Simulations of the impacts of building height layout on air quality in natural-ventilated rooms around street canyons

Effect of triangular roof angle on dispersion of gaseous pollutants and particulate matter

The Euler-Lagrangian method is adopted to simulate the dispersion of gaseous pollutants and particulate matter (PM) in isolated street canyons, and the influences of the roof angle on the flow structures and distributions of gaseous pollutants and PM are analyzed in detail. Numerical simulation results indicate that gaseous pollutants and PM in the canyons present three typical single main clockwise vortex, transition vortex, and double vortex structures, which are identified at increasing roof slopes. Gaseous pollutants and PM demonstrate the lowest concentration of pollutants when a single vortex structure exists. The concentration of gaseous pollutants and PM reaches the highest value in pedestrian-level areas when the flow field is in a transitional vortex structure. Unlike gaseous pollutants, the concentration of PM does not always decrease with increasing altitude, and higher PM concentrations sometimes occur in the mid-level areas of the canyon. A small roof incline angle is generally recommended for discharging gaseous pollutants and PM.

Analysis on the Characteristics of Air Pollution in China during the COVID-19 Outbreak

The COVID-19 pandemic poses a serious global threat to human health. In China, the government immediately implemented lockdown measures to curb the spread of this virus. These measures severely affected transportation and industrial production across the country, resulting in a significant change in the concentration of air pollutants. In this study, the Euclidean distance method was used to select the most similar meteorological field during the COVID-19 lockdown period. Changes in the concentration of air pollutants in China were analyzed under similar meteorological background conditions. Results indicate that, compared with data from 2015–2019, air quality in China significantly improved; with the exception of ozone (O3), the concentration of major air pollutants declined. Compared with baseline conditions, the reduction of air pollutants in China from 25 January to 22 February 2020 (Period 2) was the most significant. In particular, NO2 decreased by 41.7% in the Yangtze River Delta. In Period 2, the reduction of air pollutants in areas other than Hubei gradually decreased, but the reduction of NO2 in Wuhan reached 61.92%, and the reduction of air pollutants in various regions after February 23 was significantly reduced. By excluding the influence of meteorological factors and calculating the contribution of human activities to atmospheric pollutants by linear fitting, in Period 2 the effect of artificial controls on NO2 in Wuhan attained 30.66%, and reached 48.17% from 23 February to 23 March (Period 3). Results from this investigation provides effective theoretical support for pollution prevention and control in China.

Buoyant wind-driven pollutant dispersion and recirculation behaviour in wedge-shaped roof urban street canyons

The present study investigated the buoyant wind-driven pollutant plume dispersion and recirculation behaviour inside urban street canyons formed by buildings with wedge-shaped roofs. Numerical modelling was performed using a computational fluid dynamics (CFD) large eddy simulation (LES). Street canyon models with a strongly buoyant fire source located on the street and environmental winds perpendicular to the canyon were developed using the fire dynamics simulator (FDS). The complex interaction of buoyancy and wind, as well as their combined effects on the pollutant plume dispersion, was simulated inside the urban street canyon. The results showed that the flow pattern of pollutant plume dispersion inside the street canyon with increasing wind speed for different roof inclination angles could be divided into three regimes, including a recirculation regime, a quasi-recirculation regime and a non-recirculation regime. The pollutant levels in the street canyon, as indexed by carbon monoxide (CO) concentration, increased under the recirculation regime. For the quasi-recirculation regime, however, the leeward buildings primarily suffered from the higher pollutant levels. The critical wind speed needed to trigger recirculation was analysed for various roof inclination angles. A correlation was proposed to predict the critical wind speed of various wedge-shaped roof angles for recirculation regime and quasi-recirculation regimes.