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Chemical Reactions in Sunlight: What's the Hidden Relationship Between NOx and Ozone?
In air chemistry, NOx refers to nitric oxide (NO) and nitrogen dioxide (NO2). These two nitrogen oxides are closely related to air pollution. These gases contribute significantly to the formation of photochemical smog and acid rain, and also affect the concentration of tropospheric ozone. Especially in high-temperature environments, such as during the combustion process of automobile engines, the reaction of nitrogen and oxygen will produce NOx. In heavy-traffic areas of large cities, nitrogen oxide emissions often become a major source of air pollution.
Natural sources of NOx also include lightning, but the main source of pollution is still the burning of fossil fuels.
Formation and reaction of NOx
Under normal temperature and pressure, nitrogen and oxygen will not react. But in high-temperature environments, they will undergo an endothermic reaction to generate various types of nitrogen oxides. This high temperature usually occurs in internal combustion engines or power station boilers, or occurs naturally during lightning. The total concentration of NOx refers to NO and NO2 because the conversion between the two is very rapid in both the troposphere and stratosphere.
Generation of ozone
Photochemical smog, an important form of air pollution, is formed when NOx and volatile organic compounds (VOCs) react in sunlight. In summer, as solar radiation increases, the frequency of this type of smog will also increase. Emissions from industrial activities and traffic quickly react with NOx, thereby increasing the concentration of ozone and peroxides, causing human health problems.
Especially for children, asthmatics, and people who work or exercise outdoors, the negative effects of smoke cannot be underestimated.
The mechanism of acid rain
NO2 will react with OH radicals during the day and be further oxidized to generate nitric acid (HNO3), which can dissolve in liquid water to form acid rain. This process is due to the conversion of NO2 in the atmosphere, causing lasting effects on the environment.
Natural and anthropogenic sources of nitrogen oxides
Natural sources of NOx include lightning, because during the intense heating and cooling process of lightning, stable molecules (such as N2 and O2) are converted into significant amounts of NOx. According to some studies, each lightning flash produces an average of about 7 kilograms of NOx, which can reach 8.6 million tons per year. However, anthropogenic activities, especially coal and oil combustion, are the main source of nitrogen oxides, with annual emissions estimated at 28.5 million tons.
Effects on health and environment
NOx exposure is closely related to respiratory system diseases, can trigger and aggravate asthma symptoms, and may even lead to the development of asthma. These gases have also been linked to heart disease, diabetes and mortality. NOx can react with ammonia, moisture and other compounds to form nitric acid dust particles, which can have negative health effects.
According to estimates from the American Lung Association, nearly 50% of U.S. residents live in counties that do not meet ozone standards.
Impact on biodiesel
Biodiesel and its blends are generally considered to reduce harmful tailpipe emissions. Although initial studies showed that biodiesel may increase NOx emissions, subsequent studies have shown that up to 20% biodiesel blends have an insignificant impact on NOx. .
Regulation and emission control technology
Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are commonly used technologies to reduce NOx emissions and rely on the reaction with urea or ammonia to generate nitrogen and water. In the United States, emission levels from modern diesel vehicles have been significantly reduced, but the challenge of controlling NOx emissions remains.
Conclusion
The relationship between NOx and ozone is not only reflected in the impact on air quality, but also involves multiple dimensions of human health and the ecological environment. As we seek to solve these problems, we should pay attention to deeper chemical reactions and environmental changes. So, can we find more effective ways to reduce the impact of NOx on the environment?
