Comment on egusphere-2023-652

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 Cl chemistry of the community atmospheric chemistry box model CAABA/MECCA has been extended. In particular, an explicit mechanism for ClNO₂ formation following N₂O₅ uptake to aerosols has been developed. The updated model has been applied to two urban environments with different concentrations of NO_x (NO and NO₂): New Delhi (India) and Leicester (United Kingdom). The model shows a sharp build-up of Cl at sunrise through Cl₂ photolysis in both environments. Besides Cl₂ photolysis, ClO+NO reaction, and photolysis of ClNO₂ and ClONO are prominent sources of Cl in Leicester. High-NO_x conditions in Delhi tend to suppress the night-time build-up of N₂O₅ due to titration of O₃ and thus lead to lower ClNO₂, in contrast to Leicester. Major loss of ClNO₂ is through its uptake on chloride, producing Cl₂ , which consequently leads to the formation of Cl through photolysis. The reactivities of Cl and OH are much higher in Delhi, however, the Cl/OH ratio is up to &asymp;7 times greater in Leicester. The contribution of Cl to the atmospheric oxidation capacity is significant and even exceeds (by &asymp;2.9 times) that of OH during the morning hours in Leicester. Sensitivity simulations suggest that the additional consumption of VOCs due to active gas and aqueous-phase chlorine chemistry enhances OH, HO₂, RO₂ near the sunrise. The simulation results of the updated model have important implications for future studies on atmospheric chemistry and urban air quality.