Review on the pollution characteristics and light absorption effects of Nitrophenol-like compounds in ambient air

Nitrophenol-like compounds (NPCs) have attracted considerable scientific attention due to their ecotoxicity and light-absorbing capacity. Research studies on the chemical composition, spatiotemporal variations, particle size distributions, gas-particle partitioning, primary emissions and secondary processes, and light absorption properties of NPCs are summarized in this work. The NPCs identified in ambient air consist mainly of 4-nitrophenol, 4-nitrocatechol, methyl-nitrophenols, and nitro-salicylic acids. Due to intensive emissions of primary particles (e.g. biomass burning) and gaseous precursors (e.g. aromatic hydrocarbons and nitrogen oxides), concentrations of NPCs were higher in urban areas than in rural areas. The temporal variations in NPC concentrations were characterized by a pattern of high winter and low summer values, which could be attributed to the influence of primary emissions, meteorological conditions, and the “photobleaching” effect. While its diurnal pattern strongly depends on the season. The particulate NPCs from primary emissions are mainly distributed in fine particles with Dp x. Currently, correlation analysis and receptor modeling based on racer data have been the primary methods for determining the sources of NPCs. The results of positive matrix factorization (PMF) showed that secondary formation is the main source of NPCs in summer, while biomass burning and coal combustion dominates in winter. Traffic emissions contribute significantly in all seasons in urban areas. NPCs have strong light absorption in both the near UV and visible wavelength ranges. Besides source emissions, the contribution of NPCs to the total light absorption of organic aerosols was also impacted by the pH of the aqueous phase of the aerosol and was 1—2 orders of magnitude higher than their mass contribution. Future research is warranted to improve the characterization of gaseous NPCs and to investigate high molecular weight NPCs, so as to deepen our understanding of the environmental behaviors, ecological and climate impacts of NPCs.