Nitrogen dioxide photochemical production

Nitrosothiols decompose photochemically and thermally to give the corresponding disulphides and nitric oxide18 23 14 24 (equation 6). In most cases the nitric oxide has not been identified as the primary product but rather as its oxidized form, nitrogen dioxide. 

The reaction in water at pH 7.4 has been much studied since the discovery of the importance of nitric oxide. The products are as for the thermal and photochemical reactions, except that the final product is nitrite ion. This is to be expected since nitric oxide in aerated water at pH 7.4 also yields quantitatively nitrite ion25, by it is believed the series of equations 7-9, which involves oxidation to nitrogen dioxide, further reaction to give dinitrogen trioxide which, in mildly alkaline solution, is hydrolysed to nitrite ion. Under anaerobic conditions it is possible to detect nitric oxide directly from the decomposition of nitrosothiols using a NO-probe electrode system26. Solutions of nitrosothiols both in... 

Some pollutants fall in both categories. Nitrogen dioxide, which is emitted directly from auto exhaust, is also formed in the atmosphere photochemically from NO. Aldehydes, which are released in auto exhausts, are also formed in the photochemical oxidation of hydrocarbons. Carbon monoxide, which arises primarily from autos and stationary sources, is likewise a product of atmospheric hydrocarbon oxidation. 

Besides ozone, the main indicator of photochemical pollution, other important concomitant products are peroxyacetylnitrate (PAN), hydrogen peroxide, nitrogen dioxide, hydroxyl radicals and various aldehydes that are both products and primary pollutants, particles, sulfates, nitrates, ammonium, chloride, water, and various types of oxygenated organic compounds. The most important precursors of photochemical pollution are nitric oxide and hydrocarbons. The measurement procedures for the hydrocarbons are not as highly developed as those for ozone and the nitrogen oxides. 

Heck, W. W. Plant injury induced by photochemical reaction products of propylene-nitrogen dioxide mixtures. J. Air Pollut. Control Assoc. 14 255-261, 1. ... 

In the first step of photochemical smog production, sunlight (hv) acts on nitrogen dioxide to produce nitric oxide and free oxygen ... 

Victorin, K., Busk, L., Cederberg, H. Magnusson, J. (1990) Genotoxic activity of 1,3-butadiene and nitrogen dioxide and their photochemical reaction products in Drosophila and in mouse bone marrow micronucleus assay. Mutat. Res., 228, 203-209... 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

Photochemical production of ozone in the troposphere occurs from the photolysis of nitrogen dioxide (N02) during the daytime, producing oxygen atoms (O) ... 

Tn the presence of sunlight and oxides of nitrogen (NOa,), hydrocarbons react to form new products, some of which are called oxidants. The most commonly investigated photochemically produced oxidants in the urban atmosphere are ozone (O3), nitrogen dioxide (NO2), and peroxy-acetylnitrate (PAN). 

Nitrogen oxide is a serious air pollutant because it is oxidized to nitrogen dioxide, NO2, which reacts with water to form nitric acid and with other products of the incomplete combustion of hydrocarbons to form nitrates. The latter are eye irritants in photochemical smog. 

Photochemical transformations of PAH, adsorbed on various substrates are well documented in the literature they may involve radical initiated auto-oxidation, singlet molecular oxygen or hydroxy 1/hydroperoxyl attack. In model systems, several PAH coated on filters or simulant particles have been shown to react fast with ozone or nitrogen dioxide in the dark and to result in the formation of direct mutagens. Any future policy aimed at the reduction of PAH emission levels should therefore also be assessed in terms of possible conversion of PAH into other hazardous products. 

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