Modeling sea salt and sulfate aerosol over the global oceans to understand the origins of marine cloud condensation nuclei and the impact of pollution on them

Over the oceans, anthropogenic aerosols compete with natural aerosols from sea spray and oceanic phytoplankton-­‐derived sulfate to create cloud condensation nuclei (CCN). To understand the impact of pollution on the marine CCN, we need knowledge of both natural and anthropogenic aerosols. In this research, we model sea salt and sulfate aerosol in a coupled climate and sectional microphysical model, CAM/CARMA. We develop a sea salt source function, CMS, based upon several earlier source functions (Clarke, Monahan, and Smith). The CMS source function is capable of reproducing observed sea salt mass, optical depth and number concentration as well as the size distribution better than other source function choices we tried. However, as we note, it is also important to properly set the removal rate of the particles to reproduce the observed abundances. The simulated non-­‐sea-­‐salt sulfate mass agrees well with the observations. Direct emission of sulfate from sea spray is the largest source of marine sulfate aerosol and depends on the sea salt emission. Non-­‐sea-­‐salt sulfate from gas-­‐ and aqueous-­‐phase conversion, together with sea salt, contributes to the marine CCN over the mid-­‐latitude Northern Hemisphere, while sea salt dominates the CCN over the Southern Ocean. Human impact on marine CCN extends to 45 °S. Anthropogenic sulfur emissions are responsible for about 35% of the surface layer CCN over the global oceans. With doubling the year 2000 anthropogenic sulfur emissions. Surface layer CCN increases by about 22% over the global oceans if sulfur emissions are doubled from. With no or double anthropogenic emissions, the changes in the surface layer CCN number over the Southern Hemisphere oceans are usually less than 10%