Nitrogen dioxide, or NO2, is a reddish-brown gas with a characteristic sharp, acrid smell. It is a significant air pollutant, primarily produced from the combustion of fossil fuels in vehicles and power plants. NO2 plays a role in the formation of smog and can contribute to respiratory problems in humans. In the atmosphere, NO2 can react with other chemicals to form ozone at ground level, which is harmful to health and the environment. Monitoring and regulating NO2 levels is essential for improving air quality and protecting public health, especially in urban areas where traffic is heavy.