The modification of a semi-empirical long-range transport model to allow estimation of ambient sulfate concentrations

Abstract

Abstract This paper describes the modification of a semi-empirical long-range transport model (STATMOD) to allow the estimation of ambient SO2 and sulfate concentrations, and sulfur concentrations in rain. These substances are relevant to the chemistry of deposition and the optical properties of aerosols. The improved model incorporates a treatment of SO2 oxidation to sulfate in non-precipitating clouds. Model parameters were determined by fitting model estimates of ambient SO2 and sulfate concentrations, and sulfate concentrations in rain, to corresponding observations from the Eulerian Model Evaluation and Field Study (EMEFS) and the National Dry Deposition Network (NDDN) databases for the period July-August-September 1988. The derived value of the parameter corresponding to the in-cloud conversion of SO2 suggests that sulfate formed in non-precipitating clouds made a substantial contribution to ambient sulfate concentrations belonging to the 1988 data set used in this study. To evaluate the general applicability of the parameter value, model estimates were compared with observations obtained from the Sulfate Regional Experiment (SURE), the Eastern Regional Air Quality Study (ERAQS), and the Acid Deposition System (ADS) databases for a similar time period in 1978. The results showed that the estimated concentrations were correlated with the observed concentrations of sulfur species for a similar period of a different year. This provides a test of the realism of the model as a first-cut engineering tool to examine the relationship between SO2 emissions and ambient sulfate levels.