Semi-Lagrangian modelling of tropospheric ozone

The occurrence of high concentrations of ozone in the lower part of the troposphere is considered as one of the most important issues of tropospheric chemistry. The chemical mechanisms of tropospheric ozone formation are complex, and highly variable meteorological conditions contribute additionally to difficulties in an accurate prediction of ozone episodes. An effective way to increase our understanding of the problem and eventually improve our ability to predict the concentration of tropospheric ozone and to formulate emission control strategies is by applying a comprehensive model representing accurately the interaction between meteorological processes and chemical reactions. This paper presents a 3-dimensional semi-Lagrangian, chemical tracer model (CTM) featuring an accurate transport algorithm, comprehensive oxidants chemistry and deposition modules. The CTM is executed in off-line mode with a semi-Lagrangian, nonhydrostatic, mesoscale meteorological model that contains an extensive parameterization of physical processes (including a boundary layer scheme and clouds). The system of models was run for a time period of 6 days in order to generate a tropospheric ozone field during a smog episode observed in the eastern part of North America, in the beginning of August 1988. The numerical simulation was performed on grids with resolution of 20 and 40 km with 25 vertical levels. The emissions inventory considered in the simulation included point sources, surface biogenic sources, surface mobile sources and surface non-mobile sources. An evaluation of the model results against observations clearly indicates the ability of the system to simulate regional aspects of a tropospheric ozone episode. The model performance compares well to other models' results reported in the literature. An important achievement of this work is improving the physical realism of simulations by using highly accurate, nonoscillatory semi-Lagrangian advection transport algorithms.