A study on high ozone formation mechanism associated with change of NOx/VOCs ratio at a rural area in the Korean Peninsula

We present an analysis of high surface ozone (O3) episodes occurring in the rural area of Chuncheon, which is situated 70 km to the prevailing westerly downwind direction of Seoul Metropolitan Area. During the episode days, the daily maximum O3 concentration exceeded 120 ppb and an O3 warning was issued in Chuncheon. Although Chuncheon has much lower emission levels than Seoul, the daily mean and maximum O3 levels were higher in Chuncheon than those in Seoul during the episode days. In addition, the high O3 concentrations in Chuncheon were not dependent on the meteorological condition being generally favorable for O3 production. The inverse trajectory analysis using HYSPLIT clearly shows the features of pollutant transport from Seoul to Chuncheon. For further investigation, we conducted numerical experiments using the atmospheric dynamic Weather Research and Forecasting (WRF) model and the air quality Community Multi-scale Air Quality (CMAQ) model to examine the detail process of these high O3 episodes with the Decoupled Direct Method (DDM-3D) and Integrated Process Rate (IPR) analyses. The DDM-3D analysis results demonstrate that the O3 concentration in the air mass moving from Seoul to Chuncheon was very sensitive to the concentration of nitrogen oxide (NOx) because of abundant biogenic volatile organic compounds (BVOCs). The IPR results also show that the NOx emitted from Seoul strongly affected the high O3 levels over its downwind area, mainly with local BVOC emissions. The produced O3 was gradually accumulated during its transport downwind, leading to an O3 concentration maximum at Chuncheon.