The characteristics of atmospheric brown carbon in Xi'an, inland China: sources, size distributions and optical properties

To investigate the characteristics of atmospheric brown carbon (BrC) in the semiarid region of East Asia, PM2.5 and size-resolved particles in the urban atmosphere of Xi'an, inland China, during the winter and summer of 2017 were collected and analyzed for optical properties and chemical compositions. Methanol extracts (MeOH extracts) were more light-absorbing than water extracts (H2O extracts) in the optical wavelength of 300-600 nm and well correlated with nitrophenols, polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (r > 0.78). The light absorptions (abs(lambda=365 nm)) of H2O extracts and MeOH extracts in winter were 28 +/- 16 and 49 +/- 32Mm(-1), respectively, which are about 10 times higher than those in summer, mainly due to the enhanced emissions from biomass burning for house heating. Water-extracted BrC predominately occurred in the fine mode (< 2.1 mu m) during winter and summer, accounting for 81% and 65% of the total absorption of BrC, respectively. The light absorption and stable carbon isotope composition measurements showed an increasing ratio of abs(lambda=365 nm)-MeOH to abs(lambda=550 nm)-EC along with an enrichment of C-13 in PM2.5 during the haze development, indicating an accumulation of secondarily formed BrC (e.g., nitro-phenols) in the aerosol aging process. Positive matrix factorization (PMF) analysis showed that biomass burning, fossil fuel combustion, secondary formation, and fugitive dust are the major sources of BrC in the city, accounting for 55 %, 19 %, 16 %, and 10% of the total BrC of PM2.5, respectively.