On factors controlling marine boundary layer aerosol optical depth

Sea spray aerosol is one of the largest natural contributors to the global aerosol loading and thus plays an important role in the global radiative budget through both direct and indirect effects. Previous studies have shown either strong or weak relationships between marine boundary layer (MBL) aerosol optical depth (τ) and the near-surface wind speed. However, the marine τ is influenced by a wide range of factors. This study attempts to examine extra contributing factors beyond wind to better characterize MBL τ variations over the global ocean by using 4 year A-train data (2006–2010). The results show that among many factors controlling MBL τ, surface wind speed and MBL depth are the two most important factors. This suggests that not only mechanical production of sea spray particles driven by near-surface wind processes but also vertical redistribution driven by turbulent and shallow convective mixing in the MBL controls MBL τ variations. A new two-parameter parameterization of τ was derived based on satellite measurements. Evaluations with independent data show that the new parameterization improves the prediction of MBL τ. The comparisons between the Fu-Liou radiative transfer model calculations and Aqua Clouds and the Earth’s Radiant Energy System observations showed that the new parameterization improves the estimation of aerosol radiative forcing.