Seasonal Changes of Brown Carbon in an Industrial City of Central China: Importance of Water-Insoluble BrC

A total of 125 samples were collected to investigate the light absorption characteristics and formation mechanism of brown carbon (BrC) in Luoyang. The seasonal range of BrC light absorption was from 2.34 Mm^− 1 to 26.60 Mm^− 1, with extremely high in autumn and winter specifically. The seasonal water-soluble BrC light absorption (2*b_{abs, WS−BrC, 405 nm}/b_{abs, BrC, 405 nm}) varied from 13 to 73%. All the seasonal mean values of the Absorption Ångström Exponent (AAE_BrC) of brown carbon were greater than 3. Secondary organic carbon (SOC) had significantly positive correlation to b_{abs, BrC, 405 nm} when SOC to elemental carbon ratio (SOC/EC) > 1 (R = 0.45, p < 0.05) and water-soluble organic carbon to organic carbon (WSOC/OC) < 0.45 (R = 0.49, p < 0.05), indicating that water-insoluble SOC potentially contributed to BrC in autumn and winter. In autumn, secondary transformations are dominant when NO₃⁻/EC > 3 (R = 0.48, p < 0.01), and the secondary organic aerosols formed by the oxidation of organic gases are the main cause of BrC light absorption, therefore the formation of water-insoluble SOC is related to secondary transformations of nitrogen oxides (NO_x) and volatile organic compounds (VOCs). In winter, the correlation between BrC light absorption and SOC was higher than in autumn, this may be related to the lower winter temperatures, which benefit the condensation/coalescence of semi-volatile organic compounds into organic particulate matter, therefore more water-insoluble SOC is formed in winter. Overall, the results highlighted that it is crucial to alleviate BrC light absorption to keep eyes on water-insoluble SOC.