The Numerical Simulation Of Comprehensive Air Pollution Characteristics In A Typical City-cluster

Due to the high non-linear relationship between ozone, fine particulate matterand their precursors, together with the functions of vertical transportation, horizontaltransportation, cloud process, gas-phase chemistry and aerosol chemistry, theproduction and cleanup processes of O₃and PM_2.5in the major city-clusters of Chinaare quite complicated. Therefore, to understand the production rate and analyze themain control factors of O₃and PM_2.5is not only a core issue of understanding theproduction mechanism of comprehensive air pollution, but also very important forpolicymakers to establish pollution control strategies.In this thesis, the O₃and PM_2.5pollution episodes in the typical city-cluster ofChina–the Yangtze River Delta （YRD） in2010are selected based on the onlineaerosol and photochemical observational data. The regional emission inventory in theYRD is updated to2010based on the latest activity data. The MM5-CMAQ modelingsystem is then applied to reproduce the typical air pollution episodes, and to study thepollution level and characteristics in the YRD region. In addition, the IntegratedProcess Rate Analysis （IPR） method coupled in the CMAQ model is applied to studythe contributions of various physical and chemical processes to atmospheric O₃andPM_2.5in typical cities at different heights, and the indicators including O₃/NO_zandH₂O₂/HNO₃are used to analyze the major control factors of O₃in different areas.Results of the2010regional emission inventory update show that the totalemissions of SO₂、NO_x、PM₁₀、PM_2.5、VOC、NH₃and BC in the YRD are1964.5、2702.5、1429.8、911.8、3705.0、1197.2å’Œ44.6kilo tons. The emission intensities inShanghai, Suzhou, Wuxi, Ningbo etc are relatively high, which will make regional airpollution transportation very significant under specific meteorological conditions.Results of the indicator analysis （including O₃/NO_zand H₂O₂/HNO₃） show thatthe urban Shanghai area belongs to VOC control region, while Suzhou belongs to NO_xcontrol area. In contrast, Hangzhou and Zhoushan belong to transition area. Thus，themajor control factors of O₃differs with region due to the emission characteristics.Therefore, the control of O₃should be different with area and time.The O₃process analysis studied based on IPR method indicates that themaximum concentration of photochemical pollutants occurs due to transportphenomena, including vertical diffusion, vertical advection and horizontal advection.The gas-phase chemistry producing O₃mainly occurs at the height of300-1500m,making a strong vertical O₃transportation from upper level to the surface layer. In thedownwind area, the high surface O₃levels come from horizontally advected flowsduring the morning and gas-phase chemical contributions occurring aloft. The urbandomain behavior slightly differs: the horizontal advection is also the main contributor to the surface concentrations of O₃. The gas-phase chemistry is an important sink forO₃in the lowest layer, coupled with vertical diffusion flows and dry deposition. Thecontributions of diffusive processes to net O₃concentrations under the planetaryboundary layer （PBL） are relatively low, and in particular the horizontal diffusion isnegligible compared to other atmospheric processes. Vertical diffusion compensatesthe loss of O₃in surface layers due to NO titration, contributing positively to net O₃concentrations in urban areas. At the surface layer, the maximum O₃concentration inthe suburban area is higher than in the urban region.The PM_2.5process analysis indicates that the emission of fine particles is the oneof the major sources of high PM_2.5concentrations in the cities of the YRD likeShanghai, Nanjing, Hangzhou, etc, following horizontal transportation and aerosolchemistry. The PM_2.5concentration could be reduced due to the vertical advection anddiffusion from lower level to upper air. The aerosol chemistry producing PM_2.5mainlyoccurs in the height of upper300m; dry deposition can significantly reduce theparticulates in the lower level, and wet deposition can reduce particles below the PBL.In the whole YRD region, during the high PM_2.5pollution episode, the concentrationsof secondary organic aerosol （SOA） and nitrate increase. In the whole simulationperiod, the major components of PM_2.5including nitrate （NO₃-）, ammonia （NH4+）,sulfate (SO₄^2-) organic carbon （OC） and element carbon （EC）. During the highpollution episode, the number and surface area of accumulation-mode particlesincrease significantly.The formation of regional comprehensive air pollution in the YRD is not onlyrelated to air pollutant emissions, but also is significantly influenced by the localmetrological conditions like wind direction, wind speed and atmospheric stability. Thesolar radiation, temperature and humidity are more important to the secondarypollution, and they are the most significant external factors affecting the chemicalreactions among NO_x, VOC, O₃and SOA. The formation mechanism of regional airpollution is quite complicated. The most important point to improve regional airpollution is to grasp the key control factors, so as to effectively reduce anthropogenicprecursor emissions.