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

Abstract

Abstract. 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 (OHRext, e.g., from volatile organic compounds (VOCs), NOx, 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 (OHRO3 = 10 s−1), 10-fold OH suppression is observed at OHRext ~ 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 OHexp measurements acquired under different conditions could lead to over a 1-order-of-magnitude error in the estimated OHexp. 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. OHexp 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 OHexp. However, in the models with RTDs, OHexp estimated from SO2 is systematically lower than directly integrated OHexp in the case of significant SO2 consumption. We thus recommended using OHexp estimated from the decay of the species under study when possible, to obtain the most appropriate information on photochemical aging in the OFR. Using HOx-recycling vs. destructive external OH reactivity only leads to small changes in OHexp under most conditions. Changing the identity (rate constant) of external OH reactants can result in substantial changes in OHexp 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 OHexp 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 HOx and Ox chemistry in these reactors, and enables better experiment planning and interpretation as well as improved design of future reactors.

Assessing the degree of plug flow in oxidation flow reactors (OFRs): a study on a potential aerosol mass (PAM) reactor

“…The mercury lamps are left in place with their housing to mimic simple OFR internals; they have not been turned on during this study. Details of their mode of operations for oxidant formation can be found elsewhere Peng et al, 2015Peng et al, , 2016. OFRs like the WU-PAM have removable internals, face plates, and peripheral inlets and outlets that allow a wide variety of configurations.…”

“…However, recent work incorporates the effect of non-ideal RTDs on model outputs , 2018, 2017. Peng et al (2015) show that for three OFR operational modes (that is, modes of different oxidant formation mechanisms denoted by OFR185, OFR254-70, and OFR254-7), a comparison between model output for ideal plug flow vs. non-ideal RTDs (using the RTD experimentally obtained by Lambe et al, 2011a) for OH exposure (OH exp ) generally agree within a factor of 2 for low OH exp ; the model disagreement is exacerbated at high OH exp beyond a factor of ∼ 4. then extend OFR operational modes to include N-containing chemistry (in modes referred to therein as OFR185-iNO, OFR185-7-iNO, and OFR185-70-iNO) where at moderate-to-high OH exp , the deviations are exacerbated significantly, although the authors argue those conditions represent unrealistic chemical pathways. It is worthwhile noting that the chemistry modeled by Peng and Jimenez may find a workaround by utilizing N 2 O as NO precursor rather than NO itself, potentially minimizing RTD-related errors.…”

“…Because the mechanism of exposure between traditional chambers OFRs was different, validating the OFR concept began by replicating data obtained from traditional chambers Lambe et al, 2015;Liu et al, 2015), and assessing whether the chemistry was realistic McNeill et al, 2008;Peng et al, 2015;Renbaum and Smith, 2011). Consequently, much modeling work has focused on pure chemical reactions and comparison of SOA yields between the two (Bruns et al, 2015;Lambe et al, 2015;Li et al, 2015;Ortega et al, 2016;Peng et al, 2015). However, little modeling work has been done on understanding hydrodynamics or flow fields inside OFRs so that the flow patterns can be improved.…”

“…This application therefore altered the study of SOA formation, previously dominated by the traditional large, well-mixed reactors (Kroll and Seinfeld, 2008;Rudich et al, 2007;Turpin et al, 2000), by allowing the generation of laboratory data beyond first simulated day of exposure. Because the mechanism of exposure between traditional chambers OFRs was different, validating the OFR concept began by replicating data obtained from traditional chambers Lambe et al, 2015;Liu et al, 2015), and assessing whether the chemistry was realistic McNeill et al, 2008;Peng et al, 2015;Renbaum and Smith, 2011). Consequently, much modeling work has focused on pure chemical reactions and comparison of SOA yields between the two (Bruns et al, 2015;Lambe et al, 2015;Li et al, 2015;Ortega et al, 2016;Peng et al, 2015).…”

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modelling

“…Further OFR sampling and measurement details for the data used in this work can be found in Palm et al (2016Palm et al ( , 2018. The chemical regime was relevant to ambient OH oxidation, as discussed in detail in Peng et al (2015Peng et al ( , 2016. 5…”