Particulate-phase and gaseous elemental mercury emissions during biomass combustion:: Controlling factors and correlation with particulate matter emissions

Mercury emissions from wildfires are significant natural sources of atmospheric mercury, but little is known about what controls speciation of emissions important to mercury deposition processes. The goal of this study was to quantify gaseous elemental mercury (GEM) and particulate-phase mercury (PHg) emissions from biomass combustion to identify key factors controlling the speciation. Emissions were characterized in an exhaust stack 17 m above fires using a gaseous mercury analyzer and quartz-fiber filters. Fuels included fresh and air-dried leaves, needles, and branches with different fuel moistures (9-95% of dry weight) and combustion properties (e.g., from <10 to 90% of fire durations characterized by flaming phases). Fuel moisture was the overall driving factor defining emissions,with GEM being the dominant fraction (>= 95%) in low moisture fuels and substantial PHg contributions-up to 50% of total mercury emissions-in fresh fuels. High PHg emissions were observed during smoldering combustion whereas flaming-dominated fires showed insignificant PHg emissions. PHg mass emissions were correlated with particulate matter (PM; r(2) = 0.67), organic carbon (OC; r(2) = 0.63) and sulfur (S; r(2) = 0.46) mass emissions, but not with elemental carbon (EC) nor with the total mercury emissions. These data suggest that the formation of PHg involves similar processes as the formation of particulate OC, for example condensation of volatile species onto preexisting smoke particles during cooling and dilution. Based on the observed relationship between PM and OC mass concentrations and published emission inventories, we estimate global PHg emissions by wildfires of 4-5 Mg yr(-1).