Impact of urban population on concentrations of nitrogen dioxide(NO_2) and fine particles(PM_(2.5)) in China

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,NO_2 and PM_(2.5),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 NO_2and PM_(2.5) concentrations in 114 major cities in China during the winter of2013-2014 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 NO_2concentration beyond the air quality guideline of World Health Organization(AQG of WHO; 40 [mu]g/m),and no city was found with PM_(2.5) concentration within the AQG of WHO(10 [mu]g/m).(2) The spatial distributions of the two pollutants had distinct regional characteristics; compared with NO_2,PM_(2.5) 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 NO_2 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 PM_(2.5) 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 NO_2 and PM_(2.5) concentration of69.28[mu]g/m and 119.58[mu]g/m,respectively.(4) Significant positive correlations were obtained between the urban population and the concentrations of NO_2(r = 0.35,P 0.01),and PM_(2.5)(r = 0.39,P 0.05) for the cities with popu