An abbreviation commonly used to describe the combined local atmospheric concentration of the compounds nitric/nitrous oxide (NO) and nitrogen dioxide (). is formed naturally in the atmosphere, especially at high altitudes, by the intense heat of lightning (Levine et al. 1984). Near the surface, is formed largely by combustion (automobiles, industry, etc.), microbial activity in soils, and widespread application of nitrogen-rich, agricultural fertiliser (Harrison et al. 1995; Shepherd et al. 1991; Zhang et al. 2012). The reaction of, particularly, with other anthropogenic chemicals contributes to the formation of tropospheric ozone . In the stratosphere, has been shown to reduce ozone concentrations and most of this is a result of emissions (Revell et al. 2012)., specifically, has been shown to have negative effects on the human respiratory system (Chauhan et al. 1998).