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

Abstract

<p><strong>Abstract.</strong> Rate coefficients (<i>k</i><sub>5</sub>) 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 eighty determinations of <i>k</i><sub>5</sub> 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 HNO<sub>3</sub> reactant and NO<sub>2</sub> impurity. The rate coefficients show both temperature and pressure dependence with a rapid increase in <i>k</i><sub>5</sub> at low temperatures. The pressure dependence was weak at room temperature but increased significantly at low temperatures. The entire dataset was combined with selected literature values of <i>k</i><sub>5</sub> 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 <i>k</i><sub>5</sub> rather than those presently accepted, indicated small but significant latitude and altitude dependent changes in the HNO<sub>3</sub> / NO<sub><i>x</i></sub> ratio of between −6 % and +6 %. Effective HNO<sub>3</sub> absorption cross sections (184.95 and 213.86 nm, units of cm<sup>2</sup> molecule<sup>−1</sup>) were obtained as part of this work: σ<sub>213.86</sub> = 4.52<span style="border-bottom: 0px solid #68; vertical-align: 50%; font-size: .7em; color: #68;">+0.23</span><span style="margin-left: -2.9em; margin-right: 0.2em; vertical-align: -15%; font-size: .7em; color: #68;">−0.12</span> × 10<sup>−19</sup> and σ<sub>184.95</sub> = 1.61<span style="border-bottom: 0px solid #68; vertical-align: 50%; font-size: .7em; color: #68;">+0.08</span><span style="margin-left: -2.9em; margin-right: 0.2em; vertical-align: -15%; font-size: .7em; color: #68;">−0.04</span> × 10<sup>−17</sup>.</p>

Reaction kinetics of OH + HNO3 under conditions relevant to the upper troposphere/lower stratosphere

“…The absorption cross-section for HNO3 at 185 nm, σ185nm, has been determined several times in the literature [15][16][17] . More recently, Dulitz, et al, 10 have confirmed the previous measurements using a meticulous apparatus to account for a variety of impurities (NO2, NO3, N2O5, and H2O) and measure at two wavelengths simultaneously. Based on these studies, σ185nm = (1.6 ± 0.1) × 10 -17 cm 2 was used here.…”

“…as a function of temperature and pressure: Margitan and Watson, 8 over the pressure range of 20 -100 Torr and temperature range of 225-415 K, Stachnik, et al, 9 over the pressure range of 10 -730 Torr and at two temperatures of 248 and 297 K, Brown, et al, 5 between 50 and 500 Torr over the temperature range of 200-375 K and the most recent study by Dulitz, et al, 10 over the pressure range of 18 -696 Torr and the temperature range of 208 -318 K. The three earlier studies are in fair agreement which led to the current JPL recommended uncertainty in k1 of 20% (±1σ) at STP.…”

“…To compensate for this problem, HNO3 characterization using 2-photon photolysis was implemented. 10,14,18 Briefly, the 248 nm output of the excimer laser was focused into the center of the reaction cell (UV-fused silica planoconvex, f = 1000 mm). When a molecule of HNO3 was pumped with two photons of 248 nm light, fluorescence was observed at ~308 nm from the photodissociation products.…”

“…All previous studies have been performed under pseudo first order conditions and require accurate determination of HNO3 concentration in order to convert experimentally observed decay constants into bimolecular rate coefficients. Until recently 10 , former studies have relied on ex-situ measurements of the HNO3 concentration after the reaction cell, which could lead to greater uncertainties in the overall determination of the rate coefficient from heterogeneous uptake of the low volatility HNO3 onto cold surfaces of the reactors used. The work by Dulitz, et al, 10 utilizes a two-photon photolysis induced fluorescence detection method for HNO3 and has shown that at low temperatures k1 may be smaller than previously thought.…”

“…Until recently 10 , former studies have relied on ex-situ measurements of the HNO3 concentration after the reaction cell, which could lead to greater uncertainties in the overall determination of the rate coefficient from heterogeneous uptake of the low volatility HNO3 onto cold surfaces of the reactors used. The work by Dulitz, et al, 10 utilizes a two-photon photolysis induced fluorescence detection method for HNO3 and has shown that at low temperatures k1 may be smaller than previously thought. Photolysing the HNO3 and measuring fluorescence in the center of the reactor allows a more accurate determination of [HNO3] in the kinetic measurement region (i.e.…”