Brown Carbon in Primary and Aged Coal Combustion Emission

Smog chamber experiments were conducted to characterize the light absorption of brown carbon (BrC) from primary and photochemically aged coal combustion emissions. Light absorption was measured by the UV–visible spectrophotometric analysis of water and methanol extracts of filter samples. The single-scattering albedo at 450 nm was 0.73 ± 0.10 for primary emissions and 0.75 ± 0.13 for aged emissions. The light absorption coefficient at 365 nm of methanol extracts was higher than that of water extracts by a factor of 10 for primary emissions and a factor of 7 for aged emissions. This suggests that the majority of BrC is water-insoluble even after aging. The mass absorption efficiency of this BrC (MAE₃₆₅) for primary OA (POA) was dependent on combustion conditions, with an average of 0.84 ± 0.54 m² g^–1, which was significantly higher than that for aged OA (0.24 ± 0.18 m² g^–1). Secondary OA (SOA) dominated aged OA and the decreased MAE₃₆₅ after aging indicates that SOA is less light absorbing than POA and/or that BrC is bleached (oxidized) with aging. The estimated MAE₃₆₅ of SOA (0.14 ± 0.08 m² g^–1) was much lower than that of POA. A comparison of MAE₃₆₅ of residential coal combustion with other anthropogenic sources suggests that residential coal combustion emissions are among the strongest absorbing BrC organics.