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 O, NO, NO, SO, CO, and volatile organic compounds (VOCs), photochemical byproducts such as HO 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 O 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 and VOC as well as O. During such episodes, the nighttime NO 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 O episodes. This study revealed that toluene was the most abundant VOC in Seoul, and its concentration increased greatly with NO due to the large local influence under stagnant conditions. When O was highly elevated concurrently with PM under dominant westerlies, NO and VOCs were relatively lower and CO was noticeably higher than in other episodes. Additionally, the O production efficiency was the highest due to a low NO with the highest NO/NO ratio among the four episodes. When westerlies were dominant in transport-south episode, the nighttime concentration of O remained as high as 40~50 ppbv due to the minimum level of NO titration. Overall, the Seoul Metropolitan Area is at NO-saturated and VOC-limited conditions, which was diagnosed by indicator species and VOC/NO ratios.