Marine biogenic aerosols explain seasonal and spatial patterns of Southern Ocean cloud droplet number concentration

Cloud droplets form on preexisting aerosol particles, so in pristine air with low particulate concentrations, the number concentration of cloud droplets is constrained by their availability. The size and concentration of these cloud droplets plays a significant role in determining cloud albedo. Over the Southern Ocean (35S-55S), which is far removed from continental or anthropogenic aerosol sources, cloud droplet number density (Nd) is controlled by natural oceanic aerosol such as sea salt or phytoplanktonic sources. While the Southern Ocean cloud albedo plays a significant role in determining global climate sensitivity, there is not a strong consensus on what these sources are and how strongly they affect Nd and, consequently, cloud albedo. Here we show from observations that Nd is correlated with chlorophyll-a, a proxy of ocean primary productivity, and that the marine biogenic aerosol calculated using the framework of Burrows et al. (2014) and modeled sulfate aerosol from the Aerocom multi-model median product explain more than half of the variability in observed Nd. We attribute enhancement of Nd over regions of high biological activity to high concentrations of sulfate aerosol at lower latitudes (35-45 S) and to organic matter in sea spray aerosol at higher latitudes (45-55 S). The enhancement in reflected shortwave owing to biogenic sources influencing Nd is estimated using offline radiative transfer performed in the Community Atmospheric Model (CAM5).