Brown carbon absorption linked to organic mass tracers in biomass burning particles
Abstract. Traditional gas and particle phase chemical markers used to identify the presence of biomass burning (BB) emissions were measured for a large forest fire near Boulder, Colorado. Correlation of the organic matter mass spectroscopic m/z 60 with measured particle light absorption properties found no link at 532 nm, and a strong correlation at 404 nm. Non-black carbon absorption at 404 nm was well correlated to the ratio of the mass fractions of particulate organic matter (POM) that was m/z 60 (f60) to m/z 44 (f44). The f60 to f44 ratio did not fully explain the variability in non-BC absorption, due to contributions of brown carbon (BrC) absorption and absorption due to internal mixing of POM with black carbon (BC). The absorption Ångstrom exponent (ÅAbs) showed a good correlation to f60/f44; however the best correlation resulted from the mass absorption efficiency (MAE) of BrC at 404 nm (MAEPOM-404 nm) and f60/f44. This result indicates that the absorption of POM at low visible and UV wavelengths is linked to emissions of organic matter that contribute to the m/z 60 mass fragment, although they do not contribute to 532 nm absorption. m/z 60 is often attributed to levoglucosan and related compounds. The linear relationship between MAEPOM-404 nm and f60/f44 suggests that the strength of BrC absorption for this fire can be predicted by emissions of f60-related organic matter.
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