Abstract
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.
Highlights
Several short-lived chemical species present in the atmosphere affect air quality as well as the level of climate forcing
We have shown that turbulent motions affect the rate at which chemical species react together and their mean concentration in the planetary boundary layer
Segregation effects can only be captured in high-resolution models and quantified, for example, through large eddy simulations
Summary
Several short-lived chemical species present in the atmosphere affect air quality as well as the level of climate forcing. Because the reaction rates between OH and some of those chemical compounds, such as isoprene, are fast, these reactions can be affected by turbulence in the boundary layer [2,3]. It is the case of ozone (O3), which is a secondary air pollutant and a radiatively active trace constituent. Emissions of different ozone precursors are often not co-located, and turbulent motions in the atmosphere may not completely mix in the same air masses as the molecules that potentially react with each other. The analysis is based on a LES approach formulated at medium spatial resolution and without detailed representation of the urban canopy
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