| With the continuous development of human society, more and more people begin to pay close attention to the city air quality problem. Air pollution diffusion problem has always been a research hot spot in the field of scientific research. Relatively mesoscale pollution, the study of the small scale in the city and the neighborhood scale is relatively few, however, pollution accidents happening in small scale and neighborhood scale are relatively common. Urban land as the most complex underlying surface, which combines the features of a variety of underlying surface buildings, bare ground, grass, garden, etc., the urban flow field is very complex, because it is simultaneously subjected to weather-scale and small-scale(1-2km) and the role of neighborhood scale (tens of meters), but also affected by human activities. Integrated control for multi-scale elements caused great distress for study of urban air pollution dispersion, therefore, gripping the law of pollutant dispersion in urban research, timely and accurately predicting the temporal and spatial distribution of pollutants,which can help to control pollution or contamination when things take emergency measures, has great significance.This paper first elaborated the basic theory of diffusion transport of pollutants and the main influential factors on the diffusion of pollutants transmission. Use the field experiment (Urban Meteorological and Tracer Experiment, UMTE2011) data in Yuzhong County, Lanzhou City, which conducted by the project named" transport and transformation laws of atmospheric accident substance ", analyzed the complexity of the underlying surface of the flow field and the feature of pollutant distribution. Subsequently, constructed WRF-fluent nested model, simulated this field experiment, test the applicability of the model in complex underlying surface. Through researching, following results were obtained:(1) The urban flow field is simultaneously subjected to weather-scale and small-scale (1-2km) and the role of neighborhood scale (tens of meters), but also affected by human activities, so the flow field is more complex; average wind speed in suburbs is61%larger than urban, urban buildings drag effect on the wind speed is apparent, and wind direction inside the city is quite different.(2) The average daily change of turbulent kinetic energy is significantly, and turbulent kinetic energy of urban canopy is larger than the surface layer of the inner city near the experiment area; differences of the average turbulent kinetic energy is small between urban and suburbs; undulating of turbulent kinetic energy is determined by both of wind speed and temperature; relative turbulence intensity component in the horizontal direction is significantly greater than component in the vertical, the relative turbulence intensity in urban is significantly greater than the suburbs; in urban areas, relative urban turbulence intensity component in the horizontal direction of the top of the city canopy is greater than the near-surface component, and the relative urban vertical turbulence intensity near the ground is greater than the urban canopy roof.(3) The main axis of tracer concentration changes with the wind direction, advection effect is obvious; When the wind speed increases, the advection effect of wind is enhanced, and subsidence is relative weakened; After stopping the release of tracer, tracer concentration decreased rapidly, and gradually becomes uniform.(4) The basic WRF simulation results coincide with the observations, simulation results of vertical wind field and temperature field is better than the humidity field. Simulation results show that WRF simulation results can be used as background field into Fluent.(5) The simulation results of Fluent wind field shows that the drag effect in is very obvious, and the urban flow field is very complicated because of the streets’ vertical and horizontal distribution will generate streaming and convergence of flow field distribution; The mean absolute error of average simulation wind direction is less than42.1°, the relative error of average simulation wind speed is less than45%; while the wind speed is small, wind direction error is large because the flow field disturbed by the complex turbulence, with the wind speed increasing, wind power force is enhanced, so that simulation wind direction error is reduced.(6) Fluent can simulate the change of tracer plume axis with the main wind direction, asymmetric on both sides of the axis of the ribbon edges and irregular shape of the tracer concentration show that city buildings impacts tracer diffusion obviously; comparison between simulated and observed tracer, they have similar diffusion patterns, tracer fan-shaped distribution are transmitted over the main wind direction; Concentration level of the ladder for Fluent simulation value is larger than observations, relative to the observed values, the distribution is more concentrated. Simulated and observed values have magnitude of the gap.Preliminary studies revealed the physical mechanisms of atmospheric pollutants diffusion in urban surface, initially built WRF-fluent nested pollution forecasting system model, which can provide a theoretical basis for the relief and disaster assessment for the accident pollution incidents. In the ever advancing urbanization background, the significance of atmospheric pollutants diffusion in urban surface is clear. Currently, laws of atmospheric pollutants diffusion in urban surface are many unknown, it is necessary to do more field experiment and numerical model test for in-depth understanding the laws of pollutants diffusion in complex underlying surface, the numerical model will join these laws, and thus continuously improve the accuracy of the prediction results. The numerical model can provide a scientific basis for disaster relief and post-disaster assessment of atmospheric pollution incidents. It makes more rapid and effective response to air pollution incidents with collaborating effectively with various departments, and provides strong protection for human life and property safety. |
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