The covalent compound of \(NO_2\) is nitrogen dioxide. It is a reddish-brown gas with a pungent, irritating odor. It is a major air pollutant and a greenhouse gas. Nitrogen dioxide is produced by the combustion of fossil fuels, such as in汽车engines, power plants, and industrial boilers. It is also produced by the oxidation of nitric oxide (NO) in the atmosphere. Nitrogen dioxide can cause respiratory problems, such as asthma, bronchitis, and emphysema. It can also contribute to the formation of acid rain.