Peroxyacetyl radical reaction with

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. 

Because of the mixture of VOCs in the atmosphere, the composition of smog reaction products and intermediates is extremely complex. formed via reaction 16, is important because when dissolved in cloud droplets it is an important oxidant, responsible for oxidising SO2 to sulfuric acid [7664-93-9] H2SO4, the primary cause of acid precipitation. The oxidation of many VOCs produces acetyl radicals, CH CO, which can react with O2 to produce peroxyacetyl radicals, CH2(C0)02, which react with NO2... 

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. 

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 ... 

NO2 conversion is virtually completed. Thus, the competitive reactions of peroxyacetyl radicals with NO (Reaction 20) and with NO2 (Reaction 21) were postulated, and their relative rate constants were chosen to match the experimental data. 

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. 

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. 

A thermal decomposition reaction of much greater atmospheric importance is that of PAN, in thermal equilibrium with NO2 and the peroxyacetyl radical " ... 

This is, of course, the reverse of Reaction 19.3 The overall atmospheric lifetime of PAN depends on the ratio of NO to NO2 and the abundance of peroxyacetyl radical, because the reverse reaction to form PAN is also important. The forward rate for the unimolecular decomposition reaction (Reaction 19.4) is 3.3 X 10 " sec at 298 The temperature dependence of the thermal equilibrium is quite strong, with an activation energy of approximately 25 kcal. At the cold temperatures found at higher altitudes and in winter time, PAN is quite stable in the atmosphere, while at lower altitudes in the summer PAN has a fairly short lifetime (< 1 h). These observations have implications for sampling and chromatographic analysis of PAN in warm temperatures. 

Because PAN is in thermal equilibrium with NO2 and the peroxyacetyl radical, it can act as a means of transporting these more reactive species over long distances. The NO2 released by thermal decomposition of PAN is photolyzed rapidly in the troposphere to form O3 by Reaction 19.1 and Reaction 19.2. Ozone is a criteria air pollutant and is a major health concern. Thus, the PANs play important roles as a chemical means of transporting key species such as NO2 and formaldehyde to remote locations. As such, PANs are globally important atmospheric molecules, as well as urban air pollutants. Since the original observation of PANs in Los Angeles photochemical smog, PANs have been measured in every corner of the world. 

The rate constant of the reaction of NO3 with another peroxy radical, the peroxyacetyl radical (CH3C(0)02) was also measured in a slow discharge flow reactor at 300-423 K (Wayne), using thermal decomposition of PAN as a source of the organic radicals. 

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 important reaction of the acylperoxy radical is with NO2 to form an acylperoxy nitrate. In the example shown, the oxidation of acetaldehyde gives acetyl peroxy radicals which can react with NO2 to form peroxyacetyl nitrate, CH3C(0)02N02, generally known as PAN ... 

The atmospheric lifetime of peroxyacetyl nitrate with respect to the removal by reaction with OH radicals is estimated to be more than 1 year. The wet and dry depositions are minor removal processes (Roberts, 1990). The thermal decomposition remains the most important loss process up to around 7 km, above which photolysis takes... 

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). 

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). 

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. 

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]. 

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). 

Magneron et al. (2003) observed acetone and peroxyacetyl nitrate as products following the reaction of OH radicals with (CH3)2C(OH)CH2CH(OH)CH3, which enabled them to derive a branching ratio for H-atom abstraction from the group of (47 4)% and a lower limit of (33 7)% for abstraction from the —CH2 group. Table II-C-2 summarizes the formation yields of selected hydroxyketones from the reaction of OH with diols. 

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