The letter X is used in the formula as more than one oxide is involved, for example, NO and NO2. Aforementioned, NOx is a general term for the mono – nitrogen oxides like NO and NO2 (i.e., nitric oxide and nitrogen dioxide). These gases are produced from the interaction of nitrogen and oxygen gases in the air while combusting, especially at high temperatures. In many place, like large cities, where the traffic is large, the amount of nitrogen oxides transmitted into the atmosphere as air pollution is very much significant. NOx gases are formed as and when the combustion occurs in the presence of nitrogen; for example, in air – breathing engine and they also are produced naturally by lightning. This term means the total concentration of NO and NO2, in atmospheric chemistry. NOx gases are the major cause of smog and acid rain and play a major role in the formation of troposphere ozone. Mono – nitrogen oxides form nitric acid eventually when dissolved in atmospheric moisture, thus, forming a major ingredient of acid rain. This nitric acid may end up in the soil where it forms nitrate which is useful for growing plants.
NOx should not be mistaken to be nitrous oxide (N2O) which is a greenhouse gas and can be used in many ways such as a food additive, an oxidizer and an anesthetic. NOy is reactive in nature and has odd number of nitrogen. It is defined as the sum of NOx and the compounds produced by the oxidation of NOx which also includes nitric acid.