Impact of Shanghai urban land surface forcing on downstream city ozone chemistry

The urban land surface has a significant impact on local urban heat island effects and air quality. In addition, it influences the atmospheric conditions and air quality in the downwind cities. In this study, the impact of Shanghai urban land surface forcing on weather conditions and air quality over Kunshan was investigated using the Weather Research and Forecasting model coupled with a multilayer urban canopy model and the Community Multiscale Air Quality model. Two simulations were conducted to identify the impact of upstream effects with and without upstream urban land surfaces in control and sensitivity experiments. The results show that the near-surface temperature and boundary layer height over Kunshan increased significantly with the appearance of the upstream urban land surface. Horizontal transport of O-3 and its precursors, from Shanghai to Kunshan, are suppressed in the lower boundary layer but are strengthened in the upper boundary layer because of strong urban heat island circulation. As a result, O-3 chemical production is decreased in the lower boundary layer of Kunshan but is increased in the upper boundary layer. On average, daytime O-3 concentrations over Kunshan are decreased by approximately 2ppbv in the lower boundary layer but are increased by as much as 40ppbv in the upper air.