Oxides of nitrogen

Overview

In the context of this criteria document, the term oxides of nitrogen is understood to include nitric oxide (NO) and nitrogen dioxide (NO2). Other oxides of nitrogen which exist in the atmosphere are not known to have any biological significance and have not been referred to in this document. At the point of discharge from man-made sources, the predominant oxide of nitrogen is nitric oxide which is readily converted to nitrogen dioxide by chemical reactions in the atmosphere.

The major source of man-made emissions of oxides of nitrogen into the atmosphere is the combustion of fossil fuels in stationary sources (heating, power generation) and in motor vehicles (internal combustion engines). Other contributions to the atmosphere come from specific non-combustion industrial processes, such as the manufacture of nitric acid and explosives. Indoor sources include smoking, gas-fired appliances, and oil stoves. Differences in the nitrogen dioxide emission of various countries are mainly due to differences in fossil fuel consumption.