogen isotopic fractionation ((15)epsilon(photo)) of -55.8 +/- 12.0% from the difference in the derived apparent isotopic fractionations of the two experimental fields, as both pits were exposed to similar physical processes except exposure to solar UV. This value is in close agreement with the (15)epsilon(photo) value of -47.9 +/- 6.8% derived in a laboratory experiment simulated for Dome C conditions (Berhanu et al., 2014). We have also observed an insensitivity of (15)epsilon with depth in the snowpack under the given experimental setup. This is due to the uniform attenuation of incoming solar UV by snow, as (15)epsilon is strongly dependent on the spectral distribution of the incoming light flux. Together with earlier work, the results presented here represent a strong body of evidence that solar UV photolysis is the most relevant post-depositional process modifying the stable isotope ratios of snow nitrate at low-accumulation sites, where many deep ice cores are drilled. Nevertheless, modeling the loss of nitrate in snow is still required before a robust interpretation of ice core records can be provided.

Abstract: Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. [“Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry,” J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate (N-15, O-17, and O-18) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (delta N-15, delta O-18, and Delta O-17). From these measurements an average photolytic isotopic fractionation of (15)epsilon = (-15 +/- 1.2)parts per thousand was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of (15)epsilon = (-47.9 +/- 6.8)parts per thousand in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from -40 to -74.3 parts per thousand. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of (NO3-)-N-14 and (NO3-)-N-15 in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Abstract: On the Antarctic plateau, precipitation quantities are so low that the surface mass budget is for an important part determined by exchanges of water vapor between the snow surface and the atmosphere surface. At Dome C (75 degrees S, 123 degrees E), we have frequently observed the growth of crystals on the snow surface under calm sunny weather. Here we present the time variations of specific surface area (SSA) and density of these crystals. Using the detailed snow model Crocus, we conclude that the formation of these crystals was very likely due to the nighttime formation of surface hoar crystals and to the daytime formation of sublimation crystals. These latter crystals form by processes similar to those involved in the formation of frost flowers on young sea ice. The formation of these crystals impacts the albedo, mass and energy budget of the Antarctic plateau. In particular, the SSA variations of the surface layer can induce an instantaneous forcing at the snow surface up to -10 W m(-2) at noon, resulting in a surface temperature drop of 0.45 K. This result confirms that snow SSA is a crucial variable to consider in the energy budget and climate of snow-covered surfaces.

Abstract: Ice core nitrate concentrations peak in the summer in both Greenland and Antarctica. Two nitrate concentration peaks in one annual layer have been observed some years in ice cores in Greenland from samples dating post-1900, with the additional nitrate peak occurring in the spring. The origin of the spring nitrate peak was hypothesized to be pollution transport from the mid-latitudes in the industrial era. We performed a case study on the origin of a spring nitrate peak in 2005 measured from a snowpit at Summit, Greenland, covering 3 years of snow accumulation. The effect of long-range transport of nitrate on this spring peak was excluded by using sulfate as a pollution tracer. The isotopic composition of nitrate (delta N-15, delta O-18 and Delta O-17) combined with photochemical calculations suggest that the occurrence of this spring peak is linked to a significantly weakened stratospheric ozone (O-3) layer. The weakened O-3 layer resulted in elevated UVB (ultraviolet-B) radiation on the snow surface, where the production of OH and NOx from the photolysis of their precursors was enhanced. Elevated NOx and OH concentrations resulted in enhanced nitrate production mainly through the NO2 + OH formation pathway, as indicated by decreases in delta O-18 and Delta O-17 of nitrate associated with the spring peak. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and found 19 years with double nitrate peaks after the 1950s. Out of these 19 years, 14 of the secondary nitrate peaks were accompanied by sulfate peaks, suggesting long-range transport of nitrate as their source. In the other 5 years, low springtime O-3 column density was observed, suggesting enhanced local production of nitrate as their source. The results suggest that, in addition to direct transport of nitrate from polluted regions, enhanced local photochemistry can also lead to a spring nitrate peak. The enhanced local photochemistry is probably associated with the interannual variability of O-3 column density in the Arctic, which leads to elevated surface UV radiation in some years. In this scenario, enhanced photochemistry caused increased local nitrate production under the condition of elevated local NOx abundance in the industrial era.

Abstract: Post-depositional processes alter nitrate concentration and nitrate isotopic composition in the top layers of snow at sites with low snow accumulation rates, such as Dome C, Antarctica. Available nitrate ice core records can provide input for studying past atmospheres and climate if such processes are understood. It has been shown that photolysis of nitrate in the snowpack plays a major role in nitrate loss and that the photolysis products have a significant influence on the local troposphere as well as on other species in the snow. Reported quantum yields for the main reaction spans orders of magnitude -apparently a result of whether nitrate is located at the air-ice interface or in the ice matrix constituting the largest uncertainty in models of snowpack NOx emissions. Here, a laboratory study is presented that uses snow from Dome C and minimizes effects of desorption and recombination by flushing the snow during irradiation with UV light. A selection of UV filters allowed examination of the effects of the 200 and 305 nm absorption bands of nitrate. Nitrate concentration and photon flux were measured in the snow. The quantum yield for loss of nitrate was observed to decrease from 0.44 to 0.003 within what corresponds to days of UV exposure in Antarctica. The superposition of photolysis in two photochemical domains of nitrate in snow is proposed: one of photolabile nitrate, and one of buried nitrate. The difference lies in the ability of reaction products to escape the snow crystal, versus undergoing secondary (recombination) chemistry. Modeled NOx emissions may increase significantly above measured values due to the observed quantum yield in this study. The apparent quantum yield in the 200 nm band was found to be similar to 1%, much lower than reported for aqueous chemistry. A companion paper presents an analysis of the change in isotopic composition of snowpack nitrate based on the same samples as in this study. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Abstract: The unique and distinctive O-17-excess (Delta O-17) of ozone (O-3) provides a conservative tracer for oxidative processes in both modern and paleo-atmospheres and has acted as the primary driver of theoretical and experimental research into non-mass-dependent fractionation (NMDF) for over three decades. However, due to the inherent complexity of extracting O-3 from ambient air, the existing observational dataset for tropospheric O-3 isotopic composition remains quite small. Recent analytical developments have provided a robust and reliable means for determining Delta O-17(O-3)(trans.), the transferrable Delta O-17 signature of ozone in the troposphere (Vicars et al., 2012). We have employed this new methodology in a systematic investigation of the spatial and seasonal features of Delta O-17(O-3)(trans.) in two separate field campaigns: a weekly sampling effort at our laboratory in Grenoble, France (45 degrees N) throughout 2012 (n = 47) and a four-week campaign onboard the Research Vessel (R/V) Polarstern along a latitudinal transect from 50 degrees S to 50 degrees N in the Atlantic Ocean (n = 30). The bulk O-17-excess of ozone, denoted Delta O-17(O-3)(bulk), exhibited mean (+/- 1 sigma) values of 26.2 +/- 1.3 parts per thousand (Delta O-17(O-3)(trans.) = 39.3 +/- 2.0 parts per thousand) and 25.9 +/- 1.1 parts per thousand (Delta O-17(O-3)(trans.) = 38.8 +/- 1.6 parts per thousand) for the Grenoble and R/V Polarstern collections, respectively. This range of values is in excellent quantitative agreement with the two previous studies of ozone triple-isotope composition, which have yielded mean (+/- 1 sigma) Delta O-17(O-3)(bulk) values of 25.4 +/- 9.0 parts per thousand (n = 89). However, the magnitude of variability detected in the present study is much smaller than that formerly reported. In fact, the standard deviation of Delta O-17(O-3)(bulk) in each new dataset is lower than the uncertainty previously estimated for the filter technique (+/- 1.7 parts per thousand), indicating a low level of natural spatial and temporal variation in the O-17-excess of surface ozone. For instance, no clear temporal pattern in Delta O-17(O-3) is evident in the annual record from Grenoble despite dramatic seasonal variations in ozone and atmospheric reactive nitrogen (NOx = NO + NO2) concentrations. However, a small but statistically significant difference is distinguishable in the R/V Polarstern record when comparing samples collected in the Southern and Northern Hemispheres, which possessed average Delta O-17(O-3)(bulk) values of 25.2 +/- 1.0 parts per thousand and 26.5 +/- 0.7 parts per thousand, respectively. The implications of these results are discussed in the context of the tropospheric ozone budget and the use of oxygen isotope ratios of secondary atmospheric species to derive information regarding oxidation pathways from modern and paleo-atmospheres. (C) 2014 Elsevier Ltd. All rights reserved.

Abstract: Here we report the measurement of the comprehensive isotopic composition (delta N-15, Delta O-17 and delta O-18) of nitrate at the air-snow interface at Dome C, Antarctica (DC, 75 degrees 06'S, 123 degrees 19'E), and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66 degrees S and 78 degrees S. In most of the snow pits, nitrate loss (either by physical release or UV photolysis of nitrate) is observed and fractionation constants associated are calculated. Nitrate collected from snow pits on the plateau (snow accumulation rate below 50 kg m(-2)a(-1)) displays average fractionation constants of (-59 +/- 10) parts per thousand, (+2.0 +/- 1.0)parts per thousand and (+8.7 +/- 2.4)parts per thousand for delta N-15, Delta O-17 and delta O-18, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average fractionation constants of (-16 +/- 14) parts per thousand, (-0.2 +/- 1.5)parts per thousand and (+3.1 +/- 5.8) parts per thousand, for delta N-15, Delta O-17 and delta O-18, respectively. Our observations corroborate that photolysis (associated with a N-15/N-14 fractionation constant of the order of -48 parts per thousand according to Frey et al. (2009)) is the dominant nitrate loss process on the East Antarctic Plateau, while on the coast the loss is less pronounced and could involve both physical release and photochemical processes. Year-round isotopic measurements at DC show a close relationship between the Delta O-17 of atmospheric nitrate and Delta O-17 of nitrate in skin layer snow, suggesting a photolytically driven isotopic equilibrium imposed by nitrate recycling at this interface. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift of the order of 25 parts per thousand between the delta N-15 values in the atmospheric and skin layer nitrate at DC. Asymptotic delta N-15(NO3-) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(delta N-15(as.) + 1) = (5.76 +/- 0.47) . (kg m(-2) a(-1)/A)+(0.01 +/- 0.02)), confirming the strong relationship between the snow accumulation rate and the degree of isotopic fractionation, consistent with previous observations by Freyer et al. (1996). Asymptotic Delta O-17(NO3-) values on the plateau are smaller than the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photoproducts and water molecules from the surrounding ice. However, the apparent fractionation in Delta O-17 is small, thus allowing the preservation of a portion of the atmospheric signal.

Abstract: RATIONALEThe oxygen-17 excess (O-17) of nitrate and sulfate contains valuable information regarding their atmospheric formation pathways. However, the current pyrolysis method to measure O-17 requires large sample amounts (>4 μmol for nitrate and >1 μmol for sulfate). We present a new approach employing a Gas Bench interface which cryofocuses O-2 produced from sample pyrolysis, enabling the analysis of sub-micromole size samples. METHODSSilver nitrate or sulfate at sub-micromole levels in a sample container was thermally decomposed to O-2 and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Byproducts (mainly NO2 for silver nitrate and SO2 for silver sulfate) were removed in a liquid nitrogen trap and the sample O-2 was carried by ultra-pure helium (He) gas to a Gas Bench II interface where it was cryofocused prior to entering an isotope ratio mass spectrometer. RESULTSAnalysis of the international nitrate reference material USGS35 (O-17=21.6) within the size range of 300-1000nmol O-2 gave a mean O-17 value of (21.6 +/- 0.69) parts per thousand (mean +/- 1 sigma). Three inter-laboratory calibrated sulfate reference materials, Sulf-, Sulf- and Sulf-epsilon, each within the size range of 180-1000nmol O-2, were analyzed and shown to possess mean O-17 values of (0.9 +/- 0.10) parts per thousand, (2.1 +/- 0.25) parts per thousand and (7.0 +/- 0.63) parts per thousand, respectively. CONCLUSIONSThe analyses of nitrate and sulfate reference materials at sub-micromole levels gave O-17 values consistent with their accepted values. This new approach of employing the Gas Bench to cryofocus O-2 after the pyrolysis of AgNO3 and Ag2SO4 particularly benefits the effort of measuring O-17 in sample types with a low abundance of nitrate and sulfate such as ice cores. Copyright (c) 2013 John Wiley & Sons, Ltd.

Abstract: A multi-isotope approach has been used in the Marano lagoon (NE Italy) and parts of its catchment area to identify causes of increased NO3- pollution. The hydrogeochemical features of different water types and potential sources of NO3- were characterized using the isotopic composition of NO3- (delta N-15, delta O-18, and Delta O-17) and other source-related species such as B (delta B-11), water (delta H-2 and delta O-18) and SO42- (delta S-34 and delta O-18). Water samples from the lagoon, its tributary rivers, the groundwater up-welling line, groundwater, sewer pipes, and open sea water have been collected at quarterly intervals in the years 2009-2010. The results indicate that the NO3- load in the lagoon was not only derived from agricultural activities but also from other sources such as urban waste water, in situ nitrification and atmospheric deposition. The delta S-34 signature in the lagoon clearly denotes the largely prevailing origin of aqueous SO42- from seawater, and practically points to the absence of any appreciable redox process involving S species in the lagoon. It also supports the existence of a connection between the lagoon and the nearby Tagliamento river. (c) 2013 Elsevier Ltd. All rights reserved.

Abstract: The ability of sulfate aerosols to reflect solar radiation and simultaneously act as cloud condensation nuclei renders them central players in the global climate system. The oxidation of S(IV) compounds and their transport as stable S(VI) in the Earth's system are intricately linked to planetary scale processes, and precise characterization of the overall process requires a detailed understanding of the linkage between climate dynamics and the chemistry leading to the product sulfate. This paper reports a high-resolution, 22-y (1980-2002) record of the oxygen-triple isotopic composition of sulfate (SO4) aerosols retrieved from a snow pit at the South Pole. Observed variation in the O-isotopic anomaly of SO4 aerosol is linked to the ozone variation in the tropical upper troposphere/lower stratosphere via the Ozone El-Ni o Southern Oscillations (ENSO) Index (OEI). Higher Delta O-17 values (3.3%, 4.5%, and 4.2%) were observed during the three largest ENSO events of the past 2 decades. Volcanic events inject significant quantities of SO4 aerosol into the stratosphere, which are known to affect ENSO strength by modulating stratospheric ozone levels (OEI = 6 and Delta O-17 = 3.3%, OEI = 11 and Delta O-17 = 4.5%) and normal oxidative pathways. Our high-resolution data indicated that Delta O-17 of sulfate aerosols can record extreme phases of naturally occurring climate cycles, such as ENSOs, which couple variations in the ozone levels in the atmosphere and the hydrosphere via temperature driven changes in relative humidity levels. A longer term, higher resolution oxygen-triple isotope analysis of sulfate aerosols from ice cores, encompassing more ENSO periods, is required to reconstruct paleo-ENSO events and paleotropical ozone variations.

Abstract: The ozone molecule possesses a unique and distinctive O-17 excess (Delta O-17), which can be transferred to some of the atmospheric molecules via oxidation. This isotopic signal can be used to trace oxidation reactions in the atmosphere. However, such an approach depends on a robust and quantitative understanding of the oxygen transfer mechanism, which is currently lacking for the gas-phase NO2 + O-3 reaction, an important step in the nocturnal production of atmospheric nitrate. In the present study, the transfer of Delta O-17 from ozone to nitrate radical (NO3) during the gas-phase NO2 + O-3 -> NO3 + O-2 reaction was investigated in a series of laboratory experiments. The isotopic composition (delta O-17, delta O-18) of the bulk ozone and the oxygen gas produced in the reaction was determined via isotope ratio mass spectrometry. The Delta O-17 transfer function for the NO2 + O-3 reaction was determined to be: Delta O-17(O-3*) = (1.23 +/- 0.19) x Delta O-17(O-3)(bulk) + (9.02 +/- 0.99). The intramolecular oxygen isotope distribution of ozone was evaluated and results suggest that the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The results obtained in this study will be useful in the interpretation of high Delta O-17 values measured for atmospheric nitrate, thus leading to a better understanding of the natural cycling of atmospheric reactive nitrogen. (C) 2012 American Institute of Physics. [doi:10.1063/1.3666852]

Abstract: Historic records and research have suggested that the 1783-1784 eruption of the Laki fissure volcano in Iceland impacted Northern Hemisphere climate significantly, probably as a result of the direct injection of volcanic materials into the stratosphere where the volcanic aerosols would linger for years to cause surface cooling across the Northern Hemisphere. However, recent modeling work indicates the Laki climatic impact was limited to the Northern Hemisphere and only in the second half of 1783. We measured sulfur-33 isotope excess (Delta(33)S) in volcanic sulfate of historical eruptions including Laki found in Summit, Greenland ice cores. No Delta(33)S excess is found in sulfate of apparently tropospheric eruptions, while sulfate of stratospheric eruptions is characterized by significant Delta(33)S excess and a positive-to-negative change in Delta(33)S during its gradual removal from the atmosphere. Because the same characteristics have been previously found in volcanic sulfate in Antarctica snow, the results from Greenland indicate similar global processes of stratospheric chemical conversion of SO(2) to sulfate. The isotopic composition of Laki sulfate is essentially normal and shows no characteristics of sulfate produced by stratospheric photochemical reactions. This clearly indicates that the Laki plume did not reach altitudes of the stratospheric ozone layer. Further, the short aerosol residence time (<6 months) suggests that the bulk of the Laki plume and subsequent aerosols were probably confined to the middle and upper troposphere. These conclusions support the hypothesis of D'Arrigo and colleagues that the unusually cold winter of 1783-1784 was not caused by Laki. Citation: Lanciki, A., J. Cole-Dai, M. H. Thiemens, and J. Savarino (2012), Sulfur isotope evidence of little or no stratospheric impact by the 1783 Laki volcanic eruption, Geophys. Res. Lett., 39, L01806, doi:10.1029/2011GL050075.

Abstract: RATIONALE Triple oxygen isotopes of sulfate and nitrate are useful metrics for the chemistry of their formation. Existing measurement methods, however, do not account for oxygen atom exchange with quartz during the thermal decomposition of sulfate. We present evidence for oxygen atom exchange, a simple modification to prevent exchange, and a correction for previous measurements. METHODS Silver sulfates and silver nitrates with excess 17O were thermally decomposed in quartz and gold (for sulfate) and quartz and silver (for nitrate) sample containers to O2 and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Helium carries O2 through purification for isotope-ratio analysis of the three isotopes of oxygen in a Finnigan MAT253 isotope ratio mass spectrometer. RESULTS The Delta 17O results show clear oxygen atom exchange from non-zero 17O-excess reference materials to zero 17O-excess quartz cup sample containers. Quartz sample containers lower the Delta 17O values of designer sulfate reference materials and USGS35 nitrate by 15% relative to gold or silver sample containers for quantities of 210 μmol O2. CONCLUSIONS Previous Delta 17O measurements of sulfate that rely on pyrolysis in a quartz cup have been affected by oxygen exchange. These previous results can be corrected using a simple linear equation (Delta 17Ogold=Delta 17Oquartz * 1.14 + 0.06). Future pyrolysis of silver sulfate should be conducted in gold capsules or corrected to data obtained from gold capsules to avoid obtaining oxygen isotope exchange-affected data. Copyright (c) 2012 John Wiley & Sons, Ltd.

Abstract: Measurements of e-folding depth, nadir reflectivity and stratigraphy of the snowpack around Concordia station (Dome C, 75.10 degrees S, 123.31 degrees E) were undertaken to determine wavelength dependent coefficients (350 nm to 550 nm) for light scattering and absorption and to calculate potential fluxes (depth-integrated production rates) of nitrogen dioxide (NO(2)) from the snowpack due to nitrate photolysis within the snowpack. The stratigraphy of the top 80 cm of Dome C snowpack generally consists of three main layers:- a surface of soft windpack (not ubiquitous), a hard windpack, and a hoar-like layer beneath the windpack(s). The e-folding depths are similar to 10 cm for the two windpack layers and similar to 20 cm for the hoar-like layer for solar radiation at a wavelength of 400 nm; about a factor 2-4 larger than previous model estimates for South Pole. The absorption cross-section due to impurities in each snowpack layer are consistent with a combination of absorption due to black carbon and HULIS (HU-mic LIke Substances), with amounts of 1-2 ng g(-1) of black carbon for the surface snow layers. Depth-integrated photochemical production rates of NO(2) in the Dome C snowpack were calculated as 5.3 x 10(12) molecules m(-2) s(-1), 2.3 x 10(12) molecules m(-2) s(-1) and 8 x 10(11) molecules m(-2) s(-1) for clear skies and solar zenith angles of 60 degrees, 70 degrees and 80 degrees respectively using the TUV-snow radiative-transfer model. Depending upon the snowpack stratigraphy, a minimum of 85% of the NO(2) may originate from the top 20 cm of the Dome C snowpack. It is found that on a multi-annual time-scale photolysis can remove up to 80% of nitrate from surface snow, confirming independent isotopic evidence that photolysis is an important driver of nitrate loss occurring in the EAIS (East Antarctic Ice Sheet) snowpack. However, the model cannot completely account for the total observed nitrate loss of 90-95% or the shape of the observed nitrate concentration depth profile. A more complete model will need to include also physical processes such as evaporation, re-deposition or diffusion between the quasi-liquid layer on snow grains and firn air to account for the discrepancies.

Abstract: The isotope anomaly (Delta O-17) of secondary atmospheric species such as nitrate (NO3-) or hydrogen peroxide (H2O2) has potential to provide useful constrains on their formation pathways. Indeed, the Delta O-17 of their precursors (NOx, HOx etc.) differs and depends on their interactions with ozone, which is the main source of non-zero Delta O-17 in the atmosphere. Interpreting variations of Delta O-17 in secondary species requires an in-depth understanding of the Delta O-17 of their precursors taking into account non-linear chemical regimes operating under various environmental settings. This article reviews and illustrates a series of basic concepts relevant to the propagation of the Delta O-17 of ozone to other reactive or secondary atmospheric species within a photochemical box model. We present results from numerical simulations carried out using the atmospheric chemistry box model CAABA/MECCA to explicitly compute the diurnal variations of the isotope anomaly of short-lived species such as NOx and HOx. Using a simplified but realistic tropospheric gas-phase chemistry mechanism, Delta O-17 was propagated from ozone to other species (NO, NO2, OH, HO2, RO2, NO3, N2O5, HONO, HNO3, HNO4, H2O2) according to the mass-balance equations, through the implementation of various sets of hypotheses pertaining to the transfer of Delta O-17 during chemical reactions. The model results confirm that diurnal variations in Delta O-17 of NOx predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the Delta O-17 of NOx is “frozen” at night as soon as the photolytical lifetime of NOx drops below ca. 10 min. We introduce and quantify the diurnally-integrated isotopic signature (DIIS) of sources of atmospheric nitrate and H2O2, which is of particular relevance to larger-scale simulations of Delta O-17 where high computational costs cannot be afforded.

Abstract: During the electrolysis of water in an acidified medium, ozone is produced, in association with oxygen, at the anode. This ozone is found to be depleted in heavy isotopes (O-18 and O-17), with respect to the source water, following a strict mass-dependent rule. Our experiments also suggest that the isotopes are distributed at the apex and base positions of the bent ozone molecule in a random fashion, without obeying the zero-point energy constraint. Endowed with these characteristics, the electrolytic ozone provides a source of reference that has a known internal heavy isotope distribution for spectroscopic studies. In addition, this ozone, when subjected to photolytic decomposition, can be used as a source of atomic oxygen with mass-dependent isotope ratios that can be varied by simply changing the water composition. Such an oxygen source is important for studying isotope effects in gas-phase recombination/exchange reactions such as COO + O* -> [COOO*] -> COO* + O.

Abstract: The nitrogen (delta N-15) and triple oxygen (delta O-17 and delta O-18) isotopic composition of nitrate (NO3-) was measured year-round in the atmosphere and snow pits at Dome C, Antarctica (DC, 75.1 degrees S, 123.3 degrees E), and in surface snow on a transect between DC and the coast. Comparison to the isotopic signal in atmospheric NO3- shows that snow NO3- is significantly enriched in delta N-15 by > 200 parts per thousand and depleted in delta O-18 by < 40 parts per thousand. Post-depositional fractionation in delta O-17(NO3-) is small, potentially allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find fractionation constants epsilon of -60 +/- 15 parts per thousand, 8 +/- 2 parts per thousand and 1 +/- 1 parts per thousand, for delta N-15, delta O-18 and delta O-17, respectively. A photolysis model yields an upper limit for the photolytic fractionation constant (15)epsilon of delta N-15, consistent with lab and field measurements, and demonstrates a high sensitivity of (15)epsilon to the incident actinic flux spectrum. The photolytic (15)epsilon is process-specific and therefore applies to any snow covered location. Previously published (15)epsilon values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al., 2005; Jacobi et al., 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, N-15 enrichment of the NO3- fraction in the snow implies N-15 depletion of emissions. Indeed, delta N-15 in atmospheric NO3- shows a strong decrease from background levels (4 +/- 7 parts per thousand) to -35 parts per thousand in spring followed by recovery during summer, consistent with significant snowpack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis is an important process driving fractionation and associated NO3- loss from snow. The delta O-17 signature confirms previous coastal measurements that the peak of atmospheric NO3- in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO3- from the snowpack photic zone to the atmosphere and a snow surface skin layer, thereby concentrating NO3- at the surface. Little NO3- appears to be exported off the EAIS plateau, still snow emissions from as far as 600 km inland can contribute to the coastal NO3- budget.

Abstract: The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: N-15/N-14, O-17/O-16 and O-18/O-16) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65 degrees S (Weddell Sea) to 79 degrees N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the delta N-15 of nitrate mostly deriving from anthropogenically emitted NOx is found to be significantly different (from 0 to 6%) from nitrate sampled in locations influenced by natural NOx sources (-4 +/- 2)%. The effects on delta N-15(NO3-) of different NOx sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Delta O-17 = delta O-17 – 0.52 x delta O-18) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NOx sinks. Different Delta O-17 between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low delta N-15 values (down to -40%) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NOx from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.

Abstract: The reconstruction of past volcanism from glaciological archives is based on the measurement of sulfate concentrations in ice. This method does not allow a proper evaluation of the climatic impact of an eruption owing to the uncertainty in classifying an event between stratospheric or tropospheric. This work develops a new method, using anomalous sulfur isotope composition of volcanic sulfate in order to identify stratospheric eruptions over the last millennium. The advantages and limits of this new method are established with the examination of the 10 largest volcanic signals in ice cores from Dome C and South Pole, Antarctica. Of the 10, seven are identified as stratospheric eruptions. Among them, three have been known to be stratospheric (Tambora, Kuwae, the 1259 Unknown Event) and they exhibit anomalous sulfur isotope compositions. Three unknown events ( circa 1277, 1230, 1170 A. D.) and the Serua eruption have been identified as stratospheric eruptions, which suggests for the first time that they could have had significant climatic impact. However, the Kuwae and the 1259 Unknown Event stratospheric eruptions exhibit different anomalous sulfur isotope compositions between South Pole and Dome C samples. Differences in sulfate deposition and preservation patterns between the two sites can help explain these discrepancies. This study shows that the presence of an anomalous sulfur isotope composition of volcanic sulfate in ice core indicates a stratospheric eruption, but the absence of such composition does not necessarily lead to the conclusion of a tropospheric process because of differences in the sulfate deposition on the ice sheet.

Abstract: In springtime, the polar marine boundary layer exhibits drastic ozone depletion events (ODEs), associated with elevated bromine oxide (BrO) mixing ratios. The current interpretation of this peculiar chemistry requires the existence of acid and bromide-enriched surfaces to heterogeneously promote and sustain ODEs. Sander et al. (2006) have proposed that calcium carbonate (CaCO3) precipitation in any seawater-derived medium could potentially decrease its alkalinity, making it easier for atmospheric acids such as HNO3 and H2SO4 to acidify it. We performed simulations using the state-of-the-art FREZCHEM model, capable of handling the thermodynamics of concentrated electrolyte solutions, to try to reproduce their results, and found that when ikaite (CaCO3 center dot 6H(2)O) rather than calcite (CaCO3) precipitates, there is no such effect on alkalinity. Given that ikaite has recently been identified in Antarctic brines (Dieckmann et al., 2008), our results show that great caution should be exercised when using the results of Sander et al. (2006), and reveal the urgent need of laboratory investigations on the actual link(s) between bromine activation and the pH of the surfaces on which it is supposed to take place at subzero temperature. In addition, the evolution of the Cl/Br ratio in the brine during freezing was computed using FREZCHEM, taking into account Br substitutions in Cl-containing salts.

Abstract: Atmospheric nitrate shows a large oxygen isotope anomaly (Delta O-17), characterized by an excess enrichment of O-17 over O-18, similar to the ozone molecule. Modeling and observations assign this specific isotopic composition mainly to the photochemical steady state that exists in the atmosphere between ozone and nitrate precursors, namely, the nitrogen oxides (NOx=NO+NO2). However, this transfer is poorly quantified and is built on unverified assumptions about which oxygen atoms of ozone are transferred to NOx, greatly weakening any interpretation of the nitrate oxygen isotopic composition in terms of chemical reaction pathways and the oxidation state of the atmosphere. With the aim to improve our understanding and quantify how nitrate inherits this unusual isotopic composition, we have carried out a triple isotope study of the reaction NO+O-3. Using ozone intramolecular isotope distributions available in the literature, we have found that the central atom of the ozone is abstracted by NO with a probability of (8 +/- 5)%(+/- 2 sigma) at room temperature. This result is at least qualitatively supported by dynamical reaction experiments, the non-Arrhenius behavior of the kinetic rate of this reaction, and the kinetic isotope fractionation factor. Finally, we have established the transfer function of the isotope anomaly of O-3 to NO2, which is described by the linear relationship Delta O-17(NO2)=Ax Delta O-17(O-3)+B, with A=1.18 +/- 0.07(+/- 1 sigma) and B=(6.6 +/- 1.5)%(+/- 1 sigma). Such a relationship can be easily incorporated into models dealing with the propagation of the ozone isotope anomaly among oxygen-bearing species in the atmosphere and should help to better interpret the oxygen isotope anomaly of atmospheric nitrate in terms of its formation reaction pathways. (c) 2008 American Institute of Physics.

Abstract: It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3-4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NOx flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.

Abstract: The triple oxygen isotopic composition of atmospheric inorganic nitrate was measured in samples collected in the Arctic in springtime at Alert, Nunavut and Barrow, Alaska. The isotope anomaly of nitrate (Delta O-17 = delta O-17 – 0.52 delta O-18) was used to probe the influence of ozone (O-3), bromine oxide (BrO), and peroxy radicals (RO2) in the oxidation of NO to NO2, and to identify the dominant pathway that leads to the production of atmospheric nitrate. Isotopic measurements confirm that the hydrolysis of bromine nitrate (BrONO2) is a major source of nitrate in the context of ozone depletion events (ODEs), when brominated compounds primarily originating from sea salt catalytically destroy boundary layer ozone. They also show a case when BrO is the main oxidant of NO into NO2.

Abstract: 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.

Abstract: The oxygen isotopic composition of atmospheric nitrate was measured in samples collected in the High Canadian Arctic at Alert (Nunavut, 82.5 degrees N). Focusing on the polar sunrise period in early spring, when “ozone depletion events” are known to occur every year, we show a significant correlation between the isotope anomaly of nitrate (Delta O-17) and the ozone mixing ratio at the surface. This allows establishing a relationship between the magnitude of local atmospheric nitrogen oxides and ozone cycling, and the isotopic composition of nitrate. This isotopic fingerprint of the ozone activity appears promising in the perspective of using the isotopic composition of nitrate embedded in polar ice cores as a paleo-record of the ozone atmospheric mixing ratios. This may yield an indicator for the oxidative power of past atmospheres.

Abstract: [1] Both sulfur and oxygen isotopes of sulfate preserved in ice cores from Greenland and Antarctica have provided information on the relative sources of sulfate in the ice and their chemical transformation pathways in the atmosphere over various time periods. The mass-independent fractionation in the oxygen isotopes of sulfate from the Vostok ice core from east Antarctica suggests that gas-phase oxidation by the hydroxyl radical (OH) was relatively greater than aqueous-phase oxidation by O-3 and H2O2 during the last glacial period than during the Eemian and preindustrial Holocene. The complete sulfur isotopic composition (delta(33) S, delta(34)S, delta(36)S) from the same Vostok ice core samples along with delta(34)S measurements from the Dome C, east Antarctic ice core from this study lend support to these conclusions and reveal significant isotopic fractionation of delta(34)S during chemical transformation and transport to east Antarctica. These findings reveal that conservation of sulfur isotopic signatures upon transport cannot be assumed for the East Antarctic plateau over the time periods considered.

Abstract: [1] Sulfuric acid aerosols produced in the stratosphere following massive volcanic eruptions possess a mass-independent sulfur isotopic signature, acquired when volcanic SO2 experiences UV photooxidation. The volcanic data are consistent with laboratory SO2 photooxidation experiments using UV light at 248 nm ( maximum absorption of ozone), whereas sulfur isotopic anomalies previously observed in Archean samples are consistent with photodissociation at 190 – 220 nm. A mechanism of SO2 photooxidation, occurring in the early stage of a stratospheric volcanic plume, in the range of 220 – 320 nm ( weak band absorption of SO2), is also proposed. Since mass- independent sulfur isotope anomalies in stratospheric volcanic sulfate appear to depend on the exposure of SO2 to UV radiation, their measurements might therefore offer the possibility to determine the degree of UV penetration in the ozone- absorption window for the present and past atmospheres. They can also be used to determine the stratospheric or tropospheric nature of volcanic eruptions preserved in glaciological records, offering the possibility to reassess the climatic impact of past volcanic eruptions.

Abstract: [1] Ice cores have provided a wealth of information about past atmospheric composition and climate variability. However, relatively little is known about how the chemistry of the atmosphere has responded to natural climate change and anthropogenic influences. The oxygen isotopes (delta(17)O and delta(18)O) of sulfate serve as a recorder of the relative amounts of gas and aqueous-phase oxidation pathways in the atmosphere. This quality, along with its stability, renders sulfate an ideal proxy to investigate changes in oxidation pathways of S(IV) species in present and ancient atmospheres. The oxygen isotopic composition of sulfate in eight samples from the Vostok, Antarctica ice core, covering one full climate cycle, is presented. Assuming tropospheric-derived sulfate only, isotope data reveal that the ratio of gas-phase over aqueous-phase oxidation of S(IV) species was greater during the last glacial than the surrounding interglacial periods.

Abstract: A thermal decomposition method was developed and tested for the simultaneous determination of delta(18)O and delta(17)O in nitrate. The thermal decomposition of AgNO3 allows for the rapid and accurate determination of O-18/ O-16 and O-17/O-16 isotopic ratios with a precision of +/-1.5parts per thousand for delta(18)O and +/-0.11parts per thousand for Delta(17)O (Delta(17)O = delta(17)O – 0.52 x delta(18)O). The international nitrate isotope reference material IAEA-NO3 yielded a delta(18)O value of +23.6parts per thousand and Delta(17)O of -0.2parts per thousand, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO3 from the Atacama Desert Chile, was found to have Delta(17)O = 21.56 +/- 0.11parts per thousand and delta(18)O = 54.9 +/- 1.5parts per thousand, demonstrating a substantial mass-independent isotopic composition consistent with the proposed atmospheric origin of the desert nitrate. It is suggested that this sample (designated USGS-35) can be used to generate other gases (CO2, CO, N2O, O-2) with the same Delta(17)O to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.

Abstract: A high-resolution profile covering the last two centuries and a discontinuous study spanning the complete last glacial-interglacial cycle of methanesulfonate (MSA) (CH,SO,) and sulfate were obtained along Summit (central Greenland) ice cores. MSA concentrations were close to 4 +/- 1.4-ng g(-1) from 1770 to 1870 A.D. and 3 ng g(-1) in 1900, and exhibited a well-marked decreasing trend from 1945 to the present. These changes of Summit snow MSA concentrations between 1770 and 1945 are discussed in terms of possible modulation of dimethylsulfide (DMS) marine emissions influencing the Greenland Ice Sheet by past climatic fluctuations in these regions. The decrease of MSA levels in Summit snow layers deposited since 1945 suggests either a decline in marine biota at high northern latitudes or a changing yield of MSA from DMS oxidation driven by modification of the oxidative capacity of the atmosphere in response to increasing anthropogenic NO, and hydrocarbon emissions. While interglacial ice concentrations of MSA and sulfate are close to 2.9 +/- 1.9 ng g(-1) and 27 +/- 10 ng g(-1), respectively, reduced MSA (1.2 +/- 0.7 ng g(-1)) and enhanced sulfate (55 +/- 19 ng g(-1)) levels characterized the early Holocene stage (9000 to 11,000 years B.P.). MSA concentrations in glacial ice remain similar to the ones observed during interglacial stages. In contrast, sulfate levels are strongly enhanced (243 +/- 84 ng g(-1)) during the last glacial maximum (14,400 to 15,700 B.P.) compared with the interglacial ones. These variations of sulfur-containing species in response to past climatic conditions are similar to those found in other Greenland cores. In contrast, they are different from those revealed in the Antarctic Vostok ice core, where colder climates were associated with an increase by a factor of 5 and 2 in MSA and sulfate concentrations, respectively. These glacial-interglacial changes are discussed in terms of present and past contributions of marine DMS emissions versus other sulfate sources such as volcanic emissions and continental dust to the Greenland precipitation.

Abstract: Short-term (seasonal) changes in Pb, Cd, Cu and Zn concentrations in snow from central Greenland have been investigated by analysing a detailed sequence of 19 samples covering the years 1989-1990. Pronounced variations are observed for all four metals, with low concentrations in fall-winter and high concentrations during spring-summer. The anthropogenic contribution is found to always be predominant, but the natural contributions can, however, be significant, especially in spring. Although there is a similar behaviour between the changes for the four metals, some differences are observed (especially in summer) and are tentatively interpreted in term of sources and source regions.