Why is it necessary to control the nitrogen oxide content in exhaust gases?

　　Nitrogen oxides include various compounds such as dinitrogen monoxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2), dinitrogen trioxide (N2O3), dinitrogen tetroxide (N2O4), and dinitrogen pentoxide (N2O5), among others. Except for nitrogen dioxide, all other nitrogen oxides are highly unstable; they turn into nitrogen dioxide and nitric oxide when exposed to light, moisture, or heat, with nitric oxide subsequently converting back into nitrogen dioxide. 　　The nitrogen oxides that cause air pollution are mainly nitric oxide (NO) and nitrogen dioxide (NO2); therefore, in environmental science, nitrogen oxides generally refer to these two compounds together. Nitrogen oxides pose hazards to varying degrees. 　　Nitrogen oxides combine with water in the air to eventually turn into nitric acid and nitrates, and nitric acid is one of the causes of acid rain ； Under certain conditions, it can generate photochemical smog pollution together with other pollutants. 　　The increase in nitrogen oxide concentrations in the atmosphere has led to an increase in nitrogen deposition. According to acid rain monitoring data, the ratio of the equivalent concentrations of NO3－ to SO42－ in precipitation has shown an upward trend since 1999. An increasing ratio of the equivalent concentrations of NO3－ to SO42－ indicates that nitrogen oxides are contributing more to acidic precipitation, and acid rain in China is transitioning from sulfuric acid-type acid rain to a sulfuric acid/nitric acid composite type. At the same time, nitrogen deposition produces more nitrate and nitrogen oxides, which acidify the soil, as well as water, and cause eutrophication. 　　The continuous increase in nitrogen oxides also accelerates the formation of fine particulate matter and secondary aerosols. Nitrogen oxides are one of the precursors of photochemical pollution. Under sunlight, NO2 and VOCs (volatile organic compounds) undergo a series of photochemical reactions to produce O3 and various secondary pollutants such as formaldehyde and acetaldehyde, thereby increasing the oxidizing capacity of the atmosphere and leading to the formation of photochemical smog, which poses threats to the atmospheric environment and human health. In some of China’s densely populated, economically developed cities with a high number of motor vehicles, a trend toward photochemical pollution has been observed; in particular, photochemical pollution has been detected in megacities such as Beijing, Guangzhou, and Shanghai. 　　Therefore, reducing nitrogen oxides in the atmosphere plays an important role in protecting the ecosystem and maintaining people’s health. Reducing nitrogen oxides is a significant measure to protect the environment and improve people’s livelihoods. 　 The sulfur in sulfur dioxide comes mainly from the fuel, whereas the nitrogen in nitrogen oxides originates from both the fuel and the air; it is related to both the combustion temperature and the time the mixture stays in the high-temperature area. The concentration of nitrogen oxides in flue gas varies widely, making accurate measurement difficult. As fuel consumption and the number of motor vehicles increase, nitrogen oxides also increase. It is estimated that the annual growth rate of nitrogen oxide emissions nationwide is 5% to 8%. If no further measures are taken to reduce nitrogen oxide emissions, as the national economy continues to develop, the population grows, and urbanization accelerates, nitrogen oxide emissions in China will continue to rise in the future. Based on the current trends, by 2030 China’s nitrogen oxide emissions are expected to reach 35.4 million tons, which will inevitably have serious environmental impacts; therefore, it is essential to strengthen controls on nitrogen oxide emissions. And the most important policy measure for reducing nitrogen oxides is total emission control.

Nitric oxide and nitrogen dioxide, which are part of nitrogen oxides, are harmful to the human body; they are toxic substances that are also detrimental to the environment. They are key factors contributing to acid rain and photochemical pollution. If the **environmental protection agency does not impose limits on these harmful substances, then what is the point of having employees in that agency?

Nitrogen oxides are also one of the sources of photochemical pollution, right?

In our country, large cities with a population of over 1 million are facing increasingly severe nitrogen oxide pollution due to factors such as vehicle emissions and industrial emissions; this poses a risk of the formation of photochemical smog. Therefore, it is now necessary to control nitrogen oxide emissions – we cannot wait until problems arise before taking action.