In-Cloud Processes Serve as a Potential Formation Pathway of Atmospheric Chlorinated Organic Compounds

While chlorinated organic compounds (OrgCl) pose health risks, their molecular composition and formation pathways in the atmosphere remain largely unexplored. In this study, OrgCl molecules in cloudwater and aerosols collected from Mt. Tianjing in southern China were characterized using Fourier transform ion cyclotron resonance mass spectrometry. A total of 17–74 and 18–79 OrgCl formulas were identified in cloudwater and aerosols, respectively, accounting for 0.5–3.1% and 0.3–8.2% of the total detected molecular abundance. CHONCl (e.g., C6H4NO3Cl, C6H10NO6Cl) predominated in cloudwater, comprising 67% of the total OrgCl abundance, whereas CHOCl were more prevalent in aerosols (64%). OrgCl unique to cloudwater contained fewer –CH2 groups and a higher proportion of CHONCl with oxidized nitrogen functionalities. OrgCl in cloudwater and aerosols likely forms a distinct molecular group compared to the primary sources-derived OrgCl reported in previous studies. Correlation and precursor–product pair analysis indicate that the “–H + Cl” substitution pathway accounted for 70.4% and 51.5% of OrgCl in cloudwater and aerosols, respectively. The lower proportion observed in aerosols is likely attributable to limited chlorine availability. Chlorination preferentially occurred in aliphatic/peptide-like and carbohydrate-like compounds. Given the potential toxicity of OrgCl, further research into their formation mechanisms and atmospheric behaviors is essential.