Impacts of urban land-surface forcing on ozone air quality in the Seoul metropolitan area
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 ozone air quality during a high ozone (O<sub>3</sub>) episode in the Seoul metropolitan area, South Korea, are investigated using a high-resolution chemical transport model (CMAQ). Under fair weather conditions, 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 O<sub>3</sub> levels in the urban area of 16 ppb in the nighttime and 13 ppb in the daytime. Enhanced turbulence in the deep urban boundary layer dilutes pollutants such as NO<sub>x</sub>, and this contributes to the elevated O<sub>3</sub> levels through the reduced O<sub>3</sub> destruction by NO in the NO<sub>x</sub>-rich environment. The advection of O<sub>3</sub> precursors over the mountains near Seoul by the prevailing valley-breeze circulation in the mid- to late morning results in the build-up of O<sub>3</sub> 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 O<sub>3</sub>-rich air masses over the mountains are advected over the urban area. The urban-breeze circulation exerts significant influences on not only the advection of O<sub>3</sub> but also the chemical production of O<sub>3</sub> under the circumstances in which both anthropogenic and biogenic (natural) emissions play important roles in O<sub>3</sub> formation. As the air masses that are characterized by low NO<sub>x</sub> and high BVOC levels and long OH chain length are advected over the urban area from the surroundings, the ozone production efficiency increases in the urban area. The relatively strong vertical mixing in the urban boundary layer embedded in the sea-breeze inflow layer reduces NO<sub>x</sub> levels, thus contributing to the elevated O<sub>3</sub> levels in the urban area.