A study on high ozone formation mechanism associated with change of NO x /VOCs ratio at a rural area in the Korean Peninsula Won-Bae Jeon a , Soon-Hwan Lee b, * , Hwawoon Lee a , Changhyoun Park c , Dong-Hyuk Kim c , Soon-Young Park a a Division of Earth Environmental System, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea b Department of Earth Science Education, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea c Institute of Environment Studies, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea highlights  High ozone episodes in rural area near Seoul are often occurred in the Korea.  Precursors’ transformation at high-O 3 episode in rural area was clarified.  Different precursor distribution at air pathway is associated with ozone formation.  Transported NO x can contribute chemical O 3 production with local biogenic VOCs. article info Article history: Received 12 August 2013 Received in revised form 3 February 2014 Accepted 5 February 2014 Available online 6 February 2014 Keywords: Precursors Ozone formation WRF CMAQ Chemical process analysis abstract We present an analysis of high surface ozone (O 3 ) 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 O 3 concentration exceeded 120 ppb and an O 3 warning was issued in Chuncheon. Although Chuncheon has much lower emission levels than Seoul, the daily mean and maximum O 3 levels were higher in Chuncheon than those in Seoul during the episode days. In addition, the high O 3 concentrations in Chuncheon were not dependent on the meteorological condition being generally favorable for O 3 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 O 3 episodes with the Decoupled Direct Method (DDM-3D) and Integrated Process Rate (IPR) analyses. The DDM-3D analysis results demonstrate that the O 3 concentration in the air mass moving from Seoul to Chuncheon was very sensitive to the concentration of nitrogen oxide (NO x ) because of abundant biogenic volatile organic compounds (BVOCs). The IPR results also show that the NO x emitted from Seoul strongly affected the high O 3 levels over its downwind area, mainly with local BVOC emis- sions. The produced O 3 was gradually accumulated during its transport downwind, leading to an O 3 concentration maximum at Chuncheon. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In general, high surface ozone (O 3 ) episodes occur in urbanized areas because of densely distributed emission sources, and most air pollution studies have therefore focused on urbanized areas (Lam et al., 2005; Tie et al., 2009; Banta et al., 2011; Jeon et al., 2012). However, severe air pollution events have also frequently occurred in rural areas (Tanimoto, 2009; Sharma et al., 2013), especially in the downwind direction from metropolitan areas (Wang et al., 2012). Several studies have investigated the mechanisms of high O 3 episodes in rural areas. Xu et al. (2008) and He et al. (2012) separately studied severe high O 3 episodes occurring in rural areas near Beijing and Shanghai, respectively, and attributed the episodes to the transport of pollutants, especially nitrogen oxide (NO x ) and volatile organic compounds (VOCs), from urbanized upwind areas. * Corresponding author. E-mail address: withshlee@pusan.ac.kr (S.-H. Lee). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv http://dx.doi.org/10.1016/j.atmosenv.2014.02.005 1352-2310/Ó 2014 Elsevier Ltd. All rights reserved. Atmospheric Environment 89 (2014) 10e21