Abstract:

Objective To collect and organize the atmospheric nitrogen dioxide（NO₂） pollution data in 31 key cities in China from 2003 to 2019，and analyze their changing trend. Methods Collect annual average mass concentration data of atmospheric NO₂ in 31 key cities from 2003 to 2019 from the official website of the National Bureau of Statistics，and evaluate the annual average mass concentration of NO₂ based on the limit standards in GB 3095-2012 "the Ambient Air Quality Standards". The Daniel trend test method was used to evaluate the annual trend of atmospheric NO₂ mass concentration，and the Kruskal-Wallis H test was used to compare the differences of NO₂ mass concentrations in different groups. Results The annual average mass concentration of atmospheric NO₂ in 31 key cities from 2003 to 2019 was （41±12）μg/m³，with an annual average decline rate of 0.79%. There was no statistically significant difference in the annual average mass concentration of NO₂ between different years（χ²=16.24，P>0.05）. NO₂ pollution was relatively mild in Southwest and South China，and more severe in North and Central China. The annual average mass concentration of NO₂ in the Northwest China showed an upward trend（r_s=0.710，P<0.01），while in the East China and South China，it showed a downward trend（r_s=-0.757，-0.815，both P<0.01）. Compared with 2003，the annual average mass concentration of NO₂ in 18 cities in 2019 showed varying degrees of decline，with a decrease of 2% to 51%（an average annual decrease rate of 0.1% to 4.3%），the annual average mass concentration of NO₂ in 8 cities（including Hefei，Taiyuan，Changchun，Xi'an，Zhengzhou，Xining，Guiyang，and Shijiazhuang） increased by 5% to 68%（an average annual growth rate of 0.3% to 3.3%），and the annual average mass concentration of NO₂ remained the same in 5 cities（Shenyang，Nanchang，Haikou，Lanzhou，and Yinchuan）. The trend test results showed that the annual average mass concentration of NO₂ in 9 cities（Shijiazhuang，Taiyuan，Hefei，Jinan，Zhengzhou，Guiyang，Xi'an，Xining，and Yinchuan） showed an upward trend（all r_s>0，all P<0.05），while in 10 cities（Beijing，Harbin，Shanghai，Nanjing，Hangzhou，Fuzhou，Guangzhou，Kunming，Lhasa，Urumqi），the annual average mass concentration of NO₂ showed a downward trend（all r_s<0，all P<0.01）. From 2003 to 2019，the annual average mass concentration of NO₂ in seven cities（Shanghai，Nanjing，Hangzhou，Wuhan，Guangzhou，Chengdu，and Urumqi） exceeded the standard limit every year，while three cities（Haikou，Guiyang，and Lhasa） had good quality and were within the standard limit every year. Conclusion There are regional and urban differences in atmospheric NO₂ pollution in 31 key cities in China，with heavy pollution in North China，Central China，and Northwest China. The situation of NO₂ prevention and control in Beijing-Tianjin-Hebei and its surrounding areas，the Yangtze River Delta，the Pearl River Delta，and western cities remains severe. In the future，comprehensive treatment measures，"tailored measures"，and regional coordinated prevention and control should continue to be taken for NO₂ pollution.

Key words: Air pollution, NO₂, Prevention and control