Sensitivity of sulphate aerosol size distributions and CCN concentrations over North America to SOx emissions and H2O2 concentrations

To assess the influence of aerosols on climate, the Northern Aerosol Region al Climate Model (NARCM) is currently being developed. NARCM includes size- segregated aerosols as prognostic and interactive constituents. In this pap er, the model is being applied to sulphate aerosol over North America durin g time periods in July and December 1994. The results give evidence for con siderable regional and seasonal variations in sulphate aerosol size distrib utions over North America. Comparisons of the results with different observ ations yield a reasonably good agreement in terms of meteorological and phy sicochemical parameters. Some of the differences in sulphate concentrations and wet deposition rates can be attributed to differences in cloud amounts and precipitation between model results and observations. Indirect tests o f the simulated aerosol mass mean diameters are also encouraging. Additiona l simulations for hypothetical decreases in anthropogenic sulphur emissions and increases in hydrogen peroxide (H2O2) background concentrations are pe rformed for the same time periods to study the responses of concentration, size distribution, and wet deposition of sulphate aerosol to these changes. Also, responses of cloud condensation nuclei (CCN) number concentrations a re investigated. The simulation results show that sulphate aerosol concentr ations respond almost linearly in both time periods to decreases in sulphur emissions but that CCN number concentrations respond nonlinearly due to de creases in sulphate mass mean diameters. Especially for the December period , increases in hydrogen peroxide background concentrations lead to increase s in CCN number concentrations at critical diameters larger than about 0.07 mu m. These results lead to the hypothesis that increased in-cloud oxidati on in convective clouds due to future increases in oxidant concentrations m ay produce larger CCN which eventually can be easily activated in subsequen tly forming stratiform clouds.