Ozone distributions over the los angeles basin: Three-dimensional simulations with the smog model

The UCLA Surface Meteorology and Ozone Generation (SMOG) model has been applied to simulate and analyze the distributions of ozone in the Los Angeles basin on 27 and 28 August 1987, during the Southern California Air Quality Study (SCAQS). SMOG is an integrated air pollution modeling system that includes the coupled effects of meteorology, pollutant sources and dispersion, photochemistry and aerosol microphysics, and radiative processes. High surface ozone concentrations are predicted along the slopes of the surrounding mountain barriers and in the eastern basin, as observed. The pattern of surface ozone concentrations is shown to be influenced by the evolution of oxidants within the temperature inversion that typically blankets the Los Angeles basin. In the past, dense layers of ozone have been observed to be embedded within the temperature inversion, but their origin has remained unexplained. The present model simulations reproduce these and other three-dimensional structures in the ozone distribution, as documented by measurements in the basin. The high concentrations of oxidants aloft are explained through a detailed dynamical/chemical mechanism. High mountains surrounding the Los Angeles basin block the dispersion of ozone and other pollutants and generally contain them within the basin. At the same time, vertical circulations associated with the interaction of the sea-breeze and mountain slope winds inject pollutants into the base of the inversion and create high concentrations of ozone, PAN, nitric acid, organic nitrates and a variety of other pollutants, including aerosols. The contrast between ozone concentrations at the surface and in these elevated layers can be enhanced in the evening by NO emissions, which titrate ozone near the surface. The elevated layers act as a reservoir for aged pollutants, which can be mixed downward on subsequent days to enhance surface concentrations. The recirculation of pollutants in the Los Angeles basin is thus reinforced by the presence of these oxidant layers, and may be responsible for the rapid increase in surface ozone concentrations often seen in the morning, particularly in the eastern basin where emissions of primary pollutants are relatively small. The impact on surface ozone concentrations of recirculated photochemically aged air is estimated to be as high as 8 pphmv in the eastern Los Angeles basin. These simulations represent the first quantitative description of the role of pollutants aloft on surface air quality.