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 a 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 sulfide marine
emissions. We present a statistical cluster analysis of the physical characteristics of
particle size distributions (PSDs) 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-mean method we find eight clusters
describing the entire aerosol population. Three clusters show pristine average low
particle number concentrations (< 121–179 cm−3) with three main
modes (30, 75–95 and 135–160 nm) and represent 57 % of the annual PSD
(up to 89 %–100 % during winter and 34 %–65 % during summer based on monthly
averages). Nucleation and Aitken mode PSD clusters dominate summer months
(September–January, 59 %–90 %), whereas a clear bimodal distribution (43 and 134 nm,
respectively; Hoppel minimum at mode 75 nm) is seen only during the December–April
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
the 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 and 160 nm, respectively), strongly 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 d overlap) shows that the dynamics of aerosol number
concentrations and distributions are more complex than the simple
sulfate–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.