HO<sub>x</sub> radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling

Abstract

<p><strong>Abstract.</strong> Oxidation flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) are commonly used in atmospheric chemistry and other fields. OFR254 requires the addition of externally formed O₃ since OH is formed from O₃ photolysis, while OFR185 does not since O₂ can be photolyzed to produce O₃, and OH can also be formed from H₂O photolysis. In this study, we use a plug-flow kinetic model to investigate OFR properties under a very wide range of conditions applicable to both field and laboratory studies. We show that the radical chemistry in OFRs can be characterized as a function of UV light intensity, H₂O concentration, and total external OH reactivity (OHR_ext, e.g., from volatile organic compounds (VOCs), NO_x, and SO₂). OH exposure is decreased by added external OH reactivity. OFR185 is especially sensitive to this effect at low UV intensity due to low primary OH production. OFR254 can be more resilient against OH suppression at high injected O₃ (e.g., 70 ppm), as a larger primary OH source from O₃, as well as enhanced recycling of HO₂ to OH, make external perturbations to the radical chemistry less significant. However if the external OH reactivity in OFR254 is much larger than OH reactivity from injected O₃, OH suppression can reach 2 orders of magnitude. For a typical input of 7 ppm O₃ (OHR_O₃ = 10 s⁻¹), 10-fold OH suppression is observed at OHR_ext ~ 100 s⁻¹, which is similar or lower than used in many laboratory studies. The range of modeled OH suppression for literature experiments is consistent with the measured values except for those with isoprene. The finding on OH suppression may have important implications for the interpretation of past laboratory studies, as applying OH_exp measurements acquired under different conditions could lead to over a 1-order-of-magnitude error in the estimated OH_exp. The uncertainties of key model outputs due to uncertainty in all rate constants and absorption cross-sections in the model are within ±25 % for OH exposure and within ±60 % for other parameters. These uncertainties are small relative to the dynamic range of outputs. Uncertainty analysis shows that most of the uncertainty is contributed by photolysis rates of O₃, O₂, and H₂O and reactions of OH and HO₂ with themselves or with some abundant species, i.e., O₃ and H₂O₂. OH_exp calculated from direct integration and estimated from SO₂ decay in the model with laminar and measured residence time distributions (RTDs) are generally within a factor of 2 from the plug-flow OH_exp. However, in the models with RTDs, OH_exp estimated from SO₂ is systematically lower than directly integrated OH_exp in the case of significant SO₂ consumption. We thus recommended using OH_exp estimated from the decay of the species under study when possible, to obtain the most appropriate information on photochemical aging in the OFR. Using HO_x-recycling vs. destructive external OH reactivity only leads to small changes in OH_exp under most conditions. Changing the identity (rate constant) of external OH reactants can result in substantial changes in OH_exp due to different reductions in OH suppression as the reactant is consumed. We also report two equations for estimating OH exposure in OFR254. We find that the equation estimating OH_exp from measured O₃ consumption performs better than an alternative equation that does not use it, and thus recommend measuring both input and output O₃ concentrations in OFR254 experiments. This study contributes to establishing a firm and systematic understanding of the gas-phase HO_x and O_x chemistry in these reactors, and enables better experiment planning and interpretation as well as improved design of future reactors.</p>