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. Emissions from biomass burning are an important source of mercury (Hg) tothe atmosphere and an integral component of the global Hg biogeochemicalcycle. In 2018, measurements of gaseous elemental Hg (GEM) were takenon board a research aircraft along with a series of co-emitted contaminantsin the emissions plume of an 88 km2 boreal forest wildfire on theGarson Lake Plain (GLP) in NW Saskatchewan, Canada. A series of four flighttracks were made perpendicular to the plume at increasing distances from thefire, each with three to five passes at different altitudes at each downwindlocation. The maximum GEM concentration measured on the flight was2.88 ng m−3, which is ≈ 2.4× background concentration. GEMconcentrations were significantly correlated with the co-emitted carbonspecies (CO, CO2, and CH4). Emissions ratios (ERs) were calculatedfrom measured GEM and carbon co-contaminant data. Using the most correlated(least uncertain) of these ratios (GEM:CO), GEM concentrations wereestimated at the higher 0.5 Hz time resolution of the CO measurements,resulting in maximum GEM concentrations and enhancements of 6.76 ng m−3and ≈ 5.6×, respectively. Extrapolating the estimated maximum 0.5 HzGEM concentration data from each downwind location back to source, 1 km and1 m (from fire) concentrations were predicted to be 12.9 and 30.0 ng m−3, or enhancements of ≈ 11× and ≈ 25×, respectively.ERs and emissions factors (EFs) derived from the measured data andliterature values were also used to calculate Hg emissions estimates onthree spatial scales: (i) the GLP fires themselves, (ii) all boreal forestbiomass burning, and (iii) global biomass burning. The most robust estimatewas of the GLP fires (21 ± 10 kg of Hg) using calculated EFs that usedminimal literature-derived data. Using the Top-down Emission Rate RetrievalAlgorithm (TERRA), we were able to determine a similar emission estimate of22 ± 7 kg of Hg. The elevated uncertainties of the other estimates andhigh variability between the different methods used in the calculationshighlight concerns with some of the assumptions that have been used incalculating Hg biomass burning in the literature. Among these problematicassumptions are variable ERs of contaminants based on vegetation type andfire intensity, differing atmospheric lifetimes of emitted contaminants, theuse of only one co-contaminant in emissions estimate calculations, and thepaucity of atmospheric Hg species concentration measurements in biomassburning plumes.

atmospheric chemistry and physics

Where there is smoke there is mercury: Assessing boreal forest fire mercury emissions using aircraft and highlighting uncertainties associated with upscaling emissions estimates