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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 (OHRext, e.g., from volatile organic compounds (VOCs), NOx, 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 (OHRO3 = 10 s−1), 10-fold OH suppressionis observed at OHRext ~ 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 OHexp measurements acquired under differentconditions could lead to over a 1-order-of-magnitude error in the estimatedOHexp. 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. OHexp 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-flowOHexp. However, in the models with RTDs, OHexp estimated fromSO2 is systematically lower than directly integrated OHexp in thecase of significant SO2 consumption. We thus recommended usingOHexp estimated from the decay of the species under study whenpossible, to obtain the most appropriate information on photochemical agingin the OFR. Using HOx-recycling vs. destructive external OH reactivityonly leads to small changes in OHexp under most conditions. Changingthe identity (rate constant) of external OH reactants can result insubstantial changes in OHexp 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 estimatingOHexp 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 HOx and Ox chemistry in these reactors, and enablesbetter experiment planning and interpretation as well as improved design offuture reactors

atmospheric measurement techniques

HO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling