Impacts of land-surface forcing on local meteorology and ozone concentrations in a heavily industrialized coastal urban area

In this study, the Weather Research and Forecasting/Community Multiscale Air Quality (WRF-CMAQ) model was used to investigate the interaction of urban land-surface forcing with local circulations and the impact on boundary layer ozone concentrations in southern Taiwan at an urban-scale resolution. Two simulations were performed with the same emissions but different land cover designations. URBAN was the baseline simulation representing the current urbanized condition, while NO-URBAN replaced all urban grid cells with cropland. The interaction of the sea-breeze with the urban-heat-island (UHI) convergent flow during the daytime in URBAN transports near-surface O3 precursors to the upper planetary boundary layer (PBL). When the UHI convergent flow stalls over the city center, a circulation flow is formed and traps the pollutants at an elevated height, increasing the reaction rate of hydroxyl radical with volatile organic compounds by 2.0–4.0 ppbv h−1 at 1000–1500 m. At nighttime, the deeper boundary layer of URBAN diluted the NOx mixing ratio by ~17 ppbv and weakened the titration effect, contributing to higher O3 by +15 ppbv in the urban area. However, once the daytime vertical mixing diminished, the O3 aloft also diffused downward to the surface level and further degraded the nighttime air quality. •NO2 plume is directed upward due to sea-breeze interaction with urban heat island.•Diluted NOx concentrations weaken O3 titration effect in the urban boundary layer.•Circulation flow forms and traps pollutants at an elevated height during daytime.•O3 aloft diffuses to the surface level throughout the evening and nighttime.