Peroxyacetyl nitrate 154 Radical

During SOAPEX-2, measurements of the free-radicals OH, HO2, HO2+XRO2, NO3, IO and OIO were supported by measurements of temperature, wind speed and direction, photolysis rates (j D) and j(N02)), water vapor, O3, HCHO, CO, CH4, NO, NO2, peroxyacetyl nitrate (PAN), a wide range of NMHCs, organic halogens, H2O2, CH3OOH and condensation nuclei (CN). 

Complex reactions involving radicals occur, giving rise to secondary pollutants such as ozone, aldehydes, peroxyacetyl nitrate (PAN) and particulate matter. 

Photolytic. Photolysis of acetone in air yields carbon monoxide and free radicals, but in isopropanol, pinacol is formed (Calvert and Pitts, 1966). Photolysis of acetone vapor with nitrogen dioxide via a mercury lamp gave peroxyacetyl nitrate as the major product with smaller quantities of methyl nitrate (Warneck and Zerbach, 1992). 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Photolytic. Synthetic air containing gaseous nitrous acid and exposed to artificial sunlight (A, = 300-450 nm) photooxidized 2-butanone into peroxyacetyl nitrate and methyl nitrate (Cox et al., 1980). They reported a rate constant of 2.6 x 10 cm /molecule-sec for the reaction of gaseous 2-butane with OH radicals based on a value of 8 x 10 cm /molecule-sec for the reaction of ethylene with OH radicals. 

The OH radical-initiated photooxidation of 2-butanone in a smog chamber produced peroxyacetyl nitrate and acetaldehyde (Cox et al., 1981). Reported rate constants for the reaction of 2-butanone with OH radicals in the atmosphere and in water are 1.15 x lO and 1.50 x 10 cmVmolecule-sec, respectively (Wallington and Kurylo, 1987 Wallington et al, 1988a). The rate constant for the reaction of 2-butanone and OH radicals in the atmosphere at 300 K is 2.0 x 10 cmVmolecule-sec (Hendry and Kenley, 1979). Cox et al. (1981) reported a photooxidation half-life of 2.3 d for the reaction of 2-butanone and OH radicals in the atmosphere. 

Major products reported from the photooxidation of o-xylene with nitrogen oxides include formaldehyde, acetaldehyde, peroxyacetyl nitrate, glyoxal, and methylglyoxal (Altshuller, 1983). The rate constant for the reaction of o-xylene and OH radicals at room temperature was 1.53 x 10 " cmVmolecule-sec (Hansen et al, 1975). A rate constant of 8.4 x 10 L/molecule-sec was reported for the reaction of o-xylene with OH radicals in the gas phase (Darnall et al., 1976). Similarly, a room temperature rate constant of 1.34 x 10 " cmVmolecule-sec was reported for the vapor-phase reaction of o-xylene with OH radicals (Atkinson, 1985). At 25 °C, a rate constant of 1.25 X 10 " cmVmolecule-sec was reported for the same reaction (Ohta and Ohyama, 1985). 

As we have seen in Chapter 1, peroxyacetyl nitrate (PAN) is a powerful lachrymator and severe plant phytotoxicant formed in irradiated VOC-NOx mixtures from the reaction of peroxyacetyl radicals with N02 ... 

Seefeld, S D. J. Kinnison, and J. Alistair Kerr, Relative Rate Study of the Reactions of Acetylperoxy Radicals with NO and N02 Peroxyacetyl Nitrate Formation under Laboratory Conditions Related to the Troposphere, J. Phys. Chem. A, 101, 55-59 (1997). 

Various oxidation products, including PAN (peroxyacetyl nitrate), are formed by the radical termination reactions... 

Peroxyacetyl nitrate (PAN), CH3C000N02, is also a source of OH radicals when it thermally decomposes ... 

Winer, A.M., Lloyd, A.C., Darnall, K.R., Atkinson, R., Pitts, Jr., J.N. (1977) Rate constants for the reactions of hydroxyl radicals with n-propyl acetate, i ec-butyl acetate, tetrahydrofuran and peroxyacetyl nitrate. Chem. Phys. Lett. 51(2), 221-226. 

However, near the Earth s surface, the hydrocarbons, especially olefins and substituted aromatics, are attacked by the free atomic O, and with NO, produce more NO2. Thus, the balance of the reactions shown in the above reactions is upset so that O3 levels build up, particularly when the Sun s intensity is greatest at midday. The reactions with hydrocarbons are very complex and involve the formation of unstable intermediate free radicals that undergo a series of changes. Aldehydes are major products in these reactions. Formaldehyde and acrolein account for 50% and 5%, respectively, of the total aldehyde in urban atmospheres. Peroxyacetyl nitrate (CH3COONO2), often referred to as PAN, and its homologs, also arise in urban air, most likely from the reaction of the peroxyacyl radicals with NO2. 

Stockwell, W.R., J.B. Milford, D. Gao and Y.J. Yang The effect of acetyl peroxy-peroxy radical reactions on peroxyacetyl nitrate and ozone concentrations, Atmos. Environ. 29 (1995) 1591-1599. 

Aldehydes are emitted by combustion processes and also are formed in the atmosphere from the photochemical degradation of other organic compounds. Aldehydes undergo photolysis, reaction with OH radicals, and reaction with N03 radicals in the troposphere. Reaction with N03 radicals is of relatively minor importance as a loss process for these compounds, but can be a minor contributor to the H02 (from formaldehyde) and peroxyacetyl nitrate (PAN) formation during nighttime hours (Stockwell and Calvert, 1983 Cantrell et al., 1985). Thus, the major loss processes involve photolysis and reaction with OH radicals. 

The first member of the peroxyacyl nitrate series is CH3C(0)00N02, peroxyacetyl nitrate (PAN). The lifetime of PAN is strongly temperature-dependent and ranges from 30 min at 303 K to 8 h at 273 K. Under urban conditions at fairly warm temperatures, the concentration of PAN is governed by the steady-state concentration of the peroxyacetyl radical, CH3C(0)00. With PAN formation proportional to NOz and competitive with peroxyacetyl radical reaction with NO, the steady-state concentration of PAN is proportional to the NOz/NO ratio. From the N0/N02/03 photostationary state relation (18), since the steady-state concentration of 03 is also proportional to the N02/N0 ratio, the steady-state PAN concentration is proportional to the 03 concentration. 

Table 9-9 includes similar data for the association products of N02 with peroxy radicals. Peroxyacetyl nitrate (PAN) is thermally the most stable of these adducts. It is important in all regions of the troposphere. Pernitric acid and methylperoxy nitrate are less stable but may become significant in the upper troposphere where temperatures are low. Model calculations by Logan et al. (1981) suggest that as much as 50% of N02 may be present as H00N02 at higher altitudes. Observational data are lacking, however, so that the real contribution of these addition products to the total reservoir of N02 in the troposphere remains to be established. 

The major products were ethyl formate and formaldehyde and the minor products were ethyl acetate, acetaldehyde, peroxyacetyl nitrate, and methyl and ethyl nitrates. The products arise from the decomposition reactions of the 1-ethoxyethoxy radical and from its reaction with molecular oxygen ... 

The reaction with NO leads to the formation of CO2 and a methyl radical that is oxidized to formaldehyde by reactions (16)-(20). In addition, the oxidation of CH3 regenerates HO c so that the oxidation cycle continues. Association with NO2 produces peroxyacetyl nitrate (PAN). Its lifetime is longer than that of alkylperoxy nitrates, but strongly temperature dependent, ranging from 1 hr at 298 K to 140 d at 250 K. Thus, PAN can be transported over a great distance before undergoing thermal decomposition. Under conditions of lowNOj concentrations acetyl peroxy radicals interact also with HO2 radicals... 

An identical process is proposed for conversion of peroxyacetyl nitrate [20]. This reaction requires high mobility of the molecular fragments to build up six-membered complexes. In contrast to R O, the chain peroxide radicals in a rigid matrix of PTFE form such transition states with difficulty [21]. 

It is nearly 40 years since the first catalytic devices were commercially produced and fitted into cars, after the recognition that car exhaust primary pollutants, that is, unburned hydrocarbons (HCs), nitrogen oxides (NO ), and carbon monoxide (CO), interact with sun light resulting in the formation of secondary pollutants (e.g., ozone, oxygenated hydrocarbons and hydrocarbon radicals, peroxyacetyl nitrate PAN, and nitric dioxide), which are responsible for the photochemical smog in capital cities [1]. The phenomenon had become of such a concern in almost all the big cities, that forced environmental legislation firstly introduced in 1970 by the US Clean Air Act (US-CAA), and practically applied in 1975 [2]. 

Here, acetylperoxy radicals formed in the reaction (5.55) reacts with NO2 in polluted atmosphere to give peculiar compound called peroxyacetyl nitrate (PAN) CH3C(0)02N02 (see Sect. 7.2.9). 

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