Mercury emissions from the August 2001 wildfires in Washington State and an agricultural waste fire in Oregon and atmospheric mercury budget estimates

Abstract

Smoke plumes from large wildfires burning in late August 2001 in the temperate forests along the eastern slopes of the Cascade Mountains in Washington State and from a wheat stubble fire on the northeastern plain of Oregon were sampled for mercury from the University of Washington's Convair 580 research aircraft. Total gaseous mercury (TGM) concentrations as high as 7.5 ng/m3, a six‐time enhancement over background, were measured. The TGM was linearly correlated with CO (R2 = 0.805, where R is a 95% confidence level regression coefficient). Mercury was speciated as elemental mercury (>95%) and as mercury contained in the particulate fraction of the smoke. Emission factors (EF) were calculated from TGM and CO emissions and from a combustion efficiency of 0.9, adjusted for mercury contained in particulates, to yield an EF for total mercury of (113 ± 59) μg Hg per kilogram consumed fuel. The amount of mercury released from temperate forests in North America was extrapolated from our EF measurements and from National Interagency Fire Center estimates of burn areas. The average release for the period 1997–2001 is estimated to be 3.7 t Hg/yr, which is small relative to anthropogenic emissions. More significant is the contribution from fires in boreal forests (∼22.1 t Hg/yr) because of their much larger burn area. A measurement in smoke from a wheat stubble fire in Oregon showed mercury emissions as well, although with a lower EF. However, since agricultural waste burning is large globally, mercury from this source is also important in the global atmospheric cycling of mercury.