Coupling dynamics and chemistry in the air pollution modelling of street canyons: A review

Abstract

Air pollutants emitted from vehicles in street canyons may be reactive, undergoing mixing and chemical processing before escaping into the overlying atmosphere. The deterioration of air quality in street canyons occurs due to combined effects of proximate emission sources, dynamical processes (reduced dispersion) and chemical processes (evolution of reactive primary and formation of secondary pollutants). The coupling between dynamics and chemistry plays a major role in determining street canyon air quality, and numerical model approaches to represent this coupling are reviewed in this article. Dynamical processes can be represented by Computational Fluid Dynamics (CFD) techniques. The choice of CFD approach (mainly the Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) models) depends on the computational cost, the accuracy required and hence the application. Simplified parameterisations of the overall integrated effect of dynamics in street canyons provide capability to handle relatively complex chemistry in practical applications. Chemical processes are represented by a chemical mechanism, which describes mathematically the chemical removal and formation of primary and secondary species. Coupling between these aspects needs to accommodate transport, dispersion and chemical reactions for reactive pollutants, especially fast chemical reactions with time scales comparable to or shorter than those of typical turbulent eddies inside the street canyon. Different approaches to dynamical and chemical coupling have varying strengths, costs and levels of accuracy, which must be considered in their use for provision of reference information concerning urban canopy air pollution to stakeholders considering traffic and urban planning policies.

Large eddy simulation of reactive pollutants in a deep urban street canyon: Coupling dynamics with O3-NO -VOC chemistry

“…The deterioration of urban air quality occurs due to the combined effects of traffic emissions, dynamics and chemistry within such an atmospheric compartment (Li et al, 2008b). Investigation of urban air pollution in street canyons has become a focus of environmental research (Zhong et al, 2016). A variety of approaches, from full-scale field measurements, reduced-scale physical modelling (e.g.…”

“…Vardoulakis et al (2003) focused on full-scale field measurements and parametric (operational) modelling; Ahmad et al (2005) examined wind tunnel experiments; Li et al (2006) used CFD models to understand the dynamical processes and Yazid et al (2014) carried out field measurements and modelling studies. Recently, Zhong et al (2016) presented a comprehensive review of numerical modelling studies that couple street canyon dynamics with chemistry of reactive pollutants. Coupling dynamics and chemistry accommodates the mixing, and photo-chemical processes of emissions.…”

Tailored Chemical Mechanisms for Simulation of Urban Air Pollution

“…There are a few reduced chemical mechanisms ocurring in the literature, some described in a comprehensive review of coupled street canyon modelling [5], some described in a review of chemical mechanisms used in global models [22]. Perhaps the first mechanism including both VOC and NO x chemistry used in urban street canyon modelling was the Generalized VOCs and NO x mechanism [2], [4], which contains 23 species and 20 reactions.…”

“…Simulations of urban air pollution on street scale can be performed either with dynamical models (e.g. CFD models) [3], chemistry box models [4], or coupled models [5]. Plain CFD models (without a chemistry module), which describes mixing and transport of species emitted within the canyon or transported into it, depending on canyon geometry and weather conditions, is arguably the most common approach thus far.…”

“…Many of the emitted species are, however, reactive on the advection time scales of the street canyon, which means that chemical composition rely also on reactivity. Hence, it has been argued lately that the complexity of the interplay between dynamic and chemical effects within a street canyon demands coupled fluid dynamic and chemistry models [5], which in turn, for computational cost reasons, demand the chemistry scheme to be sufficiently small. The crucial question is therefore: What level of detail needs to be included in the chemical kinetic model to represent the system sufficiently accurate.…”