Model implementation of marine organc aerosols

In the last decade, there has been an increase in research concerning organic aerosol emissions from the ocean. Global sources of ocean-emitted organic aerosols have been shown to be comparable in magnitude to primary organic aerosols (POA) emitted from combustion processes and are sometimes found in concentrations more typical of organic aerosols in urban areas. Due to their potential importance to air quality and climate, several attempts have been made to better quantify the emission rate of marine organics. In this work, we present results from two modeling studies in which marine organic aerosol emissions have been implemented. The Community Atmosphere Model (CAM5) climate model was used to determine the impact of marine organic aerosols on cloud condensation nuclei (CCN) concentration, cloud droplet number concentration (CDNC), and shortwave cloud forcing. The global chemical transport model GEOS-Chem was used to evaluate the ability of the latest version of all currently available marine (POA) emission schemes to predict the observed surface concentrations ranging in temporal resolution from hourly to seasonally. Using CAM5, the greatest impact on cloud condensation nuclei (CCN) concentration, cloud droplet number concentration (CDNC), and shortwave cloud forcing was found to occur when marine POA was externally-mixed (added as additional mass and number) with sea-salt. Simulations including marine POA emissions internally-mixed with sea-salt did not have any significant changes in CCN concentration, CDNC, or shortwave cloud forcing. Using the GEOS-Chem model, global marine POA emission rates were found to range from 0.1 to 11.9 Tg/yr with a top-down estimate of 6.3 Tg/yr. Compared to observations, the marine POA emission scheme based solely [chl a] was able to best simulate the seasonality of surface organic aerosol concentrations at two coastal sites. The schemes calculating an organic mass fraction of sea spray aerosol frequently had emissions rates that were too high in the winter and too low in the summer. Comparison with hourly observed concentrations revealed that none of the marine POA emissions parameterizations were able to represent short term variability in concentrations. These studies suggest that 1) understanding the emission processes and physiochemical characteristics of marine POA is important as marine organic aerosols could play considerable role in model-predicted climate; 2) more work is needed for development of parameterizations grounded in physical processes controlling marine organic aerosol emissions.