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 O-3 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 similar to 17 ppbv and weakened the titration effect, contributing to higher O-3 by +15 ppbv in the urban area. However, once the daytime vertical mixing diminished, the O-3 aloft also diffused downward to the surface level and further degraded the nighttime air quality.

Automated summary

This study investigates the interaction of urban land-surface forcing with local circulations and its impact on boundary layer ozone concentrations in southern Taiwan using the WRF-CMAQ model. The results show that the sea-breeze and urban-heat-island (UHI) flow during the daytime transport O3 precursors to higher layers, while the deeper boundary layer at night dilutes NOx and weakens the titration effect but leads to the downward diffusion of O3, resulting in worsened nighttime air quality.