A brief review of the use of environmental chambers for gas phase studies of kinetics, chemical mechanisms and characterisation of field instruments

Seakins, Paul W.

doi:10.1051/epjconf/201009012

Cited by 24 publications

(30 citation statements)

References 60 publications

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“…Table 1). For this case, OH concentrations in both OFR185 and OFR254-70 are of the order of magnitude of 10 10 molecules cm −3 and thus ∼ 4 orders of magnitude higher than typical ambient values (Stone et al, 2012). The O 3 and HO 2 concentrations are higher than ambient as well by several orders of magnitude.…”

Section: Main Species and Conversionsmentioning

confidence: 70%

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

et al. 2015

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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.

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Section: Main Species and Conversionsmentioning

confidence: 70%

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

et al. 2015

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“…They typically employ actinic wavelength (> 300 nm) light sources (e.g., outdoor solar radiation and UV black lights) to produce oxidants and radicals and have large volumes (on the order of several cubic meters or larger). However, the capability of generating sustained elevated levels of OH, the most important tropospheric oxidant, is usually limited in chambers, resulting in OH concentrations similar to those in the atmosphere (10 6 -10 7 molecules cm −3 ; Mao et al, 2009;Ng et al, 2010) and consequently long simulation times (typically hours) to reach OH equivalent ages of atmospheric relevance (George et al, 2007;Kang et al, 2007;Carlton et al, 2009;Seakins, 2010;Wang et al, 2011). The partitioning of gases and aerosols to chamber walls (usually made of Teflon) in timescales of tens of minutes to hours makes it difficult to conduct very long experiments that simulate high atmospherically relevant photochemical ages (Cocker et al, 2001;Matsunaga and Ziemann, 2010;Zhang et al, 2014;.…”

Section: Introductionmentioning

confidence: 99%

Modeling of the chemistry in oxidation flow reactors with high initial NO

Peng

Jimenez

2017

Atmos. Chem. Phys.

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Abstract. Oxidation flow reactors (OFRs) are increasingly employed in atmospheric chemistry research because of their high efficiency of OH radical production from low-pressure Hg lamp emissions at both 185 and 254 nm (OFR185) or 254 nm only (OFR254). OFRs have been thought to be limited to studying low-NO chemistry (in which peroxy radicals (RO 2 ) react preferentially with HO 2 ) because NO is very rapidly oxidized by the high concentrations of O 3 , HO 2 , and OH in OFRs. However, many groups are performing experiments by aging combustion exhaust with high NO levels or adding NO in the hopes of simulating high-NO chemistry (in which RO 2 + NO dominates). This work systematically explores the chemistry in OFRs with high initial NO. Using box modeling, we investigate the interconversion of Ncontaining species and the uncertainties due to kinetic parameters. Simple initial injection of NO in OFR185 can result in more RO 2 reacted with NO than with HO 2 and minor non-tropospheric photolysis, but only under a very narrow set of conditions (high water mixing ratio, low UV intensity, low external OH reactivity (OHR ext ), and initial NO concentration (NO in ) of tens to hundreds of ppb) that account for a very small fraction of the input parameter space. These conditions are generally far away from experimental conditions of published OFR studies with high initial NO. In particular, studies of aerosol formation from vehicle emissions in OFRs often used OHR ext and NO in several orders of magnitude higher. Due to extremely high OHR ext and NO in , some studies may have resulted in substantial non-tropospheric photolysis, strong delay to RO 2 chemistry due to peroxynitrate formation, VOC reactions with NO 3 dominating over those with OH, and faster reactions of OH-aromatic adducts with NO 2 than those with O 2 , all of which are irrelevant to ambient VOC photooxidation chemistry. Some of the negative effects are the worst for alkene and aromatic precursors. To avoid undesired chemistry, vehicle emissions generally need to be diluted by a factor of > 100 before being injected into an OFR. However, sufficiently diluted vehicle emissions generally do not lead to high-NO chemistry in OFRs but are rather dominated by the low-NO RO 2 + HO 2 pathway. To ensure high-NO conditions without substantial atmospherically irrelevant chemistry in a more controlled fashion, new techniques are needed.

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“…Although OH is the primary radical initiator of hydrocarbon oxidation in the atmosphere, atomic chlorine can be an important initiator under certain circumstances and Cl atoms are often used as a convenient method of initiating oxidation in chamber studies . Chlorine is present in the atmosphere in various forms from a range of processes; although ambient Cl concentrations are much lower than OH, chlorine atoms can play an important role in atmospheric chemistry since their reactions with hydrocarbons are typically several orders of magnitude greater than the corresponding OH reaction.…”

Section: Introductionmentioning

confidence: 99%

Gas‐phase rate coefficients for a series of alkyl cyclohexanes with OH radicals and Cl atoms

Bejan

Winiberg

Mortimer

et al. 2018

Int J of Chemical Kinetics

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The rate coefficients of the reactions of OH radicals and Cl atoms with three alkylcyclohexanes compounds, methylcyclohexane (MCH), trans-1,4-dimethylcyclohexane (DCH), and ethylcyclohexane (ECH) have been investigated at (293 ± 1) K and 1000 mbar of air using relative rate methods. A majority of the experiments were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC), a stainless steel chamber using in situ FTIR analysis and online gas chromatography with flame ionization detection (GC-FID) detection to monitor the decay of the alkylcyclohexanes and the reference compounds. The studies were undertaken to provide kinetic data for calibrations of radical detection techniques in HIRAC. The following rate coefficients (in cm 3 molecule −1 s −1 ) were obtained for Cl reactions: k (Cl+MCH) = (3.51 ± 0.37) × 10 -10 , k (Cl+DCH) = (3.63 ± 0.38) × 10 −10 , k (Cl+ECH) = (3.88 ± 0.41) × 10 −10 , and for the reactions with OH radicals: k (OH+MCH) = (9.5 ± 1.3) × 10 -12 , k (OH+DCH) = (12.1 ± 2.2) × 10 −12 , k (OH+ECH) = (11.8 ± 2.0) × 10 −12 . Errors are a combination of statistical errors in the relative rate ratio (2 ) and the error in the reference rate coefficient. Checks for possible systematic errors were made by the use of two reference compounds, two different measurement techniques, and also three different sources of OH were employed in this study: photolysis of CH 3 ONO with black lamps, photolysis of H 2 O 2 at 254 nm, and nonphotolytic trans-2-butene ozonolysis. For DCH, some direct laser flash photolysis studies were also undertaken, producing results in good agreement with the relative rate measurements. Additionally, temperature-dependent rate coefficient investigations were performed for the reaction of methylcyclohexane with the OH radical over the range 273-343 K using the relative rate method; the resulting recommended Arrhenius expression is k (OH + MCH) = (1.85 ± 0.27) × 10 -11 exp((-1.62 ± 0.16) kJ mol −1 /RT) cm 3 molecule −1 s −1 . The kinetic data are discussed in terms of OH and Cl reactivity trends, and comparisons are made with the existing literature values and with rate coefficients from structure-activity relationship methods. This is the first study on the rate coefficient determination of the reaction of ECH with OH radicals and chlorine atoms, respectively. K E Y W O R D Scalibration of OH detector, kinetics, OH and Cl structure activity relationships, substituent effect 544

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A brief review of the use of environmental chambers for gas phase studies of kinetics, chemical mechanisms and characterisation of field instruments

Cited by 24 publications

References 60 publications

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

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

Modeling of the chemistry in oxidation flow reactors with high initial NO

Gas‐phase rate coefficients for a series of alkyl cyclohexanes with OH radicals and Cl atoms

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A brief review of the use of environmental chambers for gas phase studies of kinetics, chemical mechanisms and characterisation of field instruments

Cited by 24 publications

References 60 publications

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

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

Modeling of the chemistry in oxidation flow reactors with high initial NO

Gas‐phase rate coefficients for a series of alkyl cyclohexanes with OH radicals and Cl atoms

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Product

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

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