The influences of meteorological conditions and complex topography on ozone (O₃) concentrations were evaluated at two different sites [Jung Ang (JA): a semi-closed topography and Seo Chang (SC): a closed topography] within a valley city neighboring coastal metropolitan cities (Busan and Ulsan) during days exhibiting high O₃ episodes (8 and 10 August 2006), using the MM5-CMAQ modeling system. The contributions of physical and chemical processes to the O₃ concentrations were also assessed within the mixed layers of the two sites (JA and SC) during the study period. High O₃ concentrations at site JA on 8 August were the result of combined effects of the horizontal transport (in a north-south direction) of O₃ and its precursors and their subsequent reaction with southerly (from Busan) and easterly sea breezes (from Ulsan) and their accumulation due to the convergence of these sea breezes. Such O₃ enhancements on 10 August were caused mainly by the vertical transport of O₃ and its precursors and partly by their accumulation (from local emission sources) due to weak winds. Unlike site JA, the O₃ enhancement at site SC on 8 August was mainly ascribable to horizontal transport (in an east-west direction) from the source regions of Ulsan. On 10 August, vertical transport of downslope flows from the top of mountains largely contributed to O₃ concentrations. In addition, the heights of maximum O₃ inflow due to horizontal or vertical transport between the two sites during the episodes were different depending on the differences in the heights of mountains and/or the movement directions and intensities of sea breezes.