Meteorological case studies of regional high sulfur episodes in the western United States

Meteorological conditions associated with regional scale episodes of elevated atmospheric sulfur concentrations over the regions of Arizona and New Mexico, and North and South Dakota were studied. Episodes were defined as the period when the atmospheric concentration of fine sulfur (diameters less than 2.5 μm) at each station in the region exceeded one standard deviation above the station's annual geometric mean. For the Arizona and New Mexico region, the most extraordinary episode occurred during September 1979 when an anticyclonic circulation aloft covered the entire region, while a planetary boundary layer (PBL) convergence zone existed over western Arizona and southeastern California. Convergence of pollutants in the PBL along with slow transport and dispersion by the anticyclonic flow took place during the episode. A similar flow pattern was responsible for an August 1981 episode. Copper smelters in southern Arizona and New Mexico apparently were the major cause of these episodes. Southerly or southeasterly flow during September 1982 again transported sulfur from the smelter region, causing another episode. A statistical study of backward air parcel trajectories from Grand Canyon National Park showed that southern Arizona and New Mexico, as well as the San Joaquin Valley of California, were potential source regions of atmospheric sulfur at the Grand Canyon. Long-range transport of pollutants from states east and/or southeast of North and South Dakota was the major cause of episodes in this region. These episodes occurred when high pressure systems were located over southern Canada or the northeastern United States. A statistical study of backward air parcel trajectories from Theodore Roosevelt National Park (North Dakota) showed that long-range transport from states east and/or southeast of the park was primarily responsible for high concentrations of fine sulfur. No clean air parcels arrived at the park from states to the east.