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Preprints
https://doi.org/10.5194/acp-2020-545
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-545
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  24 Jul 2020

24 Jul 2020

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This preprint is currently under review for the journal ACP.

Error induced by neglecting subgrid chemical segregation due to inefficient turbulent mixing in regional chemical-transport models in urban environments

Cathy W. Y. Li1, Guy P. Brasseur1,2, Hauke Schmidt1, and Juan Pedro Mellado3 Cathy W. Y. Li et al.
  • 1Max Planck Institute for Meteorology, Bundesstrasse 53, 20146, Hamburg, Germany
  • 2National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80305, USA
  • 3Department of Physics, Aerospace Engineering Division, Universitat Politècnica de Catalunya, C. Jordi Girona 1–3, 08034, Barcelona, Spain

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 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.

Cathy W. Y. Li et al.

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Cathy W. Y. Li et al.

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Latest update: 11 Aug 2020
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Short summary
Intense and localised emissions of pollutants are common in urban environments, in which turbulence cannot mix these segregated pollutants efficiently in the atmosphere. Despite their relatively high resolution, regional models cannot resolve such segregation and assume instantaneous mixing of these pollutants in their model grids, which potentially induces significant error in the subsequent chemical calculation, based on our calculation with a model that explicitly resolves turbulent motions.
Intense and localised emissions of pollutants are common in urban environments, in which...
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