Long-term relationships between mercury wet deposition and meteorology

Daily-event precipitation samples collected in Underhill, VT from 1995 to 2006 were analyzed for total mercury and results suggest that there were no statistically significant changes in annual mercury wet deposition over time, despite significant emissions reductions in the Northeast United States. Meteorological analysis indicates that mercury deposition has not decreased as transport of emissions from major source regions in the Midwest and East Coast have consistently contributed to the largest observed mercury wet deposition amounts over the period. In contrast, annual volume-weighted mean (VWM) mercury concentration declined slightly over the 12-years, and a significant decrease was observed from CY 2001 to 2006. An increase in the total annual precipitation amount corresponded with the decline in annual VWM mercury concentration. Analysis suggests that the increase in precipitation observed was strongly related to changes in the amount and type of precipitation that fell seasonally, and this departure was attributed to a response in meteorological conditions to climate variability and the El Niño-Southern Oscillation (ENSO) cycle. Increased amounts of rainfall and mixed precipitation (mixture of rainfall and snowfall), particularly in the spring and fall seasons, enhanced annual precipitation amounts and resulted in declining VWM mercury concentrations during these periods. Thus, declines in concentration at the more remote Underhill site appear to be more directly linked to local scale meteorological and climatological variability than to a reduction in emissions of mercury to the atmosphere.