The influence of atmospheric transport on precipitation chemistry at two sites in the midwestern United States

Abstract

Cluster analyses of two-dimensional mixed layer back trajectory data were used to determine what fraction of chemical variability in precipitation composition could be related to differences in atmospheric transport. Trajectories arriving at two different sites, Rockport, Indiana, and Gaylord, Michigan, were clustered to identify events occurring with similar transport patterns. It was found that certain transport situations resulted in significantly higher concentrations and depositions of the major ions, H +, SO 4 −, NO 3 − and NH 4 −. At Rockport, the greatest fraction of acid deposition was associated with low wind speeds. At Gaylord, transport direction played a greater role than transport speed in influencing precipitation composition. Results presented here suggest that 10%–40% of the variability in ion concentrations may be related to differences in atmospheric transport. The residual variation in concentrations was correlated with differences in the occurrence of upwind precipitation, precipitation type, and variation in precipitation amount.