Life–cycle impacts of South Korean air pollution on tropospheric ozone and methane: sensitivity to dispersion time

Abstract. We calculate the global change in the production of tropospheric ozone (O3) and loss of methane (CH4) caused by 45 d 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° × 0.1° 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 d of South Korean emissions create an excess CH4 sink of 4.3 Gmol and a net O3 source of 31.2 Gmol. A simplistic scaling of these values to annual global anthropogenic emissions suggests around 10 % of CH4 loss and 30 % of net O3 production is attributable to anthropogenic air pollution, but our Korean summertime case exaggerates these proportions. Reducing plume aging time to 2 d 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, e.g. through coarse resolution, overestimates its impact on tropospheric O3 and CH4.