Dependence of Secondary Organic Aerosol Formation on Combustion Conditions in Simulated Prescribed Fires

MUHAMMAD ABDURRAHMAN, Michael Caraway, Thomas Carroll, Kruthika Kumar, Anita Anosike, Nicholas Kusumo, Ariana Deegan, Ryan Poland, John Allen, Mac A. Callaham, Joseph O'Brien, Edward Fortner, Amanda Frossard, Geoffrey Smith, Rawad Saleh, University of Georgia

     Abstract Number: 410
     Working Group: Combustion

Abstract
We examined the dependence of secondary organic aerosol (SOA) formation on combustion conditions for burns representative of prescribed fires, as part of the second Georgia Wildland Fire Simulation Experiments (G-WISE-2). Combustion experiments were performed using fuel beds that featured fine and woody surface fuels as well as a forest floor organic layer (often termed duff by fire managers) for select experiments. We created different combustion conditions by systematically varying surface fuel compactness, mass loading (250 g/m2 – 1500 g/m²), moisture content (5% - 30%), fuel bed area (0.5 m2 - 1.0 m²), and wind speed (0.5 m/s – 1 m/s). Emissions were aged in an oxidation flow reactor and characterized using a scanning mobility particle sizer (SMPS), a soot particle high-resolution aerosol mass spectrometer (SP-AMS), and a real-time VOC analyzer (AROMA-VOC). OH exposure, estimated via Toluene decay, indicated an equivalent atmospheric photochemical age of 4 to 6 days across all experiments. Results show a strong dependence of SOA formation on combustion conditions, quantified using fire radiative energy normalized by fuel-bed mass (FREnorm). SOA formation from burns involving only surface fuels increased with both fuel moisture content and mass loading but decreased as the fuel-bed area expanded. The highest SOA yields were observed under low-efficiency combustion conditions, particularly in burns containing duff and in compacted fuel beds, producing nearly an order of magnitude more SOA compared to burns with non-compacted surface fuels alone. Overall, our results reveal a strong inverse correlation between FREnorm and SOA formation. Specifically, inefficient combustion, marked by lower FREnorm, generates higher levels of SOA precursors leading to elevated SOA formation. These findings suggest that the large variability in SOA formation in biomass-burning emissions reported in the literature are in part due to differences in combustion conditions across different studies.