Reaction Kinetics of OH + HNO₃ under conditions relevant to the Upper Troposphere/Lower Stratosphere

The OH initiated oxidation of HNO₃ in the UT/LS plays an important role in controlling the O₃ budget, removing HO_x radicals whilst driving NO_x/y partitioning chemistry by yielding NO₃ radicals: OH + HNO₃ → H₂O + NO₃. In this paper, k₁(T, P) was measured using OH (A ← X) Laser Induced Fluorescence (LIF) and the data was modelled over the 223–298 K temperature and 25–750 Torr pressure ranges, using the modified Lindemann–Hinshelwood expression, k₁=k₀+(k₃[M])/(1+(k₃[M]/k₂)) where k₀ = 5.2 × 10⁻¹⁴ exp(200/T) cm³ s⁻¹, k₂ = 8.4 × 10⁻¹⁷ exp(1900/T) cm³ s⁻¹ and k₃ = 1.6 × 10⁻³⁴ exp(1745/T) cm³ s⁻¹. A significant source of experimental uncertainty derives from accurate determination of HNO₃ concentration, which is impacted by heterogeneous uptake of the low volatility HNO₃ onto cold surfaces of the reactors. Our results represent the determination of k₁(T, P) using two different in situ [HNO₃] measurements: VUV absorption and a new two photon Photolysis Induced Fluoresence (PIF). Experimental results are discussed along with a computational master equation calculation (MESMER), which highlight the need for further theoretical study into the OH + HNO₃ mechanism and potential energy surface. The atmospheric impact of these new rate constants were modelled using the STOCHEM-CRI chemistry transport global model, which have shown a small reduction in global budgets of key atmospheric species, with more significant changes in the NO_x/HNO₃ ratio, peaking in the tropical upper troposphere regions.