Emissions of Fine Particle Fluoride from Biomass Burning

The burning of biomasses releases fluorine to the atmosphere, representing a major and previously uncharacterized flux of this atmospheric pollutant. Emissions of fine particle (PM2.5) water-soluble fluoride (F−) from biomass burning were evaluated during the fourth Fire Laboratory at Missoula Experiment (FLAME-IV) using scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) and ion chromatography with conductivity detection. F− was detected in 100% of the PM2.5 emissions from conifers (n = 11), 94% of emissions from agricultural residues (n = 16), and 36% of the grasses and other perennial plants (n = 14). When F− was quantified, it accounted for an average (±standard error) of 0.13 ± 0.02% of PM2.5. F− was not detected in remaining samples (n = 15) collected from peat burning, shredded tire combustion, and cook-stove emissions. Emission factors (EF) of F− emitted per kilogram of biomass burned correlated with emissions of PM2.5 and combustion efficiency, and also varied with the type of biomass burned and the geographic location where it was harvested. Based on recent evaluations of global biomass burning, we estimate that biomass burning releases 76 Gg F− yr−1 to the atmosphere, with upper and lower bounds of 40−150 Gg F− yr−1. The estimated F− flux from biomass burning is comparable to total fluorine emissions from coal combustion plus other anthropogenic sources. These data demonstrate that biomass burning represents a major source of fluorine to the atmosphere in the form of fine particles, which have potential to undergo long-range transport.