Turnover times and mean heights of modeled sulfate and SO2 over the North Atlantic and adjacent continental regions. Stephen E. Schwartz and Carmen M. Benkovitz. 8th Atmospheric Radiation Measurement (ARM) Science Team Meeting, Tucson, AZ, Mar. 23-26, 1998.

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 (1) and presented such comparisons for four seasonal months in 1986-87 (2). 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 from biogenic sources. Maps will be presented 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 substantial incursions of European sulfate. Primary sulfate is generally at lower altitude (mean height mainly ~1 km, except summer, ~2 km) than secondary sulfate ~3 km; 4-5 km summer). Sulfate turnover times are typically about 5 days, consistent with values that have been employed in previous estimates of sulfate aerosol forcing. Plots showing spatial and temporal variation in loading will be presented; these plots indicate short decorrelation distances (<1000 km) and times (<24 h) even in the mid North Atlantic well removed from sources, indicative of the necessity of taking this variation into account in comparison with observations and in evaluation of forcing, especially the indirect forcing, which is highly nonlinear in aerosol loading.

(1) Benkovitz C. M., Berkowitz C. M., Easter R. C., Nemesure S., Wagener R. and Schwartz S. E. (1994) Sulfate over the North Atlantic and adjacent continental regions: Evaluation for October and November 1986 using a three-dimensional model driven by observation-derived meteorology. J. Geophys. Res. 99, 20725-20756.

(2) Benkovitz C. M. and Schwartz S. E. Evaluation of Modeled Sulfate and SO2 over North America and Europe for Four Seasonal Months in 1986-87. J. Geophys. Res. 102, 25305-25338 (1997).

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