Microscale Flow, Gas-Phase Chemistry, and Dispersion in Urban Areas

Abstract

Flow, gas-phase chemistry, and dispersion in urban areas are predominantly microscale, which are directly modulated by artificial topography and land cover at surfaces. Because these features are connected to each other, an investigation in a single framework is necessary. This study develops a computational fluid dynamics (CFD) model coupled with essential physical and chemical components. At first, an urban surface and radiation model are incorporated into the CFD model to investigate the diurnal variation of flow in a street canyon with an aspect ratio of 1. In one-day simulations with various ambient wind speeds, two flow regimes are identified by vortex configuration in a street canyon. Flow regime I is characterized by a primary vortex, and flow regime II is characterized by two counter-rotating vortices. For weak ambient winds, the dependency of surface sensible heat flux on the ambient wind speed is found to play an essential role in determining the relationship between canyon wind speed and ambient wind speed. Secondly, the carbon bond mechanism Ⅳ (CBM–Ⅳ) is included in the CFD model to investigate reactive pollutant dispersion in and above a street canyon with an aspect ratio of 1. In simulations with 14 emission scenarios, dispersion types are classified into NO-type, NO2-type, and O3-type dispersion. The VOC-to-NOx emission ratio is found to be an important factor in determining the transition of dispersion type. In this transition process, OH plays a key role through a radical chain including HO2, RO, and RO2. The examination of O3 sensitivity shows that the O3 concentration is negatively correlated with the NOx emission level and weakly correlated with the VOC emission level. Therefore, the street canyon is a negatively NOx-sensitive regime. In a street canyon with a canyon aspect ratio of 2, there is a contrast between different regions in the street canyon, showing O3 chemical production in an upper region and O3 chemical loss in a lower region. Thirdly, the urban surface and radiation model along with the CBM–Ⅳ are incorporated into the CFD model to investigate the diurnal variation of NOx and O3 exchange between a street canyon and the overlying air. The temporal and spatial distribution of NOx removal and O3 entrainment across the roof level of street canyon totally depends on flow regime. While the turbulent process is generally the dominant mechanism of NOx removal and O3 entrainment, the mean process can be responsible for the mechanism when the upwind building wall is strongly heated in the afternoon. The calculated NOx and O3 exchange velocities depends not only on flow regime but also photochemistry. Finally, urban air quality simulations using the CFD model coupled with mesoscale meteorological (WRF) and chemistry-transport (CMAQ) models are performed in a high-rise building area. The simulated NO2 and O3 concentrations are evaluated with data measured at a roadside air quality monitoring station, showing significant improvements compared to those in the CMAQ simulation. The large spatial variabilities of the NO2 and O3 concentrations in the high-rise building area are attributed to the heterogeneities of building geometry and mobile emission. As a consequence, the integrated urban air quality modeling system is a leading-edge numerical approach that embraces multiscale influences on air quality in urban areas.