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

The extent to which nitrogen dioxide (NO 2 ) 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 NO 2 at street-canyon “hotspots”. 42,000 sensitivity studies — varying nitrogen oxides (NO x ) and VOC emission strength, and primary NO 2 fraction ( f_NO₂ f NO 2 ) 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 NO 2 standard. Ignoring street-canyon chemistry can lead, therefore, to false positives in regulatory air quality modelling. Neglecting VOC chemistry can underestimate NO 2 by 6–22% in regular canyons, and even more (−51–31%) in deep canyons (AR = 2), particularly with lower f_NO₂ f NO 2 values resulting from gasoline-dominated vehicle fleets or by tighter control of primary NO 2 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.