The influence of solar natural heating and NOx-O3 photochemistry on flow and reactive pollutant exposure in 2D street canyons

Abstract

This study incorporates solar radiation model and NO_x-O₃ photochemistry into computational fluid dynamics (CFD) simulations with the standard k-ε model to quantify the integrated impacts of turbulent mixing, solar heating and chemical processes on vehicular passive (CO) and reactive (NO_x, O₃) pollutant dispersion within two-dimensional (2D) street canyons. Various street aspect ratios (H/W = 1, 3, 5) and solar-radiative scenarios (LST 0900, 1200, 1500) are considered. The initial source ratio of NO₂ to NO is 1:10 and the background O₃ concentration is 100 ppb (mole fraction). The reference Reynolds numbers are ~10⁶-10⁷ and Froude number ranges from 0.23 to 1.14. Personal intake fraction (P_IF) and its spatially-averaged values at the leeward-side (⟨P_IF⟩_lee), windward-side (⟨P_IF⟩_wind) and both street sides (⟨P_IF⟩) are adopted to evaluate pollutant exposure in near-road buildings. As H/W = 1 and 3, the clockwise single vortex is formed under neutral condition. Leeward/ground solar heating at LST 0900/1200 slightly enhance such vortex and reduce ⟨P_IF⟩. However, as H/W = 3, the single dominant vortex is separated into two counter-rotating vortices by windward solar heating at LST 1500, thus this ⟨P_IF⟩_wind is significantly larger than the neutral case. As H/W = 5, the lower-level secondary anticlockwise vortex appears under neutral condition inducing much weaker wind and extremely higher pedestrian-level concentration. This two-main-vortex structure is destroyed by leeward/ground heating into single-main-vortex pattern, but dissociates into three counter-rotating vortices by windward heating. These three radiative scenarios raise pedestrian-level velocity in neutral case by about two orders, and reduce overall ⟨P_IF⟩ by two times to one order. For all cases, NO₂ exposure is generally about 40%-380% larger than passive CO exposure, which indicates the conversion of NO into NO₂ by depleting O₃ is dominant in present NO_x-O₃ titration interactions. Finally, solar heating only raises air temperature by up to 2-3 K and influences chemical rate slightly, thus this impact on reactive pollutant dispersion is less significant than its effect by the enhanced turbulent mixing.

Keywords: Aspect ratio (H/W, AR); Computational fluid dynamics (CFD) simulation; NO(x)-O(3) photochemistry; Personal intake fraction (P_IF); Solar radiation; Two-dimensional (2D) deep street canyons.