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Presents results for April‐September 1996 from the Multi‐layer Atmospheric CHemistry model, Oslo (MACHO), a regional scale photo‐chemistry model, and compares to measurements. Initial and lateral boundary concentrations are provided by a global CTM (chemical tracer model). As a base run, monthly averaged concentrations for June 1996 are used as initial and lateral boundary concentrations. The sensitivity of the interior model domain to the lateral boundaries is analyzed by comparing the base run with a model run with lateral boundary concentrations updated at six‐hour intervals. In the boundary layer over Europe, differences between the two runs are small both for accumulated excesses and for concentrations on individual days, indicating that ozone levels here are determined predominantly by local European scale sources. in the free troposphere, however, the effects of the lateral boundary concentrations are shown to be significant.