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

Elevated levels of molecular chlorine (Cl 2 ) have been observed during both daytime and nighttime based on limited short-term field observations. However, the long-term trend and sources of Cl 2 remain poorly understood, especially regarding the key factors driving nocturnal Cl 2 formation under moderately acidic conditions of aerosol. In this study, over a year of continuous Cl 2 measurements in urban Beijing revealed a continuous nocturnal increase (from 19:00 to 05:00) in Cl 2 concentration (up to 242 ppt), accompanied by increases in ClNO 2 concentration and aerosol pH. The mean heterogeneous uptake coefficient of ClNO 2 (γ ClNO 2 ) decreased from 2.2 × 10 –4 to 3.6 × 10 –5 when the mean aerosol pH varied from 2.6 to 5.2, while Cl 2 concentrations were underestimated if only the variation of aerosol pH was accounted for. Multivariate nonlinear regression revealed that although high aerosol pH hindered the Cl 2 formation via heterogeneous uptake of ClNO 2, this process is possibly promoted by elevated aerosol ionic strength at high aerosol pH values, jointly driving nocturnal Cl 2 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.