Extreme Concentrations of Nitric Oxide Control Daytime Oxidation and Quench Nocturnal Oxidation Chemistry in Delhi during Highly Polluted Episodes

Beth S. Nelson*, Daniel J. Bryant, Mohammed S. Alam, Roberto Sommariva, William J. Bloss, Mike J. Newland, Will S. Drysdale, Adam R. Vaughan, W. Joe F. Acton, C. Nicholas Hewitt, Leigh R. Crilley, Stefan J. Swift, Pete M. Edwards, Alastair C. Lewis, Ben Langford, Eiko Nemitz, None Shivani, Ranu Gadi, Bhola R. Gurjar, Dwayne E. HeardLisa K. Whalley, Ülkü A. Şahin, David C.S. Beddows, James R. Hopkins, James D. Lee, Andrew R. Rickard, Jacqueline F. Hamilton

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Delhi, India, suffers from periods of very poor air quality, but little is known about the chemical production of secondary pollutants in this highly polluted environment. During the postmonsoon period in 2018, extremely high nighttime concentrations of NOx (NO and NO2) and volatile organic compounds (VOCs) were observed, with median NOx mixing ratios of ∼200 ppbV (maximum of ∼700 ppbV). A detailed chemical box model constrained to a comprehensive suite of speciated VOC and NOx measurements revealed very low nighttime concentrations of oxidants, NO3, O3, and OH, driven by high nighttime NO concentrations. This results in an atypical NO3 diel profile, not previously reported in other highly polluted urban environments, significantly perturbing nighttime radical oxidation chemistry. Low concentrations of oxidants and high nocturnal primary emissions coupled with a shallow boundary layer led to enhanced early morning photo-oxidation chemistry. This results in a temporal shift in peak O3 concentrations when compared to the premonsoon period (12:00 and 15:00 local time, respectively). This shift will likely have important implications on local air quality, and effective urban air quality management should consider the impacts of nighttime emission sources during the postmonsoon period.

Original languageEnglish
Number of pages8
JournalEnvironmental Science and Technology Letters
Early online date3 May 2023
Publication statusE-pub ahead of print - 3 May 2023

Bibliographical note

Funding Information:
The authors acknowledge Tuhin Mandal at CSIR National Physical Laboratory for his support in facilitating the measurement sites used in this project and Gareth Stewart for VOC measurements. This work was supported by the Newton Bhabha fund administered by the UK Natural Environment Research Council through the DelhiFlux (NE/P016502/1) and ASAP projects of the Atmospheric Pollution and Human Health in an Indian Megacity (APHH-India) program. The authors gratefully acknowledge the financial support provided by the UK Natural Environment Research Council and the Earth System Science Organization, Ministry of Earth Sciences, Government of India, under the Indo-UK Joint Collaboration (DelhiFlux). B.S.N. and D.J.B. acknowledge the NERC SPHERES doctoral training program for studentships. This project was undertaken on the Viking Cluster, which is a high-performance computing facility provided by the University of York. The authors are grateful for computational support from the University of York High Performance Computing service, Viking, and the Research Computing team.

Publisher Copyright:
© 2023 The Authors. Published by American Chemical Society.


  • air quality
  • atmospheric oxidation chemistry
  • nocturnal atmospheric chemistry
  • ozone
  • VOC

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