THE MODIFICATION OF A SEMIEMPIRICAL LONG-RANGE TRANSPORT MODEL TO ALLOW ESTIMATION OF AMBIENT SULFATE CONCENTRATIONS

This paper describes the modification of a semi-empirical long-range t ransport model (STAT-MOD) to allow the estimation of ambient SO2 and s ulfate concentrations, and sulfur concentrations in rain. These substa nces are relevant to the chemistry of deposition and the optical prope rties of aerosols. The improved model incorporates a treatment of SO2 oxidation to sulfate in non-precipitating clouds. Model parameters wer e determined by fitting model estimates of ambient SO2 and sulfate con centrations, and sulfate concentrations in rain to corresponding obser vations from the Eulerian Model Evaluation and Field Study (EMEFS) and the National Dry Deposition Network (NDDN) databases for the period J uly-August-September 1988. The derived value of the parameter correspo nding to the in-cloud conversion of SO2 suggests that sulfate formed i n non-precipitating clouds made a substantial contribution to ambient sulfate concentrations belonging to the 1988 data set used in this stu dy. To evaluate the general applicability of the parameter value, mode l estimates were compared with observations obtained from the Sulfate Regional Experiment (SURE), the Eastern Regional Air Quality Study (ER AQS), and the Acid Deposition System (ADS) databases for a similar tim e period in 1978. The results showed that the estimated concentrations were correlated with the observed concentrations of sulfur species fo r a similar period of a different year. This provides a test of the re alism of the model as a first-cut engineering tool to examine the rela tionship between SO2 emissions and ambient sulfate levels.