Improved aerosol characterization from combined OMI-MODIS retrieval

The Ozone Monitoring Instrument (OMI) aboard EOS-Aura and the MODerate resolution Imaging Spectroradiometer (MODIS) on board EOS-Aqua fly in formation as part of the A-Train. Both instruments retrieve aerosol properties operationally. OMI retrieves aerosol optical depth (AOD) and aerosol absorption, but must assume aerosol layer height. The MODIS aerosol retrieval is not sensitive to aerosol layer height and with its smaller pixel size is less affected by sub-pixel clouds. However, MODIS cannot retrieve aerosol absorption with any accuracy. Here, we demonstrate an approach that uses MODIS-retrieved AOD to constrain the OMI retrieval, freeing OMI from making an a priori estimate of aerosol height and allowing a more direct retrieval of absorption. To predict UV optical depths using MODIS data we rely on the spectral curvature of the MODIS-retrieved visible and near IR aerosol spectral optical depths (AODs). Applying the joint retrieval over the north tropical Atlantic shows good agreement between OMI and MODIS-adjusted AODs in the UV, which implies that the aerosol height assumed in the OMI-only algorithm is probably correct. In contrast, over the Arabian Sea, MODIS-adjusted AOD deviated from the OMI-only retrieval, and combined OMI-MODIS retrievals substantially improved information on aerosol layer height (based on validation against air-borne lidar measurements). This implies an improvement to the aerosol absorption retrieval, but lack of UV absorption measurements prevents a true validation. Our study demonstrates the potential of multi- satellite analysis of A-train data to improve the accuracy of retrieved aerosol products and suggests that a combined OMI-MODIS-CALIPSO retrieval has large potential to further improve assessments of aerosol absorption.