Process analysis of the impact of atmospheric recirculation on consecutive high-O3 episodes over the Seoul Metropolitan Area in the Korean Peninsula

Episodes of exceptionally high surface ozone (O3) levels were measured in the Seoul Metropolitan Area (SMA) from July 6 to July 8. During the episode period, the 1-h average O3 concentration exceeded the daily maximum of 80 ppb over three consecutive days. In order to understand the mechanisms behind the increased O3 concentrations on consecutive days, several numerical experiments were conducted using the atmospheric dynamic model WRF for assessing the atmospheric flow as well as CMAQ for the estimation of the amount of photochemical ozone. lly increasing ozone concentration during episode period in SMA depends not only on the variation of meteorological conditions and the emission rate of precursors but also on the accompanying precursors within the air mass recirculation that are emitted from the SMA and its surrounding areas. The impact from the recirculation of precursors responsible for the increased ozone concentration was clarified for numerical studies using the particle dispersion model FLEXPART. Results obtained from our experimental model also demonstrated that recirculated O3 precursors such as nitrogen oxide (NOx) and volatile organic compounds (VOCs) significantly influenced O3 levels. During the episode period, NOx and VOCs concentrations in the PBL increased by an average of 2.9% and 19.7%, respectively, through recirculation. The recirculated precursors also are involved in the activation of photochemical reactions responsible for the formation of O3, leading to an average increase in the local O3 production of 10.9 ppb. And overall effect due to recirculation is dependent on the actual path and direction of recirculation.