An observational and numerical study of the effects of the late sea breeze on ozone distributions in the Busan metropolitan area, Korea

The late sea breeze (LSB), defined as a sea breeze with an onset time later than 1200 LST, and its impact on O3 concentration distributions have been investigated both observationally and numerically over the Busan metropolitan area in Korea. The observed LSB mostly occurred under weak offshore synoptic flows during mid-day, and was found with late but apparent transitions in wind direction along with lower wind speed. The observed O3 concentrations associated with the LSB demonstrated more significant enhancement of maximum O3 concentrations than those of the early sea breeze by a factor of approximately 1.5. The main meteorological feature associated with the LSB was also reproduced by MM5 for typical cases. The numerically derived backward trajectories suggested that the increase of O3 at the coastal site is mainly due to recirculated polluted air mass. The numerical simulation of O3 concentrations from MM5/UAM-V shows that offshore winds of northwesterly synoptic flow swept the precursors of O3 that were emitted by the urban area toward the sea during the early morning. As the development of the LSB was suppressed by the synoptic flow, a large near-stagnant wind field formed over the sea with intense photochemical activity at mid-day. However, the subsequent LSB with low wind speed was simulated in order to slowly re-advect the photochemically produced air parcels toward the inland area, resulting in both a significant enhancement of O3 concentrations and high concentrations over the coastal area lasting for relatively longer hours. This result implies that the LSB plays an important role in the recirculation/accumulation of O3 concentrations over the Busan metropolitan area, Korea.