Apportioning global and non-global components of mercury deposition through 210Pb indexing

Our previous work has documented a correlation between Hg concentrations and Pb-210 activity measured in wet deposition that might be used to help apportion sources of Hg in precipitation. Here we present the results of a 27-month precipitation collection effort using co-located samplers for Hg and Pb-210 designed to assess this hypothesis. Study sites were located on the east and west coasts of North America, in the continental interior, and on the Florida Peninsula. Relatively high variability in Hg/Pb-210 ratios was found at all sites regionally and seasonally (e.g., overall: 0.99-9.13 ng dpm(-1)). The ratio of average volume-weighted Hg concentrations and Pb-210 activities showed consistent trends (higher in impacted area), with Glacier Bay in southeast Alaska, exhibiting the lowest value. Assuming that Glacier Bay represents a benchmark for a site with no regional contribution, we estimate less than 50% of the Hg input was global at the Seattle and Florida sites. Differences in Hg/Pb-210 in wet deposition could be due to either a regional/local source contribution of Hg, or a regional/local enhancement in the removal of Hg from the atmosphere (i.e., oxidants), however, this approach is not capable of discerning between these two possibilities. Thus, this method of source apportionment represents an estimate of the maximal amount of Hg contributed by regional sources and may be limited in regions of deep convective mixing. (C) 2012 Elsevier B.V. All rights reserved.