Impacts of aerosol size-dependent below-cloud scavenging on tropospheric aerosol in the NASA GEOS model

Wet deposition is the major sink for a suite of tropospheric aerosols, and thus largely affects the global distribution and lifetime of aerosols as well as the formation of clouds. We implement here size-dependent below-cloud scavenging (BCS; washout) parameterizations of Croft et al. (Atmos. Chem. Phys., 2009; hereafter referred to as C09) and study the impacts on tropospheric aerosols in the NASA Goddard Earth Observing System (GEOS) model. The model uses the aerosol module of either the Goddard Chemistry Aerosol Radiation and Transport model (GOCART) bulk aerosol scheme or the Modal Aerosol Microphysics model (MAM-7), coupled with the GEOS-5 two-moment cloud microphysics scheme. The current BCS scheme uses a first-order removal with a constant scavenging coefficient. The C09 BCS parameterizations calculate aerosol below-cloud scavenging coefficients based on a size-dependent collision efficiency between aerosol particles and raindrops (or snow crystals). A look-up table of compiled collision efficiencies provides values for sixty aerosol sizes and nine rain flux rates. In the case of using GOCART as the aerosol module, we assume a log-normal size distribution of aerosols. The collision efficiencies are then calculated online using a bilinear interpolation from the look-up table for the given aerosol sizes and precipitation flux rates in the model. We will examine the impacts of the C09 BCS parameterizations on the simulated aerosol vertical profiles, tropospheric lifetimes, and deposition fluxes of aerosols (sulfate, nitrate, black carbon, organic carbon, dust, and sea salt,) through comparisons with NASA aircraft measurements (ATom and NAAMES), surface observations of aerosol concentrations (IMPROVE) and wet deposition fluxes (NADP and APQMP), and MODIS aerosol optical depth (AOD) retrievals. Sea salt is expected to be most sensitive to below-cloud scavenging because it has a large fraction of total mass in the coarse mode and can be efficiently scavenged by stratiform precipitation over the ocean. The impact of the C09 BCS parameterizations on the clouds and precipitation in the model will also be discussed.