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 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 lab 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 (OHRext). In OFR254, OH exposure is less sensitive to OHRext than in OFR185, because injected O3 promotes the HO2-to-OH recycling, making external perturbations to the radical chemistry less significant. The parametric uncertainties of modeled OH are typically 20%. Most of the uncertainty is contributed by photolysis plus reactions involving OH and HO2. For field studies in forested regions or the Los Angeles urban area, reactants of atmospheric interest are dominantly consumed by OH. “Nontropospheric chemistry” (photolysis at 185 or 254 nm, and/or reactions with O(1D) and O(3P)) contributes to the degradation of some speciesunder conditions of low H2O concentration and/or very high OHRext. This may have been a problem in some lab and source studies, but can be avoided by experimental planning, e.g., use of lower concentrations/source dilution, application of stronger UV light, and addition of H2O. OFR185 and OFR254 are comparable in terms of the influence of nontropospheric oxidant. Some biogenic VOCs can have substantial contributions of reaction with O3 under some operating conditions, especially for OFR254. In both OFRs, NO is rapidly oxidized, while NO2-to-NO3 conversion in OFR254 is much faster than in OFR185, and NO3 may have played an unexpectedly significant role in some past lab studies. RO2 fate is similar to that in the atmosphere under low-NOx conditions. A comparison of OFRs with typical chamber studies using UV blacklights and with the atmosphere is presented. OFRs’ key advantages are their short experimental time scales, portability to field sites, enabling a direct connection of field and lab studies, and controllable and predictable radical chemistry. This study further establishes the usefulness of these reactors and enables better experiment planning and interpretation.

Oxidation flow reactors for the study of atmospheric chemistry systematically examined by modeling

249th National Meeting (2015)

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