Effects of aerosols on tropospheric oxidants: A global model study

The global distributions of sulfate and soot particles in the atmosphere are calculated, and the effect of aerosol particles on tropospheric oxidants is studied using a global chemical/transport/ aerosol model. The model is developed in the framework of the National Center for Atmospheric Research (NCAR) global three-dimensional chemical/transport model (Model for Ozone and Related Chemical Tracers (MOZART)). In addition to the gas-phase photochemistry implemented in the MOZART model, the present study also accounts for the formation of sulfate and black carbon aerosols as well as for heterogeneous reactions on particles. The simulated global sulfate aerosol distributions and seasonal variation are compared with observations. The seasonal variation of sulfate aerosols is in agreement with measurements, except in the Arctic region. The calculated vertical profiles of sulfate aerosol agree well with the observations over North America. In the case of black carbon the calculated surface distribution is in fair agreement with observations. The effects of aerosol formation and heterogeneous reactions on the surface of sulfate aerosols are studied. The model calculations show the following: (1) The concentration of H2O2 is reduced when sulfate aerosols are formed due to the reaction Of SO2+ H2O2 in cloud droplets. The gas-phase reaction SO2 + OH converts OH to HO2, but the reduction of OH and enhancement of HO2 are insignificant (< 3%). (2) The heterogeneous reaction of HO2 on the surface of sulfate aerosols produces up to 10% reduction of hydroperoxyl radical (HO2) with an uptake coefficient of 0.2. However, this uptake coefficient could be overestimated, and the results should be regard as an upper limit estimation. (3) The N2O5 reaction on the surface of sulfate aerosols leads to an 80% reduction of NOx at middle to high latitudes during winter. Because ozone production efficiency is low in winter, ozone decreases by only 10% as a result of this reaction. However, during summer the N2O5 reaction reduces NOx by 15% and O-3 by 8-10% at middle to high latitudes. (4) The heterogeneous reaction of CH2O on sulfate aerosols with an upper limit uptake coefficient (gamma = 0.01) leads to an 80 to 90% decrease in CH2O and 8 to 10% reduction of HO2 at middle to high latitudes during winter. Many uncertainties remain in our understanding of heterogeneous chemical processes and in the estimate of kinetic parameters. This model study should therefore be regarded as exploratory and subject to further improvements before final conclusions can be made.