Nitrogen oxides - Nitrogen staghorn fern

[ I ] A general term for compounds represented by _NOx . Seven types are known: dinitrogen monoxide ( _N2O ), nitric oxide (NO), dinitrogen trioxide _{( N2O3 )} , nitrogen dioxide ( _{NO2 )} , dinitrogen tetroxide ( _{N2O4 )} , dinitrogen pentoxide (N2O5), and nitrogen trioxide (NO3 ₎ . [ II ] As a pollution term, a general term for NO and _NO2 . Abbreviated as _NOx . It is mainly produced by the combustion of fuel. Most of the initial product is NO, which is oxidized in the air to become _NO2 . Most is produced by the combination of _N2 and _O2 in the air due to the high temperature during combustion (thermal _NOx ), but some is caused by nitrogen compounds in the fuel (fuel _NOx ). It can also be emitted from nitric acid and explosives manufacturing plants. Oxidation reaction of NO to _NO2 in a pure NO- _O2 mixture

(2NO + _O2 = _2NO2 )
has long been known as an example of a ternary molecular reaction. This reaction is quadratic with respect to the NO concentration, so when the NO concentration is extremely low (around ppm), the oxidation is extremely slow, but at normal concentrations it is rapid. The oxidation reaction of NO in polluted urban air is complex and cannot be discussed in the same category as that of a pure NO- _O2 mixture. The annual average concentration of _NO2 in urban air varies from city to city, ranging from 0.01 to 0.06 ppm, while the _NO2 concentration in natural areas without artificial pollution is about 0.004 ppm. The sources of nitrogen oxides are broadly divided into mobile sources (such as automobiles) and stationary sources (such as factories and offices), and the proportion of each in the total amount of nitrogen oxides generated varies greatly from city to city. The harmful effects of nitrogen oxides are not known for NO, but _NO2 causes lesions in the peripheral airways deep in the respiratory tract and weakens resistance to infections. In animal experiments, these adverse effects were observed at 0.5 ppm, while epidemiological studies on humans have shown an increased prevalence of persistent phlegm and coughing at annual mean _NO2 concentrations of 0.02 ppm or higher. The environmental standard for _NO2 was set at 0.04-0.06 ppm or lower in 1978. In addition to its own harmfulness, _NO2 absorbs and dissociates in sunlight at wavelengths of 300-400 nm, triggering photochemical smog. Currently in Japan, the Salzman method is the main method used to measure the concentration of _NOx in the atmosphere, but chemiluminescence and other methods are also available. Techniques for reducing _NOx emissions include the use of high-quality fuel, improvements to combustion equipment and methods, and flue gas denitrification, but from the perspective of resource conservation, it is also important to devise ways to reduce the amount of fuel used itself. In areas with severe _NO2 pollution, total _NOx emissions are regulated.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

【Ⅰ】NO_xで表される化合物の総称．一酸化二窒素N₂O，一酸化窒素NO，三酸化二窒素N₂O₃，二酸化窒素NO₂，四酸化二窒素N₂O₄，五酸化二窒素N₂O₅，三酸化窒素NO₃の7種類が知られている．【Ⅱ】公害用語としては，NOとNO₂の総称．略称NO_x．おもに燃料の燃焼によって生じる．このとき最初にできるものは大部分がNOであり，NOは空気中で酸化されてNO₂となる．大部分は燃焼のときの高温により，空気中の N₂ と O₂ が化合してできるが(熱的NO_x)，一部は燃料中の窒素化合物に起因する(燃料NO_x)．そのほか，硝酸製造工場，火薬製造工場から排出されることもある．純粋なNO-O₂混合物でのNOのNO₂への酸化反応

(2NO ＋ O₂ ＝ 2NO₂)
は三分子反応の例として古くから知られている．この反応はNO濃度に関し二次であるため，NO濃度がいちじるしく低いとき(ppm 程度)，その酸化はきわめて遅いが，普通の濃度ではすみやかである．都市の汚染大気中でのNOの酸化反応は複雑であり，純粋なNO-O₂混合物の場合とは同列に論じられない．都市大気中のNO₂の環境濃度は都市により年平均値0.01～0.06 ppm で，一方，人工的汚染のない自然界でのNO₂濃度は約0.004 ppm である．窒素酸化物の発生源は移動発生源(自動車など)と固定発生源(工場，事業所など)に大別され，発生総量における両者の割合は都市により大きく異なる．窒素酸化物の有害性に関しては，NOについては知られていないが，NO₂は呼吸器深部の末梢気道に病変をもたらし，また感染症に対する抵抗力を弱める．動物実験の結果では0.5 ppm でこれらの悪影響が観察され，一方，ヒトに対する疫学調査では，NO₂濃度年平均値約0.02 ppm 以上で持続性痰・咳の有症率の増加が認められる．NO₂の環境基準は1978年に0.04～0.06 ppm，またはそれ以下と定められた．NO₂はそれ自身の有害性のほか，太陽光線中の300～400 nm の波長光を吸収して解離し，光化学スモッグの引き金反応となる．現在，わが国では，大気中のNO_xの濃度測定には，おもにザルツマン法が用いられているが，ケミルミネセンス法，そのほかの方法もある．NO_x発生の低減技術としては，良質燃料の使用，燃焼装置・方法の改良および排煙脱硝があるが，省資源の点からも，燃料の使用量そのものを節減する工夫も重要である．なお，NO₂汚染のいちじるしい地域に対しては，NO_x排出量が総量規制されている．

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