Laboratory studies of the kinetics of formation of nitrous acid from the thermal reaction of nitrogen dioxide and water vapour

Abstract

The kinetics of the formation of nitrous acid (MONO) and removal of NO 2 during the heterogeneous reaction of NO 2 and H 2O were investigated using infra-red diode laser spectroscopy and u.v./visible spectroscopy. The majority of experiments were performed at temperatures of 292 and 296 K, and at pressures of less than 10 Torr, but a limited number were performed at 277 and 311 K and at pressures of up to 297 Torr. The 19.8-ℓ pyrex reaction vessel used in this work ( s v = 13 m −1 ) had stainless steel end plates and contained gold coated multi-reflection ‘White’ optics. The initial kinetics of HONO production and NO 2 removal were found to be 1st order in both [NO 2] and [H 2O] and the observed 2nd order rate constant, independent of pressure at 296 K, was: k= d[ HONO] dt [NO 2][H 2O] = 3.2 × 10 −22 cm 3 molecule −1 s −1 . The yield of HONO during the early stages of reaction was ~ 50 % relative to NO 2 removed, but as the reaction proceeded the kinetics became more complicated, and the yield was reduced. HNO 3 is almost certainly produced, but could not be detected in the gas phase by infra-red measurement at 890 cm −1. HONO may be an important precursor to OH radicals in the atmospheric boundary layer. An estimate of the production rate of HONO from the reaction of NO 2 and H 2O in the night-time boundary layer has been made using the results of this work, and a comparison is made with those based on field observations.