Study of OH + NO/NO2 under upper troposphere/lower stratosphere conditions

The oxidation of nitrogen oxides (NO_x = NO + NO₂) by hydroxyl radicals (OH) play an important role in the upper troposphere/lower stratosphere (UT/LS). The OH-initiated oxidation of NO is the only known gas-phase reaction source for nitrous acid (HONO) and governs the daytime photo stationary state concentration of HONO in the troposphere. Despite this importance, the reaction is poorly understood and few experimental studies have been conducted over a temperature and pressure range important for Earth's lower atmosphere. The importance of OH + NO₂ on tropospheric and stratospheric chemistry cannot be understated; the reaction plays a crucial role in controlling the O₃ budget through removing oxidative capacity in the form of OH radicals, whilst simultaneously removing NO₂, the photolysis of which is responsible for O₃ production.

A recent modelling study has highlighted the significance of these reactions in affecting the global ozone radiative forcing potential, rating them as the 1^st (NO₂) and 37^th (NO) based on the current uncertainty estimation quoted in the JPL Kinetic Data Evaluation. Currently, the recommended uncertainies in k_OH+NO2 and k_OH+NO are 30% (±1σ) and 20% (±1σ) respectively at 298 K and 760 Torr; however under UT/LS conditions, these uncertainties could increase to as much as ± 50%, which would have a significant impact on atmospheric models. This work presents experimental results into the OH + NO and NO₂ rate coefficient over the matrix of temperature (T = 220 - 300 K) and pressure (p = 25 - 750 Torr) using OH (A ← X) Laser Induced Fluorescence (LIF). These results represent the first comprehensive determination of k_OH+NO over an atmospherically important range of T and p and the first determination of k_OH+NO2(T,p) using simultaneous in-situ detection of [NO₂]. Experimental results will be discussed along with atmospheric impact.

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