Characteristics of aerosol wet deposition inferred from satellite re-analysis

Aerosol wet deposition is the key aerosol loss mechanism globally, and thus a primary control on aerosol abundance and aerosol-climate interactions. However, the rates and governing parameters of wet deposition are not well-understood, in part due to the lack of appropriate data sets. In this study, we combine two independent satellite products to address this issue. Aerosol optical depth (AOD) is obtained from the ECMWF Monitoring Atmospheric Composition and Climate project, which is a re-analysis product that assimilates MODIS-retrieved AOD. Rainfall is obtained from the Tropical Rainfall Measurement Mission (TRMM) Multi-satellite Precipitation Analysis version 7. Our approach involves identifying fifteen study regions worldwide (e.g. equatorial Pacific; Amazon basin; continental Asia) and constructing AOD budgets for each region, from which we derive region-averaged wet deposition (conditioned on precipitation). We then link inferred wet deposition with TRMM-based precipitation statistics. On average, we find that raining areas (minimum rainrate of 3 mm/day) exhibit AOD scavenging rates between 0.02 and 0.05 per day, with a long tail to high scavenging rates. Using these results, we test four hypotheses: (1) An increase in precipitation rate leads to an increase in the wet deposition of AOD; (2) An increase in precipitation rate leads to a decrease in the wet deposition of AOD per mm of rain; (3) Initial AOD substantially controls AOD wet deposition variability; (4) Characteristics of wet deposition differ for oceanic vs. continental regions. We find, surprisingly, that Hypotheses 1 and 3 are not supported by this analysis even though these are the basis for common wet deposition parameterizations. We do find that Hypotheses 2 and 4 are supported by this analysis. A number of potential explanations for these results will be discussed.