NO_x is created by both natural and anthropogenic (human) processes. A naturally important factor in the production of NO_x is the altitude at which it is emitted into the atmosphere.

Natural sources of NO_x

• Lightning is the most significant source of atmospheric NO_x, most of which is generated in the troposphere.
• Microbial activity in soil also plays a part in the production of NO_x. One of these activities is the oxidation of ammonium to nitrate, which is known as nitrification, and the other, denitrification, is the anaerobic reduction of nitrate to gaseous forms of nitrogen. NO_x from these sources is released from the surface into the boundary layer.

Anthropogenic sources of NO_x

Much more NO_x is produced by anthropogenic activity than by natural processes.

Anthropogenic NO_x is principally produced by combustion processes in furnaces, vehicle engines and biomass burning. When air is heated to high temperatures the oxygen and nitrogen it contains can react to form nitrogen oxides. The primary nitrogen oxide formed is NO, which, as we have seen, can be rapidly converted into NO₂ by reaction with O₃. The amount of NOx produced during combustion is highly dependent on temperature. The main anthropogenic sources of NO_x are:

• Surface transport
• Power generation and industrial processes based on the combustion of fuels.
• Air transport
• Biomass burning, a widely-spread practice in certain regions of the world which makes a large contribution to emissions of NO_x.