Ambient observations of chlorine-containing oxygenated organic molecules in summer Nanjing using ultrahigh-resolution Orbitrap mass spectrometry

Chlorine radicals (Cl) formed from reactive chlorine compounds are highly efficient oxidants for volatile organic compounds (VOCs) in the atmosphere, both in marine influenced regions and polluted urban environments, owing to their exceptionally high reactivity. While the reaction kinetics of Cl with VOCs have been extensively investigated in laboratory studies, and several modeling and chamber studies have explored the impacts of Cl-initiated oxidation on secondary organic aerosol (SOA) formation and atmospheric composition, direct ambient observations of chlorine-containing oxygenated organic molecules (Cl-OOMs) remain extremely limited. In this study, we report comprehensive field observations of Cl-OOMs in the urban atmosphere of Nanjing during summer, using an ultrahigh-resolution Orbitrap mass spectrometer coupled with a nitrate (NO₃^-) chemical ionization source. More than 40 distinct Cl-OOMs were unambiguously identified, among which, chlorinated nitrophenol-related compounds exhibited the highest concentrations. The majority of Cl-OOMs showed pronounced daytime maxima, consistent with enhanced photochemical activity, although several species displayed elevated nighttime concentrations. These compounds are likely formed through atmospheric oxidation of VOCs involving Cl radicals, frequently in combination with other oxidants such as OH and NO₃ radicals. In addition, regional transport and the oxidation of chlorinated VOC precursors by other oxidants may also contribute to the observed Cl-OOMs. This work provides rare ambient evidence for the existence and diversity of Cl-OOMs, bridging the gap between laboratory studies and real atmospheric conditions. The results offer new constraints for understanding Cl-initiated VOC oxidation pathways and their potential role in urban atmospheric chemistry.