NOx
In atmospheric chemistry, ' is shorthand for nitric oxide and nitrogen dioxide, the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.
gases are usually produced from the reaction between nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as in car engines. In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. gases are also produced naturally by lightning.
does not include nitrous oxide, a fairly inert oxide of nitrogen that contributes less severely to air pollution, notwithstanding its involvement in ozone depletion and high global warming potential.
' is the class of compounds comprising and the compounds produced from the oxidation of which include nitric acid, nitrous acid, dinitrogen pentoxide, peroxyacetyl nitrate, alkyl nitrates, peroxyalkyl nitrates, the nitrate radical, and peroxynitric acid.
Formation and reactions
Because of energy limitations, oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash.In atmospheric chemistry, the term refers to the total concentration of NO and since the conversion between these two species is rapid in the stratosphere and troposphere. During daylight hours, these concentrations together with that of ozone are in steady state, also known as photostationary state ; the ratio of NO to is determined by the intensity of sunshine and the concentration of ozone.
In other words, the concentration of ozone in the atmosphere is determined by the ratio of these two species.
The symbol
The time that is needed to reach a steady state among and ozone is dominated by reaction, which reverses reactions +:
for mixing ratio of NO, = 10 part per billion, the time constant is 40 minutes; for = 1 ppb, 4 minutes.
Formation of smog
When and volatile organic compounds react in the presence of sunlight, they form photochemical smog, a significant form of air pollution. The presence of photochemical smog increases during the summer when the incident solar radiation is higher. The emitted hydrocarbons from industrial activities and transportation react with quickly and increase the concentration of ozone and peroxide compounds, especially peroxyacetyl nitrate.Children, people with lung diseases such as asthma, and people who work or exercise outside are particularly susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function.
Formation of nitric acid and acid rain
is further oxidized in the gas phase during daytime by reaction with OHwhere M denotes a third molecule required to stabilize the addition product. Nitric acid is highly soluble in liquid water in aerosol particles or cloud drops.
also reacts with ozone to form nitrate radical
During the daytime, is quickly photolyzed back to, but at night it can react with a second to form dinitrogen pentoxide.
reacts rapidly with liquid water to form,
These are thought to be the principal pathways for formation of nitric acid in the atmosphere. This nitric acid contributes to acid rain or may deposit to soil, where it makes nitrate, which is of use to growing plants. The aqueous phase reaction
is too slow to be of any significance in the atmosphere.
Natural sources
is produced during thunderstorms due to the extreme heating and cooling within a lightning strike. This causes stable molecules such as and to convert into significant amounts of NO similar to the process that occurs during high temperature fuel combustion. from lightning can become oxidized to produce nitric acid, this can be precipitated out as acid rain or deposited onto particles in the air. Elevated production of from lightning depends on the season and geographic location. The occurrence of lightning is more common over land near the equator in the inter-tropical convergence zone during summer months. This area migrates slightly as seasons change. production from lightning can be observed through satellite observations.Scientists Ott et al. estimated that each flash of lightning on average in the several mid-latitude and subtropical thunderstorms studied turned of nitrogen into chemically reactive. With 1.4 billion lightning flashes per year, multiplied by 7 kilograms per lightning strike, they estimated the total amount of produced by lightning per year is 8.6 million tonnes. However, emissions resulting from fossil fuel combustion are estimated at 28.5 million tonnes.
A recent discovery indicated that cosmic ray and solar flares can significantly influence the number of lightning strikes occurring on Earth. Therefore, space weather can be a major driving force of lightning-produced atmospheric. Atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere. Ott noted that the lightning-produced is typically found at altitudes greater than 5 km, while combustion and biogenic are typically found near the sources at near surface elevation.
Biogenic sources
and the use of nitrogen fixing plants also contribute to atmospheric, by promoting nitrogen fixation by microorganisms. The nitrification process transforms ammonia into nitrate. Denitrification is basically the reverse process of nitrification. During denitrification, nitrate is reduced to nitrite, then NO, then and finally nitrogen. Through these processes, is emitted to the atmosphere.A recent study conducted by the University of California Davis found that adding nitrogen fertilizer to soil in California is contributing 25 percent or more to state-wide pollution levels. When nitrogen fertilizer is added to the soil, excess ammonium and nitrate not used by plants can be converted to NO by microorganisms in the soil, which escapes into the air. is a precursor for smog formation which is already a known issue for the state of California. In addition to contributing to smog, when nitrogen fertilizer is added to the soil and the excess is released in the form of NO, or leached as nitrate this can be a costly process for the farming industry.
A 2018 study by the Indiana University determined that forests in the eastern United States can expect to see increases in and in turn, changes in the types of trees which predominate. Due to human activity and climate change, the maples, sassafras, and tulip poplar have been pushing out the beneficial oak, beech, and hickory. The team determined that the first three tree species, maples, sassafras, and tulip poplar, are associated with ammonia-oxidizing bacteria known to "emit reactive nitrogen from soil." By contrast, the second three tree species, oak, beech and hickory, are associated with microbes that "absorb reactive nitrogen oxides," and thus can have a positive impact on the nitrogen oxide component of air quality. Nitrogen oxide release from forest soils is expected to be highest in Indiana, Illinois, Michigan, Kentucky and Ohio.
Industrial sources (anthropogenic sources)
The three primary sources of in combustion processes:- thermal
- fuel
- prompt
Thermal
Thermal refers to formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1300 °C, molecular nitrogen and oxygen in the combustion air dissociate into their atomic states and participate in a series of reactions.The three principal reactions producing thermal are:
All three reactions are reversible. Zeldovich was the first to suggest the importance of the first two reactions. The last reaction of atomic nitrogen with the hydroxyl radical, •HO, was added by Lavoie, Heywood and Keck to the mechanism and makes a significant contribution to the formation of thermal.
Fuel
It is estimated that transportation fuels cause 54% of the anthropogenic . The major source of production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free, or NO. Fuel can contribute as much as 50% of total emissions through the combusting oil and as much as 80% through the combusting of coalAlthough the complete mechanism is not fully understood, there are two primary pathways of formation. The first involves the oxidation of volatile nitrogen species during the initial stages of combustion. During the release and before the oxidation of the volatiles, nitrogen reacts to form several intermediaries which are then oxidized into NO. If the volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to form nitrogen gas, rather than. The second pathway involves the combustion of nitrogen contained in the char matrix during the combustion of the char portion of the fuels. This reaction occurs much more slowly than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted as, since much of the that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.