On the retrieval of columnar aerosol mass and CCN concentration by MODIS -: art. no. 4010

A parameterization is introduced for the derivation of columnar aerosol mass concentration (AMC) and cloud condensation nucleus concentration (CCNC) from the primary aerosol products of Moderate-Resolution Imaging Spectroradiometer (MODIS). The method relies on the scaling between AMC and optical depth with a proportionality constant dependent on MODIS-derived r(eff), eta (contribution of the accumulation mode radiance to the total radiance), ambient RH, and an assumed constant aerosol composition. The CCNC is derived from a parameterization of concentration as a function of the retrieved aerosol volume. By comparing with in situ data, it is shown that retrievals in dry ambient conditions (dust) are improved when using a proportionality constant dependent on r(eff) and eta obtained in the same pixel. In high-humidity environments, the improvement in the new method is inconclusive because of the difficulty in accounting for the uneven vertical distribution of relative humidity. Additionally, two detailed comparisons of AMC and CCNC retrieved by the MODIS Airborne Simulator (MAS) algorithm and the new method are shown. The new method and MAS retrievals of AMC are within the same order of magnitude with respect to the in situ measurements of aerosol mass. However, the proposed method is closer to the in situ measurements than the MODIS retrievals. The retrievals of CCNC are also within the same order of magnitude for both methods. Finally, the new method is applied to an actual MODIS retrieval. Although no in situ data is available for comparison, it is shown that the proposed method yields more credible values than the MODIS retrievals.