In this paper, the Conditional Moment Closure is applied to simple problems of reacting flows in the atmosphere with the CBM-IV chemical mechanism in order to evaluate the effect of small-scale segregation on the evolution of the mean pollutant concentrations. The CMC method uses modelled transport equations for conditional averages of the reactive scalars, with the conditioning done on a conserved scalar. Macro-mixing is taken into account by solving the transport equation of the time-averaged conserved scalar, while micro-mixing is included in CMC theory in models for the scalar dissipation rate. The results show that the usual assumption of well-mixed reactants is valid for the species that react over long timescales, but for species afiected by fast chemistry (e.g. radicals such as HO_2), the differences between the CMC predictions and the well-mixed assumption diverge by a factor of two or more. The magnitude of these differences depends on the conserved scalar fluctuations and their dissipation rate. The CMC method offers a way to introduce segregation effects in Air Quality Modelling when the emphasis is on the small-scale features of the flow, such as in street canyons and power station plumes.
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