Turnover Times and Mean Heights of Modeled Sulfate and SO2 over the North Atlantic and Adjacent Continental Regions

Shortwave radiative forcing of climate by direct light scattering by sulfate and other aerosols is considered important in the context of anthropogenic climate change. Representation of this forcing in climate models requires a chemical model for the loading and distribution of this aerosol whose accuracy must be evaluated by comparison with observations. We have previously described a model for sulfate aerosol driven by observation-derived synoptic-scale meteorological data suitable for such comparisons (Benkovitz et al. 1994) and presented such comparisons for four seasonal months in 1986-1987 (Benkovitz and Schwartz 1997). Here we examine turnover times, mean heights, and other properties of SO2 and sulfate from anthropogenic sources in North America and Europe and of sulfate and methanesulfonic acid (MSA) from biogenic sources. We have generated maps showing the geographical distribution and mean height of sulfate from the several sources. In general, North American sulfate dominates over the mid-North Atlantic, but occasionally there are sub-stantial incursions of European sulfate. Primary sulfate is generally at lower altitude (mean height mainly ~1 km to 2 km) than secondary sulfate (~3 km to 5 km). Sulfate turnover times are typically about 5 days. Decorrelation distances of sulfate column burden are less than 1000 km, and decorrelation times less than 24 h, even in the mid North Atlantic well removed from sources