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. The Southern Ocean and Antarctic region currently best represent one of thefew places left on our planet with conditions similar to the preindustrialage. Currently, climate models have a low ability to simulate conditionsforming the aerosol baseline; a major uncertainty comes from the lack ofunderstanding of aerosol size distributions and their dynamics. Contrastingstudies stress that primary sea salt aerosol can contribute significantly tothe aerosol population, challenging the concept of climate biogenicregulation by new particle formation (NPF) from dimethyl sulfide marineemissions. We present a statistical cluster analysis of the physical characteristics ofparticle size distributions (PSDs) collected at Halley (Antarctica) for theyear 2015 (89 % data coverage; 6–209 nm size range; daily sizeresolution). By applying the Hartigan–Wong k -mean method we find eight clustersdescribing the entire aerosol population. Three clusters show pristine average lowparticle number concentrations (  121–179 cm −3 ) with three mainmodes (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 monthlyaverages). 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–Aprilperiod (6 %–21 %). Major findings of the current work include: (1) NPF andgrowth events originate from both the sea ice marginal zone and theAntarctic plateau, strongly suggesting multiple vertical origins, includingthe marine boundary layer and free troposphere; (2) very low particle numberconcentrations are detected for a substantial part of the year (57 %),including summer (34 %–65 %), suggesting that the strong annual aerosolconcentration cycle is driven by a short temporal interval of strong NPFevents; (3) a unique pristine aerosol cluster is seen with a bimodal sizedistribution (75 and 160 nm, respectively), strongly associated with highwind speed and possibly associated with blowing snow and sea spray sea salt,dominating the winter aerosol population (34 %–54 %). A brief comparisonwith two other stations (Dome C – Concordia – and King Sejong Station) duringthe year 2015 (240 d overlap) shows that the dynamics of aerosol numberconcentrations and distributions are more complex than the simplesulfate–sea-spray binary combination, and it is likely that an array ofadditional chemical components and processes drive the aerosol population. Aconceptual illustration is proposed indicating the various atmosphericprocesses related to the Antarctic aerosols, with particular emphasis on theorigin of new particle formation and growth.

On the annual variability of Antarctic aerosol size distributions at Halley Research Station