High Aerosol Ionic Strength Possibly Affecting Nocturnal Chlorine Formation via Heterogeneous Reaction in Urban Beijing

Elevated levels of molecular chlorine (Cl₂) have been observed during both daytime and nighttime based on limited short-term field observations. However, the long-term trend and sources of Cl₂ remain poorly understood, especially regarding the key factors driving nocturnal Cl₂ formation under moderately acidic conditions of aerosol. In this study, over a year of continuous Cl₂ measurements in urban Beijing revealed a continuous nocturnal increase (from 19:00 to 05:00) in Cl₂ concentration (up to 242 ppt), accompanied by increases in ClNO₂ concentration and aerosol pH. The mean heterogeneous uptake coefficient of ClNO₂ (γ_ClNO2) decreased from 2.2 × 10^-4 to 3.6 × 10^-5 when the mean aerosol pH varied from 2.6 to 5.2, while Cl₂ concentrations were underestimated if only the variation of aerosol pH was accounted for. Multivariate nonlinear regression revealed that although high aerosol pH hindered the Cl₂ formation via heterogeneous uptake of ClNO₂, this process is possibly promoted by elevated aerosol ionic strength at high aerosol pH values, jointly driving nocturnal Cl₂ variability. This finding suggests that aerosol chemical properties, especially ionic strength, may play an important role in promoting reactive chlorine chemistry in polluted urban atmospheres.