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Abstract
Brown carbon (BrC) particles, emitted from incomplete combustion, are characterized by a large uncertainty in their light-absorption properties (defined as the imaginary component of the index of refraction, k). In this work, we present a method to isolate different classes of BrC, capitalizing on the correlation between BrC’s light absorption properties and their solubility in organic solvents such as methanol. Therein, we divided BrC produced from biomass combustion into a methanol-soluble (MSBrC) and a methanol-insoluble (MISBrC) fraction and estimated the average light-absorption properties of each. We found that, although the MSBrC fraction dominated the BrC by mass (~90%), a major fraction of light absorption (~70% at 532 nm) was contributed by the MIBrC. Further, k values for the MIBrC were typically 2 orders of magnitude larger than those of the MSBrC at 532 nm. The physicochemical properties of BrC are associated with the combustion conditions under which they were produced. This variability in physicochemical properties could cause different toxicity outcomes. To evaluate the importance of combustion conditions, we used a highly controlled combustion setup, simulating smoldering and flaming combustion, to produce BrC from the combustion of toluene. We then exposed cells in vitro to the produced BrC at different exposures and assessed the cell viability (using WST-8) after 24 hours. We found that the BrC produced at the lower temperatures (smoldering) was significantly more toxic than the BrC produced at the higher temperature (flaming), with cell viabilities of 25% and 65%, respectively, at the highest exposure.
After they are emitted, biomass combustion emissions evolve in the atmosphere, undergoing myriad processes, such as oxidation and evaporation, that lead to fundamentally different particle compositions over time. Inside an environmental chamber, we burnt biomass fuels and initiated photooxidation reactions to simulate such atmospheric evolution. We collected fresh and aged particles and evaluated their toxicity to cells using the WST-8 cell viability assay. We found that the fresh particles were significantly more toxic to cells than the aged ones.