Natural aerosols explain seasonal and Spatial patterns of Southern Ocean Cloud albedo

Atmospheric aerosols, suspended solid and liquid particles, act as nucleation sites for cloud drop formation, affecting
clouds and cloud properties—ultimately influencing the cloud dynamics, lifetime, water path, and areal extent that
determine the reflectivity (albedo) of clouds. The concentration Nd of droplets in clouds that influences planetary
albedo is sensitive to the availability of aerosol particles on which the droplets form. Natural aerosol concentrations
affect not only cloud properties themselves but also modulate the sensitivity of clouds to changes in anthropogenic
aerosols. It is shown that modeled natural aerosols, principally marine biogenic primary and secondary aerosol
sources, explain more than half of the spatiotemporal variability in satellite-observed Nd. Enhanced Nd is spatially
correlated with regions of high chlorophyll a, and the spatiotemporal variability in Nd is found to be driven primarily
by high concentrations of sulfate aerosol at lower Southern Ocean latitudes (35o to 45oS) and by organic matter in sea
spray aerosol at higher latitudes (45o to 55oS). Biogenic sources are estimated to increase the summertime mean
reflected solar radiation in excess of 10 W m–2 over parts of the Southern Ocean, which is comparable to the annual
mean increases expected from anthropogenic aerosols over heavily polluted regions of the Northern Hemisphere.