Dynamical influences on the distribution and loading of SO2 and sulfate over North America, the North Atlantic, and Europe in April 1987

[1] The loading of tropospheric aerosols is highly variable spatially and temporally as a consequence of large spatial variability in sources, temporal variability in transport winds, and short residence times (days), and additionally for secondary sulfate, intermittent production associated with aqueous-phase reaction in clouds, and intermittent removal, mainly by precipitation. We have used a chemical transport and transformation model for atmospheric sulfur driven by observationally derived meteorological data to calculate the geographical distribution of sulfate and sulfur dioxide (SO2) over North America, the North Atlantic, and Europe as a function of time for April 1987. We present the results of these calculations, mainly as animations showing the time dependence of the column burden (vertical integral of concentration) and of sulfate wet deposition, and interpret the temporal evolution in terms of the controlling meteorological phenomena as discerned from synoptic analyzes at 925 and 500 hPa. The analyzes show highly localized and episodic buildup and removal of sulfate and SO2 and circulations under the influence of high- and low-pressure systems as well as instances of rapid meridional and zonal transport over distances of thousands of kilometers. Key meteorological phenomena giving rise to large amounts of sulfate include not only slow moving high-pressure systems, as previously recognized, but also low-pressure systems. Whether or not a given low-pressure system results in large sulfate loading depends strongly on its location relative to sources of sulfur species and of the water vapor that is required for formation of clouds and precipitation. We present a detailed analysis of the evolution of sulfate and SO2 over the eastern North Atlantic April 3–8, 1987, under the influence of a cutoff low-pressure system during which sources in northern Europe continued to feed a system that repeatedly circulated over source regions. Animations such as these facilitate identification of episodes of large aerosol loading and relating these loadings to the controlling meteorological phenomena.