Impacts of thermal circulations induced by urbanization on ozone formation in the Pearl River Delta region, China

Thermal circulations induced by urbanization could exert important effects on regional ozone (O3) formation through regulating the chemical transformations and transport of O3 and its precursors. In this study, the Weather Research and Forecasting/Chemistry (WRF/Chem) model combined with remote sensing are used to investigate the impacts of urbanization-induced circulations on O3 formation in the Pearl River Delta (PRD) region, China. The urban heat island (UHI) effect in PRD significantly enhances turbulent mixing and modifies local circulations, i.e., initiates the UHI circulation and strengthens the sea breeze, which in turn cause a detectable decrease of daytime O3 concentration (−1.3 ppb) and an increase of O3 (+5.2 ppb) around the nocturnal rush-hours. The suppressed O3 titration destruction due to NOx dilution into the deeper urban boundary layer (200-400 m) is the main reason for elevated nocturnal O3 levels. In the daytime, however, the upward transport of O3 precursors weakens near-surface O3 photochemical production and conversely enhances upper-level O3 generation. Furthermore, the surface UHI convergence flow and intensified sea breeze act to effectively trap O3 at the suburban and coastal regions.