Brown Carbon Emissions from Biomass Burning under Simulated Wildfire and Prescribed-Fire Conditions

We investigated the
light-absorption properties of brown carbon
(BrC) as part of the Georgia Wildland-Fire Simulation Experiment.
We constructed fuel beds representative of three ecoregions in the
Southeastern U.S. and varied the fuel-bed moisture content to simulate
either prescribed fires or drought-induced wildfires. Based on decreasing
fire radiative energy normalized by fuel-bed mass loading (FREnorm), the combustion conditions were grouped into wildfire
(Wild), prescribed fire (Rx), and wildfire involving duff ignition
(WildDuff). The emitted BrC ranged from weakly absorbing (WildDuff)
to moderately absorbing (Rx and Wild) with the imaginary part of the
refractive index (k) values that were well-correlated
with FREnorm. We apportioned the BrC into water-soluble
(WSBrC) and water-insoluble (WIBrC). Approximately half of the WSBrC
molecules detected using electrospray-ionization mass spectrometry
were potential chromophores. Nevertheless, k of WSBrC
was an order of magnitude smaller than k of WIBrC.
Furthermore, k of WIBrC was well-correlated with
FREnorm while k of WSBrC was not, suggesting
different formation pathways between WIBrC and WSBrC. Overall, the
results signify the importance of combustion conditions in determining
BrC light-absorption properties and indicate that variables in wildland
fires, such as moisture content and fuel-bed composition, impact BrC
light-absorption properties to the extent that they influence combustion
conditions.