Abstract: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.
