Peroxyacetyl radical

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... 

Villalta, P.W., Lovejoy, E.R., and Hanson. D.R. Reaction probability of peroxyacetyl radical on aqueous surfaces. Geophys. Res. Lett., 23(14) 1765-1768, 1996. 

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 ... 

PAN is thermally unstable, however, and decomposes at higher temperatures to reform peroxyacetyl radicals... 

IR absorbance spectra of products from, 81 methanol from, 81 peroxyalkyl nitrates from, 84 Atmospheric oxidation of peroxyacetyl radical, 96... 

Peroxyacetylnitrate (PAN) is a well-known product of the propylene ozonolysis in the presence of NO2 and O2 (57) this is strong evidence for the formation of peroxyacetyl (or acetyl) radicals in the subsequent reaction of acetyl zwitterion as given by Reaction 15. However, the peroxyacetyl radicals lead to the formation of PAN only after the NO-... 

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. 

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. 

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. 

The peroxyacetyl radical can be removed from the atmosphere by NO and hydroperoxy radical via the following reactions ... 

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. 

As do other peroxy radicals, the peroxyacetyl radical, CH3C(0)02, reacts with NO ... 

The peroxyacetyl radical also reacts with N02 to form PAN... 

In support of the proposed scheme, whereby CMesOO and CraqOO compete for the intermediate CMe3C(0)00, the change of aldehyde to CH3CHO resulted in predictable changes in kinetic behavior. Because there are no persistent radicals in the CH3CHO system, and CraqOO is the only scavenger for peroxyacetyl radicals produced in Eq. (29), the consumption of CraqOO is much larger than in the pivaldehyde reaction. As shown in Fig. 13, all the CraqOO was consumed before the reaction was completed, as shown by the sudden break in the kinetic trace at 9 s. Under comparable conditions in... 

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. 

Peroxyacetyl nitrate (PAN) is an air pollutant produced in photochemical smog by the reaction of hydrocarbons, oxides of nitrogen, and sunlight. PAN is unstable and dissociates into peroxyacetyl radicals and N02(g). Its presence in polluted air is like a reservoir for NO2 storage. 

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