Atmospheric Processing of Nitrophenols and Nitrocresols From Biomass Burning Emissions

We present the analysis of the atmospheric budget of nitrophenols and nitrocresols, a class of nitroaromatics that raise great ecosystem and health concerns due to their phytotoxic and genotoxic properties, during the spring wheat harvest season in Eastern China. Significant quantities with maximum concentrations over 100 pptv and distinct diurnal patterns that peak around midnight and maintain low levels throughout the day were observed, in coincidence with the extensive open crop residue burning activities conducted in the vicinity. An observationally constrained zero-dimension box model was constructed to assess the relative importance of various production and removal pathways at play in determining the measured surface concentrations. The NO₃-initiated dark chemistry, in concert with meteorological variations predominantly dilution and entrainment, exerts major controls over the observed diurnal behaviors of nitrophenols and nitrocresols. Structural isomerism is predicted to have a significant impact on the multiphase partitioning and chemistry of nitrophenol isomers. Furthermore, simulations show that an appreciable amount of nitrophenols is present in the aerosol water, thereby representing an important source of water-soluble brown carbon in atmospheric aerosols under the humid subtropical weather prevailing during the campaign. Sensitivity analysis performed on the model parameterizations of reaction schemes helps to further understand the chemistry underlying the diurnal cycles. Implementing NO-dependent yields of cresols from toluene photooxidation improves the model predictions of nitrocresols at low NO ranges (<1 ppb), thereby underscoring the complexity of the peroxy radical reaction pathways from toluene photooxidation under atmospheric relevant conditions.