Segregation of atmospheric oxidants in turbulent urban environments

In the atmospheric boundary layer that is affected by turbulent motions and inhomogeneous surface chemical emissions, short-lived reactive species may not be completely mixed within any given airmass. Coarse atmospheric models, which assume complete mixing within each grid-box, may overestimate the rates at which chemical species react. We used a large eddy simulation (LES) model embedded in the Weather Research and Forecasting (WRF) model to assess the influence of species segregation on the photochemistry in the convective boundary layer. We implemented our model in the vicinity of Hong Kong Island, which is subject to strong turbulent flow and spatially inhomogeneous anthropogenic and biogenic emissions. We conclude that under heavy pollution conditions, segregation reduces the rate of the reaction between anthropogenic hydrocarbons and hydroxyl radical (OH) by 25% near the surface in urban areas. Furthermore, under polluted conditions, segregation reduces the ozone production rate in the urbanized areas by 50% at about 100 m above the surface. The reduction is only equal to 20% near the surface in the forested mountain area. This highlights the need to develop grid refinement approaches in regional and global models in the vicinity of large urban areas with high pollution levels. Under clean conditions, our large eddy simulations suggest that the role of segregation is small and can be ignored in regional and global modelling approaches.