Mercury emissions from burning of biomass from temperate North American forests: laboratory and airborne measurements

The emission of mercury from biomass burning was investigated in laboratory experiments and the results confirmed in airborne measurements on a wildfire near Hearst, Ont. Mercury contained in vegetation (live, dead, coniferous, deciduous) was essentially completely released in laboratory burns in the form of gaseous elemental mercury and mercury contained in particles. Replicate burns of dry Ponderosa needles indicated a linear relationship between emitted mercury and fuel mass loss. Regionally collected fuels showed the same behavior as the replicate burns, i.e. essentially total removal of mercury. Mercury released from fuel could be accounted for as gaseous and particulate mercury in the smoke. The mercury content of regionally collected fuels varied between 14 and 70 ng/g on a dry mass (dm) basis. The smoke plume from a small wildfire was investigated with a research aircraft yielding a mean output of 0.15 +/- 0.02 ng/m(3) of elemental mercury for each ppm of CO2 emitted. The particulate mercury determined by sampling at specific points in the plume was <0.083 ng/m(3) compared to elemental mercury of 0.56 ng/m(3) for the same air, supporting the conclusion that most of the mercury was emitted in the gaseous elemental form. Emission factors for the high/low mercury content samples of the laboratory burns were 14-71 x 10(-6) and 112 x 10(-6) g Hg/kg (dm) fuel for the wildfire. The difference is believed to be the contribution of mercury released from fire-heated soil. Mercury budgets extrapolated from this single wildfire gave upper emission limits of 66 t/yr for temperate/boreal forests. This large source estimate must be refined and included in future regional and global models. Forests are sinks for mercury already in the atmosphere, thus the wildfire "source" is part of the overall cycling of mercury originating from other sources. (C) 2002 Elsevier Science Ltd. All rights reserved.