Southern Ocean latitudinal gradients of Cloud Condensation Nuclei

Abstract. The Southern Ocean region is one of the most pristine in the world, and serves as an important proxy for the pre-industrial atmosphere. Improving our understanding of the natural processes in this region are likely to result in the largest reductions in the uncertainty of climate and earth system models. While remoteness from anthropogenic and continental sources is responsible for its clean atmosphere, this also results in the dearth of atmospheric observations in the region. Here we present a statistical summary of the latitudinal gradient of aerosol and cloud condensation nuclei concentrations obtained from five voyages spanning the Southern Ocean between Australia and Antarctica from late-spring to early autumn (October to March) of the 2017/18 austral seasons. Three main regions of influence were identified: the northern sector (40–45° S) where continental and anthropogenic sources added to the background marine aerosol populations; the mid-latitude sector (45–65° S), where the aerosol populations reflected a mixture of biogenic and sea-salt aerosol; and the southern sector (65–70° S), south of the atmospheric Polar Front, where sea-salt aerosol concentrations were greatly reduced and aerosol populations were primarily biologically-derived sulfur species with a significant history in the Antarctic free-troposphere. The northern sector showed the highest number concentrations with median (25^th to 75^th percentiles) CN₁₀ and CCN_0:5 concentrations of (388–839) cm⁻³ and 322 (105–443) cm⁻³, respectively. Concentrations in the mid-latitudes were typically around 350 cm⁻³ and 160  cm⁻³ for CN₁₀ and CCN_0:5, respectively. In the southern sector, concentrations rose markedly, reaching 447 (298–446) cm⁻³ and 232 (186–271) cm⁻³ for CN₁₀ and CCN_0:5, respectively. The aerosol composition in this sector was marked by a distinct drop in sea-salt and increase in both sulfate fraction and absolute concentrations, resulting in a substantially higher CCN_0:5 / CN₁₀ activation ratio of 0.8 compared to around 0.4 for mid-latitudes. Long-term measurements at land-based research stations surrounding the Southern Ocean were found to be good representations at their respective latitudes i.e. CCN observations at Cape Grim (40°39'S) corresponded with CCN measurements from northern and mid-latitude sectors, while CN10 observations only corresponded with observations from the northern sector. Measurements from a simultaneous two year campaign at Macquarie Island (54°30'S) were found to represent all aerosol species well. The southern-most latitudes differed significantly from either of these stations and previous work suggests that Antarctic stations on the East Antarctic coastline do not represent the East Antarctic sea-ice latitudes well. Further measurements are needed to capture the long-term, seasonal and longitudinal variability in aerosol processes across the Southern Ocean.