Abstract: Multiple year-round levels of acetate and formate in gas and aerosol phases were investigated at Dumont d'Urville (DDU, a coastal Antarctic site) by using mist chamber and aerosol filter sampling. Formate and acetate aerosol levels range from <0.5 ppt in winter to 3 ppt in summer. With corresponding gas phase levels of more than a hundred of pptv, formic and acetic acids are mainly (99%) present in the gas phase, representing the 2 major acidic gases before inorganic species (HCl, HNO3 and SO2) there. Mixing ratios of formic acid are minimal from May to August (70 pptv) and increase regularly toward November–February months when levels reach ?200 pptv. Mixing ratios of acetic acid exhibit a more well-marked seasonal cycle with values remaining close to 70 pptv from April to October and strongly increase during November–February months (mean value of 400 pptv). These seasonal changes suggest that the 2 carboxylic acids mainly originate from biogenic emissions of the Antarctic ocean whose variations follow the annual cycle of sea ice extent and solar radiation via photochemical production of alkenes from dissolved organic carbon released by phytoplankton. In summer, acetic acid levels show daily variations with maxima at noon and minima at night whereas formic acid levels peaks later in the afternoon. These dial variations in summer suggest that carboxylic acids are rapidly produced during the day and lost at night due to dry deposition on wet surface. It is suggested that the reactions of peroxy acetyl radical produced from propene with HO2 and CH3O2 in these poor NOx environments represent in summer the dominant chemical mechanisms producing acetic acid whereas ozone-alkene reactions remain of minor importance at that season. Neither ozone-alkene reactions nor aqueous phase HCHO oxidation can explain the summer levels of formic acid. In winter the long range transport of alkenes emitted at more temperate oceanic regions and reactions with ozone could account for the observed level of formic acid and possibly of acetic acid.

Abstract: 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-?m-diameter particles, nitrate being often the anion causing the chloride loss. The summer SO42?/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 SO2 (or H2SO4). HCl levels range from 47 ± 28 ng m?3 in the winter to 130 ± 110 ng m?3 in the summer, being close to the mass chloride loss of sea-salt aerosols. In the winter, sea-salt particles exhibit Cl?/Na+ and SO42?/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?3 of submicron sulfate is found, confirming the existence of nssSO42? 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, nssSO42? levels range between 15 and 30 ng m?3, but its origin is still unclear (quasi-continuous SO2 emissions from the Mount Erebus volcano or local wintertime dimethyl sulfide [DMS] oxidation, in addition to long-range transported by-product of DMS oxidation).

Abstract: Surface ozone is measured since 2004 at the coastal East Antarctic station of Dumont d'Urville (DDU) and since 2007 at the Concordia station located on the high East Antarctic plateau. Ozone levels at Concordia reach a maximum of 35 ppbv in July and a minimum of 21 ppbv in February. From November to January, sudden increases of the ozone level, up to 15–20 ppbv above average, often take place. They are attributed to local photochemical ozone production as previously seen at the South Pole. The detailed examination of the diurnal ozone record in summer at Concordia suggests a local photochemical ozone production of around 0.2 ppbv h-1 during the morning. The ozone record at DDU exhibits a maximum of 35 ppbv in July and a minimum of 18 ppbv in January. Mixing ratios at DDU are always higher than those at Neumayer (NM), another coastal Antarctic station. A noticeable difference in the ozone records at the two coastal sites lies in the larger ozone depletion events occurring from July to September at NM compared to DDU, likely due to stronger BrO episodes in relation with a larger sea ice coverage offshore that site. A second difference is the large day-to-day fluctuations which are observed from November to January at DDU but not at NM. That is attributed to a stronger impact at DDU than at NM of air masses coming from the Antarctic plateau. The consequences of such a high oxidizing property of the atmosphere over East Antarctica are discussed with regard to the dimethylsulfide (DMS) chemistry.

Abstract: Multiple year-round (2006–2015) records of the bulk and size-segregated composition of aerosol were obtained at the inland site of Concordia located in East Antarctica. The well-marked maximum of non-sea-salt sulfate (nssSO₄) in January (100 ± 28 ng m⁻³ versus 4.4 ± 2.3 ng m⁻³ in July) is consistent with observations made at the coast (280 ± 78 ng m⁻³ in January versus 16 ± 9 ng m⁻³ in July at Dumont d'Urville, for instance). In contrast, the well-marked maximum of MSA at the coast in January (60 ± 23 ng m⁻³ at Dumont d'Urville) is not observed at Concordia (5.2 ± 2.0 ng m⁻³ in January). Instead, the MSA level at Concordia peaks in October (5.6 ± 1.9 ng m⁻³) and March (14.9 ± 5.7 ng m⁻³). As a result, a surprisingly low MSA-to-nssSO₄ ratio (R_MSA) is observed at Concordia in mid-summer (0.05 ± 0.02 in January versus 0.25 ± 0.09 in March). We find that the low value of R_MSA in mid-summer at Concordia is mainly driven by a drop of MSA levels that takes place in submicron aerosol (0.3 µm diameter). The drop of MSA coincides with periods of high photochemical activity as indicated by high ozone levels, strongly suggesting the occurrence of an efficient chemical destruction of MSA over the Antarctic plateau in mid-summer. The relationship between MSA and nssSO₄ levels is examined separately for each season and indicates that concentration of non-biogenic sulfate over the Antarctic plateau does not exceed 1 ng m⁻³ in fall and winter and remains close to 5 ng m⁻³ in spring. This weak non-biogenic sulfate level is discussed in the light of radionuclides (²¹⁰Pb, ¹⁰Be, and ⁷Be) also measured on bulk aerosol samples collected at Concordia. The findings highlight the complexity in using MSA in deep ice cores extracted from inland Antarctica as a proxy of past dimethyl sulfide emissions from the Southern Ocean.