Numerical study of a local PM2.5 pollution event under the typhoon Neoguri (1408) background over a coastal metropolitan city, Shenzhen, China

In this study, the transport mechanism of a regional PM2.5 pollution event over Shenzhen on 8 July 2014 was studied using the Weather Research and Forecasting (WRF) model with Four-Dimensional Data Assimilation (FDDA) techniques and the FLEXPART-WRF model to understand the effects of the local atmospheric circulations under the typhoon NEOGURI (1408) background. WRF-FDDA produces the spatiotemporal variation of winds that are consistent with the regional wind profiler observations. The air quality observations show that in this typhoon related PM2.5 pollution episode, there are significant temporal and spatial variation of PM2.5 concentrations over Shenzhen and there are two obvious PM2.5 peaks during the day of 8 July 2014. The simulation results revealed that the first peak was developed due to a wind convergence associated with the offshore winds forced by NEOGURI which was located to the west of Shenzhen at around noon. In the afternoon, the wind convergence motivated the upward motion and suppressed by the downdraft from the typhoon's peripheral circulation. The northerly wind forced by the typhoon over the land and the southerly sea breezes led to a local circulation that was favorable for air pollutant accumulation over the Pearl River area. Part of these accumulated air pollutants was recirculated into west part of Shenzhen which resulting in the second PM2.5 peak in the afternoon, and then gradually transported to the middle and east part of the city. The FLEXPART-WRF analysis confirms the recirculation of air pollutants from the Pearl River. This study demonstrated that air pollution can form over Shenzhen under the typhoon background, and the complex interaction between typhoon circulations and sea breezes development controls the spatiotemporal distribution of the air pollution in the coastal metropolitan city.