This paper researches on the method for city air quality assessment based on fractal model. Firstly,the fractal summation model is used to describe the character of the random distribution of the air pollutants in order to get the atmospheric background value and the standard for the study area. Then the air quality assessment method for anthropogenic resource is established with SO2, NO2 and PM10 indexes base on the ORAQI model. Finally, the method is used to assess the air quality for Xi'an City in 2010. The results shows that the air quality appears as a "U" shaped distribution with a character of seasonal variation. The air quality of Xi'an city in 2010 is good generally, with 83 percent of non-pollution days even affected by the dust storm. The air quality changes with the alternating seasons apparently, it's better in summer and autumn than that of winter and spring.%基于分形模型开展了城市空气质量的评价方法研究.首先利用分形求和模型分析大气污染物分布特征,用分维数定量描述其浓度分布随机程度,开展了城市空气质量评价的背景值和标准值的确定方法研究.然后在此基础上基于橡树岭大气环境质量指数评价方法,确定了基于二氧化硫、二氧化氮、PM10三项污染物的中国城市人为源空气质量评价方法模型.最后以西安市为示范区,利用本研究方法确定了西安市的大气环境背景值和评价标准值,建立了适用于西安市的大气环境质量指数模型,并开展了2010年该市全年环境质量分析评价工作.结果表明,基于分形模型的评价结果认为西安市空气质量年变化呈“U”字型分布,该方法比常规环境空气质量评价方法更能表现出空气质量的变化规律和季节分布特征.总体上,2010年西安市空气质量良好,未受到污染天数占83％,但是受沙尘天气影响比较大；西安市空气质量随季节变化特征比较明显,夏季和秋季空气质量较好,受污染程度较低,冬季和春季空气质量较差,受污染程度偏高.
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