Nitrogen dioxide is a brown-red, highly reactive gaseous substance that plays an important role in the formation of ozone. Man-made nitrogen dioxide mainly comes from the release of high-temperature combustion process, such as motor vehicle exhaust, boiler exhaust emissions. Nitrogen dioxide is also a cause of acid rain and has a variety of environmental effects, including: impacts on competition and composition changes between wetland and terrestrial plant species, reduced atmospheric visibility, acidification of surface water, eutrophication, and increased levels of toxins harmful to fish and other aquatic organisms.

Since the beginning of this summer, nitrogen dioxide has gradually become one of the main influence of summer air quality "killer". When ozone levels are low in the morning, nitrogen dioxide levels are high, and when they are high in the afternoon, nitrogen dioxide levels go down, like a seesaw. In addition, the concentration of nitrogen dioxide and the frequency of the primary pollutant increased compared with the same period of last year.

This problem is more obvious in summer. When the concentration of PM10 and other pollutants decreases, it indicates that there is no meteorological factor adverse to pollution diffusion, but nitrogen dioxide rises instead of falling, which is also a main reason for the increase of ozone pollution.