Oxidative potential of atmospheric brown carbon in six Chinese megacities: Seasonal variation and source apportionment

Abstract

Brown carbon (BrC) not only has a light-absorbing ability affecting the global radiation balance and also posing adverse health effects by catalyzing the generation of reactive oxygen species (ROS). In this work, six representative megacities in China including Beijing, Harbin, Xi'an, Chengdu, Guangzhou, and Wuhan were selected to determine the seasonal difference of molecular structures and oxidative potential (OP) levels in BrC samples. Average OP concentration levels show a distinct seasonal difference with higher values in winter (0.41 ± 0.29 nmol H2O2 equivalent μg−1) than that in summer (0.27 ± 0.14 nmol H2O2 equivalent μg−1), attributed to the differences in source contributions that characterized by molecular structures of BrC with Fourier Transform-Infrared Spectroscopy (FT-IR). Quinones that contain aromatic functional group (with wavelengths 1640 cm−1 for C-C and 1385 cm−1 for C-O) can catalyze ROS production, while organic nitrogen-containing (C−ONO2) compounds can promote oxidative activity. These components exhibit a similar trend with higher abundances in winter than in summer. Using Positive Matrix Factorization (PMF) coupled with Artificial Neural Network (ANN)-Multilayer Perceptron (MLP) models, the average contribution of secondary formation source to OP in winter (26.3%) is lower than that in summer (43.1%). However, a reverse trend is seen for the combustion sources (i.e., biomass burning and coal combustion), while their contributions in winter (56.7%) are much higher those in summer (32.1%). In addition, the contribution of biomass-burning to OP is only resolved in winter (an average of 35.6%), reflecting the biomass fuel is the major sponsor to the seasonal OP levels. The findings of this study provide a basis for formulating effective policies to control the adverse effects of OP to atmospheric environment and human health in China.