Sea spray aerosol and climate assessments: Model results and remotely sensed data (Invited Presentation)

Abundance of natural aerosols in the atmosphere strongly affects global aerosol optical depth (AOD) and influences clouds and the hydrological cycle through their ability to act as cloud condensation nuclei (CCN). As ocean-derived aerosols contribute substantially to the pre-industrial, natural background which provides the baseline on top of which anthropogenic forcing should be quantified, the representation of marine aerosols in climate models strongly influences the predicted direct and indirect climate effects of anthropogenic aerosols. This presentation will show results from two modeling studies in which marine organic aerosol emissions have been implemented. The Community Atmosphere Model (CAM5) is used to determine the impact of marine organic aerosols on CCN concentration, cloud droplet number concentration (CDNC), and shortwave cloud forcing. Using CAM5, the greatest impact on CCN concentration, CDNC, and shortwave cloud forcing was found to occur when marine Primary Organic Aerosol (POA) was externally-mixed (added as additional mass and number) with sea-salt. Specifically, emissions of externally-mixed marine POA led to a 1.3% increase in global CCN surface concentration (with regional increases of 20% in the Southern Ocean, Antarctica, and the Arctic) and a ~0.1 W m^-2 (7%) reduction in model-predicted aerosol indirect forcing. Global sub-micron marine POA emission rates were found to be ~10 ± 5 Tg yr^-1.

Marine aerosol optical properties will also be explored using remotely sensed data obtained by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the CALIPSO satellite. CALIOP-provides high resolution (~30 m) vertical profile information about different aerosol subtypes (defined as clean continental, marine, desert dust, polluted continental, polluted dust, and biomass burning), particulate depolarization ratio (or particle non-sphericity), and aerosol color ratio (the ratio of aerosol backscatter at the two wavelengths). Despite new insights into marine aerosol optical properties, past studies have shown that CALIOP underestimates AOD values over the oceans. Here we present a new method for the correction of CALIOP-derived AOD values over the ocean using the Synergized Optical Depth of Aerosols (SODA) product. It was found that due to lower sea spray lidar ratio prescribed by CALIOP the average AOD values over the ocean are underestimated by roughly 30%. In addition, our method also explains some of the physical reasons for the variability in sea spray aerosol lidar ratios over different parts of the oceans. The results of our study suggest that 1)our calculated value of the lidar ratio is consistent with past studies, 2) the CALIOP lidar ratio selection algorithm is likely biased low (our calculated median lidar ratio is ≈ 26 sr vs. 20 sr prescribed by CALIOP), and 3) the surface wind speed is an important parameter controlling sea spray aerosol lidar ratios.