Key role of nitrogen-containing organic species in haze formation and health implications: an ion mobility-analysis based study

Despite the ubiquitous impacts on ecosystem and human health, the formation mechanisms of nitrogen-containing organic compounds (NOCs) remain poorly understood in the atmosphere. In this study, wintertime PM_2.5 samples were collected in Chongqing with molecular-level composition characterized by an ion mobility spectrometry (IMS) - time of flight mass spectrometry to elucidate key formation/evolution factors and health implications of NOCs in ambient particles. Generally, nitrogen-containing organic species (CHNOs) were found to be critical PM_2.5 components for haze formation and health impacts during polluted episodes. Evidence from IMS-derived collision cross sections calculation further illustrated organic nitrates/nitrites (ONs) as the main contributors to CHNOs observed in this study. Secondary transformation process was identified as the major formation pathway for CHNOs. Significant levels of NH₄⁺ and NO₂ accompanied by high relative humidity (RH) resulted in enhanced production of particulate CHNOs during wintertime haze episodes, which was also remediated by aerosol liquid water content and aerosol acidity. In addition, CHNOs (e.g., C₈H₁₃NO_8-9, C₉H₁₃NO₈, and C₉H₁₅NO_8-9) were found to be associated with acellular and cellular oxidative potential metrics (OP^AA, OP^GSH, and in vitro ROS). Further in vitro results from human bronchial epithelial (BEAS-2B) cells confirmed that both authentic PM_2.5 sampels and atmospherically relevant nitrogen containing species, including organonitrate/nitrite, nitrosamine, nitrophenol, amide, and amines, can significantly induce intracellular ROS and the expression of proinflammatory biomarkers (e.g., IL-8, NQO1, and Nrf2). By illustrating this very initial evidence between molecular-level CHNOs and oxidative stress effects characterized for wintertime PM_2.5, our study sheds light on future regional air quality mitigation strategies based on source, composition, and health implications under complex pollution conditions.