CFD modeling of reactive pollutant dispersion in simplified urban configurations with different chemical mechanisms

摘要

An accurate understanding of urban air quality requires considering a coupledbehavior between the dispersion of reactive pollutants and atmospheric dynamics.Currently, urban air pollution is mostly dominated by traffic emission,where nitrogen oxides (NO) and volatile organic compounds (VOCs) are theprimary emitted pollutants. However, modeling reactive pollutants with alarge set of chemical reactions, using a computational fluid dynamic (CFD)model, requires a large amount of computational (CPU) time. In this sense, theselection of the chemical reactions needed in different atmosphericconditions becomes essential in finding the best compromise between CPU time andaccuracy. The purpose of this work is to assess the differences in NO andNO concentrations by considering three chemical approaches: (a) passivetracers (non-reactive), (b) the NO–O photostationary state and(c) a reduced complex chemical mechanism based on 23 species and 25reactions. The appraisal of the effects of chemical reactions focuses onstudying the NO and NO dispersion in comparison with the tracer behaviorwithin the street. In turn, the effect of including VOC reactions is alsoanalyzed taking into account several VOC ∕ NO ratios of trafficemission. Given that the NO and NO dispersion can also be affected byatmospheric conditions, such as wind flow or the background concentration fromseason-dependent pollutants, in this work the influence of wind speeds andbackground O concentrations are studied. The results show that the presence ofozone in the street plays an important role in NO and NO concentrations.Therefore, greater differences linked to the chemical approach used are foundwith higher O concentrations and faster wind speeds. This bears relation tothe vertical flux as a function of ambient wind speed since it increases thepollutant exchange between the street and the overlying air. This detailedstudy allows one to ascertain under which atmospheric conditions the inclusion ofchemical reactions are necessary for the study of NO and NO dispersion.The conclusions can be applied to future studies in order to establish thechemical reactions needed in terms of an accurate modeling of NO and NOdispersion and the CPU time required in a real urban area.