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 (O 3 ) formation through regulating the chemical transformations and transport of O 3 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 O 3 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 O 3 concentration (−1.3 ppb) and an increase of O 3 (+5.2 ppb) around the nocturnal rush-hours. The suppressed O 3 titration destruction due to NOx dilution into the deeper urban boundary layer (200–400 m) is the main reason for elevated nocturnal O 3 levels. In the daytime, however, the upward transport of O 3 precursors weakens near-surface O 3 photochemical production and conversely enhances upper-level O 3 generation. Furthermore, the surface UHI convergence flow and intensified sea breeze act to effectively trap O 3 at the suburban and coastal regions.