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A platform for research: civil engineering, architecture and urbanism
Characteristics of flow and reactive pollutant dispersion in urban street canyons
Abstract In this study, the effects of aspect ratio defined as the ratio of building height to street width on the dispersion of reactive pollutants in street canyons were investigated using a coupled CFD-chemistry model. Flow characteristics for different aspect ratios were analyzed first. For each aspect ratio, six emission scenarios with different VOC–NOX ratios were considered. One vortex was generated when the aspect ratio was less than 1.6 (shallow street canyon). When the aspect ratio was greater than 1.6 (deep street canyon), two vortices were formed in the street canyons. Comparing to previous studies on two-dimensional street canyons, the vortex center is slanted toward the upwind building and reverse and downward flows are dominant in street canyons. Near the street bottom, there is a marked difference in flow pattern between in shallow and deep street canyons. Near the street bottom, reverse and downward flows are dominant in shallow street canyon and flow convergence exists near the center of the deep street canyons, which induces a large difference in the NOX and O3 dispersion patterns in the street canyons. NOX concentrations are high near the street bottom and decreases with height. The O3 concentrations are low at high NO concentrations near the street bottom because of NO titration. At a low VOC–NOX ratio, the NO concentrations are sufficiently high to destroy large amount of O3 by titration, resulting in an O3 concentration in the street canyon much lower than the background concentration. At high VOC–NOX ratios, a small amount of O3 is destroyed by NO titration in the lower layer of the street canyons. However, in the upper layer, O3 is formed through the photolysis of NO2 by VOC degradation reactions. As the aspect ratio increases, NOX (O3) concentrations averaged over the street canyons decrease (increase) in the shallow street canyons. This is because outward flow becomes strong and NOX flux toward the outsides of the street canyons increases, resulting in less NO titration. In the deep street canyons, outward flow becomes weak and outward NOX flux decreases, resulting in an increase (decrease) in NOX (O3) concentration.
Highlights We investigate the characteristics of flow and reactive pollutant dispersion in street canyons. Near the street bottom, there is a marked difference in flow pattern between in shallow and deep street canyons. O3 concentration near the street bottom depends on street-canyon aspect ratio. In deep street canyons, canyon-height increase results in an increase (decrease) in averaged NOX (O3) concentration. At high VOC–NOX ratios, O3 is formed through the photolysis of NO2 by VOC degradation reactions.
Characteristics of flow and reactive pollutant dispersion in urban street canyons
Abstract In this study, the effects of aspect ratio defined as the ratio of building height to street width on the dispersion of reactive pollutants in street canyons were investigated using a coupled CFD-chemistry model. Flow characteristics for different aspect ratios were analyzed first. For each aspect ratio, six emission scenarios with different VOC–NOX ratios were considered. One vortex was generated when the aspect ratio was less than 1.6 (shallow street canyon). When the aspect ratio was greater than 1.6 (deep street canyon), two vortices were formed in the street canyons. Comparing to previous studies on two-dimensional street canyons, the vortex center is slanted toward the upwind building and reverse and downward flows are dominant in street canyons. Near the street bottom, there is a marked difference in flow pattern between in shallow and deep street canyons. Near the street bottom, reverse and downward flows are dominant in shallow street canyon and flow convergence exists near the center of the deep street canyons, which induces a large difference in the NOX and O3 dispersion patterns in the street canyons. NOX concentrations are high near the street bottom and decreases with height. The O3 concentrations are low at high NO concentrations near the street bottom because of NO titration. At a low VOC–NOX ratio, the NO concentrations are sufficiently high to destroy large amount of O3 by titration, resulting in an O3 concentration in the street canyon much lower than the background concentration. At high VOC–NOX ratios, a small amount of O3 is destroyed by NO titration in the lower layer of the street canyons. However, in the upper layer, O3 is formed through the photolysis of NO2 by VOC degradation reactions. As the aspect ratio increases, NOX (O3) concentrations averaged over the street canyons decrease (increase) in the shallow street canyons. This is because outward flow becomes strong and NOX flux toward the outsides of the street canyons increases, resulting in less NO titration. In the deep street canyons, outward flow becomes weak and outward NOX flux decreases, resulting in an increase (decrease) in NOX (O3) concentration.
Highlights We investigate the characteristics of flow and reactive pollutant dispersion in street canyons. Near the street bottom, there is a marked difference in flow pattern between in shallow and deep street canyons. O3 concentration near the street bottom depends on street-canyon aspect ratio. In deep street canyons, canyon-height increase results in an increase (decrease) in averaged NOX (O3) concentration. At high VOC–NOX ratios, O3 is formed through the photolysis of NO2 by VOC degradation reactions.
Characteristics of flow and reactive pollutant dispersion in urban street canyons
Park, Soo-Jin (author) / Kim, Jae-Jin (author) / Kim, Minjoong J. (author) / Park, Rokjin J. (author) / Cheong, Hyeong-Bin (author)
Atmospheric Environment ; 108 ; 20-31
2015-02-26
12 pages
Article (Journal)
Electronic Resource
English
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