PNNL

Abstract

A high-resolution (1°×1°, 27 vertical levels) Eulerian chemical transport and transformation model for sulfate, SO2, and related species driven by analyzed forecast meteorological data has been run for the Northern Hemisphere for June-July 1997 and extensively evaluated with observational data, mainly from air-quality and precipitation chemistry networks. For ~5000 evaluations, 50% of the modeled sulfate 24-h mixing ratios were within a factor of 1.85 of the observations; 50% of ~328 concurrent subgrid observations were within a factor of 1.33. Much greater subgrid variation for 24-h SO2 mixing ratios (50% of ~3552 observations were within a factor of 2.32) reflects high variability of this primary species; for ~12,600 evaluations 50% of modeled mixing ratios were within a factor of 2.54 of the observations. These results indicate that a substantial fraction of the modeled and observed differences is due to subgrid variation and/or measurement error. Sulfate mixing ratios are identified by source type (biogenic, volcanic, and anthropogenic) and production mechanism (primary and by gas-phase and aqueous-phase oxidation). Examination of key diagnostics showed substantial variation for the different types of sulfur, e.g., SO2 aqueous-phase oxidation rates of 29 to 102% day-1, sulfate residence times of 4 to 9 days. Volcanic emissions contributed 10% of the sulfate burden and 6% of emissions, because the elevated release allows 2 large fractional conversion of SO2 and long residence time. Biogenic SO2 was generally at lower concentrations than H2O2, resulting in efficient aqueous-phase oxidation; this source type contributed 13% of emissions but only 5% of sulfate burden. Anthropogenic sources were the dominant contributors to sulfur emissions, 80%, and sulfate burden, 84%.