| Summary: | The Southern Ocean (SO) exhibits some of the most pristine air on Earth, particularly during the winter season, when the lowest concentrations of cloud condensation nuclei (CCN) are observed. Historically, research has focused on the biogenic production of dimethyl sulfide as the primary explanation for the observed seasonal cycle in CCN and in support of the ‘CLAW’ hypothesis. More recent research, however, suggests that this hypothesis is incomplete and there is a need to better understand alternate sources (e.g., sea spray) and sinks (e.g., coalescence scavenging) to fully constrain the CCN budget. We examine the potential impact of the structure of marine boundary layer clouds on the CCN concentration through precipitation and wet deposition. Marine boundary layer clouds dominate the lower latitudes of the Southern Ocean, specifically the various states (open, closed, disorganised) of mesoscale cellular convection (MCC). Using a cloud climatology based on Himawari-8 observations, the relationship between CCN concentrations and precipitation from Kennaook/Cape Grim Baseline Air Pollution Station was examined. A lower median CCN concentration (68.9 cm^-3) was observed when open MCC was dominant upwind of the site under ‘baseline’ conditions, as compared to when closed MCC (88.6 cm^-3) was dominant. This difference is statistically significant. It was observed that open MCC precipitated more heavily (1.72 mm/day) than closed MCC (0.29 mm/day), establishing a negative relationship between CCN concentration and precipitation. The most pristine air is observed when open MCC is directly upwind of the Kennaook/Cape Grim station. This negative relationship was observed at both diurnal and seasonal time scale.
|
|---|