We calculate the global change in the production of tropospheric ozone (O₃) and loss of methane (CH₄) caused by 45 days of summertime South Korean anthropogenic emissions during the Korea-US Air Quality (KORUS-AQ) mission. Our modelling system consists of three stages: the boundary layer-residual layer (BL-RL) stage processes the emissions, photochemistry, deposition, aerosol reactivity, and transport over terrestrial South Korea at 0.1&deg; x 0.1&deg; with hourly resolution. The plume (PL) stage continues to integrate the chemistry of air masses from the BL-RL stage as they are transported offshore, simulating offshore pollution plumes observed by aircraft. After three days of chemical aging in non-diluting plumes, the pollution remnants are dispersed (DP stage) into the background atmosphere and integrated until the pollution disappears. Net O₃ production is diagnosed in each stage using the integrated ozone change and our calculated perturbation lifetimes. In total, these 45 days of South Korean emissions create an excess CH₄ sink of 4.3 Gmol and a net O₃ source of 31.2 Gmol. Scaling these values to annual global emissions suggests around 10 % of CH₄ loss and 30 % of net O₃ production is attributable to anthropogenic air pollution, but our Korean summertime case may exaggerate the proportions. Reducing plume aging time to 2 days increases these terms by about 10 %, and immediate dispersion (no plume aging) more than doubles them. Our model supports the typical result that rapid dispersion of pollution, <em>e.g. </em>through coarse resolution, overestimates its impact on tropospheric O₃ and CH₄.