Peroxy radicals Peroxyacetyl nitrate

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. 

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. 

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 ... 

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