The Analysis Of Two Haze Episodes From Different Sources Over Yangtze River Delta In Winter

With the speeding up of urbanization, a large number of pollutants are discharged into the atmosphere, leading to serious air pollution, which occurred frequently over Beijing-Tianjin-Hebei(BTH), Yangtze river delta(YRD), Pearl river delta(PRD). These regional continuous pollution incidents bring great harm to people’s production and life .In this paper, we analyzed two episodes over the Yangtze river delta during 21 January to 24 January, 2015 and 24 December to 27 December, 2015. By using WRF-CMAQ model, Hysplit model and observation data, we discussed the change law of quality of PM2.5 in Yangtze river delta, besides the difference between long-rage transport and local pollution impacted on the formation haze days.A haze episode, which occurred over Yangtze River Delta (YRD) from 21 January to 24 January, 2015, was investigated using the WRF-CMAQ model system.The temporal and spatial distribution characteristics and regional transport process of fine particulate matter (PM2.5) were discussed. In addition, the contribution of local accumulation and long-range transport to this haze episode were also analyzed.Results showed that the WRF-CMAQ model could reproduce the temporal and spatial distributions and evolution features of PM2.5. On 21st, the YRD was in northerly winds prevailed at the ground level, which led to short-term northern input-base pollution. Horizontal flux at the ground level during short-term pollution was larger than that during local accumulation pollution. There was a vertical transport in boundary layer. During 22nd to 24st, small wind at surface and temperature inversion led to the local accumulation of pollution. The major source processes in sequence during short-term pollution are local source emission(35.04%), horizontal advection (27.07%), aerosol chemical process (20.91%) and vertical advection (14.07%). Those during local accumulation pollution are local source emission (50.93%), aerosol chemical process (27.05%), vertical advection(17.47%) and horizontal advection (0.34%). The contribution rates of horizontal advection, source emission and aerosol chemical process changed obviously.During 24 December to 27 December, 2015, a haze episode occurred over YRD.We used observation PM2.5 data of MEPC, micaps meteorological data, combining the HYSPLIT model, sensitivity test of WRF-CMAQ to analyze the temporal and spatial distribution characteristics and local pollution process of PM2.5. Results showed that from 23 December to 24 December, YRD is in front of the cold high pressure, where the surface pressure gradient is small, wind speed is tiny, and the atmosphere is stable. There also have the lower mixing layer height, weak vertical transmission and diffusion, which is beneficial to the accumulation of pollutants.From 26～27, there is a cold waves, which make wind speed increases at surface over YRD and the atmospheric diffusion ability strengthen. Pollution dissipates gradually. In the pollution process, the YRD local pollution play an important role to the contribution of PM2.5, which accounted for 40% in the upstream city, accounted for 70% in the downstream cities. There also exists transport between upstream to downstream city. Using the sensitivity test method, source areas(Anhui, Jiangsu,Zhejiang, Shanghai) are closed, before and after the differences of atmospheric pollution in YRD is obvious. At the beginning of pollution (24 December),long-range transport occurred earlier. At the mid of stage (25 December), mainly contribution is local pollution, the average maximum contribution amounting to 73μg/m3. At the end of pollution (26～27),source area amount in high value contribution, average maximum value up to 135μg/m3.