Biogenic sulphur emissions and inferred non‐sea‐salt‐sulphate cloud condensation nuclei in and around Antarctica

Abstract

Accumulation mode aerosol properties and biogenic sulphur emissions over the South Atlantic and Antarctic Oceans are examined. Two contrasting air masses, polar and maritime, each possessing distinct aerosol properties, were encountered during the summer months. By examining aerosol volatile properties, polar air masses arriving from the Antarctic continent were shown to consist primarily Of H2SO4 in the accumulation mode size range, with inferred NH+4 to SO=4 molar ratios close to zero. By comparison, air masses of temperate maritime origin were significantly neutralized with molar ratios of ≈1. These results suggest a deficit of ammonia in polar air masses compared with that in maritime air masses. Dimethyl sulphide (DMS) exhibited no correlation with its putative aerosol oxidation products, although spatial coherence in atmospheric concentrations of DMS, methane sulphonic acid (MSA), and non‐sea‐salt (nss)‐sulphate mass was observed. Volatility analysis, used to infer nss‐sulphate cloud condensation nuclei (nss‐sCCN) active at a supersaturation of ≈0.2%, indicates that nss‐sCCN mass and number concentration were best correlated with MSA mass (r≈0.63). Aerosol volatility identified the presence of MSA in submicron non‐sea‐salt aerosol; however, its contribution to the aerosol mass was small relative to the contribution of sulphuric acid and ammonium bisulphate/sulphate aerosol. The marine sulphur cycle appears strongly coupled to the sea‐salt cycle with, typically, 80–90% of nss‐sulphate thought to be internally mixed with sea‐salt aerosol. During the austral Summer of 1992/1993, a period of strong biological productivity in the Weddell Sea and sub‐Antarctic Ocean, particularly during ice‐melt, the cruise‐average DMS flux of 61 μg m−2 d−1 corresponded to a very modest average nss‐sCCN concentration of 21 cm−3. Observed peak values of DMS flux and inferred nss‐CCN concentrations during the cruise were 477 μg m−2 d−1 and 64 cm−3, respectively. Events of new particle formation were identified in the Weddell Sea and occurred under conditions of high DMS flux and low aerosol surface area.