Understanding mercury oxidation and air–snow exchange on the East Antarctic Plateau: a modeling study

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 Hg0 (gaseous elemental mercury) measurements in thes year 2013. The model includes atmospheric Hg0 oxidation (by OH, O3, or bromine), surface snow HgII (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 Hg0 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 Hg0 (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 Hg0 is mainly driven by photoreduction of snow HgII in summer. Intermittent warming events and a hypothesized reduction of HgII 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 Hg0 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 Hg0 between these two polar inland stations.National Science Foundation (U.S.) (Grant ACP-1053648

Acquisition of isotopic composition for surface snow in East Antarctica and the links to climatic parameters

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

A new insight into the climatic impact of volcanic explosion: A lesson from the sulfur stable isotopes

International audienc

Caractérisations isotopiques des voies de formation du nitrate atmosphérique et de la photochimie du nitrate dans la neige

Le nitrate, produit de la fin de chaîne de réaction des oxydes d azotes de l atmosphère (NOx = NO +NO2), est l un des ions le plus abondant de la neige et de la glace polaire. Ses rapports isotopiques stables ( 18O, 15N et 17O) ont été abondamment utilisés pour contraindre ses sources et les chemins réactionnels. De plus, le nitrate archivé dans les carottes de glace profondes peut apporter de nouvelles contraintes sur les conditions climatiques passées. Cependant, le dépôt de nitrate dans les régions polaires à faible accumulation est réversible en raison des processus post-dépôts, compliquant l interprétation des enregistrements. Actuellement, il existe des enregistrements de nitrate issus de carottes de glace profonds couvrant de l information climatique sur plusieurs milliers d années dont leur interprétation dépend d une quantification précise ces phénomènes post-dépôts. Nous avons étudié expérimentalement le transfert d excès-17O de l ozone durant la réaction en phase gaz de NO2 + O3 NO3 + O2, qui est une réaction importante de la chimie nocturne de formation du nitrate. De cette étude nous avons déterminé la fonction de transfert du 17O donnée par : 17O(O3*) = (1.23 +- 0.19) . 17O(O3)bulk + (9.02 +- 0.99). Nous avons aussi évalué la distribution intramoléculaire des isotopes de l oxygène de l ozone et observé que l excès d enrichissement résidait de manière prépondérante sur les atomes terminaux de l ozone. Ces résultats auront une implication importante sur la compréhension de la formation du nitrate via les mécanismes d oxydation des précurseurs NOx. L impact de la photolyse sur les concentrations et les compositions isotopiques stables du nitrate est étudié dans ce travail de thèse sur la base d étude de laboratoire et de terrain. Une étude de laboratoire a été conduite en irradiant de la neige naturelle de Dôme C avec une lampe UV à xénon et en utilisant différents filtres UV (280 nm, 305 nm et 320 nm). Sur la base des mesures des rapports isotopiques de l oxygène et de l azote, la dépendance aux longueurs d onde des fractionnements isotopiques a été déterminée. En conséquence, en présence de lumière UV de haute énergie, le fractionnement isotopique est décalé vers des valeurs moins négatives et vice versa. Sur la base des fractionnements isotopiques obtenus en laboratoire, nous avons dérivé un décalage apparent de la valeur du zéro point d énergie (ZPE) qui apporte une meilleure contrainte sur la section efficace d absorption du 15NO3-. Ce décalage apparent est obtenu en minimisant les écarts entre les observations et les fractionnements isotopiques calculés basés sur un modèle de décalage ZPE, modèle qui inclut outre le décalage ZPE, le changement des largeurs, de l asymétrie et de l amplitude des sections efficaces d absorption lors de la substitution isotopique. Nous avons validé le nouveau ZPE apparent en conduisant une étude de terrain à Dôme C, Antarctique. Dans cette étude, un dispositif expérimental a été construit sur le site et l effet du rayonnement solaire UV sur la photolyse du nitrate de la neige investigué. Cette étude était basée sur la comparaison de deux puits remplis par de la neige soufflée homogénéisée dont l un des deux puits n était pas soumis aux rayonnements UV. Le fractionnement isotopique de 15N pour la neige exposée aux UV (-67.9 +- 12 ) est en bon accord avec le modèle de décalage ZPE estimé au cours de ce travail de thèse (-55.4 ). Ces valeurs sont aussi dans la gamme des fractionnements isotopiques apparents observée précédemment au Dôme C. Nous pensons que l inclusion des ces nouvelles connaissances dans un modèle prédisant l enrichissement du 15N dans les carottes de glace permettra une interprétation quantitative de l information préservée dans la glace.Nitrate, the end product of oxidation of atmospheric NOX (= NO + NO2), is one of the most abundant anions present in polar snow and ice. Its stable isotope ratios ( 18O, 15N and 17O) have been widely used to constrain its sources and oxidation pathways. In addition, the nitrate archived in deep ice cores may be an important metric to constrain past climatic conditions. However, deposition of nitrate in polar regions with low snow accumulation is reversible due to post-depositional processes, and interpretation of this record is complicated. Currently, there exist deep ice core records of nitrate encompassing climatic information of millennial time scales, and their interpretation relies on careful quantification of post-depositional effects. We have experimentally studied the 17O-excess transfer from ozone during the gas phase NO2 + O3 -> NO3 + O2 reaction, which is an important nighttime nitrate formation pathway. From this study, we have determined the 17O transfer function given by: 17O(O3*) = (1.23 +- 0.19) . 17O(O3)bulk + (9.02 +- 0.99). We have also evaluated the intramolecular oxygen isotope distribution of ozone and have observed the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The findings from this study will have an important implication for understanding nitrate formation pathways via different NOX oxidation mechanisms. The impact of photolysis on the amount and stable isotope enrichments of nitrate is investigated in this PhD study based on laboratory and field experiments. A laboratory study was conducted by irradiating a natural snow from Dome C with a Xe UV lamp and a selection of UV-filters (280 nm, 305 nm and 320 nm). Based on the oxygen and nitrogen isotope ratio measurements, wavelength dependent isotopic fractionations were determined. Accordingly, in the presence of high-energy UV light, isotopic fractionation is shifted towards less negative values and the reverse for lower energy UV photons. Based the isotopic fractionations obtained in the laboratory study, we derived an apparent ZPE-shift value, which better constrains the absorption cross-section of 15NO3-. This apparent shift is derived from the best fit between the experimental observations and calculated fractionations based on existing ZPE-shift model and it includes actual ZPE-shift and changes in width, asymmetry and amplitude in absorption cross-section during isotopic substitution. We have validated the newly derived apparent ZPE-shift by conducting a field study at Dome C, Antarctica. In this study, an experimental setup was built on-site and the effect of solar UV photolysis on snow nitrate was investigated. This study was based on a comparison of two snow pits filled with locally drifted snow and by allowing/blocking the solar UV. The 15N fractionation for the UV exposed samples (-67.9 +- 12 ) was in fairly good agreement with the ZPE-shift model estimate from this study (-55.4 ). These values are also within the range of apparent isotopic fractionation observed at Dome C in previous studies. Further calculations to better constrain the absorption cross-section of 15NO3- with the ZPE-shift are underway, and we propose that the newly derived apparent ZPE-shift value should be used in future studies. We believe that incorporating these new findings in models predicting the enrichments of 15N nitrate in ice cores will allow a quantitative interpretation of the information preserved in ice.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Chimie atmosphérique polaire : une nouvelle application des isotopes stables de l'oxygène

La mesure de la composition isotopique de l'oxygène du nitrate atmosphérique collecté dans le Haut Arctique Canadien à Alert (Nunavut, 82,5 °N) au cours d'épisodes de destruction de l'ozone des basses couches de l'atmosphère a permis de mettre en évidence un lien entre la teneur en ozone de surface et l'anomalie isotopique du nitrate (∆17O). Il est montré que cette anomalie est transmise par l'ozone aux précurseurs du nitrate atmosphérique. Ainsi, l'intensité de l'activité chimique locale de l'atmosphère, due à l'ozone, s'imprime dans la composition isotopique du nitrate. Ce mode d'enregistrement apparaît prometteur dans le cadre d'études approfondies sur les processus à l'oeuvre dans l'atmosphère polaire, mais également pour la reconstruction de la composition chimique de l'atmosphère et des climats passés à partir des enregistrements glaciologiques

Ozone and Stratospheric Chemistry

International audienceThe stratosphere is the atmospheric layer comprised between 8–18 km for its lower altitudes and 40–60 km for its upper altitudes, which correspond more or less to the stratospheric ozone layer thickness. In this part of the atmosphere, intense UV radiations from the sun fuel an active and energetic photochemical reactor leading to the formation of two important by-products: the ozone molecule (O3), an oxygen allotrope, and heat that mostly originates from absorption of UV radiation by ozone. While heat causes a strong vertical stability of the air mass in the stratosphere (stratified atmosphere), large concentrations of ozone form a UV-protective shield allowing the development of life on the Earth’s surface. The intense UV radiations combined with the ozone concentration generate a distinct stratospheric chemistry where nitrogen, oxygen, and halogen compounds are highly coupled in cycles that maintain the chemical stability of this UV-protective atmospheric laye

Analyse de la composition isotopique de l'ion nitrate dans la basse atmosphère polaire et marine

Les oxydes d azote atmosphériques (NOx=NO+NO2) sont des composés clefs en chimie de l environnement, jouant un rôle central pour la capacité oxydante de l atmosphère et le cycle de l azote. La composition isotopique du nitrate atmosphérique (NO?3 particulaire et HNO3 gazeux), constituant leur puits ultime, renseigne sur leur bilan chimique. Le rapport 15N/14N donne une indication de leurs sources, alors que l anomalie isotopique en oxygène (?17O=d17O-0.52.d18O) révèle la nature de leurs mécanismes d oxydation. Des études couplées de d15N et ?17O d échantillons de nitrate atmosphérique collectés dans l Arctique, en Antarctique et dans l atmosphère marine au dessus de l Océan Atlantique, où le bilan des NOx est souvent mal connu ont été effectuées. À ces fins, le défi que constitue la mesure simultanée des trois rapports isotopiques du nitrate (17O/16O, 18O/16O et 15N/14N) dans le même échantillon représentant moins d une micromole a été relevé. La solution adoptée tire avantage des propriétés d une bactérie dénitrifiante, utilisée pour convertir le nitrate en N2O, dont la composition isotopique totale a été mesurée en utilisant un système automatisé de chromatographie en phase gazeuse et spectrométrie de masse de rapport isotopique. Les principaux résultats obtenus via les isotopes de l oxygène permettent l identification claire de transitions saisonnières entre voies d oxydation des NOx, y compris le rôle majeur des composés halogénés réactifs au printemps polaire en régions côtières. Les isotopes de l azote ont quant à eux permis d apporter de nouvelles contraintes sur le cycle de l azote dans les régions polaires, grâce au fractionnement significatif induit par les phénomènes de remobilisation post-dépôt affectant le nitrate dans le manteau neigeux, et l émission de NOx qui en découleAtmospheric nitrogen oxides (NOx=NO+NO2) are central to the chemistry of the environment, as they play a pivotal role in the cycling of reactive nitrogen and the oxidative capacity of the atmosphere. The stable isotopes of atmospheric nitrate (in the form of particulate NO?3 or gaseous HNO3), their main ultimate sinks, provide insights in chemical budget of NOx : its nitrogen isotopes are almost conservative tracers of their sources, whereas NOx sinks are revealed by its triple oxygen isotopic composition. The long-awaited challenge of measuring all three stable isotope ratios of nitrate (17O/16O, 18O/16O and 15N/14N) in a single sample at sub-micromolar levels has been resolved. The newly developed method makes use of denitrifying bacteria to quantitatively convert nitrate to a stable species (N2O), whose isotope ratios are measured using an automated gas chromatography/isotope ratio mass spectrometry analytical system. Dual measurements of d15N and the isotope anomaly (?17O=d17O-0.52.d18O) of atmospheric nitrate samples collected in the Arctic, the Antarctic and in the marine boundary layer of the Atlantic Ocean, have been used to derive the chemical budget of NOx and atmospheric nitrate in these remote regions. Main results from oxygen isotope measurements pertain to the identification of seasonal and latitudinal shifts in NOx oxidative pathways in these environments (including the role of halogen oxides chemistry in polar regions during springtime), as a function of particle sizes. Nitrogen isotopes are found to provide strong constraints on the budget of reactive nitrogen in polar regions, due to the strong fractionation associated with snowpack photochemical loss of nitrate and its conversion to NOxPARIS-EST-Université (770839901) / SudocSudocFranceF