The formation of secondary brown carbon (BrC) is chemically complex, leading to an unclear relationship between its molecular composition and optical properties. Here, we present an in-depth investigation of molecular-specific optical properties and aging of secondary BrC produced from the photooxidation of ethylbenzene at varied NOx levels for the first time. Due to the pronounced formation of unsaturated products, the mass absorption coefficient (MAC) of ethylbenzene secondary organic aerosols (ESOA) at 365 nm was higher than that of biogenic SOA by a factor of 10. A high NOx level ([ethylbenzene]0/[NOx]0 < 10 ppbC ppb-1) was found to significantly increase the average MAC300-700nm of ESOA by 0.29 m2 g-1. The data from two complementary high-resolution mass spectrometers and quantum chemical calculations suggested that nitrogen-containing compounds were largely responsible for the enhanced light absorption of high-NOx ESOA, and multifunctional nitroaromatic compounds (such as C8H9NO3 and C8H9NO4) were identified as important BrC chromophores. High-NOx ESOA underwent photobleaching upon direct exposure to ultraviolet light. Photolysis did not lead to the significant decomposition of C8H9NO3 and C8H9NO4, indicating that nitroaromatic compounds may serve as relatively stable nitrogen reservoirs and would effectively absorb solar radiation during the daytime.
Keywords: aqueous photolysis; aromatic hydrocarbons; nitroaromatic compounds; optical properties; secondary brown carbon.