A new study of sea spray optical properties from multi-sensor spaceborne observations

Abstract

Retrievals of aerosol optical depth (AOD) from the Cloud-Aerosol Lidarwith Orthogonal Polarization (CALIOP) satellite sensor require theassumption of an extinction-to-backscatter ratio, also known as thelidar ratio. This paper evaluates a new method to calculate the lidarratio of sea spray aerosol using two independent sources: the AODfrom the Synergized Optical Depth of Aerosols (SODA) algorithm andthe integrated attenuated backscatter from CALIOP. With this method,the particulate lidar ratio can be derived for individual CALIOPretrievals in single aerosol layer columns over the ocean. Globalanalyses are carried out using CALIOP level 2, 5 km sea spray aerosollayer products and the collocated SODA nighttime data from December2007 to December 2009. The global mean lidar ratio for sea sprayaerosols was found to be 26 sr, roughly 30% higher than the currentvalue prescribed by CALIOP standard retrieval algorithm. Data analysisalso showed considerable spatiotemporal variability in the calculatedlidar ratio over the remote oceans. The calculated aerosol lidarratios are shown to be inversely related to the mean ocean surfacewind speed: increase in ocean surface wind speed (U_10) from 0to > 15 m/s reduces the mean lidar ratios for sea spray particlesfrom 32 sr (for 0 \lt U_10 \lt 4\ m\ s^{-1}) to 22 sr (for U_10\gt 15\ m\ s^{-1}). Such changes in the lidar ratio are expectedto have a corresponding effect on the sea spray AOD. The outcomesof this study are relevant for future improvements of the SODA andCALIOP operational product and could lead to more accurate retrievalsof sea spray AOD.