Estimating mercury emissions resulting from wildfire in forests of the Western United States

Abstract

Understanding the emissions of mercury (Hg) from wildfires is important for quantifying the global atmospheric Hg sources. Emissions of Hg from soils resulting from wildfires in the Western United States was estimated for the 2000 to 2013 period, and the potential emission of Hg from forest soils was assessed as a function of forest type and soil-heating. Wildfire released an annual average of 3100±1900kg-Hgy−1 for the years spanning 2000–2013 in the 11 states within the study area. This estimate is nearly 5-fold lower than previous estimates for the study region. Lower emission estimates are attributed to an inclusion of fire severity within burn perimeters. Within reported wildfire perimeters, the average distribution of low, moderate, and high severity burns was 52, 29, and 19% of the total area, respectively. Review of literature data suggests that that low severity burning does not result in soil heating, moderate severity fire results in shallow soil heating, and high severity fire results in relatively deep soil heating (<5cm). Using this approach, emission factors for high severity burns ranged from 58 to 640μg-Hgkg-fuel−1. In contrast, low severity burns have emission factors that are estimated to be only 18–34μg-Hgkg-fuel−1. In this estimate, wildfire is predicted to release 1–30gHgha−1 from Western United States forest soils while above ground fuels are projected to contribute an additional 0.9 to 7.8gHgha−1. Land cover types with low biomass (desert scrub) are projected to release less than 1gHgha−1. Following soil sources, fuel source contributions to total Hg emissions generally followed the order of duff>wood>foliage>litter>branches.