Chemistry, street canyon geometry, and emissions effects on NO2 “hotspots” and regulatory “wiggle room”

Yuqing Dai, Xiaoming Cai, Jian Zhong, A. Robert MacKenzie*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)
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The extent to which nitrogen dioxide (NO2) undergoes complex chemical-transport processes near strong nitrogen-oxide sources in street canyons is not fully understood. A multi-box framework with volatile organic compound (VOC) chemistry has been evaluated against large-eddy simulation (LES) data and observations, and then used to simulate NO2 at street-canyon “hotspots”. 42,000 sensitivity studies — varying nitrogen oxides (NOx) and VOC emission strength, and primary NO2 fraction (fNO2) emitted within each of five streetscape cases — show the importance of detailed VOC chemistry, even in regular canyons (aspect ratio, AR = 1) when the ambient wind is weak. For a midsummer central London scenario, the inclusion of chemistry moves the canyon from compliance to out-of-compliance with the 1-hour NO2 standard. Ignoring street-canyon chemistry can lead, therefore, to false positives in regulatory air quality modelling. Neglecting VOC chemistry can underestimate NO2 by 6–22% in regular canyons, and even more (−51–31%) in deep canyons (AR = 2), particularly with lower fNO2 values resulting from gasoline-dominated vehicle fleets or by tighter control of primary NO2 from diesels. The very significant changes in regulatory “wiggle room” across sensitivity studies demonstrate the utility of this kind of chemistry-transport modelling for identifying efficient and effective regulatory pathways.

Original languageEnglish
Article number102
Number of pages11
Journalnpj Climate and Atmospheric Science
Issue number1
Publication statusPublished - 21 Dec 2022

Bibliographical note

Funding Information:
The authors thank the University of Birmingham’s BlueBEAR HPC service ( for offering high-performance computational resource. We thank Dr. Vivian Bright for providing the reduced chemical scheme (RCS). A.R.M.K. thanks the UK Natural Environment Research Council for support through grants NE/S00582X/1 and NE/S003487/1.

Publisher Copyright:
© 2022, The Author(s).


  • Article
  • /704/172/4081
  • /704/172/169
  • /704/172
  • article

ASJC Scopus subject areas

  • Global and Planetary Change
  • Environmental Chemistry
  • Atmospheric Science


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