Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2022 Oct 13;126(42):7639–7649. doi: 10.1021/acs.jpca.2c04968

Kinetics and Product Branching Ratio Study of the CH3O2 Self-Reaction in the Highly Instrumented Reactor for Atmospheric Chemistry

Lavinia Onel †, Alexander Brennan †, Freja F Østerstro̷m †, Ellie Cooke †, Lisa Whalley †,‡, Paul W Seakins †, Dwayne E Heard †,*
PMCID: PMC9620170  PMID: 36227778

Abstract

graphic file with name jp2c04968_0004.jpg

The fluorescence assay by gas expansion (FAGE) method for the measurement of the methyl peroxy radical (CH3O2) using the conversion of CH3O2 into methoxy radicals (CH3O) by excess NO, followed by the detection of CH3O, has been used to study the kinetics of the self-reaction of CH3O2. Fourier transform infrared (FTIR) spectroscopy has been employed to determine the products methanol and formaldehyde of the self-reaction. The kinetics and product studies were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) in the temperature range 268–344 K at 1000 mbar of air. The product measurements were used to determine the branching ratio of the reaction channel forming methoxy radicals, rCH3O. A value of 0.34 ± 0.05 (errors at 2σ level) was determined for rCH3O at 295 K. The temperature dependence of rCH3O can be parametrized as rCH3O = 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}. An overall rate coefficient of the self-reaction of (2.0 ± 0.9) × 10–13 cm3 molecule–1 s–1 at 295 K was obtained by the kinetic analysis of the observed second-order decays of CH3O2. The temperature dependence of the overall rate coefficient can be characterized by koverall = (9.1 ± 5.3) × 10–14 × exp((252 ± 174)/T) cm3 molecule–1 s–1. The found values of koverall in the range 268–344 K are ∼40% lower than the values calculated using the recommendations of the Jet Propulsion Laboratory and IUPAC, which are based on the previous studies, all of them utilizing time-resolved UV–absorption spectroscopy to monitor CH3O2. A modeling study using a complex chemical mechanism to describe the reaction system showed that unaccounted secondary chemistry involving Cl species increased the values of koverall in the previous studies using flash photolysis to initiate the chemistry. The overestimation of the koverall values by the kinetic studies using molecular modulation to generate CH3O2 can be rationalized by a combination of underestimated optical absorbance of CH3O2 and unaccounted CH3O2 losses to the walls of the reaction cells employed.

Introduction

Methyl peroxy (CH3O2) radicals are key species in atmospheric oxidation1 and the combustion of volatile organic compounds.2,3 The chemistry of CH3O2 in the troposphere is typically dominated by the reaction with NO, particularly in environments influenced by anthropogenic NOx emissions (reaction R1). The reaction is a critical step in the tropospheric production of ozone in the presence of NO and converts NO into NO2:

graphic file with name jp2c04968_m001.jpg R1

The subsequent reaction of CH3O with O2 produces HO2, which then oxidizes another NO to NO2:

graphic file with name jp2c04968_m002.jpg R2
graphic file with name jp2c04968_m003.jpg R3

During the daytime the NO2 photolysis is the dominant tropospheric source of O3. In addition, the CH3O2 + NO reaction leads to the propagation of HOx and NOx radical chains. However, under low NOx levels the self-reaction of CH3O2 and the reactions of CH3O2 with HO2 and other organic peroxy (RO2) species are significant losses of CH3O2 and terminate the radical chain. The CH3O2 self-reaction occurs through two channels, reactions R4.a and R4.b:4

graphic file with name jp2c04968_m004.jpg R4.a
graphic file with name jp2c04968_m005.jpg R4.b

The methoxy radicals generated by channel R4.b subsequently react with oxygen (reaction R2) to form CH2O and HO2.

Despite its importance, the reported values for the rate coefficient of reaction R4, k4, at room temperature lie in a wide range from (2.7–5.2) × 10–13 cm3 molecule–1 s–1,5 with IUPAC5 and the Jet Propulsion Laboratory (JPL)6 giving 20–40% and 40–50%, respectively, uncertainties at the 2σ level for k4 in the temperature range 270–350 K. The previous kinetic studies used either the flash photolysis (FP) technique7−11 or the molecular modulation (MM) method12−14 to generate CH3O2 radicals, which were coupled to time-resolved UV–absorption spectroscopy to detect CH3O2 at fixed wavelengths in the range ∼210–270 nm (typically at 250 nm). As UV-absorption is a relatively insensitive technique, the detection limits of CH3O2 were high, for example around 4 × 1012 molecules cm–3,8,11 and the UV-absorption studies used high initial concentrations of CH3O2, 1013–1014 molecules cm–3 orders of magnitude.7−11

The kinetic studies of the CH3O2 self-reaction used photolytic mixtures of CH4/Cl2/O2 to generate CH3O2.7−14 CH3O formed by the reaction R4.b is rapidly removed via the reaction with O2 (reaction R2) in high concentrations (1017–1018 molecules cm–3 orders of magnitude)7−14 to generate HO2, which quickly reacts further with another CH3O2 radical (reaction R5).

graphic file with name jp2c04968_m006.jpg R5

As each HO2 radical consumes rapidly one CH3O2 species on the time scale of the CH3O2 self-reaction, the determination of the overall rate coefficient of the reaction R4, k4, requires knowledge of the branching ratios for reaction R4 (eq 1)7,11

graphic file with name jp2c04968_m007.jpg 1

where kobs is the second-order observed rate coefficient and rCH3O is the branching ratio of the reaction channel producing CH3O (reaction R4.b).

The branching ratios in the CH3O2 self-reaction were the subject of a number of experimental studies performed from the mid 1970s to 1990 inclusive, which were followed by the study of Tyndall et al. in 1998.15 The studies used photolysis of CH4/Cl2/O2 or (CH3)2N2/O2 and either end product detection employing mass spectrometry (MS),16 GC-MS,17 and infrared spectroscopy,15,18−20 or kinetic measurements using time-resolved UV–absorption spectroscopy.7

Some of the early studies reported a third channel of the self-reaction leading to CH3OOCH3 (reaction R4.c) with an insignificant contribution to the overall reaction rate coefficient at all temperatures—such as ≤0.08 at 297 K19 and ≤0.07 at 298 K18—with other studies finding no evidence for any contribution of peroxide formation.15,20

graphic file with name jp2c04968_m008.jpg R4.c

IUPAC5 and JPL6 use the evaluation of Tyndall et al.21 that recommends considering reactions R4.a and R4.b as the sole reaction channels of the self-reaction.

The majority of the branching ratios studies were carried out at room temperature15,16,18−20 and resulted in a range of values for the branching ratio of the reaction channel leading to CH3O (reaction R4.b): rCH3O = 0.22–0.45. Tyndall et al.21 revised the room temperature results to obtain rCH3O = 0.37 ± 0.06 at 298 K, which is recommended by IUPAC5 and JPL6. There have been two experimental studies of the temperature dependence of the branching ratios, which were conducted over different temperature ranges.7,20 Lightfoot et al.7 found a positive temperature dependence for rCH3O between 388 and 573 K using flash photolysis in combination with time-resolved UV–absorption spectroscopy. Horie et al.20 performed the only branching ratio experimental study covering temperatures below room temperature using matrix isolation Fourier transform infrared spectroscopy. The results were obtained from 223–333 K to show a positive temperature dependence for rCH3O. Tyndall et al.21 combined the results of Lightfoot et al.7 and Horie et al.20 with their recommended value at 298 K, rCH3O = 0.37 ± 0.06, and results published prior to 1990 to describe the ratio of the rate coefficients of the two reaction channels R4.a and R4.b asInline graphic. The evaluation of Tyndall et al.21 is recommended by JPL.6

This work reports on the determination of the branching ratios and the overall rate coefficient of the CH3O2 self-reaction in the temperature range of 268–344 K at 1000 mbar of synthetic air. The kinetic and branching ratio measurements were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC). Fourier Transform Infrared (FTIR) spectroscopy was employed to monitor the time profiles of the concentrations of CH2O and CH3OH produced by the CH3O2 self-reaction to determine the branching ratios of the two reaction channels R4.a and R4.b.

The fluorescence assay by gas expansion (FAGE) method for the selective and sensitive detection of CH3O2 radicals was used to study the kinetics of the self-reaction.22,23 The method involves the titration of CH3O2 to CH3O by reaction with added NO, followed by the detection of the resultant CH3O by off-resonant LIF with laser excitation at ca. 298 nm.22 The FAGE instrument was calibrated for CH3O2 using the 184.9 nm photolysis of water vapor in air to generate OH followed by the conversion of OH to known concentrations of CH3O2 by reaction with CH4 and O2.22,23 The 184.9 nm photolysis of water vapor is a well-established method of FAGE calibration for OH and HO2.24−26 The FAGE method for the CH3O2 detection has been validated previously using the direct and absolute near-IR Cavity Ring Down Spectroscopy (CRDS) method to detect CH3O2.23

FAGE measurements were carried out in the HIRAC chamber to determine the observed rate coefficient of the CH3O2 self-reaction, kobs, at 1000 mbar and temperatures in the range of 268–344 K. Using kobs and the branching ratio of the reaction channel leading to methoxy radicals, rCH3O, the overall rate coefficient of the self-reaction, k4, was derived. This is the first kinetic study of the CH3O2 self-reaction using a different detection method to that of UV–absorption spectroscopy.

Experimental Section

CH3O2 Generation in HIRAC

The HIRAC chamber is a stainless steel cylinder with an internal volume of ∼2.25 m3 and has been described in detail elsewhere.22,23,27,28 Four circulation fans mounted in pairs at each end of HIRAC are used to homogenize the gas mixture contained in the chamber. The photochemistry is initiated by eight UV lamps, each of them housed in a quartz tube. The quartz tubes are mounted radially inside the chamber (aligned parallel to the chamber longitudinal axis). In order to perform experiments at temperatures different to the room temperature a thermofluid (HUBE6479 DW-therm oil) is circulated from a high capacity thermoregulator (Huber Unistat 390W) through a series of stainless steel channels welded to the outside of the chamber. To ensure that the temperature is homogeneous within the chamber, the layout of these channels is evenly distributed on the chamber outer surface and HIRAC is lagged in a 20-mm-thick expanded neoprene.

The experiments were carried out at 268, 284, 295, 323, and 344 K and 1000 mbar of synthetic air obtained by mixing high purity oxygen (BOC, > 99.999%) and nitrogen (BOC, > 99.998%) in the ratio of O2:N2 = 1:4. CH4 (BOC, CP grade, 99.5%) and Cl2 (Sigma-Aldrich, ≥ 99.5%) were delivered to the chamber. Initial reagent concentrations in HIRAC were [CH4] = (2.0–3.0) × 1017 molecules cm–3 and [Cl2] = (0.3–5.5) × 1014 molecules cm–3. After adding the reagents into the chamber, the lamps (Phillips, TL-D36W/BLB, λ = 350–400 nm) were turned on to generate CH3O2 by Cl2 photolysis at ∼365 nm (reaction R6) followed by reactions R7 and R8. In the kinetic experiments the lamps were turned on for about 5 min, and then they were turned off to record the generated CH3O2 kinetic decay. In the experiments performed to determine the product branching ratio the lamps were turned on to measure CH3OH and CH2O using FTIR spectroscopy for a typical time of 20 min.

graphic file with name jp2c04968_m010.jpg R6
graphic file with name jp2c04968_m011.jpg R7
graphic file with name jp2c04968_m012.jpg R8

Fourier Transform Infrared (FTIR) Measurements

An in situ multipass FTIR (Bruker IFS66) arrangement along the long axis of HIRAC was used to measure the concentrations of CH2O and CH3OH produced during the times with the lamps turned on. The multipass Chernin arrangement within the chamber was optimized for 72 internal reflections giving an approximate total path length of 128.5 m.27,29 IR spectra were recorded every 30–60 s as the average of 30–100 scans at 1 cm–1 resolution. The concentration–time profiles for CH2O and CH3OH were obtained using the absorption at around 1740 cm–1 due to the stretch of the C=O bond of CH2O and at around 1030 cm–1 due to the C–O stretch of CH3OH and using reference spectra taken of formaldehyde and methanol. Reference spectra were taken delivering CH2O and CH3OH in known concentrations to the chamber under the same conditions as those used for the CH3O2 self-reaction experiments. The reference compound, either CH2O or CH3OH, was delivered in the vapor phase by direct heating of either liquid CH3OH or para-formaldehyde powder in a glass finger connected to a one liter stainless steel cylinder to achieve a gas pressure of a few mbar. Then the gas was delivered from the cylinder to the chamber using a flow of N2 (pN2 = 2000 mbar).

The reference spectra of CH2O and CH3OH were fitted to the observed IR absorbance recorded as a function of wavelength (λ) and time (t), Aobsλ,t, between ∼1600–1900 cm–1 and ∼900–1120 cm–1, respectively, at each time point to determine the changes to concentrations of CH2O and CH3OH vs time during the period of time with the lamps switched on (eq 2).

graphic file with name jp2c04968_m013.jpg 2

Here Arefλ(i) is the reference IR absorbance of species i as a function of λ and Inline graphic, where [i]t is the concentration of species i at reaction time t and [i]ref is the concentration of species i giving the reference spectrum. The species i is CH2O in the fit using λ ≅ 1600–1900 cm–1 and CH3OH in the fit over ∼900–1120 cm–1. The concentrations [i]t (i = CH2O or CH3OH) were then derived (eq 3).

graphic file with name jp2c04968_m015.jpg 3

FAGE Instrument and Calibration for CH3O2

Details on the HIRAC FAGE instrument are provided in previous publications.22,23,30 The instrument sampled gas through a 1-mm-diameter pinhole mounted on one end of a 50-mm-i.d. flow tube at a rate of ∼3 slpm. The pressure inside the sampling tube was maintained at 3.3 mbar for a chamber pressure of 1000 mbar of synthetic air. A CH3O fluorescence detection cell was integrated in the tube at ∼600 mm distance from the pinhole. About 25 mm prior to the detection cell, high purity NO (BOC, N2.5 nitric oxide) was injected at 2.5 sccm using a mass flow controller (Brooks 5850S) into the center of the gas flow to convert CH3O2 radicals into CH3O. CH3O radicals were subsequently detected by LIF spectroscopy, directing laser light at λonline ≅ 297.79 nm to excite the A2A1(ν3′ = 3) ← X2E(ν3″ = 0) transition of CH3O with a 5 kHz pulse repetition frequency through the cell at a right angle to the gas flow. The off-resonant red-shifted LIF (320–430 nm) was monitored using photon counting. The laser background was measured at a wavelength of λonline + 2.5 nm and then subtracted to obtain the fluorescence signal.

The FAGE technique requires calibration to convert the measured fluorescence signal into CH3O2 concentration. The calibration procedure has been described in detail previously22,23 and hence only the important points are presented here. OH radicals were generated photolyzing water vapor in synthetic air at 184.9 nm to react with methane in excess (BOC, CP grade, 99.5%) to generate CH3O2. The produced air/radical mixture was then sampled by the FAGE instrument. The concentration of CH3O2 was determined using eq 4.

graphic file with name jp2c04968_m016.jpg 4

Here σ is the absorption cross section of water vapor at 184.9 nm, (7.2 ± 0.2) × 10–20 cm2 molecule–1;31,32 Φ is the photodissociation quantum yield of OH at 184.9 nm (unity), t is the photolysis time, and F is the lamp flux at 184.9 nm, which was varied to generate a range of CH3O2 radical concentrations. The product F × t was determined employing chemical actinometry.28

The FAGE calibration factor was utilized to determine [CH3O2] in the HIRAC experiments:

graphic file with name jp2c04968_m017.jpg 5

where SCH3O2 (counts s–1 mW–1) is the recorded signal. Previous studies have shown that the FAGE sensitivity toward OH does not depend on the chamber temperature in the range 263–344 K,33 and thus the calibration factor determined at a room temperature of 295 K, CCH3O2 = (5.0 ± 1.7) × 10–10 counts cm3 molecule–1 s–1 mW–1, was used in the kinetic analysis at all temperatures.

Results

Product Branching Ratios in the CH3O2 Self-Reaction

To determine the branching ratios in the CH3O2 self-reaction (reactions R4.a and R4.b) the time profiles of the concentrations of the self-reaction products CH3OH and CH2O generated by turning the lamps on were recorded employing FTIR spectroscopy at a time intervals of 30–60 s. Over the first few minutes of the reaction [CH3OH] and [CH2O] increased linearly in time, showing that the removal of CH3OH and CH2O by secondary reactions is negligible. Figure S5 (Supporting Information) shows that at later reaction times [CH3OH] vs time and [CH2O] vs time curve down due to the secondary reactions of the products, predominantly the CH3OH + Cl and CH2O + Cl reactions. Numerical simulations carried out using a chemical mechanism described in the Supporting Information for the reaction system at 298 K show that, following about 25 s induction time, [CH3OH] and [CH2O] increase linearly during the first few minutes of the reaction (Figure S6).

Using the integrating rate ratio for the two parallel reactions R4.a and R4.b, eq 6 is obtained.

graphic file with name jp2c04968_m018.jpg 6

Here [CH2O]b is the concentration of CH2O formed by reaction R4.b followed by reaction R2. Taking into account that FTIR measures the sum of the concentrations of CH2O produced by the two channels, i.e., [CH2O]a produced by reaction R4.a and [CH2O]b obtained by reactions R4.b + R2, and [CH2O]a = [CH3OH], eq 7 is derived from eq 6. Equation 7 was employed in previous FTIR product studies of the CH3O2 self–reaction.15,19

graphic file with name jp2c04968_m019.jpg 7

Using eq 7 the branching ratio of the reaction channel R4.b, which produces CH3O, is given by

graphic file with name jp2c04968_m020.jpg 8

The branching ratio rCH3O was determined using [CH2O]overall and [CH3OH] measured at early reaction times, when the product concentrations increased linearly in time (Figure S5 in the Supporting Information). A number (6–20) of values of rCH3O were obtained at each temperature, 268, 284, 295, 323, and 344 K. Figure S7 (Supporting Information) shows that there was no trend with time in the extracted values over the initial few minutes used to determine rCH3O, and thus the secondary reactions of CH2O and CH3OH can be neglected in the analysis.

The mean values of rCH3O are shown in Figure 1 and Table S2 (Supporting Information). The results show a positive temperature dependence which can be characterized by rCH3O = 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}, i.e., k4.b/k4.a = (3.9 ± 1.1) × exp(−600 ± 85)/T. There have been two temperature dependence studies of the branching ratios previously, in the range 388–573 K7 and between 223–333 K.20 The result obtained by Horie et al.,20rCH3O = 1/{1 + [exp(1131 ± 30/T)]/(19 ± 5)}, is shown in Figure 1, as the temperature range used by these authors overlaps with the range of temperatures where the measurements reported in the present work were carried out. In addition, Figure 1 shows rCH3O derived from the evaluation of Tyndall et al.,21k4.b/k4.a = (26.2 ± 6.6) × exp[−(1130 ± 240)/T], which is recommended by the Jet Propulsion Laboratory (JPL) evaluation.6

Figure 1.

Figure 1

Open in a new tab

Product branching ratio for the channel giving CH3O of the CH3O2 self-reaction, rCH3O, as a function of temperature, T. The data obtained in this work are shown as open circles with the fit result, 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}, shown in black. The blue line and shading show the result of Horie et al.,20rCH3O = 1/{1 + [exp(1131 ± 30/T)]/(19 ± 5)} and the red line and shading show rCH3O vs T derived from k4.b/k4.a vs T evaluated by Tyndall et al.,21 (26.2 ± 6.6) × exp[−(1130 ± 240)/T] and recommended by the Jet Propulsion Laboratory evaluation.6 All the errors are given at the 2σ level.

The values found by this work have overlapping error limits with the results reported by Horie et al.20 and the values given by the recommendation of Tyndall et al.21 at the 2σ level. However, the temperature dependence measured by Horie et al.20 and the temperature dependence recommended by Tyndall et al.21 are steeper than the increase in the value of rCH3O with the temperature found in this study. The result at 295 K, rCH3O(this work) = 0.34 ± 0.05, is between the result of Horie et al.,20rCH3O(Horie et al.) = 0.29 ± 0.08, and the value recommended by Tyndall et al.,21rCH3O(Tyndall at al.) = 0.36 ± 0.12 (uncertainties at 2σ level).

The evaluation of Tyndall et al.21 is based on the results of Horie et al.20 between 223–333 K, Lightfoot et al.7 in the range 388–573 K, results published prior to 1990 at T ≥ 373 K, and the result of the evaluation of the room temperature values, rCH3O(298 K) = 0.37 ± 0.06.15 The study of Horie et al.20 is the single experimental study performed at temperatures in a range around room temperature, i.e., (293–70+40) K. The results of the present study agree well with the values reported by Horie et al.20 at 323 and 344 K (Figure 1). However, going down in temperature rCH3O(this work) is increasingly higher than rCH3O(Horie et al.).20

Horie et al.20 carried out flow tube experiments using photolysis of CH4/Cl2/O2 mixtures to measure the ratio [CH2O]/[CH3OH], where CH2O and CH3OH were produced by the CH3O2 self-reaction, employing matrix isolation Fourier transform infrared spectroscopy. The authors performed numerical simulations based on a complex model to vary the rate coefficients of the self-reaction channels to match the measured values for [CH2O]/[CH3OH]. The authors found no evidence for the formation of CH3OOCH3 by the reaction channel R4.c. However, two sets of numerical simulations were performed: assuming a branching ratio of 0.1 for reaction R4.c and excluding reaction R4.c from the chemical mechanism used in the numerical simulations. Figure 1 shows the reported temperature dependence derived averaging the results generated by the two sets of simulations.20 Considering a zero contribution for reaction R4.c, in line with the present recommendations,5,6 the reported values of rCH3O(Horie et al.) increase by 5% over the temperature range 268–344 K and are lower than rCH3O(this work) by 7% at 295 K, 14% at 284 K, and 22% at 268 K.

The present work measured [CH2O] and [CH3OH] in situ to determine rCH3O (eq 8), while Horie et al.20 trapped the reaction products outside the reaction cell in a CO2 matrix at 50 K to analyze them by IR spectroscopy to obtain [CH2O]/[CH3OH], which was then used in the determination of rCH3O. The concentrations of CH2O produced by the self-reaction were corrected taking into account CH2O formed in the matrix using a correction factor less than 10%. The lower values obtained for rCH3O(Horie et al.) relative to rCH3O(this work) at T ≤ 295 K can be explained by a process leading to the CH2O removal enhanced by reducing the reaction temperature which was not included in the reaction mechanism employed in the analysis performed by the authors.20 Horie et al.20 reported evidence of aerosol formation at 213 K resulting in unaccounted removal of CH2O leading to a value of [CH2O]/[CH3OH] lower than unity, a result which was not expected based on the reaction mechanism; the results obtained at 213 K were thus excluded from the analysis. The experiments below room temperature used in the determination of rCH3O—i.e., in the range 223–298 K—were reported “free” of aerosols. However, [CH2O] and [CH3OH] in the experiments were relatively large, a few times higher than in the present work, increasing the potential of oligomers/particle formation at low temperatures.

Kinetics of the CH3O2 Self-Reaction

Figure 2 shows examples of CH3O2 decay generated by turning the HIRAC lamps off following the production of CH3O2 by the Cl atom initiated oxidation of CH4 in the presence of O2 (reactions R6 and R7) at 323 K. Kinetic decays were obtained in a similar fashion at all temperatures. The CH3O2 decays at each temperature, 268, 284, 295, 323, and 344 K, measured using FAGE were fitted simultaneously to the integrated second-order rate law equation describing the CH3O2 self-reaction (R4)):

graphic file with name jp2c04968_m021.jpg 9

where [CH3O2]t is the methyl peroxy concentration at reaction time t, [CH3O2]0 is the initial concentration when the lights are switched off and kobs is the observed rate coefficient. In line with previous analysis,23 this work found that the loss of CH3O2 to the walls of HIRAC was negligible over the time scale of 0.5–1 min of the kinetic measurements at all temperatures employed, and hence a wall loss was not included in the kinetic analysis. Typically about 20 CH3O2 decays were fitted simultaneously at each temperature to obtain kobs with the results shown in Table 1.

Figure 2.

Figure 2

Open in a new tab

Examples of observations of CH3O2 (open circles) and fits to the data (solid lines) generated employing eq 9. The experiments used Cl2/CH4/O2 and black lamps (see main text for details); 323 K and 1000 mbar mixture of N2:O2 = 4:1. At time zero the lamps were turned off. [CH4]0 = 2.5 × 1017 molecules cm–3 for all the kinetic decays. Initial Cl2 concentrations: 3.3 × 1014 molecules cm–3 (red), 2.4 × 1014 molecules cm–3 (blue), and 7.5 × 1013 molecules cm–3 (magenta).

Table 1. Observed Rate Coefficient, kobs for the CH3O2 Self-Reaction Measured in This Work.

Temperature/K kobs (this work)a/cm3molecule–1 s–1
268 (3.2 ± 1.1) × 10–13
284 (2.9 ± 1.0) × 10–13
295 (2.7 ± 0.9) × 10–13
323 (3.4 ± 1.4) × 10–13
344 (2.6 ± 0.9) × 10–13
Open in a new tab
a

errors are 2σ.

The observed rate coefficient is larger than the second-order rate coefficient of just the CH3O2 recombination reaction (R4), k4, as the methoxy radicals generated by channel R4.b react rapidly with molecular oxygen, which is present in large excess, 5 × 1018 molecules cm–3, to produce HO2 (reaction R2), which in turn reacts with another CH3O2 radical (reaction R5). As each HO2 radical consumes one CH3O2 species (reaction R5) on the time scale of reaction R4, k4 is derived from kobs as follows:7,11

graphic file with name jp2c04968_m022.jpg 1

where rCH3O is the branching ratio for the reaction channel R4.b. The applicability of eq 1 in the analysis of the kinetic data generated by the HIRAC experiments was demonstrated by modeling the observed temporal decays using a variety of CH3O2 and HO2 concentrations representative for the HIRAC experiments and incorporating a heterogeneous loss of HO2 in the model relevant for the experiments:22,23kloss = 0.01–0.1 s–1. The results showed that the removal of HO2 by wall loss is negligible and thus can be excluded from the model.22

Figure 3 shows the determined temperature dependence for k4. At all temperatures employed, the values of k4 obtained using both kobs and rCH3O determined in this work are practically the same as the k4 values obtained using kobs determined in this work and rCH3O given by the evaluation of Tyndall et al.,21 which is recommended by JPL.6 Using the value of rCH3O = 0.34 ± 0.05 determined at 295 K in this work the rate coefficient of the overall reaction k4(295 K) = (2.0 ± 0.7) × 10–13 cm3 molecule–1 s–1 (uncertainties at 2σ level). Using the value of rCH3O(295 K) = 0.36 ± 0.12 recommended by Tyndall et al.21 does not change the result at this level of precision: k4(295 K) = (2.0 ± 0.9) × 10–13 cm3 molecule–1 s–1 (uncertainties quoted at 2σ level). The negative temperature dependence obtained employing rCH3O determined in this work can be characterized by k4 = (9.1 ± 5.3) × 10–14 × exp((252 ± 174)/T) cm3 molecule–1 s–1. Figure 3 compares the result of this work with k4 vs T recommended by JPL and IUPAC. The JPL and IUPAC recommendations are similar to each other: k4(JPL) = 9.5 × 10–14 × exp(390/T) cm3 molecule–1 s–16 and k4(IUPAC) = 1.03 × 10–13 × exp((365 ± 200)/T) cm3 molecule–1 s–1.5 On average, the results of this work are ∼40% lower than the values calculated using the JPL and IUPAC recommendations. However, the results of this work have overlapping error limits at the 2σ level with both recommendations.

Figure 3.

Figure 3

Open in a new tab

Temperature dependence of the overall rate coefficient of the CH3O2 self-reaction (R4), k4. The data generated using kobs and rCH3O obtained in this work (black circles) are plotted with the data generated using kobs measured by this work and rCH3O given by the evaluation of Tyndall et al.21 (open red circles) and k4 recommended by JPL (blue line and shading)6 and IUPAC (orange dashed line and shading).5 The blue and orange shadings show the 2σ uncertainties in the JPL and IUPAC recommendations. The results of the fit to the data are k4 = (9.1 ± 5.3) × 10–14 × exp((252 ± 174)/T) cm3 molecule–1 s–1 (black line) and k4 = (6.8 ± 4.1) × 10–14 × exp((335 ± 179)/T) cm3 molecule–1 s–1 (red line). The parametrization of the temperature dependence of k4 recommended by JPL and IUPAC are k4(JPL) = 9.5 × 10–14 × exp(390/T) cm3 molecule–1 s–16 and k4(IUPAC) = 1.03 × 10–13 × exp((365)/T) cm3 molecule–1 s–1.5

The previous studies upon which the JPL6 and IUPAC5 recommendations are based utilized the UV-absorption of CH3O2 at fixed wavelengths in the range ∼210–270 nm (typically 250 nm) usually to determine the ratio between the observed rate coefficient and the absorption cross-section of CH3O2, kobs/σCH3O2.5,6 The values used for σCH3O2 by the previous UV-absorption studies vary significantly, between (2.5–4.8) × 10–18 cm2 molecule–1 at 250 nm, leading to a large variation in kobs across the studies, which at 298 K ranges between kobs = (3.0–5.9) × 10–13 cm3 molecule–1 s–1.7−14 The 2020 JPL6 evaluation report recommends the cross sections obtained by the re-evaluation of Tyndall et al. in 200121 of the previous reported UV-absorption spectra. At 250 nm Tyndall et al.21 recommend σCH3O2(250 nm) = 3.8 × 10–18 cm2 molecule–1. Our calculations show that using σCH3O2 reported by Tyndall et al.21 and the ratios kobs/σCH3O2 found in the previous kinetic studies,7−11,13,14 the range of the values of kobs at 298 K is reduced to (4.1–5.1) × 10–13 cm3 molecule–1 s–1. The present result at 298 K, kobs = 2.9 × 10–13 cm3 molecule–1 s–1, is 30% smaller than the lowest value of 4.1 × 10–13 cm3 molecule–1 s–1 and 40% lower than the JPL6 and IUPAC5 recommendations of kobs = 4.8 × 10–13 cm3 molecule–1 s–1.

There is a significant difference between the results obtained here and the recommendations,5,6 and here we explore possible reasons for this discrepancy. The previous studies used either the flash photolysis (FP) technique7−11 or the molecular modulation (MM) method12−14 to generate CH3O2 radicals employing photolytic mixtures of CH4/Cl2/O2. The discrepancy between kobs determined in here and the results reported in the FP studies7−11 could be due to unaccounted secondary chemistry of CH3O2 due to the high radical concentrations, on the order of [CH3O2] of 1013–1014 molecules cm–3, and/or unaccounted spectral interferences. The MM experiments12−14 used 1–2 orders of magnitude lower concentrations of CH3O2 than [CH3O2] in the FP studies,7−11 i.e., concentrations on the order 1012 molecules cm–3, to minimize the impact of the secondary chemistry on kobs. However, as this method consisted of modulating the photolysis and hence the production of radicals by alternating the time with the lamps switched on and the time with the lamps turned off, a potential important source of error was the buildup of products absorbing in the UV range of the measurements (see below). The contributions of the absorbing products (see below) were subtracted from the overall absorbance measured by the MM experiments12,14 to extract the absorbance of CH3O2, and thus the extracted absorbance depended on the concentrations and the cross sections attributed to the products. Note that the LIF method is selective and more sensitive, with a limit of detection for CH3O2 of 2.0 × 109 molecules cm–3 for a signal-to-noise ratio of 2, 1 s averaging time of the online data points measured during the kinetic decay, and 60 s averaging period for the offline data points recorded at the end of the experiment. Therefore, the LIF method requires significantly lower radical concentrations than the FP and MM studies; here, [CH3O2]0 = (0.1–1) × 1012 molecules cm–3, which helps to minimize potential secondary chemistry.

The FP studies7−11 typically derived the kobs/σCH3O2 ratio fitting either eq 10 or eq 11 to the measured optical absorbance (At) or absorption coefficient (αt) at/around 250 nm.

graphic file with name jp2c04968_m023.jpg 10
graphic file with name jp2c04968_m024.jpg 11

Here A0 and α0 are the absorbance and the absorption coefficient at the time zero of the reaction, respectively, and l is the total optical path length.

To investigate the potential impact of the secondary chemistry on the CH3O2 kinetic decays in the FP studies7−11 numerical simulations were performed using a reaction system at 298 K described in the Supporting Information (Table S1). The previous kinetic studies of the CH3O2 recombination reaction7−14 did not investigate the impact of the CH3O2 reaction with the ClO radicals produced by the CH3O2 + Cl reaction on the CH3O2 kinetic decay. The kinetics of the CH3O2 + Cl reaction34,35 was studied in the years around 1995 and thus after all the previous kinetic studies of the CH3O2 self-reaction (1980–1990).5,6 The CH3O2 + Cl reaction is fast producing ClO (rate coefficient of 7.7 × 10–11 cm3 molecule–1 s–1 at 298 K).34 The generated ClO radicals predominantly react with CH3O2 with an overall rate coefficient of 2.4 × 10–12 cm3 molecule–1 s–1 at 298 K.6

The simulations used k4 = 2.1 × 10–13 cm3 molecule–1 s–1 as determined in this work and rCH3O = 0.37.5,6 The Supporting Information shows examples of the results of the numerical simulations using concentrations representative for the FP studies:7−11 [CH4]0 = 5 × 1017 molecules cm–3 and [Cl]0 = 1.4 × 1014 molecules cm–3. As the Cl + CH4 reaction is relatively slow (rate coefficient = 1.0 × 10–13 cm3 molecule–1 s–1 at 298 K)36 about 7% of the Cl atoms react with CH3O2 to produce ClO. Subsequently ClO predominantly reacts with CH3O2 (peak [CH3O2] = 1.0 × 1014 molecules cm–3 as shown in the Supporting Information). Therefore, fitting eq 8 to the CH3O2 temporal decay simulated using the mechanism which includes the secondary chemistry results in kobs(fit) = (3.8 ± 0.1) × 10–13 cm3 molecule–1 s–1. Using eq 9, k4(fit) = 2.8 × 10–13 cm3 molecule–1 s–1 is obtained, which is 33% higher than the value of k4 determined from this work and used as an input in the numerical simulations, k4(simulations) = 2.1 × 10–13 cm3 molecule–1 s–1. To investigate the sensitivity of the results to the presence of the CH3O2 + ClO reaction in the chemical mechanism used, the simulations with [CH4]0 = 5 × 1017 molecules cm–3 and [Cl]0 = 1.4 × 1014 molecules cm–3 were repeated, but this time in the absence of the CH3O2 + ClO reaction. Without the CH3O2 + ClO reaction, kobs(fit) = (3.0 ± 0.1) × 10–13 cm3 molecule–1 s–1 resulting in k4(fit) = 2.2 × 10–13 cm3 molecule–1 s–1 being obtained, which is almost the same with the value used as input in the simulations, k4(simulations) = 2.1 × 10–13 cm3 molecule–1 s–1. Hence, for [CH4]0 = 5 × 1017 molecules cm–3 and [Cl]0 = 1.4 × 1014 molecules cm–3, the values of kobs and k4 obtained from the fit to the simulated CH3O2 decay are considerably larger if the secondary chemistry is included to generate the decay. As the studies using the flash photolysis technique7−11 typically employed ∼[CH4]0 = 1017 molecules cm–3 and ∼[Cl]0 = 1014 molecules cm–3 the studies significantly overestimated kobs and thus k4.

In the present study the concentrations of [CH3O2], [Cl], and [ClO] were in a steady-state with the lamps turned on, and the CH3O2 decays were generated by switching the lamps off. Numerical simulations were performed over 5 min to mimic the chemistry with the lamps turned on using [CH4]0 = 3.0 × 1017 molecules cm–3 with [Cl2]0 = 3.0 × 1014 molecules cm–3 representative for the present study and adding the Cl2 photolysis to the chemistry mechanism including the CH3O2 + Cl and CH3O2 + ClO reactions (Supporting Information). After 5 min with the lamps on, [CH3O2] = 5 × 1011 molecules cm–3, [Cl] = 7.0 × 106 molecules cm–3, and [ClO] = 2.4 × 108 molecules cm–3. The concentrations of all the species in the chemistry mechanism obtained after 5 min were input into numerical simulations using the same chemistry mechanism except without Cl2 photolysis to mimic the chemistry during the time with the lamps turned off. Virtually all Cl atoms and ClO radicals were removed on a time scale of hundreds of microseconds and seconds, respectively. As [Cl]0 = 7.0 × 106 molecules cm–3 and [ClO]0 = 2.4 × 108 molecules cm–3, and hence orders of magnitude lower than [Cl]0 = 1.4 × 1014 molecules cm–3 and a peak of [ClO] = 1.0 × 1013 molecules cm–3 in the simulations using concentrations representative for the FP studies,7−11 the simulated CH3O2 decays were not impacted by the chlorine species secondary chemistry (Figure S3). The fit of eq 9 to the CH3O2 decay provided a value of kobs(fit) = (3.0 ± 0.1) × 10–13 cm3 molecule–1 s–1 at 298 K, which results in k4(fit) = 2.2 × 10–13 cm3 molecule–1 s–1, i.e., practically the same as the value measured by the present experiments k4 = 2.1 × 10–13 cm3 molecule–1 s–1. Therefore, no impact by the secondary chemistry of CH3O2 included in the simulations (Supporting Information) on the value determined for k4 was found in the present study. Hence in the absence of secondary chemistry the value of k4, as determined in this work, is considerably lower than k4 when the secondary chemistry is present as a result of much higher initial [Cl] concentrations.

We now consider potential spectral interferences in previous work which monitored CH3O2 concentrations using UV absorption. At the typical λ = 250 nm used to monitor the CH3O2 absorption by the FP studies,7−11 the cross-section of ClO, σClO = 3.5 × 10–18 cm2 molecule–16 and the cross-section of CH3O2, σCH3O2 = 3.8 × 10–18 cm2 molecule–121 are similar, and hence numerical simulations were used to investigate the impact of any ClO spectral interference on the results of the FP studies. The concentrations of the species generated by numerical simulations using the representative concentrations for the FP studies of [CH4]0 = 5 × 1017 molecules cm–3 and [Cl]0 = 1.4 × 1014 molecules cm–3 in the model that includes the CH3O2 + ClO reaction were multiplied with their respective cross sections6,21 to obtain the absorption coefficients of CH3O2 and ClO:

graphic file with name jp2c04968_m025.jpg 12

where αi, t is the absorption coefficient of species i (CH3O2 or ClO) at reaction time t, σi is the absorption cross-section of species i at 250 nm and [i]t is the concentration of species i at time t. Figure S4 in the Supporting Information shows the generated αCH3O2, t and αClO, t and their sum, Σαi, t = αCH3O2, t+ αClO, t. The results (Figure S4) show that there is a minor contribution of αClO, t to Σαi, t at the start of the reaction, i.e., ∼8% in the first millisecond of the reaction, which drops to 3% at t = 10 ms. The fit of the eq 11 (see above) to Σαi, t vs time resulted in kobs/σCH3O2. Using σCH3O2 = 3.8 × 10–18 cm2 molecule–121 a value of kobs = (3.9 ± 0.1) × 10–13 cm3 molecule–1 s–1 is obtained, which is 3% higher than kobs given fitting eq 9 to the temporal decay of [CH3O2] generated by the same numerical simulations (Figure S1). The result suggests that there was no significant optical interference due to the ClO absorption which impacted the previous determinations of kobs using the FP technique.7−11

The molecular modulation (MM) studies12−14 used the time-resolved modulated UV-absorption (absorption waveform) generated in the range 210–270 nm via switching the photolysis lamps on and off with a typical frequency of 10–1 Hz (order of magnitude) to determine kobs and σCH3O2. The absorption waveform consisted of an initial rise followed by a pseudo-steady-state and then a decay during the dark phase of the modulation period.12−14 The modulated absorption components depended on a relatively large number of parameters: illumination time, rate of Cl2 photolysis, kinetic parameters, and absorbing species cross sections.12−14 Photolytic mixtures of CH4/Cl2/O2 were flowed through the reactor to minimize the buildup of the products. However, the residence times of the gases in the reaction cell were relatively long, for example, 35 and 60 s.13,14 The contributions of the absorbing products accumulating over the photolysis cycles was calculated and subtracted from the observed absorption to derive the modulated absorption of CH3O2.12,14 The contribution of the products were important in the range 210–250 nm where the cross sections of HO2 (formed by reaction R2 and the reactions of Cl with the products CH3OH and CH2O) and CH3OOH (produced by the CH3O2 + HO2 reaction) increases rapidly with decreasing λ.6 However, the cross sections of HO2 and/or CH3OOH were significantly overestimated in the MM studies.12−14 The references cited for the cross-section of HO2 by Cox and Tyndall12 reported σHO2 larger than the JPL recommendations6 by 60–70% in the range 210–250 nm37 and by ∼10% between 210–220 nm.38 The cross-section of HO239 used by Simon et al.14 is ∼30% larger than the JPL recommendation6 between 210–240 nm and by ∼60% at 250 nm. Both the studies of Cox and Tyndall12 and Jenkin et al.13 employed values for σCH3OOH at least 40–50% higher than the JPL recommendation in the range 210–250 nm.6 An overestimation of the contributions of these species to the measured absorbance results in an underestimation of the CH3O2 absorbance, ACH3O2, which in turn results in an overestimation of kobs/σCH3O2 (eq 10).

In addition, the CH3O2 loss to the walls of the reaction cell were not accounted for by neither the FP studies7−11 nor the MM studies12−14 and could also result in an overestimation of kobs. As the CH3O2 self-reaction is slow, the wall-loss could significantly contribute to the overall CH3O2 removal in the previous studies.

Discussion of the FAGE Instrument Calibration

As LIF is not an absolute detection method, the FAGE instrument required calibration and a calibration factor, CCH3O2, is used to convert the measured signal, SCH3O2, to the CH3O2 concentration:

graphic file with name jp2c04968_m026.jpg 5

To calibrate FAGE, CH3O2 radicals were generated in known concentrations employing the 184.9 nm photolysis of water vapor in synthetic air followed by the complete conversion of the generated OH radicals to CH3O2 by reaction with CH4 in a large excess in the presence of O2. Previous work described in detail the water vapor method of calibration and the uncertainties in the calibration factor, CCH3O2(water vapor method).22 As seen previously,22 in this work an overall 34% error at 2σ level was obtained for CCH3O2(water vapor method) combining the systematic and statistical uncertainties. Similar overall errors, 31% and 36%, were reported for CHO2(water vapor method) previously.28,30

The photolysis of water vapor at 184.9 nm represents the most common method used to generate accurate concentrations of OH and HO2. The method has been applied for many years for the calibration of FAGE instruments.24−26 The reliability of the method has been confirmed by intercomparisons with alternative methods of calibration for OH and HO2. The calibration of OH using the water vapor photolysis and the OH calibration based on the generation of OH by ozone reactions with alkenes have been found to agree within their experimental uncertainties.40 Very good agreement (difference within 1–13%) has been obtained by comparing the OH measurements in the SAPHIR atmospheric simulation chamber using a number of FAGE instruments and instruments employing differential optical laser absorption spectroscopy (DOAS) and chemical ionization mass spectrometry (CIMS).41,42 In the case of HO2, CHO2(water vapor method) and the calibration factor obtained analyzing the kinetic decay of HO2 by its self-reaction generated in HIRAC, CHO2(kinetic method), were found in a very good agreement (difference within 8%).28,30 However, a discrepancy within ∼40% was found between CCH3O2(water vapor method) and the CH3O2 calibration factor determined using the kinetics of the second-order recombination of CH3O2 observed in HIRAC and kobs(298 K) = 4.8 × 10–13 cm3 molecule–1 s–1,5,6CCH3O2(kinetic method).22,23 As the error in the fraction of OH which is converted to CH3O2 upon the addition of methane in the water vapor method is minor (4% at 2σ level),22 the discrepancy between the two calibration methods can be attributed to an overestimation of the reported value of kobs for the CH3O2 self-reaction at 298 K.5,6 The ∼40% difference in CCH3O2(kinetic method) and CCH3O2(water vapor method) resulted in different values for the gradient of the correlation plot of [CH3O2] measured by FAGE (y-axis) as a function of [CH3O2] measured by near-infrared cavity ring down spectroscopy, CRDS (x-axis), at 1000 mbar of synthetic air using the sensitivities from the two methods of calibration of FAGE: 1.35 ± 0.07 (water vapor calibration) and 0.92 ± 0.05 (kinetic method of calibration).23 The results show a significantly better agreement with the kinetic method than with the water vapor method. A very good level of agreement between the FAGE and CRDS measurements of CH3O2 was also obtained using the kinetic method for FAGE calibration at 100 mbar of synthetic air and 80 mbar of 3:1 He:O2 mixture. The very good agreement achieved under all conditions when the kinetic method was employed for the FAGE calibration was expected as the kinetic method was also used to determine the absorption cross section of CH3O2 from the temporal decays of the optical absorption coefficient of CH3O2 and hence calibrate the CRDS method. Therefore, with both FAGE and CRDS calibrated using the same method, the intercomparison was not subject to any error in the rate coefficient, kobs for the CH3O2 self-reaction, and the obtained very good agreement provides a validation of the FAGE (water vapor) method to determine concentrations of CH3O2. The present result at 298 K, kobs = 2.9 × 10–13 cm3 molecule–1 s–1, shows a 40% reduction in the reported value of kobs = 4.8 × 10–13 cm3 molecule–1 s–1.5,6 A reduction of 40% in the reported kobs would bring [CH3O2]CRDS generated using the kinetic method of calibration into agreement with [CH3O2]FAGE determined using the water vapor method.23 Therefore, the FAGE–CRDS intercomparison also suggests that kobs = 2.9 × 10–13 cm3 molecule–1 s–1 and thus k4 = 2.1 × 10–13 cm3 molecule–1 s–1 at 298 K, consistent with the values found in the present study.

Conclusions

Experiments were carried out in the range 268–344 K and at 1000 mbar to measure CH3OH and CH2O generated by the CH3O2 self-reaction in HIRAC using in situ FTIR detection to determine the product branching ratios in the self-reaction. The chemistry was initiated using photolysis of Cl2/CH4/N2/O2 mixtures (photolysis range: λ = 350–400 nm). The temperature dependence of the product branching ratio of the reaction channel producing CH3O can be described as rCH3O = 1/{1 + [exp(600 ± 170)/T]/(3.9 ± 2.2)}. At 295 K rCH3O(this work) = 0.34 ± 0.05, in agreement with the recommendations of JPL6 and IUPAC:5rCH3O(JPL) = 0.36 ± 0.12 and rCH3O(IUPAC) = 0.36 ± 0.17. This is the second experimental study of the temperature dependence of the product branching ratios in a range including temperatures relevant for atmospheric chemistry. The positive temperature dependence found for rCH3O is less marked than the increase in rCH3O with temperature measured by the previous study performed in a temperature range around 298 K (223–333 K), which used matrix isolation FTIR.20 The results of the present work for product branching agree well with the values obtained by Horie et al.20 at 295, 323, and 344 K. By decreasing the temperature, the present results are increasingly higher than the results reported by Horie et al.20 with a positive deviation of 22% at 268 K.

The kinetics of the CH3O2 self-reaction has been studied coupling a FAGE instrument to HIRAC to carry out time-resolved measurements of the CH3O2 concentrations during the reaction. Second-order decays of CH3O2 were generated by turning the chamber lamps off. The observed rate coefficient at 295 K and 1000 mbar was kobs = (2.7 ± 0.9) × 10–13 cm3 molecule–1 s–1. Using kobs(295 K) = k4(1 + rCH3O) with rCH3O(this work, 295 K) = 0.34 ± 0.05 the second-order rate coefficient for the self-reaction at 295 K and 1000 mbar is k4 = (2.0 ± 0.7) × 10–13 cm3 molecule–1 s–1; employing the recommended value of 0.36 for rCH3O at 295 K by JPL6 and IUPAC5 does not change the result. The result at 295 K is is ∼40% lower than the IUPAC and JPL recommendations: k4 = 3.5 × 10–13 cm3 molecule–1 s–1. The temperature dependence of the overall rate coefficient can be parametrized as k4 = (9.1 ± 5.3) × 10–14 × exp((252 ± 174)/T) cm3 molecule–1 s–1. The present results have overlapping error limits at the 2σ level with both JPL and IUPAC recommendations. However, on average the results of this work are ∼40% lower than the values calculated using the JPL6 recommendation for the temperature dependence (k4 = 9.5 × 10–14 × exp(390/T) cm3 molecule–1 s–1) and the values obtained employing the temperature dependence recommended by IUPAC,5k4 = 1.03 × 10–13 × exp((365 ± 200)/T) cm3 molecule–1 s–1.

The previous kinetic studies utilized UV–absorption spectroscopy and may be impacted by secondary chemistry owing to the high radical concentrations generated in the reaction mixtures. Chemical modeling using the conditions of the previous studies and which included secondary chemistry of Cl species showed that the secondary chemistry increases the value of k4 obtained significantly. The FAGE method detects CH3O2 sensitively, with a limit of detection of 2.0 × 109 molecules cm–3 for a signal-to-noise ratio of 2, 1 s online averaging time, and 60 s offline averaging period. Therefore, the experiments reported here required a few orders of magnitude lower concentrations of CH3O2 than [CH3O2] used in the UV–absorption studies and the impact of the secondary chemistry on the kinetic decays obtained by this work is negligible. In addition, the FAGE method probes CH3O2 selectively, in the absence of any interference from other species.

Numerical models predict that CH3O2 is the most abundant RO2 species in the atmosphere. Even though CH3O2 has not been selectively measured in the atmosphere so far, its concentration at daytime has been estimated to peak in the range of daytime peak [HO2], i.e., at 107–108 molecules cm–3.43−45 The atmospheric fate of CH3O2 is typically dominated by the reaction with NO, with the CH3O2 + HO2 reaction becoming the main daytime loss of CH3O2 under low NOx levels. As CH3O2 and HO2 reach similar levels at daytime and 298K, kCH3O2+HO2 (5.2 × 10–12 cm3 molecule–1 s–1) is ∼15 times faster than the JPL and IUPAC recommendations for k4 (3.5 × 10–13 cm3 molecule–1 s–1)5,6 and ∼25 times higher than k4(CH3O2+CH3O2) determined in this work (2.1 × 10–13 cm3 molecule–1 s–1) the inclusion of the present kobs(CH3O2+CH3O2) value in the atmospheric models might not impact significantly the daytime radical budget predicted by the models. However, at night-time it has been predicted that the self-reaction is the dominant removal of CH3O2 due to a rapid loss of HO2 under dark conditions46 and thus the atmospheric impacts of the present result need to be investigated.

Acknowledgments

The authors would like to thank the Natural Environment Research Council (NERC) for funding (grant reference NE/M011208/1) and the National Centre for Atmospheric Science. AB thanks to NERC for a studentship awarded in the framework of the SPHERES doctoral training programme (NE/L002574/1). F.F.Ø. thanks the Carlsberg Foundation for support through the Carlsberg Foundation Internationalisation Fellowship, grant numbers CF16-0493 and CF17-0608.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.2c04968.

  • Results of numerical simulations using a complex chemistry system and details of determination of the branching ratios in the CH3O2 self-reaction as a function of temperature (PDF)

Author Present Address

# School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA and Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark

The authors declare no competing financial interest.

Special Issue

Published as part of The Journal of Physical Chemistry virtual special issue “Advances in Atmospheric Chemical and Physical Processes”.

Supplementary Material

jp2c04968_si_001.pdf (685.9KB, pdf)

References

  1. Orlando J. J.; Tyndall G. S. Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance. Chem. Soc. Rev. 2012, 41 (19), 6294–6317. 10.1039/c2cs35166h. [DOI] [PubMed] [Google Scholar]
  2. Kohse-Hoinghaus K. Combustion in the future: The importance of chemistry. Proc. Combust. Inst. 2021, 38 (1), 1–56. 10.1016/j.proci.2020.06.375. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Zador J.; Taatjes C. A.; Fernandes R. X. Kinetics of elementary reactions in low-temperature autoignition chemistry. Prog. Energy Combust. 2011, 37 (4), 371–421. 10.1016/j.pecs.2010.06.006. [DOI] [Google Scholar]
  4. Tyndall G. S.; Wallington T. J.; Ball J. C. FTIR product study of the reactions CH3O2+CH3O2 and CH3O2+O3. J. Phys. Chem. A 1998, 102 (15), 2547–2554. 10.1021/jp972784h. [DOI] [Google Scholar]
  5. Atkinson R.; Baulch D. L.; Cox R. A.; Crowley J. N.; Hampson R. F.; Hynes R. G.; Jenkin M. E.; Rossi M. J.; Troe J. Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II - gas phase reactions of organic species. Atmos. Chem. Phys. 2006, 6, 3625–4055. 10.5194/acp-6-3625-2006. [DOI] [Google Scholar]
  6. Burkholder J. B.; Sander S. P.; Abbatt J. P. D.; Barker J. R.; Cappa C.; Crounse J. D.; Dibble T. S.; Huie R. E.; Kolb C. E.; Kurylo, et al. Chemical kinetics and photochemical data for use in atmospheric studies - Evaluation number 19. available at http://jpldataeval.jpl.nasa.gov/, Access 10 May 2022.
  7. Lightfoot P. D.; Lesclaux R.; Veyret B. Flash photolysis study of the CH3O2 + CH3O2 reaction: Rate constants and branching ratios from 248 to 573 K. J. Phys. Chem. 1990, 94 (2), 700–707. 10.1021/j100365a035. [DOI] [Google Scholar]
  8. Sander S. P.; Watson R. T. Kinetic studies of the reactions of CH3O2 with NO, NO2 and CH3O2 at 298 K. J. Phys. Chem. 1980, 84 (13), 1664–1674. 10.1021/j100450a002. [DOI] [Google Scholar]
  9. McAdam K.; Veyret B.; Lesclaux R. UV absorption spectra of HO2 and CH3O2 radicals and the kinetics of their mutual reactions ant 298 K. Chem. Phys. Lett. 1987, 133 (1), 39–44. 10.1016/0009-2614(87)80049-0. [DOI] [Google Scholar]
  10. Kurylo M. J.; Wallington T. J. The temperature dependence of the rate constant for the gas phase disproportionation reaction of CH3O2 radicals. Chem. Phys. Lett. 1987, 138 (6), 543–547. 10.1016/0009-2614(87)80121-5. [DOI] [Google Scholar]
  11. Sander S. P.; Watson R. T. Temperature dependence of the self-reaction of CH3O2 radicals. J. Phys. Chem. 1981, 85 (20), 2960–2964. 10.1021/j150620a023. [DOI] [Google Scholar]
  12. Cox R. A.; Tyndall G. S. Rate constants for the reactions of CH3O2 with HO2, NO and NO2 using molecular modulation spectrometry. J. Chem. Soc. Faraday Trans II 1980, 76, 153–163. 10.1039/f29807600153. [DOI] [Google Scholar]
  13. Jenkin M. E.; Cox R. A.; Hayman G. D.; Whyte L. J. Kinetic study of the reactions CH3O2 + CH3O2 and CH3O2 + HO2 using molecular modulation spectroscopy. J. Chem. Soc. Faraday Trans. II 1988, 84, 913–930. 10.1039/f29888400913. [DOI] [Google Scholar]
  14. Simon F. G.; Schneider W.; Moortgat G. K. UV absorption spectrum of the methylperoxy radical and the kinetics of its disproportionation reaction at 300 K. Int. J. Chem. Kinet. 1990, 22 (8), 791–812. 10.1002/kin.550220802. [DOI] [Google Scholar]
  15. Tyndall G. S.; Wallington T. J.; Ball J. C. FTIR product study of the reactions CH3O2 + CH3O2 and CH3O2 + O3. J. Phys. Chem. A 1998, 102 (15), 2547–2554. 10.1021/jp972784h. [DOI] [Google Scholar]
  16. Weaver J.; Meagher J.; Shortridge R.; Heicklen J. Oxidation of acetyl radicals. J. Photochem. 1975, 4 (5–6), 341–360. 10.1016/0047-2670(75)85017-9. [DOI] [Google Scholar]
  17. Selby K.; Waddington D. J. Reactions of oxygenated radicals in the gas-phase 0.4. reactions of methylperoxyl and methoxyl radicals. J. Chem. Soc.-Perkin Trans. 2 1979, 9, 1259–1263. 10.1039/p29790001259. [DOI] [Google Scholar]
  18. Kan C. S.; Calvert J. G.; Shaw J. H. Reactive channels of the CH3O2-CH3O2 reaction. J. Phys. Chem. 1980, 84 (25), 3411–3417. 10.1021/j100462a020. [DOI] [Google Scholar]
  19. Niki H.; Maker P. D.; Savage C. M.; Breitenbach L. P. Fourier transform infrared studies of the self-reaction of CH3O2 radicals. J. Phys. Chem. 1981, 85 (7), 877–881. 10.1021/j150607a028. [DOI] [Google Scholar]
  20. Horie O.; Crowley J. N.; Moortgat G. K. Methylperoxy relf-reaction: Products and branching ratio between 223 and 333 K. J. Phys. Chem. 1990, 94 (21), 8198–8203. 10.1021/j100384a040. [DOI] [Google Scholar]
  21. Tyndall G. S.; Cox R. A.; Granier C.; Lesclaux R.; Moortgat G. K.; Pilling M. J.; Ravishankara A. R.; Wallington T. J. Atmospheric chemistry of small organic peroxy radicals. J. Geophys. Res. -Atmos. 2001, 106 (D11), 12157–12182. 10.1029/2000JD900746. [DOI] [Google Scholar]
  22. Onel L.; Brennan A.; Seakins P. W.; Whalley L.; Heard D. E. A new method for atmospheric detection of the CH3O2 radical. Atmos. Meas. Technol. 2017, 10 (10), 3985–4000. 10.5194/amt-10-3985-2017. [DOI] [Google Scholar]
  23. Onel L.; Brennan A.; Gianella M.; Hooper J.; Ng N.; Hancock G.; Whalley L.; Seakins P. W.; Ritchie G. A. D.; Heard D. E. An intercomparison of CH3O2 measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry). Atmos. Meas. Technol. 2020, 13 (5), 2441–2456. 10.5194/amt-13-2441-2020. [DOI] [Google Scholar]
  24. Heard D. E.; Pilling M. J. Measurement of OH and HO2 in the troposphere. Chem. Rev. 2003, 103 (12), 5163–5198. 10.1021/cr020522s. [DOI] [PubMed] [Google Scholar]
  25. Stone D.; Whalley L. K.; Heard D. E. Tropospheric OH and HO2 radicals: field measurements and model comparisons. Chem. Soc. Rev. 2012, 41 (19), 6348–6404. 10.1039/c2cs35140d. [DOI] [PubMed] [Google Scholar]
  26. Wang G. Y.; Iradukunda Y.; Shi G. F.; Sanga P.; Niu X. L.; Wu Z. J. Hydroxyl, hydroperoxyl free radicals determination methods in atmosphere and troposphere. J. Environ. Sci. 2021, 99, 324–335. 10.1016/j.jes.2020.06.038. [DOI] [PubMed] [Google Scholar]
  27. Glowacki D. R.; Goddard A.; Hemavibool K.; Malkin T. L.; Commane R.; Anderson F.; Bloss W. J.; Heard D. E.; Ingham T.; Pilling M. J.; et al. Design of and initial results from a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC). Atmospheric Chemistry and Physics 2007, 7 (20), 5371–5390. 10.5194/acp-7-5371-2007. [DOI] [Google Scholar]
  28. Winiberg F. A. F.; Smith S. C.; Bejan I.; Brumby C. A.; Ingham T.; Malkin T. L.; Orr S. C.; Heard D. E.; Seakins P. W. Pressure-dependent calibration of the OH and HO2 channels of a FAGE HOx instrument using the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC). Atmos. Meas. Technol. 2015, 8 (2), 523–540. 10.5194/amt-8-523-2015. [DOI] [Google Scholar]
  29. Winiberg F. A. F.; Dillon T. J.; Orr S. C.; Gross C. B. M.; Bejan I.; Brumby C. A.; Evans M. J.; Smith S. C.; Heard D. E.; Seakins P. W. Direct measurements of OH and other product yields from the HO2 + CH3C(O)O2 reaction. Atmos. Chem. Phys. 2016, 16 (6), 4023–4042. 10.5194/acp-16-4023-2016. [DOI] [Google Scholar]
  30. Onel L.; Brennan A.; Gianella M.; Ronnie G.; Aguila A. L.; Hancock G.; Whalley L.; Seakins P. W.; Ritchie G. A. D.; Heard D. E. An intercomparison of HO2 measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry). Atmos. Meas. Technol. 2017, 10 (12), 4877–4894. 10.5194/amt-10-4877-2017. [DOI] [Google Scholar]
  31. Cantrell C. A.; Zimmer A.; Tyndall G. S. Absorption cross sections for water vapor from 183 to 193 nm (vol 24, pg 2195, 1997). Geophys. Res. Lett. 1997, 24 (21), 2687–2687. 10.1029/97GL02803. [DOI] [Google Scholar]
  32. Creasey D. J.; Heard D. E.; Lee J. D. Absorption cross-section measurements of water vapour and oxygen at 185 nm. Implications for the calibration of field instruments to measure OH, HO2 and RO2 radicals. Geophys. Res. Lett. 2000, 27 (11), 1651–1654. 10.1029/1999GL011014. [DOI] [Google Scholar]
  33. Winiberg F. A. F.Characterisation of FAGE apparatus for HOx detection and application in an environmental chamber, PhD. thesis. University of Leeds, Leeds, UK, 2014. [Google Scholar]
  34. Maricq M. M.; Szente J. J.; Kaiser E. W.; Shi J. C. Reaction of chlorine atoms with methylperoxy and ethylperoxy radicals. J. Phys. Chem. 1994, 98 (8), 2083–2089. 10.1021/j100059a017. [DOI] [Google Scholar]
  35. Jungkamp T. P. W.; Kukui A.; Schindler R. N. Determination of rate constants and product branching ratios for the reactions of CH3O2 and CH3O with cl atoms at room temperature. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 1995, 99 (8), 1057–1066. 10.1002/bbpc.199500031. [DOI] [Google Scholar]
  36. Atkinson R.; Baulch D. L.; Cox R. A.; Crowley J. N.; Hampson R. F.; Hynes R. G.; Jenkin M. E.; Rossi M. J.; Troe J.; Wallington T. J. Evaluated kinetic and photochemical data for atmospheric chemistry: Volume IV - gas phase reactions of organic halogen species. Atmos. Chem. Phys. 2008, 8 (15), 4141–4496. 10.5194/acp-8-4141-2008. [DOI] [Google Scholar]
  37. Hochanadel C. J.; Ogren P. J.; Ghormley J. A. Absorption spectrum and reaction kinetics of HO2 radical in gas-phase. J. Chem. Phys. 1972, 56 (9), 4426. 10.1063/1.1677885. [DOI] [Google Scholar]
  38. Paukert T. T.; Johnston H. S. Spectra and kinetics of hydroperoxyl free radical in gas phase. J. Chem. Phys. 1972, 56 (6), 2824–2838. 10.1063/1.1677613. [DOI] [Google Scholar]
  39. Moortgat G.; Veyret B.; Lesclaux R. Absorption spectrum and kinetics of reactions of the acetylperoxy radical. J. Phys. Chem. 1989, 93 (6), 2362–2368. 10.1021/j100343a032. [DOI] [Google Scholar]
  40. Dusanter S.; Vimal D.; Stevens P. S. Technical note: Measuring tropospheric OH and HO2 by laser-induced fluorescence at low pressure. A comparison of calibration techniques. Atmos. Chem. Phys. 2008, 8 (2), 321–340. 10.5194/acp-8-321-2008. [DOI] [Google Scholar]
  41. Schlosser E.; Brauers T.; Dorn H. P.; Fuchs H.; Haseler R.; Hofzumahaus A.; Holland F.; Wahner A.; Kanaya Y.; Kajii Y.; et al. Technical Note: Formal blind intercomparison of OH measurements: results from the international campaign HOxComp. Atmospheric Chemistry and Physics 2009, 9 (20), 7923–7948. 10.5194/acp-9-7923-2009. [DOI] [Google Scholar]
  42. Fuchs H.; Dorn H. P.; Bachner M.; Bohn B.; Brauers T.; Gomm S.; Hofzumahaus A.; Holland F.; Nehr S.; Rohrer F.; T; et al. Comparison of OH concentration measurements by DOAS and LIF during SAPHIR chamber experiments at high OH reactivity and low NO concentration. Atmos. Meas. Technol. 2012, 5 (7), 1611–1626. 10.5194/amt-5-1611-2012. [DOI] [Google Scholar]
  43. Whalley L. K.; Edwards P. M.; Furneaux K. L.; Goddard A.; Ingham T.; Evans M. J.; Stone D.; Hopkins J. R.; Jones C. E.; Karunaharan A.; et al. Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest. Atmos. Chem. Phys. 2011, 11 (14), 7223–7233. 10.5194/acp-11-7223-2011. [DOI] [Google Scholar]
  44. Whalley L. K.; Furneaux K. L.; Goddard A.; Lee J. D.; Mahajan A.; Oetjen H.; Read K. A.; Kaaden N.; Carpenter L. J.; Lewis; et al. The chemistry of OH and HO2 radicals in the boundary layer over the tropical Atlantic Ocean. Atmos. Chem. Phys. 2010, 10 (4), 1555–1576. 10.5194/acp-10-1555-2010. [DOI] [Google Scholar]
  45. Whalley L. K.; Stone D.; Dunmore R.; Hamilton J.; Hopkins J. R.; Lee J. D.; Lewis A. C.; Williams P.; Kleffmann J.; Laufs S.; et al. Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo). Atmos. Chem. Phys. 2018, 18 (4), 2547–2571. 10.5194/acp-18-2547-2018. [DOI] [Google Scholar]
  46. Monks P. S.; Carpenter L. J.; Penkett S. A.; Ayers G. P. Night-time peroxy radical chemistry in the remote marine boundary layer over the Southern ocean. Geophys. Res. Lett. 1996, 23 (5), 535–538. 10.1029/96GL00306. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

jp2c04968_si_001.pdf (685.9KB, pdf)

Articles from The Journal of Physical Chemistry. a are provided here courtesy of American Chemical Society

RESOURCES