A CFD modeling study of the impacts of NOx and VOC emissions on reactive pollutant dispersion in and above a street canyon

A computational fluid dynamics (CFD) model that includes the carbon bond mechanism IV (CBM-IV) is developed and used to investigate reactive pollutant dispersion in and above a street canyon with an aspect ratio of 1. Fourteen emission scenarios of NOx and volatile organic compounds (VOCs) are considered. Dispersion types are classified into NO-type, NO2-type, and O3-type dispersion that exhibit concentration maxima at the street bottom, near the center of the street canyon, and above the street canyon, respectively. For the base emission scenario, the number of reactive species is 9 in the NO-type dispersion, 10 in the NO2-type dispersion, and 15 in the O3-type dispersion. As the NOx emission level decreases or the VOC emission level increases, some species in the O3-type dispersion are shifted to the NO2-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. Because of their high OH reactivities, XYL (xylene) and OLE (olefin carbon bond) among VOCs are largely responsible for the transition of dispersion type. The O3 sensitivity is examined by reducing NOx or VOC emission level by a half. Because the NO titration of O3 is more pronounced than the NO2 photolysis and the radical chain process in the street canyon, the O3 concentration therein is negatively correlated with the NOx emission level and weakly correlated with the VOC emission level. As a result, the street canyon is a negatively NOx-sensitive regime.