Particulate Hg (pHg) is a component of smoke from biomass burning and has the potential for local redeposition. Throughfall (precipitation collected beneath a conifer or deciduous canopy) and open precipitation samples were collected pre- and postfire in 2005 and 2006 using passive precipitation collectors across the Superior National Forest, located in northern Minnesota, USA. Samples were collected approximately every two weeks and analyzed for total Hg (THg) and methyl Hg (MeHg). THg concentrations increased significantly postfire in conifer throughfall (> 4x increase), open precipitation (2.5x), and when all canopy types were considered (2.9x). MeHg concentrations also increased after fire regardless of the covertype (conifer throughfall: 10x increase; open precipitation: 3.5x increase; deciduous throughfall: 1.7x increase; all canopy types analyzed together: 8x increase). Total Hg deposition increased significantly under conifer cover (3.8x). Methyl Hg deposition increased significantly after fire when all canopy types were analyzed together (4.6x) and in conifer throughfall (5.9x). Canopy type influenced the magnitude of postfire THg and MeHg increase and the duration of elevated MeHg levels. Particulate Hg present in forest fire smoke represents a short-term source of increased Hg in the atmosphere that is available for local redeposition during and following fire.

Atmospheric mercury deposition by wet and dry processes contributes mercury to terrestrial and aquatic systems. Factors influencing the amount of mercury deposited to boreal forests were identified in this study. Throughfall and open canopy precipitation samples were collected in 2005 and 2006 using passive precipitation collectors from pristine sites located across the Superior National Forest in northern Minnesota, USA. Samples were collected approximately every 2 weeks and analyzed for total (THg) and methyl mercury (MeHg). Forest canopy type and density were the primary influences on THg and MeHg deposition. Highest THg and MeHg concentrations were measured beneath conifer canopies (THg mean=19.02 ng L −1 ; MeHg mean=0.28 ng L −1 ) followed by deciduous throughfall (THg mean= 12.53 ng L −1 ; MeHg mean=0.19 ng L −1 ) then open precipitation (THg mean=8.19 ng L −1 ; MeHg mean= 0.12 ng L −1 ). The greater efficiency of conifers at scavenging THg and MeHg from the atmosphere may increase the risk of mercury related water quality issues in conifer-dominated systems.

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