Impacts of urban land-surface forcing on air quality in the Seoul metropolitan area
1School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea
2Division of Environmental, Civil and Transportation Engineering, Ajou University, Suwon, South Korea
3Korea Environment Institute, Seoul, South Korea
Abstract. Modified local meteorology owing to heterogeneities in the urban-rural surface can affect urban air quality. In this study, the impacts of urban land-surface forcing on air quality during a high ozone (O3) episode in the Seoul metropolitan area, South Korea, are investigated using a high-resolution chemical transport model (CMAQ). Under a fair weather condition, the temperature excess (urban heat island) significantly modifies boundary layer characteristics/structures and local circulations. The modified boundary layer and local circulations result in an increase in O3 levels in the urban area of 16 ppb in the nighttime and 13 ppb in the daytime. Enhanced turbulence in the deepened urban boundary layer dilutes pollutants such as NOx, and this contributes to the elevated O3 levels through the less O3 destruction by NO in the NOx-rich environment. The advection of O3 precursors over the mountains near Seoul by the prevailing valley-breeze circulation in the mid- to late morning results in the build-up of O3 over the mountains in conjunction with biogenic volatile organic compound (BVOC) emissions there. As the prevailing local circulation in the afternoon changes to urban-breeze circulation, the O3-rich air masses over the mountains are advected over the urban area. The urban-breeze circulation exerts significant influences on not only the advection process but also the chemical process under the circumstances in which both anthropogenic and biogenic (natural) emissions play important roles in forming O3. The intrusion of the air masses, characterized by low NOx and high BVOC levels and long OH chain length, from surroundings increases ozone production efficiency in the urban area, thus leading to more O3 production. The relatively strong vertical mixing in the urban boundary layer embedded in the sea-breeze inflow layer reduces NOx levels, thus contributing to the elevated O3 levels in the urban area.