Light-absorbing organic carbon from prescribed and laboratory biomass burning and gasoline vehicle emissions

Light-absorbing organic carbon (OC), also termed brown carbon (BrC), from laboratory-based biomass burning (BB) has been studied intensively to understand the contribution of BB to radiative forcing. However, relatively few measurements have been conducted on field-based BB and even fewer measurements have examined BrC from anthropogenic combustion sources like motor vehicle emissions. In this work, the light absorption of methanol-extractable OC from prescribed and laboratory BB and gasoline vehicle emissions was examined using spectrophotometry. The light absorption of methanol extracts showed a strong wavelength dependence for both BB and gasoline vehicle emissions. The mass absorption coefficients at 365 nm (MAC₃₆₅, m² g^-1C) - used as a measurement proxy for BrC - were significantly correlated (p < 0.05) to the elemental carbon (EC)/OC ratios when examined by each BB fuel type. No significant correlation was observed when pooling fuels, indicating that both burn conditions and fuel types may impact BB BrC characteristics. The average MAC₃₆₅ of gasoline vehicle emission samples is 0.62 ± 0.76 m² g^-1C, which is similar in magnitude to the BB samples (1.27 ± 0.76 m² g^-1C). These results suggest that in addition to BB, gasoline vehicle emissions may also be an important BrC source in urban areas.