26 Apr 2023
 | 26 Apr 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Comprehensive multiphase chlorine chemistry in the box model CAABA/MECCA: Implications to atmospheric oxidative capacity

Meghna Soni, Rolf Sander, Lokesh K. Sahu, Domenico Taraborrelli, Pengfei Liu, Ankit Patel, Imran A. Girach, Andrea Pozzer, Sachin S. Gunthe, and Narendra Ojha

Abstract. 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 NOx (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-NOx 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 ≈7 times greater in Leicester. The contribution of Cl to the atmospheric oxidation capacity is significant and even exceeds (by ≈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.

Meghna Soni et al.

Status: open (until 07 Jun 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-652', Anonymous Referee #1, 21 May 2023 reply
  • RC2: 'Comment on egusphere-2023-652', Anonymous Referee #2, 22 May 2023 reply
  • RC3: 'Comment on egusphere-2023-652', Anonymous Referee #3, 24 May 2023 reply

Meghna Soni et al.


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Short summary
The study presents the implementation of comprehensive multiphase chlorine chemistry in the box model CAABA/MECCA. Simulations for contrasting urban environments of Asia and Europe highlight the significant impacts of chlorine on atmospheric oxidation capacity and composition. Chemical processes governing the production and loss of chlorine-containing species have been discussed. The updated chemical mechanism will be useful to interpret field measurements and for future air quality studies.