Model Simulation Of Air Pollution Characteristics At Representative Areas In China

During the past several decades, rapid economic development and urbanization process led to deterioration of air quality in China. High concentrations of air pollutants caused serious risk to human health and ecological environment damage. Therefore, it is urgent to improve air quality through reducing emission in China. The primary objectives of this study are to evaluate seasonal characteristics of major pollutants and to understand formation mechanisms of major air pollutants in China via air quality model simulation. Such information would provide useful perspectives for developing local and regional emission control strategies.Models-3/CMAQ v4.7 was applied to simulate the concentrations of the major pollutants (PM10, O3, SO2 and NO2) in China. Temporal and spatial distributions of these pollutants and the impact of cold front weather system on PM10 concentrations were investigated. Process analysis (PA) in CMAQ (including integrated process analysis (IPR) and integrated reaction rate analysis (IRR)) were used to identify the most influential processes and chemical reactions (such as horizontal/vertical transport, aerosol processes, clouds/fog processes and wet/dry deposition) that led to the accumulation or loss of O3 and PM10 in atmosphere. Secondary inorganic aerosol and O3 chemistry regime analysis were also analyzed based on the CMAQ-PA results. Model evaluation results indicated that our simulated results were acceptable and could fulfill the need of air quality impact and control scenario analysis, O3 and PM chemistry regime analysis and process analysis.CMAQ simulations showed that higher concentrations of SO2, NO2, and PM10 occurred over Jing-Jin-Ji Bohai sea rim region, Yangtze River Delta (YRD) area, Pearl River Delta (PRD) area and Sichuan Basin. Highest surface concentrations of SO2, NO2, and PM10 were found in winter and lowest in Jul due to the variations of weather condition and pollutants emissions. Temporal and spatial variations of O3 concentrations were different from SO2, NO2, and PM10. Highest 1-h O3 concentrations (>70 ppb) were found over North China Plain in summer. April also had relatively high O3 mixing ratios of about 60-70 ppb over large areas in the North of Yangzi River (～30°N-42°N), which extended from east to west of China while 55-65 ppb over most other areas. Compared with other seasons, Jan had the lowest O3 mixing ratios (<50 ppb) over most area of China due to weaker solar radiation in winter, except Tibetan Plateau in the western part of China (O3 mixing ratios were 50-60 ppb). Case study of PM10 concentrations influenced by cold front system indicated that our simulation captured this cold front weather system and cold front weather system could reduce PM10 concentrations significantly.Indicator analysis of O3 chemistry regime indicated that VOC-limited chemistry covered the central and eastern China including the North China Plain, YRD, PRD and the northeastern China, as well as some major cities in most provinces, while other areas were NOx-limited O3 chemistry in winter. NOx-limited O3 chemistry dominated over almost entire China during summer except several metropolitan areas (such as Beijing and Shanghai). Differences between HOx radical termination reactions in urban and rural areas also indicated VOCs-limited chemistry over Beijing and Shanghai, and NOx-limited chemistry at Xiaoping and Waliguan sites. Indicator analysis of PM chemistry regime suggested that PM formation was more sensitive to SO2 reduction than NOx and NH3. Scenario analysis indicated that SO2 control would be more effective to PM10 reduction than NOx and NH3 besides primary emissions of PM10. AVOCs reduction could lead to O3 reduction over metropolitan areas in summer while NOx reduction could lead to O3 reduction over rest areas. However, if SO2 and NOx reduction were used to control aerosol pollution over metropolitan areas, O3 mixing ratios will increase. These results indicated that different emission control strategies for air quality improvement (separate NOx or VOC emission control, or integrated control of NOx and VOCs emissions) should be taken over different regions and during different seasons to effectively control ambient O3 and PM10 air pollution.Diurnal variations of O3 concentrations in Beijing at surface layer were larger than that at higher altitude, indicating that O3 stored at higher layers at nighttime and contributed to the O3 mixing ratios of next day. Obvious discrepancy of O3 formation processes existed between surface layer and upper level. Highest O3 production rate was found at 200-1500 m layer, and vertical transport was the dominate factor that contributed to O3 accumulation at surface layer. O3 reaction with NO and dry deposition process were two major factors that contributed to O3 loss at surface layer. Above 1500 m, gas-phase chemistry contributed much less to O3 formation compared with lower layers.Primary particulate species mainly contributed to PM10 increase at low layers (< 300 m). Aerosol processes had positive and negative contributions to PM10 concentrations, e.g. the positive contributions included the formation of sulfate, nitrate, second organic aerosol, the condensation of gas phase H2SO4 and HNO3 on pre-existing particles, while the negative contribution was due to the thermal decomposition of nitrate. Wet and dry deposition processes were two main sinks of PM10, and dry deposition only occurred at surface layer. Horizontal transport helped transport particles from heavily-polluted areas to downwind areas. Vertical transport contributions to PM10 concentrations changed with layers.