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The impact that biogenic emissions have on aerosol-cloud interactions across the Southern Ocean is poorly quantified. Here we use satellite and ship observations during austral summer to study these interactions. We present observational evidence that biogenic aerosols increase cloud condensation nuclei and cloud droplet number concentrations over the Southern Ocean off East Antarctica, coinciding with very low concentrations of ice-nucleating particles and higher occurrences of supercooled liquid-containing low-level clouds.

Dimethyl sulfide (DMS) and methanethiol (MeSH) are biologically co‐produced marine volatile sulfur compounds, which play a critical role in climate‐cooling aerosol formation. The spatio‐temporal distributions of MeSH are poorly constrained, especially over the Southern Ocean. DMS and MeSH atmospheric concentrations and relative contributions to volatile methylated sulfur (VMS) were measured across the Southern Ocean, spanning all seasons and latitudes from 37°S to 67°S. Highest absolute mixing ratios of MeSH occurred in summer (up to 250 ppt), over biologically productive waters at 45°S to 52°S and close to the Antarctic coast (>${ >} $ 62°S). Highest MeSH/VMS occurred in spring and winter (up to 35%), and at the Subtropical Front and Antarctic coast. These results constrain MeSH contributions to VMS over the Southern Ocean, explore mechanisms driving these dynamics, and support recently modeled MeSH importance to the atmospheric sulfur burden, with significant implications for modeling climate‐cooling aerosols.