Nitric oxide is a colorless, odorless gas whose molecules are composed of one nitrogen atom and one oxygen atom. The largest source is the combustion of fossil fuels, particularly in vehicle engines and electric power plants. Some chemical production processes, such as the production of nitric acid, also generate nitric oxide as a byproduct.
Nitric oxide is also produced by the action of microbes in the soil, which has increased due to the large-scale use of synthetic and organic nitrogen-based fertilizers. Other sources include microbial activity in the oceans, lightning, and the burning of crop residues, wood and other organic matter.
Because nitric oxide is a free radical with an unpaired electron, it is highly reactive and quickly oxidizes in the atmosphere to form nitrogen dioxide, which plays a key role in the formation of ground-level ozone and acid rain, both of which harm human health and damage crops and ecosystems.
Significant progress has been made on slowing the growth of emissions of nitric oxide as a result of widespread recognition of the problem and the enactment and enforcement of national measures and international agreements aimed at reducing the output of nitrogen oxides as a whole. However, this progress varies greatly according to the country, with substantial decreases in output in the United States and the European Union, but continued increases in some developing countries including India, Pakistan and Brazil.
The most important means of reducing the output of nitric oxide has been the installation of selective catalytic reduction systems in power plants, automobiles and industrial facilities to convert nitrogen oxides into harmless nitrogen and water. Another has been switching to low-nitrogen fuels, especially natural gas, which produce much lower levels of nitrogen oxide emissions.