The effect of acetyl peroxy-peroxy radical reactions on peroxyacetyl nitrate and ozone concentrations

Recent field studies have shown that peroxyacetyl nitrate (PAN) decomposes faster than ozone at night even when the ratio of [NO2] to [NO] is high. PAN is important because it is a reservoir of nitrogen dioxide and peroxy radicals. Nighttime PAN decomposition results from the loss of acetyl peroxy radical through either reaction with nitric oxide or peroxy radical-peroxy radical (R02- RO2) reactions. Peroxy radical-peroxy radical reactions have a strong effect on PAN concentrations and the atmospheric odd nitrogen balance under conditions of low nitric oxide concentrations. Box model simulations, counter species analysis and sensitivity analysis were used to determine the relative importance of the reactions of acetyl peroxy radical with nitric oxide, hydroperoxy radical, acetyl peroxy radical (self-reaction), methyl peroxy radical and other peroxy radicals and the effect of these reactions on PAN concentrations. The base simulation conditions were typical of an aged NOx containing plume mixing with rural air containing non-methane hydrocarbon (NMHC) which is typical of cities in the southern United States and Canada. This sensitivity analysis shows that the self-reaction of acetyl peroxy radical and its reaction with methyl peroxy radical are especially important at night. The self-reaction of acetyl peroxy radical is a significant nighttime sink of acetyl peroxy radical, PAN and a nighttime source of methyl peroxy radical. These RO2---RO2 reactions are shown to be important for the modeling of nighttime PAN concentrations.