Nitrous oxide
Nitrous oxide (N2O) is a colorless molecular gas that has a faintly sweet odor. This gas is a greenhouse gas, with forcing power stronger than carbon dioxide; nitrous oxide has also been implicated as a neurotoxin. Nitrous oxide has solubility in water as well as organic solvents and has molar mass is 44.013 grams per mole. Nitrous oxide is emitted by soil bacteria in soils and oceans, and thus has been a part of Earth's atmosphere for millions of years.
Nitrous oxide is a powerful greenhouse gas produced both naturally and via human activities. Its concentration in the Earth's atmosphere has risen by around 15% since the Industrial Revolution. Atmospheric mixing ratios for nitrous oxide now stand at around 315 parts per billion (ppb) compared to a pre-industrial high of 275 ppb. Though its concentration the atmosphere is much smaller than that of carbon dioxide, N2O is a much more powerful greenhouse gas having a Global Warming Potential of 298 over a 100-year time span, meaning that nitrous oxide produces 296 times the atmospheric warming than CO2. This means that one kilogram (kg) of N2O released into the atmosphere has a global warming effect equivalent to 296 kg of carbon dioxide over a 100-year period. Thus, nitrous oxide is a very important contributor to Radiative forcing, a measure used by atmospheric scientists to compare relative importance of different gas species to atmospheric warming phenomena.
Sources of Nitrous oxide
Agriculture is the chief source of anthropogenic nitrous oxide; soil cultivation, nitrogen fertilizers, and animal wastes all accelerate naturally occurring bacteria in the production of massive amounts of nitrous oxide. Livestock production (e.g. cattle, chickens, pigs, goats) comprises two thirds of human-related nitrous oxide. Industrial sources contribute around one fifth of all anthropogenic sources, and include the production of nylon, and the burning of fossil fuel in internal combustion engines. Human activity accounts for around 30%; tropical soils and oceanic release account for the balance.
Natural
Natural sources of nitrous oxide are dominated by temperate and tropical soils, with the world's oceans also being an important contributor. Total annual emissions of N2O from natural sources are estimated to be about 10 million tonnes of N2O-N.
Human-made
The human-made (anthropogenic) emission of N2O is of a slightly higher magnitude relative to natural sources. In the USA, agricultural soils are by far the biggest source. When nitrogen-rich fertilizers are added to fields, some of the nitrogen can end up being emitted as N2O through the processes of nitrification and denitrification. Both these processes are usually a result of micro-organisms (nitrifiers and denitrifiers). Nitrification is common where the soil is well aerated, while denitrification tends to predominate where the soils are more water-logged and the supply of oxygen is limited. Globally, between two and four million tonnes of N2O-N are thought to be released into the atmosphere each year via this pathway.
Industry, in particular the production of nitrogen fertilizers and nylon, is an important source of global N2O emissions, as is fossil fuel burning in power generation. Other important human-made sources of N2O include biomass burning, cattle and cattle feed production.
References
- James Hansen, Dorothy Koch, Andrew Lacis, Reto Ruedy, Oleg Dubovik, Brent Holben, Mian Chin, and Tica Novakov. 2003. Global Atmospheric Black Carbon Inferred from AERONET, 100 Proc of the Natl Acad.of Sci. 6319, at 6323
- Shine et al., An alternative to radiative forcing for estimating the relative importance of climate change mechanisms, Geophysical Research Letters, Vol 30, No. 20, 2047
- D.T.Shindell, G.Faluvegi, D.M.Koch, G.A.Schmidt, N.Unger and S.E. Bauer. 2009. Improved Attribution of Climate Forcing to Emissions. Science 326 (5953): 716.
- Greenhouse Gas Sinks. Reay et al. (eds). CABI Publishing (in press).
- V.Jevtovic-Todorovic, Beals J, Benshoff N, Olney JW (2003). Prolonged exposure to inhalational anesthetic nitrous oxide kills neurons in adult rat brain. Neuroscience 122 (3): 609–16
