Temperature-(208–318 K) and pressure-(18–696 Torr) dependent rate coefficients for the reaction between OH and HNO<sub>3</sub>

Abstract

Abstract. Rate coefficients (k5) for the title reaction were obtained using pulsed laser photolytic generation of OH coupled to its detection by laser-induced fluorescence (PLP–LIF). More than 80 determinations of k5 were carried out in nitrogen or air bath gas at various temperatures and pressures. The accuracy of the rate coefficients obtained was enhanced by in situ measurement of the concentrations of both HNO3 reactant and NO2 impurity. The rate coefficients show both temperature and pressure dependence with a rapid increase in k5 at low temperatures. The pressure dependence was weak at room temperature but increased significantly at low temperatures. The entire data set was combined with selected literature values of k5 and parameterised using a combination of pressure-dependent and -independent terms to give an expression that covers the relevant pressure and temperature range for the atmosphere. A global model, using the new parameterisation for k5 rather than those presently accepted, indicated small but significant latitude- and altitude-dependent changes in the HNO3 ∕ NOx ratio of between −6 and +6 %. Effective HNO3 absorption cross sections (184.95 and 213.86 nm, units of cm2 molecule−1) were obtained as part of this work: σ213.86 = 4.52−0.12+0.23 × 10−19 and σ184.95 = 1.61−0.04+0.08 × 10−17.

Kinetics of the OH + NO2 reaction: Effect of water vapour and new parameterisation for global modelling

“…In the same experiment, we also recorded the optical density at 285 185 nm where H2O, NO2 and HNO3 all absorb. Despite the large HNO3 absorption cross-section at this wavelength (1.6 × 10 -17 cm 2 molecule -1 , Dulitz et al (2018)) we found no evidence for HNO3 formation, indicating that the NO2 lost was not converted to gas-phase HNO3. Given its great affinity for glass in the presence of H2O, we expect that any HNO3 formed is strongly partitioned to the walls of the reactor.…”

Reactive quenching of electronically excited NO<sub>2</sub><sup>∗</sup> and NO<sub>3</sub><sup>∗</sup> by H<sub>2</sub>O as potential sources of atmospheric HO<sub><i>x</i></sub> radicals

“…NO 2 concentrations were calculated using a literature reference spectrum (Vandaele et al, 1998). Concentrations of HNO 3 and H 2 O were calculated using cross sections of 1.61×10 −17 cm 2 molecule −1 (Dulitz et al, 2018) and 7.22× 10 −20 cm 2 molecule −1 (Creasey et al, 2000).…”

“…More than 10 years later, Li et al (2008) carried out similar experiments but at longer wavelengths (560-640 nm) and reached very different conclusions, deriving a yield of OH (and thus also HONO) close to 1×10 −3 , a factor of 14 larger than the upper limit of Crowley and Carl (1997). Calculations of the impact of Reactions (R4)-(R5) using the large yield reported by Li et al (2008) led to the conclusion that Reaction (R5) is important for air quality under highly polluted conditions; use of the lower yield from Crowley and Carl (1997) resulted in minimal impact (Wennberg and Dabdub, 2008;Ensberg et al, 2010). Subsequent to the work of Li et al (2008), two further experimental studies (Carr et al, 2009;Amedro et al, 2011) appeared to confirm the conclusions of Crowley and Carl (1997) and suggested that the high yield reported by Li et al (2008) was an experimental artefact, resulting from multiphoton laser excitation of NO 2 in their focussed laser beam (Amedro et al, 2011).…”

“…The rate constant and the yield of NO 3 (unity) for Reaction (R15) are well known (Brown et al, 1999(Brown et al, , 2001Carl et al, 2001;Dulitz et al, 2018), enabling the timedependent NO 3 concentration profile to be calculated if the initial amount of OH is known. This initial concentration of OH depends on the 248 nm laser fluence (measured by a Joule meter, uncertainty 30 %) and the HNO 3 concentration (measured by optical absorption at 185 nm, uncertainty 10 %).…”

Kinetics of the OH + NO2 reaction: Rate coefficients (217–333 K, 16–1200 mbar) and fall-off parameters for N2 and O2 bath-gases

“…secondary reactions of OH on the determination of k5. For the OH + NO2 reaction, the use of OH concentrations as high as 10 12 molecule cm -3 is not expected to have a significant impact on the OH decay rates because the major product, HNO3, reacts only slowly with OH with k(OH + HNO3) = 1.6 × 10 -13 cm 3 molecule -1 s -1 at 296 K and 250 Torr (Dulitz et al, 2018). Even if the minor product, HOONO, were to react with OH with a rate coefficient of close to 2 × 10 -10 cm 3 molecule -1 s -1 (i.e.…”