Temporal and Spatial Distribution Characteristics of PM2.5 Concentration in Tianjin and Simulation Analysis of Sources of Heavy Pollution Process

In Tianjin, there are many heavy pollution events with PM_2.5 as the primary pollutant. It is clear that the spatial and temporal distribution of PM_2.5 concentration and the source of heavy pollution process have far-reaching significance for the comprehensive management of PM_2.5. Using Tianjin's environmental data for 2014-2017 and in 2016, meteorological data, combined with WRF-Chem model, studied the spatial and temporal distribution characteristics of PM_2.5 concentration in Tianjin and the source of heavy pollution process. The results show that since 2014, the concentration of PM_2.5 in Tianjin has been decreasing year by year. The monthly variation curve of PM_2.5 concentration showed a "U"-shaped distribution, which was seasonal in winter and spring, and low in autumn and summer. The daily variation of PM_2.5 concentration is bimodal, with the main peak appearing at 08:00-09:00 and the second peak occurring at 21:00-00:00. The spatial distribution of PM_2.5 concentration in Tianjin is different in each season. The spring, summer, autumn and winter highvalue centers are located in Jinghai District in the southwest of Tianjin, Beichen District in the north of the central city, Wuqing District in the west and Yinzhou District in the north. The source of pollutants in the autumn and winter seasons of Tianjin simulated by WRF-Chem model showed that the local source contribution rate was 56%, and the external source transportation contribution rate was 44%, including transportation in Hebei Province and Shandong Province.