Composition and sources of brown carbon aerosols in megacity Beijing during the winter of 2016

Brown carbon (BrC) is a class of atmospheric particles that can strongly absorb visible and near-ultraviolet radiation, and it has an impact on global climate change. BrC is still poorly understood because of its complex sources and compositions. In this research, the characteristics of the light absorption, chromophores and sources of BrC were explored from PM_2.5 collected in urban Beijing during the winter of 2016. Fairly high levels of the light absorption coefficient (Abs_λ) of methanol-extracted BrC and two categories of chromophores, i.e., nitro-aromatic compounds (NACs) and polycyclic aromatic hydrocarbons (PAHs), were found during haze episodes, with averages of 112.4 ± 8.31 M m⁻¹ (λ=365 nm), 513 ± 370 ng m⁻³ and 429 ± 244 ng m⁻³, respectively, which were approximately 5 times higher than those during clean periods. Both methanol-extracted BrC and chromophores showed distinct diurnal variations that were twice as high during the nighttime than during the daytime. The average contributions of NACs and PAHs to the methanol-extracted BrC bulk light absorption at λ = 365 nm (Abs₃₆₅) were 2.30% and 1.43%, respectively, which were approximately 4.6 and 3.9 times higher than the corresponding total organic mass fractions, respectively. Correlation analyses conducted on the NACs, Abs₃₆₅, NO₂ and relative humidity (RH) for the clean and haze episodes indicated that the aqueous-phase reaction with higher RH had a significant impact on the BrC during haze episodes in Beijing. The source apportionment of BrC by positive matrix factorization (PMF) indicated that coal combustion (39%), secondary formation (24%), and biomass burning (28%) were the major sources of BrC in the winter in Beijing. The contribution of secondary BrC sharply increased from 16% during clan periods to 29% during haze periods, indicating that secondary formation was an important source of BrC during haze episodes. The present research provides evidence that aqueous-phase transformation has a significant impact on BrC aerosols during haze periods in urban Beijing during the winter.