Numerical studies of passive and reactive pollutant dispersion in high-density urban models with various building densities and height variations

Abstract

Vehicular pollutant exposure in the near-road buildings of high-density urban areas has been rarely studied. This paper investigates the impacts of high-density building morphology on passive (CO) and reactive (NOx-O3) pollutant dispersion in three-dimensional (3D) urban-like models, e.g. medium (street width W = 30 m,λp = 0.25, λf = 0.6) and compact (W = 18 m,λp = 0.39,λf = 0.94) layouts with uniform (H = 72 m) or various heights (H1 = 48 m,H2 = 96 m). Personal intake fraction P_iF and its spatially-averaged value for the entire building wall (<P_iF>W) are utilized for exposure assessment.Some meaningful findings are proposed: 1) When wind blows through neighborhood-scale (~1 km) urban models, it decelerates rapidly and then reaches a flow balance where pedestrian-level velocity and pollutant exposure vary depending on building densities and height variations. 2) Uniform-height cases attain overall <P_iF>W of ~0.3–1.3 ppm in target streets. The compact layout (λf = 0.94) experiences weaker vertical exchange where downward helical flows transport more pollutants to windward wall than medium-type (λf = 0.6), inducing two-order larger windward-side <P_iF>W (~1 ppm) than leeward-side (~0.01 ppm). 3) Building height variations obtain greater velocity and smaller <P_iF>W (~0.2–1.0 ppm) in front of taller buildings than those behind them (<P_iF>W~2.6–4.2 ppm). 4) No matter with or without height variations, densifying building arrays (λf = 0.6 to 0.94) raises windward-side <P_iF>W and mitigates leeward-side <P_iF>W, but overall <P_iF>W for both sides of target streets only change slightly. 5) With present NOx-O3 photochemistry (source emissions NO:NO2 = 10:1, background [O3] = 20 ppbv), NO2 exposure is 40%–230% larger than passive CO exposure, but NO is near to CO. This study implies the near-road pollutant exposure in high-density urban models is sensitive to building morphology which should be carefully designed.