DECOMPOSITION OF PERNITRIC ACID IN AQUEOUS-SOLUTION

The unimolecular decay of pernitric acid (PNA), HO2NO2, in aqueous solutions is strongly pH dependent and yields predominantly nitrite above pH = 5. This observation is of particular interest for atmospheric processes involving aerosol and cloud water chemistry. The detailed kinetic analysis of the underlying processes shows good agreement with the kinetics of a related phenomenon, the oxygen production from PNA solutions as observed in 1981 by Kenley, Trevor, and Lan. Both sets of data are analyzed on the basis of a common mechanism, and a number of important and hitherto unknown rate constants are evaluated. For reaction 1 HO2NO2 ⇄ H+ + O2NO2-, the equilibrium constant is (1-2.5) × 10-5 M. The rate-limiting step at large pH appears to be reaction 2, O2NO2- → O2 + NO2-, which must be rather temperature sensitive. Values found for k2 were 0.014 and 0.045 s-1 at 284 and 295 K, respectively. Below a pH of 3-4 the unimolecular decomposition into neutral products (3), HO2NO2 → HO2 + NO2, is the main reaction path with k3 = 4.6 × 10-3 s-1. © 1990 American Chemical Society.