"Introduction The term ""nitrogen oxides"" (NOx) is applied to varying mixtures of nitrogen oxide (NO) and nitrogen dioxide (NO2) encountered in the atmosphere. Usually the concentrations of these gases are expressed as mass of equivalent NO2. The same unit is used to express criteria of toxicity and standards for emissions and ambient concentrations. The reason is that in the atmosphere NO is converted to NO2 which is the much more active form and can have more undesirable effects on the environment. There are several other forms of nitrogen oxides which have little influence on the environment: 1. Nitrous oxide (N2O) is the most abundant form occurring in concentrations up to 0.25 parts per million (ppm). It is formed by the decomposition of nitrogen compounds by soil bacteria@ is chemically inert@ and is not an air pollutant. 2. The remaining oxides N2O3@ N4O5@ and NO3 are formed only under unusual conditions. N2O4 is the dimer of NO2 and there may be a trimer. N2O3 and N2O3 are the anhydrides of nitrous and nitric acids. NO3 can be isolated only at the temperature of liquid air."