Wednesday, 25 January 2012: 2:00 PM
Sea Spray Aerosols and Climate Assessments
Room 244 (New Orleans Convention Center )
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). Because the anthropogenic contribution to climate forcing represents the difference between the total forcing and that from natural aerosols, understanding background aerosols is necessary to evaluate the influences of anthropogenic aerosols on cloud reflectivity and persistence (so-called indirect radiative forcing). The effects of marine aerosols are explored using remotely sensed data obtained by Cloud-aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and the NCAR Community Atmosphere Model (CAM5.0), coupled with the PNNL Modal Aerosol Model. CALIPSO provided high resolution vertical profile information about different aerosol subtypes (defined as clean continental, marine, desert dust, polluted continental, polluted dust, and biomass burning) is employed to compare marine aerosol AOD with anthropogenic aerosol AOD downwind from large urban cities as well as over remote oceanic regions. Model-predicted abundance of CCN in remote marine atmosphere is also evaluated using satellite and in-situ data. Simulations show that over biologically productive ocean waters primary organic aerosols of marine origin can contribute up to 20% increase in CCN (at a supersaturation of 0.2%) number concentrations. Corresponding changes associated with cloud properties (liquid water path and droplet number) can decrease global annual mean indirect radiative forcing of anthropogenic aerosol by 0.1 Wm-2 or 8%. This study suggests that abundance and chemical composition of sea spray (largely comprised of sea salt and primary organic aerosol of marine origin) can be considerably different from that of sea salt and neglecting these differences could result in overprediction of anthropogenic aerosol indirect effect.