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

The loading of tropospheric aerosols is highly variable spatially and tempo rally as a consequences of large spatial variability in sources, temporal v ariability in transport winds, and short residence times (days), and additi onally for secondary sulfate, intermittent production associated with aqueo us-phase reaction in clouds, and intermittent removal, mainly by precipitat ion. We have used a chemical transport and transformation model for atmosph eric sulfur driven by observationally derived meteorological data to calcul ate the geographical distribution of sulfate and sulfur dioxide (SO2) over North America, the North Atlantic, and Europe as a function of time for Apr il 1987. We present the results of these calculations, mainly as animations showing the time dependence of the column burden (vertical integral of con centration) and of sulfate wet deposition, and interpret the temporal evolu tion in terms of the controlling meteorological phenomena as discerned from synoptic analyzes of 925 and 500 hPa. The analyzes show highly localized a nd episodic buildup and removal of sulfate and SO2 and circulations under t he influence of high- and low-pressure systems as well as instances of rapi d meridional and zonal transport over distances of thousands of kilometers. Key meteorological phenomena giving rise to large amounts of sulfate inclu de not only slow moving high-pressure systems, as previously recognized, bu t also low-pressure systems. Whether or not a given low-pressure system res ults in large sulfate loading depends strongly on its location relative to sources of sulfur species and of the water vapor that is required for forma tion of clouds and precipitation. We present a detailed analysis of the evo lution 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 epis odes of large aerosol loading and relating these loadings to the controllin g meteorological phenomena.