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

Abstract. An accurate understanding of urban air quality requires considering a coupled behavior between the dispersion of reactive pollutants and atmospheric dynamics. Currently, urban air pollution is mostly dominated by traffic emission, where nitrogen oxides (NO_x) and volatile organic compounds (VOCs) are the primary emitted pollutants. However, modeling reactive pollutants with a large set of chemical reactions, using a computational fluid dynamic (CFD) model, requires a large amount of computational (CPU) time. In this sense, the selection of the chemical reactions needed in different atmospheric conditions becomes essential in finding the best compromise between CPU time and accuracy. The purpose of this work is to assess the differences in NO and NO₂ concentrations by considering three chemical approaches: (a) passive tracers (non-reactive), (b) the NO_x–O₃ photostationary state and (c) a reduced complex chemical mechanism based on 23 species and 25 reactions. The appraisal of the effects of chemical reactions focuses on studying the NO and NO₂ dispersion in comparison with the tracer behavior within the street. In turn, the effect of including VOC reactions is also analyzed taking into account several VOC ∕ NO_x ratios of traffic emission. Given that the NO and NO₂ dispersion can also be affected by atmospheric conditions, such as wind flow or the background concentration from season-dependent pollutants, in this work the influence of wind speeds and background O₃ concentrations are studied. The results show that the presence of ozone in the street plays an important role in NO and NO₂ concentrations. Therefore, greater differences linked to the chemical approach used are found with higher O₃ concentrations and faster wind speeds. This bears relation to the vertical flux as a function of ambient wind speed since it increases the pollutant exchange between the street and the overlying air. This detailed study allows one to ascertain under which atmospheric conditions the inclusion of chemical reactions are necessary for the study of NO and NO₂ dispersion. The conclusions can be applied to future studies in order to establish the chemical reactions needed in terms of an accurate modeling of NO and NO₂ dispersion and the CPU time required in a real urban area.