Comprehensive multiphase chlorine chemistry in the box model CAABA/MECCA: implications for atmospheric oxidative capacity

Tropospheric chlorine chemistry can strongly impact the atmospheric oxidation capacity and composition, especially in urban environments. To account for these reactions, the gas- and aqueous-phase inline-formulaCl chemistry of the community atmospheric chemistry box model Chemistry As A Boxmodel Application/Module Efficiently Calculating the Chemistry of the Atmosphere (CAABA/MECCA) has been extended. In particular, an explicit mechanism for inline-formulaClNO₂ formation following inline-formulaN₂O₅ uptake to aerosols has been developed. The updated model has been applied to two urban environments with different concentrations of inline-formulaNO_x (inline-formulaNO + inline-formulaNO₂): New Delhi (India) and Leicester (United Kingdom). The model shows a sharp build-up of inline-formulaCl at sunrise through inline-formulaCl₂ photolysis in both the urban environments. Besides inline-formulaCl₂ photolysis, inline-formulaClO+inline-formulaNO reaction and photolysis of inline-formulaClNO₂ and inline-formulaClONO are also prominent sources of inline-formulaCl in Leicester. High-inline-formulaNO_x conditions in Delhi tend to suppress the nighttime build-up of inline-formulaN₂O₅ due to titration of inline-formulaO₃ and thus lead to lower inline-formulaClNO₂, in contrast to Leicester. Major loss of inline-formulaClNO₂ is through its uptake on chloride, producing inline-formulaCl₂, which consequently leads to the formation of inline-formulaCl through photolysis. The reactivities of inline-formulaCl and inline-formulaOH are much higher in Delhi; however, the inline-formulaCl/inline-formulaOH reactivity ratio is up to inline-formula≈ 9 times greater in Leicester. The contribution of inline-formulaCl to the atmospheric oxidation capacity is significant and even exceeds (by inline-formula≈ 2.9 times) that of OH during the morning hours in Leicester. Sensitivity simulations suggest that the additional consumption of volatile organic compounds (VOCs) due to active gas- and aqueous-phase chlorine chemistry enhances inline-formulaOH, inline-formulaHO₂, and inline-formulaRO₂ near sunrise. The simulation results of the updated model have important implications for future studies on atmospheric chemistry and urban air quality.