An Investigation of the Dark Formation of Nitrous Acid in Environmental Chambers

The formation of nitrous acid (HONO) in the dark from initial concentrations of NO2 of 0.1-20 ppm in air, and the concurrent disappearance of NO2, were monitored quantitatively by UV differential optical absorption spectroscopy in two different environmental chambers of ca.4300- and 5800-L volume (both with surface/volume ratios of 3.4 m-1). In these environmental chambers the initial HONO formation rate was first order in the NO2 concentration and increased with the water vapor concentration. However, the HONO formation rate was independent of the NO concentration and relatively insensitive to temperature. The initial pseudo-first-order consumption rate of NO2 was (2.8 ± 1.2) × 10-4 min-1 in the 5800-L Teflon-coated evacuable chamber and (1.6 ± 0.5) × 10-4 min-1 in a 4300-L all-Teflon reaction chamber at ca.300 K and ca.50% RH. The initial HONO yields were ca.40-50% of the NO2 reacted in the evacuable chamber and ca.10-30% in the all-Teflon chamber. Nitric oxide formation was observed during the later stages of the reaction in the evacuable chamber, but ca.50% of the nitrogen could not be accounted for, and gas phase HNO3 was not detected. The implications of these data concerning radical sources in environmental chamber irradiations of NOx- organic-air mixtures, and of HONO formation in polluted atmospheres, are discussed.