Evaluation of modeled sulfate and SO2 over North America and Europe for four seasonal months in 1986–87

Abstract. A three-dimensional Eulerian transport and transformation model driven by observa-tion-derived synoptic meteorological data has been applied to calculate mixing ratios (MRs) of sulfate and SO 2 and wet deposition of sulfate over the North Atlantic and adjacent continental regions for 1-month periods in each of four seasons in 1986-1987. Model performance is evalu-ated by comparison of grid-cell average (1.125 ø) modeled MRs for sulfate (24-hour average) and SO2 (6- and 24-hour average) in the lowest model level (surface to-65 m) to surface MRs observed at monitoring stations in North America and Europe. For su!fate-8000 model-observa-tion comparisons were made employing- 10,000 individual measurements; for 24-hour SO 2, 21,000 comparisons (54,000 measurements) and for 6-hour SO2,-71,000 comparisons (211,000 measurements). Subgrid variation of observed MRs is inferred from the spread of multiple simul-taneous measurements within individual grid cells. The median spread of the observed MRs is a factor of 1.5 for 24-hour sulfate and 2.2 for 24-hour SO2. The median spread between observed and modeled MRs is a factor of 2.3 for sulfate and 2.1 for 24-hour SO2, comparable to that for the observations themselves. This suggests that much of the departure between modeled and observed MRs can be attributed to subgrid spatial variation and nonrepresentative sampling of model gri