HOx radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling

[摘要] Oxidation flow reactors (OFRs) using OH produced fromlow-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) arecommonly used in atmospheric chemistry and other fields. OFR254 requires theaddition of externally formed O₃ since OH is formed from O₃photolysis, while OFR185 does not since O₂ can be photolyzed to produceO₃, 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 verywide range of conditions applicable to both field and laboratory studies. Weshow that the radical chemistry in OFRs can be characterized as a functionof UV light intensity, H₂O concentration, and total external OHreactivity (OHR_ext, e.g., from volatile organic compounds (VOCs), NO_x, and SO₂). OHexposure is decreased by added external OH reactivity. OFR185 is especiallysensitive to this effect at low UV intensity due to low primary OHproduction. OFR254 can be more resilient against OH suppression at highinjected O₃ (e.g., 70 ppm), as a larger primary OH source from O₃,as well as enhanced recycling of HO₂ to OH, make external perturbationsto the radical chemistry less significant. However if the external OHreactivity in OFR254 is much larger than OH reactivity from injectedO₃, OH suppression can reach 2 orders of magnitude. For a typicalinput of 7 ppm O₃ (OHR_O3= 10 s⁻¹), 10-fold OH suppressionis observed at OHR_ext ~ 100 s⁻¹, which is similar orlower than used in many laboratory studies. The range of modeled OHsuppression for literature experiments is consistent with the measuredvalues except for those with isoprene. The finding on OH suppression mayhave important implications for the interpretation of past laboratorystudies, as applying OH_exp measurements acquired under differentconditions could lead to over a 1-order-of-magnitude error in the estimatedOH_exp. The uncertainties of key model outputs due to uncertainty in allrate constants and absorption cross-sections in the model are within ±25 % for OH exposure and within ±60 % for other parameters. Theseuncertainties are small relative to the dynamic range of outputs.Uncertainty analysis shows that most of the uncertainty is contributed byphotolysis rates of O₃, O₂, and H₂O and reactions of OH andHO₂ with themselves or with some abundant species, i.e., O₃ andH₂O₂. OH_exp calculated from direct integration and estimatedfrom SO₂ decay in the model with laminar and measured residence timedistributions (RTDs) are generally within a factor of 2 from the plug-flowOH_exp. However, in the models with RTDs, OH_exp estimated fromSO₂ is systematically lower than directly integrated OH_exp in thecase of significant SO₂ consumption. We thus recommended usingOH_exp estimated from the decay of the species under study whenpossible, to obtain the most appropriate information on photochemical agingin the OFR. Using HO_x-recycling vs. destructive external OH reactivityonly leads to small changes in OH_exp under most conditions. Changingthe identity (rate constant) of external OH reactants can result insubstantial changes in OH_exp due to different reductions in OHsuppression as the reactant is consumed. We also report two equations forestimating OH exposure in OFR254. We find that the equation estimatingOH_exp from measured O₃ consumption performs better than analternative equation that does not use it, and thus recommend measuring bothinput and output O₃ concentrations in OFR254 experiments. This studycontributes to establishing a firm and systematic understanding of thegas-phase HO<

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