28 Mar 2023
 | 28 Mar 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Night-time NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi

Sophie L. Haslett, David M. Bell, Varun Kumar, Jay G. Slowik, Dongyu S. Wang, Suneeti Mishra, Neeraj Rastogi, Atinderpal Singh, Dilip Ganguly, Joel Thornton, Feixue Zheng, Yuanyuan Li, Wei Nie, Yongchun Liu, Wei Ma, Chao Yan, Markku Kulmala, Kaspar R. Daellenbach, David Hadden, Urs Baltensperger, Andre S. H. Prevot, Sachchida N. Tripathi, and Claudia Mohr

Abstract. Atmospheric pollution in urban regions is highly influenced by oxidants due to their important role in the formation of secondary organic aerosol (SOA) and smog. These include the nitrate radical (NO3), which is typically considered a night-time oxidant, and the chlorine radical (Cl), an extremely potent oxidant that can be released in the morning in chloride-rich environments as a result of nocturnal build-up of nitryl chloride (ClNO2). Chloride makes up a higher percentage of particulate matter in Delhi than has been observed anywhere else in the world, which results in Cl having an unusually strong influence in this city. Here, we present observations and model results revealing that atmospheric chemistry in Delhi exhibits an unusual diel cycle, controlled by high concentrations of NO during the night. As a result of this, the formation of both NO3 and dinitrogen pentoxide (N2O5), a precursor of ClNO2 and thus Cl, are suppressed at night and increase to unusually high levels during the day. Our results indicate that a substantial reduction in night-time NO has the potential to increase both nocturnal oxidation via NO3 and the production of Cl during the day.

Sophie L. Haslett et al.

Status: open (until 09 Jun 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-497', Anonymous Referee #1, 03 May 2023 reply
  • RC2: 'Comment on egusphere-2023-497', Anonymous Referee #2, 17 May 2023 reply

Sophie L. Haslett et al.

Sophie L. Haslett et al.


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Short summary
In Delhi, some aspects of daytime and night-time atmospheric chemistry are “flipped,” and parodoxically, vehicle emissions may be limiting other forms of particle production. This is because the night-time emissions of nitrogen oxide (NO) by traffic, as well as biomass burning, prevents some chemical processes that would otherwise create even more particles, and worsen the urban haze.