Factors controlling surface ozone in the Seoul Metropolitan Area during the KORUS-AQ campaign

To understand the characteristics of air quality in the Seoul Metropolitan Area, intensive measurements were conducted under the Korea-United States Air Quality (KORUS-AQ) campaign. Trace gases such as O₃, NO_x, NO_y, SO₂, CO, and volatile organic compounds (VOCs), photochemical byproducts such as H₂O₂ and HCHO, aerosol species, and meteorological variables including planetary boundary layer height were simultaneously measured at Olympic Park in Seoul. During the measurement period, high O₃ episodes that exceeded the 90 ppbv hourly maximum occurred on 14 days under four distinct synoptic meteorological conditions. Furthermore, local circulation such as land-sea breeze and diurnal evolution of the boundary layer were crucial in determining the concentrations of precursor gases, including NO_x and VOC as well as O₃. During such episodes, the nighttime NO_x and VOC and daytime UV levels were higher compared to non-episode days. The overall precursor levels and photochemical activity were represented fairly well by variations in the HCHO, which peaked in the morning during the high O₃ episodes. This study revealed that toluene was the most abundant VOC in Seoul, and its concentration increased greatly with NO_x due to the large local influence under stagnant conditions. When O₃ was highly elevated concurrently with PM_2.5 under dominant westerlies, NO_x and VOCs were relatively lower and CO was noticeably higher than in other episodes. Additionally, the O₃ production efficiency was the highest due to a low NO_x with the highest NO_z/NO_y ratio among the four episodes. When westerlies were dominant in transport-south episode, the nighttime concentration of O ₃ remained as high as 40~50 ppbv due to the minimum level of NO_x titration. Overall, the Seoul Metropolitan Area is at NO_x-saturated and VOC-limited conditions, which was diagnosed by indicator species and VOC/NO_x ratios.