Study On The Formation Mechanism And Forecasting Methodology On Haze Weather In Yangtze River Delta

Haze weather which is characterized by air pollution and visibility decrease is an urban and regional phenomenon induced by a lot of suspended fine particles in the atmosphere. Because haze weather would significantly affect atmospheric environment, visibility, human healthy and regional and global climate change, it has been attracting more and more attentions. With the very fast economic development and urbanization, the Yangtze River Delta (YRD) has been one of the four heaviest haze regions in China, and the frequency of haze weather in YRD has been increasing in recent years. Based on Regional Atmospheric Environment Modeling System (RegAEMS), in this study, Regional Haze Weather Forecasting System (RegHWFS) was developed by adding multiphase chemical processes module, visibility calculation module, the emission inventory of fugitive dust from paved road and updating the inventory of vehicle emission to RegAEMS. In addition with data analysis, the mechanism and the temporal and spatial distributions of haze weather in YRD was studied and numerical forecasting on haze weather was also carried out with RegHWFS.Firstly, characterization of visibility and the relationship between visibility and its affecting factors over typical cities (i.e. Nanjing, Shanghai, Hangzhou and Hefei) in YRD were investigated. Results showed that the ratios of low visibility weather (i.e. visibility<10km) over different cities in YRD during2004-2009was44.22%-83.41%, and increased with the time (0.8%/a-2.5%/a). The ratio of low visibility weather was highest (50%-90%) in about the early morning and lowest (20%-70%) in the afternoon. It was more likely to be with low visibility when the prevailing wind direction was northwest. The characterization of haze weather in Nanjing was particularly analyzed, and results showed that the annual mean visibility in Nanjing was8.8km, and visibility exhibited a significant diurnal variation with6.6km at7:00and maximum of10.7km at16:00. Seasonal variations were also observed:9.2km in spring,9.8km in summer,8.2km in autumn, and8.1km in winter. No significant weekend effect in visibility was found, and significant periodic variations, with major periods of synoptic scale (2-4days) and quasi-two-week scale (8-16days) were found. The correlation coefficients of extinction with PM10, relative humidity (RH), wind speed, air temperature and pressure were0.371,0.666,-0.290,0.152and-0.280, respectively. Synoptic weather showed significant impact on visibility in Nanjing. The average mass concentration of PM2.5over Nanjing during2008-2009was73μg/m3. Visibility showed significant negative correlation with PM2.5concentration, especially in low visibility weather. According to statistics, the judgment index of haze weather in YRD was that:visibility less than10km, RH less than80%, and PM25concentration higher than80μg/m3. Based on the level of visibility, haze weather was classified into four grades:super heavy haze (visibility<2km), heavy haze (2km≤visibility<4km), moderate haze (4km≤visibility<7km), and light haze (7km≤visibility<10km).Secondly, the characterization and formation mechanism of a typically continuous haze pollution case in YRD from October14,2009to October31,2009was studied. Results showed that, characterized by relative high PM concentration, low visibility and RH, the haze pollution affected a wide range of YRD during this period. The most polluted regions were Nanjing, Yangzhou, Xuzhou, Suqian and Hefei et al., which were mostly located in the center and north of YRD, with their mean Air Pollution Index (API) all exceeding100. During the continuous haze weather period, in Nanjing, the average value of RH was61%, the mean concentration of PM2.5and PM10were95-136μg/m3and192-244μg/m3, which were64%-143%and92%-94%higher than those during the non-haze days, respectively. High-pressure system with relative uniform pressure pattern, the inversion temperature, relative low PBL height, continuous light wind, low RH and weak vertical velocity were not favor for the air pollutants diffusion and supply favorable weather conditions to this severe pollution case, while the air pollutants releasing from crop residual burning during the period could also contribute to the haze pollution.Thirdly, the possible impact on atmospheric trace components of the multiphase chemical processes were studied by setting up coupled box models and the primary affecting factors of the multiphase chemistry were also investigated by sensitivity tests. Results indicated that heterogeneous chemical processes on the aerosol surface in the atmosphere could decrease the concentrations of trace gases except NO2and increase the concentrations of secondary aerosols such as SO42-, NO3-and NH4+, Sensitivity tests suggested that the magnitude of the impact of heterogeneous processes strongly depends on aerosol concentration and the surface uptake coefficients used in the box model. However, the impact of temperature on heterogeneous processes is considerably less. Aqueous chemical processes would decrease the concentrations of water-soluble gases such as SO2and O3, and increase the concentrations of SO42-, NO3-and NH4+. Sensitivity test suggested that the magnitude of the impact of aqueous processes strongly depends on the liquid water content (LWC), liquid drop radius, liquid temperature and initial gaseous concentrations.Fourthly, based on AP-42method from EPA and COPERT IV from EEA, the emission inventory of fugitive dust from paved road on city scale and vehicle emission on province scale in addition with their spatial distributions during2006-2008were developed. Results showed that the emissions of PM2.5, PM10and TSP in fugitive dust from paved road in YRD were539,2165and5388kt, respectively, showing an obvious increase trend (15.6%/a) with time. The emissions of PM10, PM2.5, BC, OC, SO2and NOX from vehicle emissions in YRD in2008were56.2,48.9,21.7,16.4,960.4and58.9kt. The fugitive dust emission and vehicle emission both show significant spatial variations, with the higher value mostly concentrated in the region that were fairly developed economically such as Shanghai and South Jiangsu.Lastly, RegHWFS applied in YRD was developed by adding multiphase chemical process module, visibility calculation module to RegAEMS and updating the inventory of anthropogenic emission. One year retrospective forecast starting from Jan.1,2009to Dec,31,2009were carried out to evaluate the forecasting ability of the improved modeling system. Results showed that RegHWFS had good performance in weather, PM and visibility forecast. The accurate ratio of grade forecast was70.8%. For haze weather, the TS scores of ungraded and graded forecast were73.2%and70.2%, respectively. In addition, modeling results showed that in2009haze weather in YRD exhibited obvious spatial and temporal variations. Haze weather occurred mainly in Taizhou, Yangzhou and Suzhou, which were located in the center and south of Jiangsu Province. The heaviest haze weather was in autumn, with the largest cover area, while the haze in summer was lightest. The forecasting system had a relative good performance on the continuous haze weather, and it could predict the meteorological parameters, PM2.5and visibility well.