Abstract
Abstract. We employed direct numerical simulations to estimate the error on chemical calculation in simulations with regional chemical-transport models induced by neglecting subgrid chemical segregation due to inefficient turbulent mixing in an urban boundary layer with strong and heterogeneously distributed surface emissions. In simulations of initially segregated reactive species with an entrainment-emission configuration with an A–B–C second-order chemical scheme, urban surface emission fluxes of the homogeneously emitted tracer A result in a very large segregation between the tracers and hence a very large overestimation of the effective chemical reaction rate in a complete-mixing model. This large effect can be indicated by a large Damköhler number (Da) of the limiting reactant. With heterogeneous surface emissions of the two reactants, the resultant normalised boundary-layer-averaged effective chemical reaction rate is found to be in a Gaussian function of Da, and it is increasingly overestimated by the imposed rate with an increased horizontal scale of emission heterogeneity. Coarse-grid models with resolutions commensurable to regional models give reduced yet still significant errors for all simulations with homogeneous emissions. Such model improvement is more sensitive to the increased vertical resolution. However, such improvement cannot be seen for simulations with heterogeneous emissions when the horizontal resolution of the model cannot resolve emission heterogeneity. This work highlights particular conditions in which the ability to resolve chemical segregation is especially important when modelling urban environments.
Highlights
Turbulence mixes initially segregated reactive species in the boundary layer and allows chemical reactions to occur
As more tracer A is consumed, more tracer B can flow down the boundary layer without being consumed; more tracer C is produced at a lower altitude in the boundary layer where tracer A is available
As past studies mainly examined scenarios in forestal areas, we focus on urban conditions, with strong emission fluxes and heterogeneous emissions, as both factors can potentially increase chemical segregation
Summary
Turbulence mixes initially segregated reactive species in the boundary layer and allows chemical reactions to occur. In an Eulerian chemical-transport model, these processes and characteristics often occur at a length scale smaller than the size of a model grid box that the resultant chemical segregation cannot be represented, as the chemical species are assumed to be instantaneously and homogeneously mixed within a grid box. The negligence of such subgrid segregation induces potential errors to the calculation of the chemical transformation within a model grid of a large-scale model. Earliest studies can be dated back to Damköhler (1940) and Danckwerts (1952), which focused on quantifying the effect of turbulence on combustion processes, applicable to other chemical transformations such as chemical reactions, by introducing quantitative definitions of scaling parameters including the Damköhler number
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
