A global aerosol model to study the impact of anthropogenic activities on marine cloud condensation nuclei

Description

A critical aspect of the uncertainty of aerosol indirect effects on climate is the contribution of anthropogenic sulfur emissions to CCN over the oceans, since the largest cloud forcing is from marine stratus. The production of sulfate aerosol from anthropogenic origins has to complete with natural aerosol sources. To take into account the complexity of the CCN forming processes in the marine boundary layer, processes that are essential for the marine aerosol system are introduced to construct a global sea salt/sulfate aerosol model based on a coupled climate and aerosol model CAM5/CARMA3. The processes include the emission of precursor gases from both anthropogenic and natural sources, the emission of sea salt and primary sulfate, gas-phase oxidation of DMS and SO2, nucleation, condensation, aqueous-phase oxidation of SO2, cloud droplet activation, cloud resuspension, and dry/wet removals. The model is capable of capturing the observed non-sea-salt sulfate mass concentration. The sulfate budget indicates that 55% of the sulfate particles are produced by gas-phase conversion, and 45% are produced by cloud processing. Although the number concentration is dominated by Aitken-mode aerosols (radius smaller than 0.1 µm) that are dominated by sulfate, only a small fraction of CCN are large coarse mode particles (radius larger than 1 µm), which is dominated by sea salt over the remote ocean. A test run is carried out with anthropogenic sulfate emission turned off to identify the anthropogenic effects of sulfate on global marine CCN.