The OH initiated oxidation of HNO3 in the UT/LS plays an important role in controlling the O3 budget, removing HOx radicals whilst driving NOx/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

k3 ½M pressure ranges, using the modified Lindemann–Hinshelwood expression k1 ¼ k0 þ , where

k3 ½M þ

k2 k0 = 5.2  1014 exp(200/T) cm3 s1, k2 = 8.4  1017 exp(1900/T) cm3 s1 and k3 = 1.6  1034 exp(1745/T) cm3 s1. 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 NOx/HNO3 ratio, peaking in the tropical upper troposphere regions.