High-resolution Simulations of Urban Flow and Reactive pollutants’ dispersion

Abstract

The flow and reactive pollutants distribution according to building aspect ratio, building-roof cooling, and VOCs-NOx emission ratio were investigated in detail in idealized street canyons using a CFD-Chem coupled model. The multiscale modeling system was developed to analyze the urban air quality in real-region. The numerical model was validated first against experimental wind-tunnel results and analyzed flow characteristics around step-up street canyon. Although the CFD-Chem model underestimated the sizes of the corner vortices near the ground in the deep street canyons, it reproduces the main flow features measured in the wind-tunnel experiments well. The simulated results showed that the in-canyon flows experience two stages (development and mature) as the building-length ratio increased. The main structure of the mean flows in the deep step-up street canyon was similar to that in the shallow step-up street canyon. During the mature stage, the primary vortex stabilized in one position, and the incoming flow no longer followed the building sidewalls. Flow characteristics for different aspect ratios were analyzed. 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. 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. In the presence of building-roof cooling, winds and temperature fields inside the building canopy of the street canyon were significantly modified. Building-roof cooling intensified the street-canyon vortex strength. Building-roof cooling also decreased air temperature in the street canyon by supplying cooler air near the building roof. The changes in the in-canopy distributions of primary pollutants (NOx, VOCs and CO) due to building-roof cooling were mainly caused by the modified mean flow rather than the chemical reactions. Building-roof cooling decreased primary pollutant concentrations by approximately –2.4% compared to those under non-cooling conditions. By contrast, building-roof cooling increased O3 concentrations by about 1.1% by reducing NO concentrations in the street canyon compared to concentrations under non-cooling conditions. We also developed a Multiscale modeling system, including a two-way nesting method and a coupling module that hourly provides the CFD-Chem model, as the initial and boundary conditions, the output of the air quality predicting modeling system based on the WRF-CMAQ models operated by the National Institute of Environmental Research in Korea. We simulated the winds, temperatures, and concentrations of the gas-phase pollutants and validated them against the observations at an automated weather station (AWS) and the measured concentrations at an air quality monitoring station (AQMS) located at a building-congested district in Yeongdeungpo-gu, Seoul, Republic of Korea. We investigated the effects of differential surface heating on airflow and reactive pollutants' dispersion in the target area. The surface heating intensified the vertical (both the upward and downward) motions compared to those in no heating case and improved the reproduction of the observed wind speeds and temperatures. Despite the relatively large difference between the measured and simulated pollutant concentrations, the CFD-Chem model simulated the similar variation trends of the CO, NO2, and O3 concentrations to the measured ones. The air temperature near the surface was high along the road and the high-density building area, which increased the positive buoyancy, resulting in the upward motions strengthened. The intensified upward motions resulted in enhancing transport of the CO, NO2, and VOCs from the emission source level to the AQMS measurement height, increasing the CO, NO2, and VOCs concentrations but decreasing O3 concentrations.