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.