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Abstract:
The chlorine (Cl) chemistry can strongly affect the oxidation capacity and, therefore, the atmospheric composition, especially in urban environments. Notably, the reaction rate constants of some volatile organic compounds (VOCs) are up to two orders of magnitude greater with Cl radical than with hydroxyl (OH) radical. However, studies dealing with detailed Cl chemistry and its impacts on atmospheric composition remain limited. In this regard, gas and aqueous-phase Cl chemistry have been comprehensively updated in the community atmospheric chemistry box model - CAABA / MECCA. In particular, an explicit mechanism for ClNO2 formation following N2O5 uptake to aerosols has been developed. The updated model has been applied to simulate urban environments with contrasting NOx-conditions in South Asia (Delhi, India) and Europe (Leicester, United Kingdom). Constraining the nitrogen oxides (NOx) in the model using observational data helped in better reproducing the diurnal variations of some VOCs (e.g. isoprene, pinene) and ozone over Delhi. Sensitivity simulation suggests that the Cl chemistry can lead to additional consumption of about 20 ppbv of total VOCs. The model shows a sharp build-up of Cl (~ 3 ppq) with sunrise through Cl2 photolysis and then enhancements in the afternoon due to oxidation of HCl by OH. The OH, HO2, and RO2 were also substantially enhanced near the sunrise due to Cl chemistry. These findings are in agreement with observations showing high yields of secondary organic aerosol (SOA) formation in the morning hours at Delhi. High-NOx conditions suppress the night-time build-up of N2O5 and thus ClNO2 in Delhi, whereas, ClNO2 production is stronger in the low-NOx conditions of Leicester. The model updates and simulation results have important implications for future studies on urban air quality, aerosol formation, and occurrences of haze episodes.
