Peroxyacyl nitrates Photochemically

PANs Abbreviation for peroxyacyl nitrates, photochemical oxidants in smog. 

Photochemical air pollution consists of a complex mixture of gaseous pollutants and aerosols, some of which are photochemically produced. Among the gaseous compounds are the oxidizing species ozone, nitrogen dioxide, and peroxyacyl nitrate ... 

Primary pollutants are those emitted directly to the atmosphere while secondary pollutants are those formed by chemical or photochemical reactions of primary pollutants after they have been admitted to the atmosphere and exposed to sunlight. Unbumed hydrocarbons, NO, particulates, and the oxides of sulfur are examples of primary pollutants. The particulates may be lead oxide from the oxidation of tetraethyllead in automobiles, fly ash, and various types of carbon formation. Peroxyacyl nitrate and ozone are examples of secondary pollutants. 

The worst irritants in photochemical smogs are peroxyacyl nitrates, PAN. They produce singlet oxygen when they hydrolyze, and this may account for their biological action. Other objectionable components of photochemical smogs include aldehydes, organic hydroperoxides (ROOH), and peroxynitrates. 

Another debatable approach to pollution control involves the methods currently used to reduce hydrocarbons and CO in automotive exhausts. The need to control CO is based on its direct health effects while the need to control the hydrocarbons is based on their interactions with the N02 photolytic cycle which leads to elevated concentrations of N02, 03, peroxyacyl nitrates, and aerosols. The solution adopted was to increase the efficiency of the combustion process, thereby reducing hydrocarbon and CO emissions. Unfortunately, the method adopted also leads to dramatic increases in NO emissions. When this increase in NO was objected to, the answer came back that increased NO in the atmosphere is beneficial since it rapidly reacts with and destroys ozone, one of the very health-related substances requiring control. This is another example of failure to view the total air pollution system. Of course NO destroys 03, but one product of this reaction is N02 which is also detrimental to health. Furthermore, this N02 is the beginning point of sunlight absorption which leads to all the products of photochemical interactions. In a certain location excess NO will tend to reduce 03 levels. However, downstream of these locations excess N02 will promote more photochemical reactions and perhaps even higher ozone levels. In part this nonsolution to automotive pollution may be a major cause of the substantial increases in ozone in many areas during the past few years. This automotive example clearly illustrates the need for in-depth analysis when plans are made to change any part of the system of air pollution. Decisions based on such an analysis are all the more important because the tradeoffs involve human health and welfare. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Ozone is a powerful oxidizing agent that damages rubber, plastic materials, and all plant and animal life. It also reacts with hydrocarbons from automobile exhaust and evaporated gasoUne to form secondary organic pollutants such as aldehydes and ketones (Section 27-11). The peroxyacyl nitrates (PANs), perhaps the worst of the secondary pollutants, are especially damaging photochemical oxidants that are very irritating to the eyes and throat. 

Other harmful compounds, such as peroxyacyl nitrates (PANs), respiratory and eye irritants, are also produced in photochemical smog ... 

Coincidentally, the place Plutarch suggested as best for writing a history, a city, or urban area, is where photochemical smog and associated air pollutants were first observed and peroxyacetyl nitrate (PAN) was first identified as a potentially important air pollutant. The discovery of PAN has an interesting history. It is a story based on collaboration and discussion between biologists and chemists, leading to identification of a unique family of molecules, the peroxyacyl nitrates (PANs), which are now just beginning to be appreciated as important compounds for measurement in the free troposphere, as well as in urban air. 

The class of compounds of general formula RC(0)00N02 called peroxyacyl nitrates (PANs) was first discovered in the early 1950s as components of photochemical smog. The first two compounds in the series are... 

Secondary products, formed from in situ atmospheric photochemical reactions, include the alkyl nitrates, peroxyacyl nitrate, and mat r other compounds. In general, the contribution of these species is usually fairly modest (a few percents) compared to the inoiganic species already mentioned. However, because some of these compounds have well-documented health implications (e g., PAN, ni-trosamines, lutroarenes) and because some of these can play roles in regulating the chemistiy of ozone (eg., PAN),... 

Peroxyacyl nitrates (PANs) Photochemical oxidants in smog. 

Gaseous mixtures of N O, and aldehydes give as a major reaction product peroxyacyl nitrates RCOOjNO [21, 37]. In view of the potential role of these reactions in the formation of photochemical smogs, the kinetics and mechanism of the + aldehyde system have been studied. The RCOO NO formation involves the reactions of NO and NO radicals in equilibrium with N O ... 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

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