A scheme for process-tagged SO4 and BC aerosols in NCAR CCM3:: Validation and sensitivity to cloud processes -: art. no. 4751

[1] A life cycle scheme for sulfate (SO4) and black carbon (BC) is implemented in an extended version of the National Center for Atmospheric Research (NCAR) Community Climate Model 3 (CCM3). The scheme includes emissions of dimethyl sulfide (DMS), SO2, and sulfate of natural and anthropogenic origins and emissions of BC from biomass burning and fossil fuel combustion. Chemistry and aerosol physics are parameterized based on prescribed oxidant levels and background aerosols of marine, continental, and polar origins. Aqueous chemistry depends on estimated exchange rate of cloudy and clear air. Particulate SO4 and BC are tagged by-production mechanisms for off-line reconstruction of aerosol optical and water activity properties. With emissions from International Panel on Climate Change (IPCC), calculations without feedback produce atmospheric turnover times (days) of 1.5 (SO2), 3.5 (SO4), and 4.7 (BC) for the year 2000 and 1.6 (SO2), 4.0 (SO4), and 4.7 (BC) for the year 2100 A2 emission scenario. The modeled SOx compounds compare within a factor 2 with observations at ground level in North America and Europe and for SO4 in the free troposphere. For BC, the ground-level concentrations are well within a factor 10 from observations over several regions. BC and SO4 are a factor 10 too low in Arctic winter, which can partly be linked to spurious low-level winter cloudiness. SO4 and BC are a factor 10 too high at ground-level low latitudes, and upper tropospheric SO2 is largely missing. These major model biases are caused by neglected transport and low scavenging efficiency in cumulus clouds. Cloud processes are discussed by sensitivity tests. SO4 and BC are found very sensitive to the vertical transport and scavenging in convective clouds. More research should aim at improved cloud parameterization schemes that address key processes associated with aerosols to reduce uncertainties associated with climate effects of anthropogenic aerosols.