Kinetics of the gas phase reaction OH + NO(+M)→HONO(+M) and the determination of the UV absorption cross sections of HONO

The reaction OH + NO(+M) → HONO(+M) with M = SF6 as a third body has been employed as a clean source for recording the near-ultraviolet absorption spectrum of HONO without interference from other absorbing species. The reaction was initiated by the pulse radiolysis of SF6/H2O/NO mixtures with total pressures in the range 10–1000 mbar at 298 K. The pressure dependence of the rate coefficient was studied by time-resolved UV and IR spectroscopy. By analysis of the fall-off curve we have derived a value for the limiting low pressure rate constant k0/[SF6] = (1.5 ± 0.1) × 10−30 cm6molecule−2 s−1 at 298 K, using the values of k∞ = (3.3±0.3)×10−11 cm3 molecule−1 s−1 and Fcent = 0.81 reported by Troe and co-workers. The UV spectrum of HONO was recorded in the range 320–400 nm and an absolute absorption cross section of σ = (5.02 ± 0.76) × 10−19 cm2 molecule−1 has been determined for the strongest band of HONO located at 354.2 nm. Differential absorption cross sections to be used for field measurements of HONO were also investigated.