Analysis Of Typical Haze Pollution Process In Nanjing Based On WRF-Chem Mode

Based on the observational data of major pollutants in Nanjing from 2017 to 2021,this paper comprehensively analyzed the temporal variation characteristics of major pollutants and air quality.Three typical continuous heavy haze pollution processes occurred during the selection period were used to analyze the influence of meteorological conditions on haze pollution processes by using meteorological elements monitoring data and boundary layer conditions.The HYSPLIT backward trajectory model was used to analyze the source and nature of pollution air mass,and the potential source contribution factor method（PSCF）and concentration weight trajectory method（CWT）were combined to explore the potential source area and source area contribution of PM_2.5.Finally,numerical simulation was carried out based on WRF-Chem model to reproduce the typical haze pollution process in Nanjing,and the simulation results were evaluated,verified and analyzed by using the observed data.Sensitivity experiments were set up by clearing MEIC emissions in Nanjing to explore the influence of local and external sources on PM_2.5.The results are as follows:（1）From 2017 to 2021,the air quality in Nanjing was slightly polluted for 374d,moderately polluted for 55d,severely polluted for 9d and severely polluted for 1d.For major pollutants,PM_2.5 exceeded the standard 138d,PM₁₀ exceeded the standard 62d,O₃ exceeded the standard 273d and NO₂ exceeded the standard 52d.（2）The peaks of the AQI index of the three processes were 304（severe pollution）,227（severe pollution）and 175（moderate pollution）respectively.The relative humidity was almost higher than 70%,and the lowest wind speed was about 1m/s.The high humidity environment,stable air temperature and still small wind provided favorable meteorological conditions for the haze pollution process.At the same time,during the pollution period,there is a low height of the mixed layer,combined with high frequency and intensity of temperature inversion,which inhibits the vertical diffusion of pollutants,resulting in pollution intensification.（3）HYSPLIT backward trajectory and potential source analysis show that the haze pollution process in Nanjing is mainly affected by the emission of pollutants from the local and surrounding developed areas.Meanwhile,the transport of pollutants by air mass also has a certain impact,especially the long-distance transport in the northwest.PSCF and CWT show that Nanjing is seriously affected by regional transport.To the northwest of Nanjing,Anhui and other developed cities have made significant contributions.（4）On the whole,WRF-Chem model can simulate the changes of PM_2.5 and meteorological field well,and basically reflect the real pollution process.The correlation coefficient between the simulated value and the monitored value of PM_2.5 reaches 0.92 and 0.82,but there are different degrees of overestimation.The sensitivity experiment found that the extreme value difference of PM_2.5 concentration before and after zero emission in Nanjing was large,indicating that the local source had a great impact on PM_2.5 emission in Nanjing and was the main source of PM_2.5 in Nanjing,and the regional transmission of developed cities in the northwest was the main external source of PM_2.5 in Nanjing.