Particulate matter and black carbon optical properties and emission factors from prescribed fires in the southeastern United States

Aerosol optical properties of biomass burning emissions are critical parameters determining how these emissions impact the Earth's climate. Despite their importance, field measurements of aerosol optical properties from fires remain scarce. Aerosol emissions from prescribed fires of forested and grass plots in the southeastern United States were measured and compared to emissions from laboratory simulations. Fine particulate matter (PM2.5), black carbon (BC), and aerosol light scattering and absorption were characterized for all fires. Refractory BC emission factors (EFs) measured at ground level (~2 m) were 0.76 ± 0.15 g/kg, comparable to the 0.93 ± 0.32 g/kg measured aloft (~100–600 m). However, PM EFs measured by aircraft were only 18% (5.4 ± 2.0 g/kg) of those measured on the ground (28.8 ± 9.8 g/kg). Such large differences in PM EFs for the same fire have not been previously reported and may plausibly be due to the differing particle measurement methodologies being applied but also likely related to partitioning of organic compounds to the gas phase as the plume dilutes aloft. Higher PM EFs on the ground may also be related to a higher contribution from smoldering combustion. The absorption Ångström exponents (αa) for the high intensity South Carolina fires were 3.92 ± 0.6, which was larger than prescribed forest fire in Florida (2.84) and the grass fire in Florida (2.71), implying a larger absorption contribution from brown carbon from higher‐intensity fires. Aerosol optical properties from laboratory simulations did not represent field measurements. Key Points Lab fires do not accurately reproduce field measures of biomass burning aerosol optical properties PM, but not BC, emissions factors vary greatly in the lofted plume compared to ground‐level Biomass burning emissions are an external mixture with at most 10% rBC particles