The effect of sulfate aerosol and organic matter in sea spray aerosol on Southern Ocean cloud droplet number concentration and 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. In this study 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 effect of the enhancement in Nd due to biogenic aerosols in the Southern Ocean region on the reflected shortwave flux is estimated using offline radiative transfer performed in the Community Atmospheric Model (CAM5) and will be discussed in the context of aerosol indirect effects from other natural and anthropogenic sources. We show from observations that Nd is correlated with chlorophyll-a, a proxy of ocean primary productivity. Biogenic aerosols are simulated using state-of-the art techniques and compared to observations of Nd. Primary biogenic aerosols are modeled using the physically-based framework of Burrows et al. (2014) and sulfate aerosol is taken from the Aerocom multi-model median product. Together organic matter in sea spray aerosol and sulfate aerosol are found to explain more than half of the variability in observed Nd indicating a significant biogenic effect on Southern Ocean albedo.