Chemistry, transport, emission, and shading effects on NO₂ and O_x distributions within urban canyons

The capacity to predict NO₂ and the total oxidant (O_x = NO₂ + O₃) within street canyons is critical for the assessment of air quality regulations aimed at enhancing human wellbeing in urban hotspots. However, such assessment requires the coupling of numerous processes at the street-scale, such as vehicular emissions and tightly coupled transport and photochemical processes. Photochemistry, in particular, is often ignored, heavily simplified, or parameterized. In this study, MBM-FleX — a process-based street canyon model that allows fast computation of various emission profiles and sun-lit conditions with tightly coupled physical (transport and mixing) and chemical processes and without loss of sufficient spatial resolution — was used to simulate shading effects on reactive species within urban canyons. Driven by pre-generated large-eddy simulation of flow, MBM-FleX results show that shading effects on volatile organic compound (VOC) free-radicals significantly affect the interconversion of odd-oxygen species that cannot be captured by the simple NO_x-O₃ chemistry, for example, reducing NO₂ by limiting the formation of hydroperoxyl radicals. Consistent with previous results in simpler model systems, the inclusion of VOC free-radical chemistry did not appreciably alter the sensitivity of NO₂ to shading intensity in regular canyons, but a non-linear relationship between NO₂ and shading intensity is found in deep canyons when the air residence time grew. When solar incidence simultaneously passes through multiple vortices in street canyons, VOC chemistry and shade may considerably influence model results, which may therefore affect the development of urban planning strategies and personal exposure evaluation.