Temporal Trends in Mercury Wet Deposition Across the U.S. and Canada

Peter Scott Weiss-Penzias, University of California Santa Cruz, Santa Cruz, CA, United States, David Allen Gay, National Atmospheric Deposition Program, Madison, United States, Mark E Brigham, United States Geological Survey, Mounds View, MN, United States, Matthew Parsons, Environment Canada, Edmonton, AB, Canada and Arnout ter Schure, Electric Power Research Institute, Palo Alto, CA, United States

Contact First Author: Peter Scott Weiss-Penzias; pweiss@ucsc.edu

Previously Published Material: some of these early results were reported at the National Atmospheric Deposition Meeting 2014 October, but we have moved well beyond these reported results.

Abstract ID#: 36861

 

English Abstract:
To assess whether reductions in mercury (Hg) emissions from electrical generation units and other sources in the United States and Canada have translated into changes in Hg concentrations in precipitation, we assessed spatial gradients and temporal trends in mercury (Hg) wet deposition and Hg air concentration data. We obtained wet deposition Hg data from 185 National Atmospheric Deposition Program (NADP) sites across the U.S. and Canada, 128 of which have records of at least 5 years, and 27 of which have complete records from 1998-2013.

Annual rates of change in Hg concentration and Hg deposition rates were calculated using two different methods; (1) the Seasonal Kendall-Mann non-parametric method, and (2) a Fourier transform linear regression parametric method using precipitation depth as an input variable. For this presentation, we will also show results from both methods, where they agree, where they disagree, and potential reasons for this disagreement.

The results from both methods agreed in showing widespread decreasing Hg concentration trends (p<0.05) across most regions in the eastern half of the continent (-0.5 to < -3 %/yr) and significant increasing Hg concentration trends (+1.5 to > +3 %/yr), primarily located in the U.S. Great Plains and Rocky Mountain/Desert regions. Some trends are consistent region-wide, while other regions show significant between–site and –season variation. The downward trend in precipitation Hg concentrations observed at many sites with data beginning before 1998 is becoming significantly less steep in recent years. Finally, best estimates of trends over North America will be provided for use by others.

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