Physio-chemical modeling of the NO_x-O₃ photochemical cycle and the air pollutants’ reactive dispersion around an isolated building

A numerical physio-chemical model of the NO_x-O₃ photochemical cycle in the near-wake region of an isolated residential/office building has been presented in this study. The investigation delves into the dispersion of reactive air pollutants through the lens of fluid phenomenology and its impact on chemical reactivity, formation, transport, deposition, and removal. Computational fluid dynamics (CFD) simulations were conducted for the ground-point-source (GES) and roof-point-source (RES) scenarios. Results show that the Damköhler number (Da), which quantifies pollutants’ physio-chemical timescales, displays a strong inverse proportionality with the magnitude and spread of NO-increasing Da reduces human exposure to the toxic NO and NO₂ substantially. When different wind directions were considered, the dispersion range of NO exhibited varying shrinking directions as Da increased. Furthermore, as Da increases, the concentration ratio K_NO2/K_NOx which quantifies the production of NO₂ resulting from NO depletion, forms sharp high-low gradients near emission sources. For GES, the dispersion pattern is governed by the fluid’s phenomenological features. For RES, the intoxicated area emanates from the building’s leading-edge, with the lack of shielding inhibiting pollutant interactions in the near-wake, resulting in scant physio-chemical coupling. The NO₂/NO_x distribution follows a self-similar, stratified pattern, exhibiting consistent layering gradients and attributing to the natural deposition of the already-reacted pollutants rather than in-situ reactions. In the end, building design guidelines have been proposed to reduce pedestrian and resident exposure to NO_x-O₃. © Tsinghua University Press 2023.