The Conditional Moment Closure Approach For Atmospheric PollutionProblems

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 aftected by fast chemistry (e.g. radicals such as HOJ, 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.