An IRFTS spectroscopic study of the kinetics and the mechanism of the reactions in the gaseous system, HONO, NO, NO 2, H 2O

Abstract

A kinetic study of the nitrous acid decay reaction, 2HONO → NO + NO 2 + H 2O (2), and the formation reaction, NO + NO 2 + H 2O → 2HONO (1), has been made using essentially continuous FT-IR monitoring of the reactants and products in mixtures at the ppm level. The data suggest that the reactions are homogeneous for the conditions employed in this study. The results give: k 1 = (2.2 ± 0.7) × 10 −9 ppm −2 min −1 and k 2 = (1.4 ± 0.4) × 10 −3 ppm −1 min −1 at 23°C. It is concluded that the homogeneous generation of HONO may be important in power plant plumes and in auto exhaust gases during the early stages of the dilution of these NO x -H 2O-rich mixtures. Reactions (1) and (2) are a negligible source and sink of HONO in the atmosphere once the usual low ambient levels of NO and NO 2 are achieved. These reactions may influence smog formation in urban atmospheres during the early morning hours when nitrous acid formation by alternative pathways is slow. For these conditions the sunlight photolysis of HONO may be the major source of HO-radicals which, in combination with hydrocarbons and their oxidation products, lead to NO → NO 2 conversion, O 3 development, and other manifestations of photochemical smog.