Urban atmosphere alkenes

Assume an accidental release of a mixture of gaseous alkanes and alkenes into an urban atmosphere early in the morning. If the atmosphere at the release site is monitored for these compounds, what can be said about their total and relative concentrations at the end of the day Explain. 

Photolysis of atmospheric pollutants by solar radiation results in an increase of ozone concentration in certain urban areas and is the cause of a sequence of oxidation reactions with polymers. Ozone reacts with practically all organic materials especially with alkenes. The rate of its reaction with alkene is several orders of magnitude higher than that with alkane. The ratio of the rate constants of ozone with ethene/ethane is 1.5 x 105, with propene/propane 1.6 x 106, and with butene- 1/butane 1.1 x 106, at room temperature [5],... 

Experimental studies have shown that ozone-alkene reactions in the gas phase likewise proceed via intermediate formation of 1,2,3-trioxolanes (primary ozonides) whose spontaneous decomposition then as a rule leads to a variety of subsequent products, including 1,2,4-trioxolanes (81JA3807). Because ozone is present in the atmosphere (from 0.02p.p.m. at sea level up to 0.2 p.p.m. and more in industrial and urbanized areas), reactions of... 

In addition to OH radicals, unsaturated bonds are reactive towards O3 and NO3 radicals and reaction with these species is an important atmospheric degradation mechanism for unsaturated compounds. Table 4 lists rate constants for the reactions of 03 and NO3 radicals with selected alkenes and acetylene. To place such rate constants into perspective we need to consider the typical ambient atmospheric concentrations of O3 and NO3 radicals. Typical ozone concentrations in pristine environments are 20-40 ppb while concentrations in the range 100-200 ppb are experienced in polluted air. The ambient concentration of NO3 is limited by the availability of NO sources. In remote marine environments the NO levels are extremely low (a few ppt) and NO3 radicals do not play an important role in atmospheric chemistry. In continental and urban areas the NO levels are much higher (up to several hundred ppb in polluted urban areas) and NO3 radicals can build up to 5-100 ppt at night (N03 radicals are photolyzed rapidly and are not present in appreciable amounts during the day). For the purposes of the present discussion we have calculated the atmospheric lifetimes of selected unsaturated compounds in Table 4 in the presence of 100 ppb (2.5 x 1012 cm 3) of O3 and 10 ppt (2.5 x 108 cnr3) of NO3. Lifetimes in other environments can be evaluated by appropriate scaling of the data in Table 4. As seen from Table 4, the more reactive unsaturated compounds have lifetimes with respect to reaction with O3 and NO3 radicals of only a few minutes ... 

The number of hydrocarbons in the atmosphere is potentially very large, since vapor pressures are favorable and the heavier species admit many isomers. In urban areas several hundred different hydrocarbons have been identified by gas chromatography (Appel et al, 1979 Louw el al, 1977). They include saturated compounds (alkanes) unsaturated species with one carbon-carbon double bond (alkenes) or two double bonds (alkadienes), acetylene type compounds (alkynes), and benzene derivatives or aromatic compounds (arenes). To separate that many different compounds requires... 

Alkenes and aromatic compounds have atmospheric lifetimes shorter than those of the low-weight alkanes. After 2 days of transport, propene and the butenes are reduced to a few percent of their original abundances relative to acetylene. If propene and the butenes are found at rural measurement sites, they must have local sources. Ethene, toluene, and ethylbenzene are less reactive. Their abundances relative to acetylene are reduced by a factor of about two after 2 days of transport. Toluene and ethylbenzene are primarily anthropogenic compounds. Both may serve as tracers for urban hydrocarbons in the same way as acetylene. In the cities the abundance... 

Volatile organic compounds include many products such as volatile hydrocarbons (alkanes, alkenes, aromatic compounds), carbonyl compounds (ketones, aldehydes), etc. In urban areas, they originate from motor vehicle exhaust gases, the evaporation of gasoline at filling stations, liquid fuels and industrial activities using solvents. Then-concentration may reach 50 ag-m in the atmosphere of large urban areas. 

Combining the rate coefficients for reactions with OH, NO3 radicals, and O3 with daytime average [OH] = 2.5 x 10, nighttime polluted urban area [NO3] = 3 x 10 , and a diurnal average [O3] = 10 molecule cm concentrations gives lifetime estimates of 9 h, 35 days, and 11 days for reaction with OH and NO3 radicals and O3, respectively. Clearly, reaction with OH dictates the atmospheric lifetime of methyl acrylate. As with alkenes (Calvert et al., 2000) and methacrylates (Blanco et al., 2006), the reaction of OH... 

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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