# Southern Ocean latitudinal gradients of cloud condensation nuclei

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 is 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 (condensation nuclei larger than 10 nm, CNinline-formula10) and cloud condensation nuclei (CCN at various supersaturations) 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 (40–45inline-formula S), where continental and anthropogenic sources coexisted with background marine aerosol populations; the mid-latitude sector (45–65inline-formula S), where the aerosol populations reflected a mixture of biogenic and sea-salt aerosol; and the southern sector (65–70inline-formula 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 (25th to 75th percentiles) CNinline-formula10 and CCNinline-formula0.5 concentrations of 681 (388–839) cminline-formula−3 and 322 (105–443) cminline-formula−3, respectively. Concentrations in the mid-latitudes were typically around 350 cminline-formula−3 and 160 cminline-formula−3 for CNinline-formula10 and CCNinline-formula0.5, respectively. In the southern sector, concentrations rose markedly, reaching 447 (298–446) cminline-formula−3 and 232 (186–271) cminline-formula−3 for CNinline-formula10 and CCNinline-formula0.5, 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 CCNinline-formula $M17inlinescrollmathml{}_{normal 0.5}/$ 20pt14ptsvg-formulamathimg1ca0f7f0cbab2293f1d1e6d5184f3377 acp-21-12757-2021-ie00001.svg20pt14ptacp-21-12757-2021-ie00001.png CNinline-formula10 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; however this study highlighted the need for more long-term measurements in the region. CCN observations at Cape Grim (inline-formula $M19inlinescrollmathmlnormal 40{}^{\circ }{normal 39}^{\prime }$ 34pt11ptsvg-formulamathimg666ce17390caa2e9437e22a67b9baad6 acp-21-12757-2021-ie00002.svg34pt11ptacp-21-12757-2021-ie00002.png  S) corresponded with CCN measurements from northern and mid-latitude sectors, while CNinline-formula10 observations only corresponded with observations from thepage12758 northern sector. Measurements from a simultaneous 2-year campaign at Macquarie Island (inline-formula $M21inlinescrollmathmlnormal 54{}^{\circ }{normal 30}^{\prime }$ 34pt11ptsvg-formulamathimg0918a3ee3731f893142f18257857e8cd acp-21-12757-2021-ie00003.svg34pt11ptacp-21-12757-2021-ie00003.png  S) were found to represent all aerosol species well. The southernmost latitudes differed significantly from both 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.

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Humphries, Ruhi S. / Keywood, Melita D. / Gribben, Sean / et al: Southern Ocean latitudinal gradients of cloud condensation nuclei. 2021. Copernicus Publications.

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