Abstract. Global and regional chemical transport models of the
atmosphere are based on the assumption that chemical species are completely
mixed within each model grid box. However, in reality, these species are
often segregated due to localized sources and the influence of
topography. In order to investigate the degree to which the rates of
chemical reactions between two reactive species are reduced due to the
possible segregation of species within the convective boundary layer, we
perform large-eddy simulations (LESs) in the mountainous region of Hong
Kong Island. We adopt a simple chemical scheme with 15 primary and secondary
chemical species, including ozone and its precursors. We calculate the
segregation intensity due to inhomogeneity in the surface emissions of
primary pollutants and due to turbulent motions related to topography. We
show that the inhomogeneity in the emissions increases the segregation
intensity by a factor of 2–5 relative to a case in which the emissions are
assumed to be uniformly distributed. Topography has an important effect on
the segregation locally, but this influence is relatively limited when
considering the spatial domain as a whole. In the particular setting of our
model, segregation reduces the ozone formation by 8 %–12 % compared to the
case with complete mixing, implying that the coarse-resolution models may
overestimate the surface ozone when ignoring the segregation effect.
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