Brown Carbon Emissions\nfrom Biomass Burning under\nSimulated Wildfire and Prescribed-Fire Conditions

We investigated the\nlight-absorption properties of brown carbon\n(BrC) as part of the Georgia Wildland-Fire Simulation Experiment.\nWe constructed fuel beds representative of three ecoregions in the\nSoutheastern U.S. and varied the fuel-bed moisture content to simulate\neither prescribed fires or drought-induced wildfires. Based on decreasing\nfire radiative energy normalized by fuel-bed mass loading (FRE<sub>norm</sub>), the combustion conditions were grouped into wildfire\n(Wild), prescribed fire (Rx), and wildfire involving duff ignition\n(WildDuff). The emitted BrC ranged from weakly absorbing (WildDuff)\nto moderately absorbing (Rx and Wild) with the imaginary part of the\nrefractive index (<i>k</i>) values that were well-correlated\nwith FRE<sub>norm</sub>. We apportioned the BrC into water-soluble\n(WSBrC) and water-insoluble (WIBrC). Approximately half of the WSBrC\nmolecules detected using electrospray-ionization mass spectrometry\nwere potential chromophores. Nevertheless, <i>k</i> of WSBrC\nwas an order of magnitude smaller than <i>k</i> of WIBrC.\nFurthermore, <i>k</i> of WIBrC was well-correlated with\nFRE<sub>norm</sub> while <i>k</i> of WSBrC was not, suggesting\ndifferent formation pathways between WIBrC and WSBrC. Overall, the\nresults signify the importance of combustion conditions in determining\nBrC light-absorption properties and indicate that variables in wildland\nfires, such as moisture content and fuel-bed composition, impact BrC\nlight-absorption properties to the extent that they influence combustion\nconditions.