Microscale Simulations of Reactive Pollutant Dispersion in a High-rise Building Area Using a CFD Model Coupled with Mesoscale Meteorological and Air Quality Models

Air quality in an urban area is controlled by not only microscale factors such as building configuration and traffic emission but also mesoscale factors such as mesoscale flow and background concentration. We have developed a computational fluid dynamics (CFD) model coupled with a complex chemical mechanism and investigated reactive pollutant dispersion by considering microscale factors in idealized street canyons. To extend the capability of the CFD model to include mesoscale influences, the CFD model is recently coupled with a mesoscale meteorological model (WRF) and an air quality model (CMAQ). The WRF-CMAQ-CFD modeling system is used to investigate reactive pollutant dispersion in a built-up area of Seoul, South Korea, where high-rise buildings are densely located. In a summer case with clear skies and moderate synoptic winds, the simulation results show the distinct characteristics of reactive pollutant dispersion in the upwind, in-canyon, and downwind areas. While channeling flows between high-rise buildings are apparent, recirculation flows behind high-rise buildings result in the significant vertical pollutant transport. As a result, the NOx concentrations in the in-canyon and downwind areas are higher than that in the upwind area below the building-top levels (up to 110 m above the street level). The O3 concentration is the lowest at the street level (lower than 10 ppb for the in-canyon area) due to the NO titration of O3 and generally increases with height in all areas. Interestingly, NOx and O3 concentration differences among the upwind, in-canyon, and downwind areas are not negligible even around building-top levels due to the significant vertical pollutant transport. This result reveals the capability of WRF-CMAQ-CFD modeling system in studying air quality in such a high-rise building area, which cannot be properly simulated within the lowest 2 or 3 vertical model levels in larger scale air quality models. In addition to the simulation with the time-independent boundary conditions of flow and pollutant concentrations, other simulations with time-dependent boundary conditions will be examined and presented.