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

Abstract

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 (f_{{{{mathrm{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 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 f_{{{{mathrm{NO}}}}_2} 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.