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

Abstract

The late sea breeze (LSB), defined as a sea breeze with an onset time later than 1200 LST, and its impact on O 3 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 O 3 concentrations associated with the LSB demonstrated more significant enhancement of maximum O 3 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 O 3 at the coastal site is mainly due to recirculated polluted air mass. The numerical simulation of O 3 concentrations from MM5/UAM-V shows that offshore winds of northwesterly synoptic flow swept the precursors of O 3 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 O 3 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 O 3 concentrations over the Busan metropolitan area, Korea.