RESUMEN

We employ computational fluid dynamics (CFD) simulations with NO-NO2-O3 chemistry to investigate the impacts of aspect ratios (H/W = 1,3,5), elevated-building design, wind catchers and two background ozone concentrations ([O3]b = 100/20 ppb) on reactive pollutant dispersion in two-dimensional (2D) street canyons. Personal intake fraction of NO2 (P_IFNO2) and its spatial mean value in entire street (i.e. street intake fraction ) are calculated to quantify pollutant exposure in near-road buildings. Chemical reaction contribution of NO2 exposure (CRC), O3 depletion rate (dozone) and photostationary state defect (δps) are used to analyze the interplay of turbulent and chemical processes. As H/W increases from 1, 3 to 5 with [O3]b = 100 ppb, the flow pattern turns from single-main-vortex structure to two-counter-rotating-vortex structure, and pedestrian-level velocity becomes 1-2 orders smaller. The high-dozone regions and low-|δps| regions get larger with more complete chemical reactions. Consequently, passive  rises 1 order (4.09-5.71 ppm to 41.76 ppm), but reactive only increases several times (17.80-21.28 ppm to 58.50 ppm) and the contribution of chemistry (CRC) decreases with higher H/W. Thus, chemistry raises  more effectively in shallow street canyons (H/W = 1-3). In deep street canyons (H/W = 5), elevated-building design and wind catchers destroy two-counter-rotating-vortex structure, improve street ventilation and reduce passive by 2 and 1 orders (41.76 ppm to 0.38-5.16 ppm), however they only reduce reactive by about 97.5% and 75% (58.50 ppm to 1.61-14.48 ppm). Such building techniques induce lower O3 depletion rate but greater chemical contribution. Finally, raising [O3]b from 20 to 100 ppb causes greater O3 depletion rate and chemical contribution and produces larger . For deep street canyons, the impact of higher [O3]b on is weaker than that in shallow street canyons, while it becomes stronger when fixing elevated-building design and wind catchers. This study provides some innovative findings on reactive pollutant exposure in 2D street canyons and offers effective CFD methodologies to evaluate pollutant exposure with more complicated chemistry and urban configurations.

RESUMEN

High-rise deep street canyons usually experience poor ventilation and large vehicular pollutant exposure to residents in near-road buildings. Investigations are still required to clarify the flow and dispersion mechanisms in deep street canyons and explore techniques to reduce such large pollutant exposure. By conducting computational fluid dynamics (CFD) simulations validated by wind tunnel data and scale-model outdoor field measurements, we investigate the integrated impacts of aspect ratios, first-floor and second-floor elevated building designs, viaduct settings, height variations and wind catchers on the flow, personal intake fraction (P_IF) of CO (carbon dioxide) and its spatial mean value 〈P_IF〉 in two-dimensional (2D) street canyons. Results show that cases with H/W = 5 experience two counter-rotating vortices, much poorer ventilation and 1-2 orders larger 〈P_IF〉 (43.6-120.8 ppm) than H/W = 1 and 3 (3.8-4.3 and 5.6-5.8 ppm). Moreover, in cases with H/W = 5 the height variation results in three vertically-aligned vortices and much weaker wind, subsequently produces greater 〈P_IF〉 (1402-2047 ppm). To reduce 〈P_IF〉 with H/W = 5, various urban designs are evaluated. The first-floor elevated building design creates more effective ventilation pathways than the second-floor elevated type does and reduces 〈P_IF〉 at H/W = 5 by five orders (1402 to ~0.01 ppm) or two orders (43.6 to ~0.1 ppm) in cases with or without the height variation. However, such reductions at H/W = 1 and 3 are only 76.8%-81.4% and 22.4%-36.2% respectively. Wind catchers destroy the multi-vortex flow pattern as H/W = 5, produce a contra-clockwise main vortex and reduce 〈P_IF〉 by 1-2 orders for cases with or without the height variation.

Asunto(s)

Contaminación del Aire/análisis , Contaminación por Tráfico Vehicular/prevención & control , Emisiones de Vehículos/análisis , Contaminación del Aire/prevención & control , Ciudades , Exposición a Riesgos Ambientales , Hidrodinámica , Modelos Teóricos , Ventilación , Viento