Temporal trends of mercury in precipitation from the Mercury Deposition Network: 2008-2015

Peter Weiss-Penzias1 and David Gay2

We present an analysis of temporal trends in weekly mercury (Hg) concentrations in precipitation collected at 84 sites by the Mercury Deposition Network (MDN) over 2008-2015. Our previous work with MDN data over time periods ending in 2013 showed that 53%/0% of sites in the MDN had significant negative/positive trends in mercury wet deposition for 1997–2013, whereas for 2008–2013, 6%/30% of sites had significant negative/positive trends in mercury wet deposition.  This work concluded that over the more recent time period, there were many more sites with significant positive trends than sites with negative trends.  The increase in the number of sites with positive trends was primarily found in the Plains and Rocky Mountain regions.  In this current work we have included the most recent MDN Hg concentration data through 2015 to assess whether this pattern has continued.  For Hg concentration data in precipitation from 2008-2015, we found that 12%/19% of the sites in the MDN had significant negative/positive trends in Hg concentration in precipitation (i.e. there were fewer sites with significant positive trends using the newer data set).  The Plains region continued to have the highest proportion of MDN sites with significant positive trends (6 of 12), and a regional statistical test showed that the Plains was the only region having a significant upward trend in Hg concentration in precipitation.  In general, the western half of the continent contained the majority of sites with trends (both significant and not significant) > +2% per year (12 out of 27) compared to the sites in the east (5 out of 57).  Preliminary analysis also shows that he regional temporal patterns in Hg concentrations are correlated with patterns in the Palmer Drought Severity Index (PDSI); maximum regional Hg concentrations correspond to the maximum number of states in a region with at least one county having a PDSI designation of “extreme drought”.


1UC Santa Cruz, pweiss@ucsc.edu
2NADP, University of Illinois, dgay@illinois.edu