Role of turbulence in ozone chemistry: Segregation effect implicated from multiscale modeling over Hong Kong

By influencing the mixing of reactive species in the atmosphere, turbulence affects the rates at which chemical species react, specifically in the planetary boundary layer. To analyze the turbulent segregation effect on the atmospheric ozone photochemistry, coupled meso-to micro-scale simulations are performed over Hong Kong, a polluted megacity. The comparison of the ozone production rates between the mesoscale model and the large-eddy simulations (LES) indicates that the resolved turbulent processes inhibit the mixing of the reactants, and thereby slow down the reaction rate between NO and O₃, resulting in a smaller net O₃ production when represented by a LES simulation. The calculated segregation intensities show clear correlations with the emission distribution, revealing that the segregation effect is significant in urban areas with inhomogeneous pollution sources, including traffic emissions. The evolution of the eddy structure plays a significant role in the diurnal variation of the segregation intensity. Applying the obtained segregation intensities to the mesoscale output, the NO–O₃ reaction rate decreases with results closer to those provided by the LES model. This confirms the importance of the segregation effect on fast chemical reactions and provides insights into the parameterization of the segregation effect in mesoscale models, which do not resolve turbulent eddies.