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. Global and regional chemical transport models of theatmosphere are based on the assumption that chemical species are completelymixed within each model grid box. However, in reality, these species areoften segregated due to localized sources and the influence oftopography. In order to investigate the degree to which the rates ofchemical reactions between two reactive species are reduced due to thepossible segregation of species within the convective boundary layer, weperform large-eddy simulations (LESs) in the mountainous region of HongKong Island. We adopt a simple chemical scheme with 15 primary and secondarychemical species, including ozone and its precursors. We calculate thesegregation intensity due to inhomogeneity in the surface emissions ofprimary pollutants and due to turbulent motions related to topography. Weshow that the inhomogeneity in the emissions increases the segregationintensity by a factor of 2–5 relative to a case in which the emissions areassumed to be uniformly distributed. Topography has an important effect onthe segregation locally, but this influence is relatively limited whenconsidering the spatial domain as a whole. In the particular setting of ourmodel, segregation reduces the ozone formation by 8 %–12 % compared to thecase with complete mixing, implying that the coarse-resolution models mayoverestimate the surface ozone when ignoring the segregation effect.

The impact of inhomogeneous emissions and topography on ozone photochemistry in the vicinity of Hong Kong Island