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<p>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, CN<span class="inline-formula">₁₀</span>) 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 (40–45<span class="inline-formula">^∘</span> S), where continental and anthropogenic sources coexisted with background marine aerosol populations; the mid-latitude sector (45–65<span class="inline-formula">^∘</span> S), where the aerosol populations reflected a mixture of biogenic and sea-salt aerosol; and the southern sector (65–70<span class="inline-formula">^∘</span> 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) CN<span class="inline-formula">₁₀</span> and CCN<span class="inline-formula">_0.5</span> concentrations of 681 (388–839) cm<span class="inline-formula">⁻³</span> and 322 (105–443) cm<span class="inline-formula">⁻³</span>, respectively. Concentrations in the mid-latitudes were typically around 350 cm<span class="inline-formula">⁻³</span> and 160 cm<span class="inline-formula">⁻³</span> for CN<span class="inline-formula">₁₀</span> and CCN<span class="inline-formula">_0.5</span>, respectively. In the southern sector, concentrations rose markedly, reaching 447 (298–446) cm<span class="inline-formula">⁻³</span> and 232 (186–271) cm<span class="inline-formula">⁻³</span> for CN<span class="inline-formula">₁₀</span> and CCN<span class="inline-formula">_0.5</span>, 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<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mn mathvariant="normal">0.5</mn></msub><mo>/</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1ca0f7f0cbab2293f1d1e6d5184f3377"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-12757-2021-ie00001.svg" width="20pt" height="14pt" src="acp-21-12757-2021-ie00001.png"/></svg:svg></span></span>CN<span class="inline-formula">₁₀</span> 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 (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">40</mn><msup><mi/><mo>∘</mo></msup><msup><mn mathvariant="normal">39</mn><mo>′</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="666ce17390caa2e9437e22a67b9baad6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-12757-2021-ie00002.svg" width="34pt" height="11pt" src="acp-21-12757-2021-ie00002.png"/></svg:svg></span></span> S) corresponded with CCN measurements from northern and mid-latitude sectors, while CN<span class="inline-formula">₁₀</span> observations only corresponded with observations from the<span id="page12758"/> northern sector. Measurements from a simultaneous 2-year campaign at Macquarie Island (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">54</mn><msup><mi/><mo>∘</mo></msup><msup><mn mathvariant="normal">30</mn><mo>′</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="0918a3ee3731f893142f18257857e8cd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-12757-2021-ie00003.svg" width="34pt" height="11pt" src="acp-21-12757-2021-ie00003.png"/></svg:svg></span></span> 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.</p>