On the annual variability of Antarctic aerosol size distributions at Halley research station

Abstract

The Southern Ocean and Antarctic region currently best represent one of the few places left on our planet with conditions similar to the preindustrial age. Currently, climate models have low ability to simulate conditions forming the aerosol baseline; a major uncertainty comes from the lack of understanding of aerosol size distributions and their dynamics. Contrasting studies stress that primary sea-salt aerosol can contribute significantly to the aerosol population, challenging the concept of climate biogenic regulation by new particle formation (NPF) from dimethyl sulphide marine emissions. We present a statistical cluster analysis of the physical characteristics of particle size distributions (PSD) collected at Halley (Antarctica) for the year 2015 (89% data coverage, 6-209 nm size range, daily size resolution). By applying the Hartigan-Wong k-Means method we find 8 clusters describing the entire aerosol population. Three clusters show pristine average low particle number concentrations (< 121-179 cm-3) with three main modes (30 nm, 75-95 nm, 135-160 nm) and represent 57% of the annual PSD (up to 89-100% during winter, 34-65% during summer based upon monthly averages). Nucleation and Aitken mode PSD clusters dominate summer months (Sep-Jan, 59-90%), whereas a clear bimodal distribution (43 and 134 nm, respectively, min Hoppel mode 75 nm) is seen only during the Dec-Apr period (6-21%). Major findings of the current work include: (1) NPF and growth events originate from both the sea ice marginal zone and the Antarctic plateau, strongly suggesting multiple vertical origins, including marine boundary layer and free troposphere; (2) very low particle number concentrations are detected for a substantial part of the year (57%), including summer (34-65%), suggesting that the strong annual aerosol concentration cycle is driven by a short temporal interval of strong NPF events; (3) a unique pristine aerosol cluster is seen with a bimodal size distribution (75 nm and 160 nm, respectively), strongly correlating associated with high wind speed and possibly associated with blowing snow and sea spray sea salt, dominating the winter aerosol population (34-54%). A brief comparison with two other stations (Dome C Concordia and King Sejong Station) during the year 2015 (240 days overlap) shows that the dynamics of aerosol number concentrations and distributions are more complex than the simple sulphate-sea spray binary combination, and it is likely that an array of additional chemical components and processes drive the aerosol population. A conceptual illustration is proposed indicating the various atmospheric processes related to the Antarctic aerosols, with particular emphasis on the origin of new particle formation and growth.