The two most important oxides of nitrogen with regard to air pollution are nitric oxide (NO) and nitrogen oxide (NO2), jointly referred to as NOx in the context of the environment. Global emissions of NOx and of another oxide of nitrogen, nitrous oxide (N20), are increasing. These gases play important roles in the environment through acidification, forest damage, smog formation direct and indirect damage to human health, depletion of the stratospheric ozone layer and the greenhouse effect. Although NOx arise from natural sources, the majority of emissions are due to combustion and fossil fuels. Combustion of coal, oil, gas and . . .