Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 1: Fractionation of sea-salt particles

Abstract

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.

Year-round record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol

“…Examination of the size-segregated composition of aerosol present at Concordia indicates significant sulfate depletion relative to sodium with respect to the seawater composition from May to September (i.e., a k SO 4 /Na value of 0.16 ± 0.09 instead of 0.25 in seawater) (Legrand et al, 2017), resulting from the presence of sea-salt aerosol at the site emitted from both open ocean and sea ice. This value of 0.16 was used in Eq.…”

“…located near Dome C (DC; 1100 km away from the nearest coast of East Antarctica). Working conditions are detailed in Legrand et al (2017). Given the weekly sampling time, a large air volume was sampled (∼ 8000 m 3 ), permitting the blank values to remain well below 1 ng m −3 (0.17 ± 0.15 ng m −3 for sodium, 0.4 ± 0.3 ng m −3 for sulfate, and 0 for MSA).…”

“…Ozone measurements were also occasionally disturbed under very low wind speed conditions (< 2 m s −1 ). The effect of such sporadic contamination of the station activities on the sulfate levels was here examined in the light of weekly denuder tube sampling of acidic gases done at the site, as detailed by Legrand et al (2017) for HCl and HNO 3 . Indeed, the denuder tube sampling of acidic gases conducted at Concordia also documents SO 2 by measuring sulfate on the extracts.…”

“…2. In addition, the size-segregated aerosol composition was investigated by doing 105 samplings between March 2006 and January 2012 by using a small deposit area impactor, equipped with a 20 µm cutoff diameter inlet (Legrand et al, 2017). Applying a sampling interval of 2 weeks, eight runs per year were done in 2006 and 2007 and a more continuous sampling (25 runs) from 2009 to 2012.…”

“…The record of bulk aerosol is complemented by a study of the size-segregated aerosol composition conducted by running a 12-stage impactor over 3 years (2009)(2010)(2011). As discussed in the companion paper (Legrand et al, 2017), these impactor data are essential to evaluate the degree of sulfate depletion relative to sodium of sea-salt aerosol in winter and consequently to accurately calculate here the nssSO 4 level and the R MSA ratio. Here we present and discuss the temporal variability of the composition of sulfur-derived aerosol (MSA, non-sea-salt sulfate, and R MSA ) and its dependence to the aerosol size in relation to seasonal change of marine biogenic DMS emissions and the contribution of non-biogenic sources of sulfate.…”