We studied the temporal variations of CO 2 , O 2 , and dimethylsulfide (DMS) concentrations within three environments (sea-ice brine, platelet ice-like layer, and underlying water) in the coastal area of Adélie Land, Antarctica, during spring 1999 before ice breakup. Temporal changes were different among the three environments, while similar temporal trends were observed within each environment at all stations. The underlying water was always undersaturated in O 2 (around 85%) and oversaturated in CO 2 at the deepest stations. O 2 concentrations increased in sea-ice brine as it melted, reaching oversaturation up to 160% due to the primary production by the sea-ice algae community (chlorophyll a in the bottom ice reached concentrations up to 160 mg L 21 of bulk ice). In parallel, DMS concentrations increased up to 60 nmol L 21 within sea-ice brine and the platelet ice-like layer. High biological activity consumed CO 2 and promoted the decrease of partial pressure of CO 2 (pCO 2 ). In addition, melting of pure ice crystals and calcium carbonate (CaCO 3 ) dissolution promoted the shift from a state of CO 2 oversaturation to a state of marked CO 2 undersaturation (pCO 2 , 30 dPa). On the whole, our results suggest that late spring land fast sea ice can potentially act as a sink of CO 2 and a source of DMS for the neighbouring environments, i.e., the underlying water or/and the atmosphere.Sea ice covers about 7% of Earth's surface at its maximum seasonal extent, representing one of the largest biomes on the planet. For decades, sea ice was assumed to be an impermeable and inert barrier for air-sea exchanges of CO 2 , and global climate models did not include CO 2 exchanges between this compartment and the atmosphere. However, there is a growing body of evidence that sea ice exchanges CO 2 with the atmosphere. While estimating permeation constants of sulfur hexafluoride (SF 6 ) and CO 2 within sea ice, Gosink et al. (1976) stressed that sea ice is a permeable medium for gases. These authors suggested that gas migration through sea ice could be an important factor in winter ocean-atmosphere exchange at sea-ice surface temperature above 210uC. More recently, uptake of atmospheric CO 2 over sea-ice cover has been reported (Semiletov et al. 2004;Delille 2006;Zemmelink et al. 2006) supporting the need to further investigate pCO 2 dynamics in the sea-ice realm and related CO 2 fluxes.Very few studies have been carried out on the dynamics of the carbonate system within natural sea ice. They have generally been aimed at investigating CaCO 3 precipitation or dissolution (Gleitz et al. 1995), or they have focused on measurements of dissolved inorganic carbon (DIC) and total alkalinity (TA) (Anderson and Jones, 1985;Rysgaard et al. 2007) rather than on pCO 2 . As pointed out by 1 Corresponding author (Bruno.Delille@ulg.ac.be).

[1] Year-round composition of bulk and size-segregated aerosol was examined at a coastal Antarctic site (Dumont d'Urville). Sea-salt particles display a summer depletion of chloride relative to sodium, which reaches $10%. The mass chloride loss is maximum on 1-to 3-mm-diameter particles, nitrate being often the anion causing the chloride loss. The summer SO 4 2À /Na + ratio exceeds the seawater value on submicron particles due to biogenic sulfate and on coarse particles due to ornithogenic (guano-enriched soils) sulfate and to heterogeneous uptake of SO 2 (or H 2 SO 4 ). HCl levels range from 47 ± 28 ng m À3 in the winter to 130 ± 110 ng m À3 in the summer, being close to the mass chloride loss of sea-salt aerosols. In the winter, sea-salt particles exhibit Cl À /Na + and SO 4 2À /Na + mass ratios of 1.9 ± 0.1 and 0.13 ± 0.04, respectively. Resulting from precipitation of mirabilite during freezing of seawater, this sulfate-depletion-relative sodium takes place from May to October. From March to April, warmer temperatures and/or smaller sea ice extent offshore the site limit the phenomenon. A range of 14-50 ng m À3 of submicron sulfate is found, confirming the existence of nssSO 4 2À in the winter at a coastal Antarctic site, highest values being found in the winters of 1992-1994 due to the Pinatubo volcanic input. Apart from these three winters, nssSO 4 2À levels range between 15 and 30 ng m À3 , but its origin is still unclear (quasi-continuous SO 2 emissions from the Mount Erebus volcano or local wintertime dimethyl sulfide [DMS] oxidation, in addition to long-range transported byproduct of DMS oxidation).

The Sentinel Application Platform (SNAP) architecture facilitates Earth Observation data processing. In this work, we present results from a new Snow Processor for SNAP. We also describe physical principles behind the developed snow property retrieval technique based on the analysis of Ocean and Land Colour Instrument (OLCI) onboard Sentinel-3A/B measurements over clean and polluted snow fields. Using OLCI spectral reflectance measurements in the range 400–1020 nm, we derived important snow properties such as spectral and broadband albedo, snow specific surface area, snow extent and grain size on a spatial grid of 300 m. The algorithm also incorporated cloud screening and atmospheric correction procedures over snow surfaces. We present validation results using ground measurements from Antarctica, the Greenland ice sheet and the French Alps. We find the spectral albedo retrieved with accuracy of better than 3% on average, making our retrievals sufficient for a variety of applications. Broadband albedo is retrieved with the average accuracy of about 5% over snow. Therefore, the uncertainties of satellite retrievals are close to experimental errors of ground measurements. The retrieved surface grain size shows good agreement with ground observations. Snow specific surface area observations are also consistent with our OLCI retrievals. We present snow albedo and grain size mapping over the inland ice sheet of Greenland for areas including dry snow, melted/melting snow and impurity rich bare ice. The algorithm can be applied to OLCI Sentinel-3 measurements providing an opportunity for creation of long-term snow property records essential for climate monitoring and data assimilation studies—especially in the Arctic region, where we face rapid environmental changes including reduction of snow/ice extent and, therefore, planetary albedo.

To gain a better understanding of sulfate and methanesulfonate (MS−) signals recorded in central Antarctic ice cores in terms of past atmospheric changes, an atmospheric year‐round study of these aerosols was performed in 2006 at the Concordia station (75°S, 123°E) located on the high Antarctic plateau. In addition, a year‐round study of dimethyl sulfide (DMS), the gaseous precursor of sulfur aerosol, was conducted in 2007. The DMS mixing ratio remains below 1 pptv from October to January and exhibits a maximum of 10 pptv during the first half of winter (from April to July). Surprisingly, the well‐marked maximum of sulfur aerosol recorded in January at coastal Antarctic sites is observed at Concordia for sulfate but not for MS− which peaks before and after sulfate in November and March, respectively. This first study of DMS and of its by‐oxidation aerosol species conducted at inland Antarctica points out the complex coupling between transport and photochemistry of sulfur species over Antarctica. The findings highlight the complexity of the link between MS− ice core records extracted at high Antarctic plateau sites and DMS emissions from the Southern ocean.

Abstract. Concentrations of OH radicals and the sum of peroxy radicals, RO 2 , were measured in the boundary layer for the first time on the East Antarctic Plateau at the Concordia Station (Dome C, 75.10 • S, 123.31 • E) during the austral summer 2011/2012. The median concentrations of OH and RO 2 radicals were 3.1 × 10 6 molecule cm −3 and 9.9 × 10 7 molecule cm −3 , respectively. These values are comparable to those observed at the South Pole, confirming that the elevated oxidative capacity of the Antarctic atmospheric boundary layer found at the South Pole is not restricted to the South Pole but common over the high Antarctic plateau. At Concordia, the concentration of radicals showed distinct diurnal profiles with the median maximum of 5.2 × 10 6 molecule cm −3 at 11:00 and the median minimum of 1.1 × 10 6 molecule cm −3 at 01:00 for OH radicals and 1.7 × 10 8 molecule cm −3 and 2.5 × 10 7 molecule cm −3 for RO 2 radicals at 13:00 and 23:00, respectively (all times are local times). Concurrent measurements of O 3 , HONO, NO, NO 2 , HCHO and H 2 O 2 demonstrated that the major primary source of OH and RO 2 radicals at Dome C was the photolysis of HONO, HCHO and H 2 O 2 , with the photolysis of HONO contributing ∼ 75 % of total primary radical production. However, photochemical modelling with accounting for all these radical sources overestimates the concentrations of OH and RO 2 radicals by a factor of 2 compared to field observations. Neglecting the net OH production from HONO in the photochemical modelling results in an underestimation of the concentrations of OH and RO 2 radicals by a factor of 2. To explain the observations of radicals in this case an additional source of OH equivalent to about (25-35) % of measured photolysis of HONO is required. Even with a factor of 5 reduction in the concentrations of HONO, the photolysis of HONO represents the major primary radical source at Dome C. To account for a possibility of an overestimation of NO 2 observed at Dome C the calculations were also performed with NO 2 concentrations estimated by assuming steady-state NO 2 / NO ratios. In this case the net radical production from the photolysis of HONO should be reduced by a factor of 5 or completely removed based on the photochemical budget of OH or 0-D modelling, respectively. Another major factor leading to the large concentration of OH radicals measured at Dome C was large concentrations of NO molecules and fast recycling of peroxy radicals to OH radicals.

[1] The origin of sea-salt aerosol that reaches the high Antarctic plateau and is trapped in snow and ice cores remains still unclear. In particular, the respective role of emissions from the open ocean versus those from the sea-ice surface is not yet quantified. To progress on this question, the composition of bulk and size-segregated aerosol was studied in 2006 at the Concordia station (75°S, 123°E) located on the high Antarctic plateau. A depletion of sulfate relative to sodium with respect to the seawater composition is observed on sea-salt aerosol reaching Concordia from April to September. That suggests that in winter, when the sea-salt atmospheric load reaches a maximum, emissions from the sea-ice surface significantly contribute to the sea-salt budget of inland Antarctica.

Abstract. Triple oxygen isotopic compositions (Δ17O  = δ17O − 0.52  ×  δ18O) of atmospheric sulfate (SO42−) and nitrate (NO3−) in the atmosphere reflect the relative contribution of oxidation pathways involved in their formation processes, which potentially provides information to reveal missing reactions in atmospheric chemistry models. However, there remain many theoretical assumptions for the controlling factors of Δ17O(SO42−) and Δ17O(NO3−) values in those model estimations. To test one of those assumption that Δ17O values of ozone (O3) have a flat value and do not influence the seasonality of Δ17O(SO42−) and Δ17O(NO3−) values, we performed the first simultaneous measurement of Δ17O values of atmospheric sulfate, nitrate, and ozone collected at Dumont d'Urville (DDU) Station (66°40′ S, 140°01′ E) throughout 2011. Δ17O values of sulfate and nitrate exhibited seasonal variation characterized by minima in the austral summer and maxima in winter, within the ranges of 0.9–3.4 and 23.0–41.9 ‰, respectively. In contrast, Δ17O values of ozone showed no significant seasonal variation, with values of 26 ± 1 ‰ throughout the year. These contrasting seasonal trends suggest that seasonality in Δ17O(SO42−) and Δ17O(NO3−) values is not the result of changes in Δ17O(O3), but of the changes in oxidation chemistry. The trends with summer minima and winter maxima for Δ17O(SO42−) and Δ17O(NO3−) values are caused by sunlight-driven changes in the relative contribution of O3 oxidation to the oxidation by HOx, ROx, and H2O2. In addition to that general trend, by comparing Δ17O(SO42−) and Δ17O(NO3−) values to ozone mixing ratios, we found that Δ17O(SO42−) values observed in spring (September to November) were lower than in fall (March to May), while there was no significant spring and fall difference in Δ17O(NO3−) values. The relatively lower sensitivity of Δ17O(SO42−) values to the ozone mixing ratio in spring compared to fall is possibly explained by (i) the increased contribution of SO2 oxidations by OH and H2O2 caused by NOx emission from snowpack and/or (ii) SO2 oxidation by hypohalous acids (HOX  =  HOCl + HOBr) in the aqueous phase.

Abstract. Multiple year-round records of bulk and sizesegregated composition of aerosol were obtained at the inland site of Concordia located at Dome C in East Antarctica. In parallel, sampling of acidic gases on denuder tubes was carried out to quantify the concentrations of HCl and HNO 3 present in the gas phase. These time series are used to examine aerosol present over central Antarctica in terms of chloride depletion relative to sodium with respect to freshly emitted sea-salt aerosol as well as depletion of sulfate relative to sodium with respect to the composition of seawater. A depletion of chloride relative to sodium is observed over most of the year, reaching a maximum of ∼ 20 ng m −3 in spring when there are still large sea-salt amounts and acidic components start to recover. The role of acidic sulfur aerosol and nitric acid in replacing chloride from sea-salt particles is here discussed. HCl is found to be around twice more abundant than the amount of chloride lost by sea-salt aerosol, suggesting that either HCl is more efficiently transported to Concordia than sea-salt aerosol or re-emission from the snow pack over the Antarctic plateau represents an additional significant HCl source. The size-segregated composition of aerosol collected in winter (from 2006 to 2011) indicates a mean sulfate to sodium ratio of sea-salt aerosol present over central Antarctica of 0.16 ± 0.05, suggesting that, on average, the sea-ice and open-ocean emissions equally contribute to sea-salt aerosol load of the inland Antarctic atmosphere.The temporal variability of the sulfate depletion relative to sodium was examined at the light of air mass backward trajectories, showing an overall decreasing trend of the ratio (i.e., a stronger sulfate depletion relative to sodium) when air masses arriving at Dome C had traveled a longer time over sea ice than over open ocean. The findings are shown to be useful to discuss sea-salt ice records extracted at deep drilling sites located inland Antarctica.