A New Study of Sea Spray Optical Properties from Multi-Sensor Spaceborne Observations

Retrievals of aerosol optical depth (AOD) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor require the assumption of an extinction-to-backscatter ratio, also known as the lidar ratio. This paper evaluates a new method to calculate lidar ratio of sea spray aerosol using two independent sources: the AOD from Synergized Optical Depth of Aerosols (SODA) and the integrated attenuated backscatter from CALIOP. The method applied in this study allows particulate lidar ratio to be calculated for individual CALIOP retrievals of single aerosol layer columns over the ocean. Analyses are carried out using CALIOP level 2, 5km sea spray aerosol layer products and collocated SODA nighttime data from December 2007 to December 2009. The global mean lidar ratio for sea spray aerosols was found to be 26 sr, roughly 30 higher than the one prescribed by CALIOP. Data analysis also showed considerable spatiotemporal variability in calculated lidar ratio over different parts of the remote oceans. The calculated aerosol lidar ratios are shown to be inversely related to the mean ocean surface wind speed: increase in ocean surface wind speed (U10) from 0 to 15 ms-1 reduces the mean lidar ratios for sea spray particles from 32 sr (for 0 U10 4 ms-1) to 22 sr (for U10 15 ms-1). Such changes in the lidar ratio are expected to have a corresponding effect on sea spray AOD. The outcomes of this study are relevant for future improvements of the SODA and CALIOP operational product and could lead to more accurate retrievals of sea spray AOD.