Secondary Production of Gaseous Nitrated Phenols in Polluted Urban Environments

Nitrated phenols (NPs) are important atmospheric pollutants that affect air quality, radiation, and health. The recent development of the time-of-flight chemical ionization mass spectrometer (ToF-CIMS) allows quantitative online measurements of NPs for a better understanding of their sources and environmental impacts. Herein, we deployed nitrate ions as reagent ions in the ToF-CIMS and quantified six classes of gaseous NPs in Beijing. The concentrations of NPs are in the range of 1 to 520 ng m^-3. Nitrophenol (NPh) has the greatest mean concentration. Dinitrophenol (DNP) shows the greatest haze-to-clean concentration ratio, which may be associated with aqueous production. The high concentrations and distinct diurnal profiles of NPs indicate a strong secondary formation to overweigh losses, driven by high emissions of precursors, strong oxidative capacity, and high NO levels. The budget analysis on the basis of our measurements and box-model calculations suggest a minor role of the photolysis of NPs (<1 ppb h^-1) in producing OH radicals. NPs therefore cannot explain the underestimated OH production in urban environments. Discrepancies between these results and the laboratory measurements of the NP photolysis rates indicate the need for further studies aimed at understanding the production and losses of NPs in polluted urban environments.