Seasonal composition and source apportionment of nitrated aromatic compounds in atmospheric fine particulate matter in northern China over one year

Nitrated aromatic compounds (NACs) are an important chromophore component of atmospheric brown carbon (BrC) and can affect both the urban air quality and global climate. However, the compositions, sources of NACs in atmospheric particulate matter and its impact on the optical properties of BrC are still of very limited understood. In this study, the concentrations of 10 NACs and their light absorption contributions to BrC were investigated based on daily aerosol fine particle filter samples collected in Xi’an, Northwest China from 2015 to 2016. Both the concentrations and constitutions of NACs show distinct seasonal differences with average concentration of 2.05, 1.06, 12.90 and 56.32 ng m^-3 in spring, summer, fall and winter, respectively, which could be ascribed to the differences in emission sources and formation processes. The contributions of NACs to light absorption of BrC at wavelength between 300 to 500 nm was wavelength dependent and varied greatly over different seasons, with high contribution even at wavelength > 350 nm in fall and winter and contribution mainly at wavelength < 350 nm in spring and summer, which could be related to the differences in the composition of NACs and the specific light absorbing properties of each NAC. The mean contributions of NACs to the light absorption of BrC at wavelength of 365 nm were 0.14, 0.09, 0.36 and 0.91% during spring, summer, fall and winter, respectively, about 3-5 times higher than their corresponding mass fraction in total organic carbon. Further, the sources of NACs were achieved by positive matrix factorization (PMF) receptor model. The results show that the sources of NACs varies among different seasons. Specifically, vehicular emission and secondary formation are dominant sources in summer (~80%), while biomass burning and coal combustion are major sources in winter (~75%).