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

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 (NO_x) 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.