Formation of Secondary Nitroaromatic Compounds in Polluted Urban Environments

Abstract Nitroaromatic compounds (NACs) are important light absorption contributors to atmospheric brown carbon; however, their formation pathways and the key influential factors in polluted urban atmospheres are largely unknown. Herein, we present the molecular characterization of atmospheric particulate NACs in urban Shanghai by applying ultra‐high‐performance liquid chromatography coupled to Orbitrap mass spectrometry, and investigated their seasonal and diel variations and formation pathways. The total number and abundance of NACs increased significantly during air pollution episodes, and these NACs exhibited the greatest relative abundance among CHON compounds. The 12 quantified NACs exhibited higher levels in winter (52.3 ± 29.1 ng m−3) than summer (7.83 ± 3.30 ng m−3), largely owing to air masses mostly passing through northern heating regions. Among NACs, 4‐nitrophenol and 4‐nitrocatechol were the most abundant species. Most of NACs were enhanced via photooxidation during the day in summer, while the day‐night variations in winter were not as obvious. Further analysis of atmospheric formation relevance suggested that nitrophenol and its methylated derivatives were mainly formed through gas‐phase photochemical reactions, whereas aqueous‐phase oxidation processing could be an important pathway to form methyl‐nitrocatechols. Formation of NACs was relatively sensitive to NO2 under low‐NOx conditions, and became independent of NO2 under high‐NOx conditions. A larger fraction of NO2 was transformed into NACs than into inorganic nitrate products when nitrate was less abundant, implying that NACs would play more important roles in aerosol properties with the decline in nitrate concentrations.