State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences, Beijing,China,State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences, Beijing,China,State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences, Beijing,China;University of Chinese Academy of Sciences;China,State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences, Beijing,China
随着我国城市的快速发展，城市的区域性大气污染问题日益突出，尤其是大城市和超大城市的污染问题更加严重。那么城市规模的扩大是否必然导致空气污染的加剧?控制城市人口规模是否是防治空气污染的有效手段?这些问题成为空气污染防治中政府、公众和学者广泛关注的焦点。采用2013年冬季全国114个重点城市两种典型大气污染物-NO2（传统）和PM2.5（新型）-浓度的实时监测数据，首先分析了这两种大气污染物的空间分布特征，进而定量分析城市人口规模和大气污染物浓度的关系。结果显示：（1）仅有21%的城市NO2浓度达到WHO的城市年均浓度标准（40μg/m3），全部城市的PM2.5浓度高于WHO年均浓度标准（10 μg/m3）；（2）大气污染物的空间分布具有显著的集聚特征和区域性特征，表现为：NO2呈较为分散的空间分布，而PM2.5的空间分布呈现"北高南低、内陆高沿海低"的特征。NO2浓度高的区域主要分布在天津、河北东南部和山东中部地区，PM2.5浓度高的区域主要分布于河北西南部和山东西部；（3）常住人口规模同冬季NO2和PM2.5浓度呈倒"U"型关系；在1000到1200万的城市冬季平均NO2和PM2.5浓度最高（NO2：69.28μg/m3，PM2.5：119.58μg/m3）。（4）总人口低于1200万的城市，冬季NO2浓度和PM2.5浓度随着城市规模增加而显著升高（NO2：r=0.44，P < 0.01；PM2.5：r=0.43，P < 0.01）；总人口高于1200万的城市，NO2浓度同城市规模呈显著负相关关系（r=0.91，P < 0.05），PM2.5浓度随城市规模增加逐渐降低。（5）常住人口密度在1000人/ km2以下的重点城市，NO2和PM2.5浓度同城市人口密度呈显著正相关关系（NO2：r=0.23，P < 0.05；PM2.5：r=0.36，P < 0.01）。常住人口密度在1000人/km2以上的城市人口密度同NO2和PM2.5浓度呈显著负相关关系（NO2：r=-0.61，P < 0.05；PM2.5：r=0.63，P < 0.01）。以上研究结果可以为划定不同大气污染物的重点防治区域和制定联合防治行动计划提供理论依据，并为重点城市大气污染治理和城市人口规模控制理论完善提供参考。
With the accelerated urbanization, air pollution at different areas and ways to reduce it in the major cities of China has been a growing concern. Recently, research on setting the largest cutoff for the increase in population and promoting the development of small-scale cities may effectively reduce air pollution in cities. While these measurements still need validation, our research addressed this issue in cities at different scales. Additionally, NO2 and PM2.5, which represent traditional photochemical smog and new-type haze, respectively, are taken as indicators of urban air quality. We selected the real-time ground-measured air pollutant records, which are closer to the actual exposure concentrations experienced by urban inhabitants as compared to other ways of obtaining air pollution records (e.g., modeling and remote sensing). We examined the spatial patterns of NO2 and PM2.5 concentrations in 114 major cities in China during the winter of 2013-2014 with the real-time monitoring records that have seldom been applied at the national scale in previous researches. Then, we examined the spatial pattern of the two air pollutants, and the relationship between urban population size and the concentrations of the two air pollutants. We found that (1) only 21% (23 cities) of the cities that have NO2 concentration beyond the air quality guideline of World Health Organization (AQG of WHO; 40 μg/m3), and no city was found with PM2.5 concentration within the AQG of WHO (10 μg/m3). (2) The spatial distributions of the two pollutants had distinct regional characteristics; compared with NO2, PM2.5 distribution has a distinctive and clear pattern, i.e., the northern part has higher concentrations than the southern part, and the inland part is higher than the eastern coast of China. Special prevention and control measures against heavy NO2 pollution should be taken at Tianjin, the southeast of Hebei and the middle of Shandong. On the other hand, special prevention and control should be taken against heavy PM2.5 pollution at the southwest of Hebei and the west of Shandong. (3) An inverse "U" type relationship between air pollutants and urban population is observed. Cities with population between 10-12 million have the highest NO2 and PM2.5 concentration of 69.28μg/m3 and 119.58μg/m3, respectively. (4) Significant positive correlations were obtained between the urban population and the concentrations of NO2 (r=0.35, P < 0.01), and PM2.5 (r=0.39, P < 0.05) for the cities with population less than 12 million. Moreover, in the cities with population more than 12 million, the size of urban population had a significantly negative correlation with the concentration of NO2 (r=0.58, P < 0.05) and PM2.5. (5). While the positive correlation of NO2 and PM2.5 with the population density is significant for cities with population density less than 1000 persons per square kilometer (NO2: r=0.23, P < 0.05; PM2.5: r=0.36, P < 0.01), the negative correlation of NO2 and PM2.5 with the population density is significant for cities with population density more than 1000 persons per square kilometer (NO2: r=-0.61, P < 0.05; PM2.5:r=0.63, P < 0.01). The results of this study have provided important insights on areas with "traditional" and "new-type" air pollutants as well as the design for joint prevention and control unit. It also provides a theoretical basis for the opinion that many effective measures, except limiting the population of cities, should be explored and adapted to control air pollution.