Cl-Initiated oxidation of methacrolein under NO_x-free conditions studied by VUV photoionization mass spectrometry

Abstract

The Cl-initiated oxidation of methacrolein (MACR, C₄H₆O) under NO_x-free conditions has been investigated in a fast flow tube by using a home-made vacuum ultraviolet (VUV) photoionization mass spectrometer complemented by high-level theoretical calculations. The key species such as intermediates and radicals together with products involved in the oxidation are observed online and confirmed in photoionization mass spectra. The reaction potential energy surfaces of the transient C₄H₅O and C₄H₆OCl radicals, formed from the hydrogen-abstraction reaction and the addition reaction of MACR with Cl atoms, with oxygen have been theoretically calculated to illuminate the formation of the peroxy radicals of C₄H₅OO₂ and C₄H₆OClO₂. The photoionization processes of these peroxy radicals, whose cations are not stable, and their individual self-reactions as well as bimolecular reactions with HO₂ radical are studied and discussed. In addition, kinetic experiments are also performed to get the time evolution of specific products and compared with theoretical models, providing a detailed insight into the reaction mechanism of the Cl-initiated oxidation of MACR.

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1. Introduction

Methacrolein (MACR, C₄H₆O) is a key reaction intermediate in atmospheric chemistry and plays an essential role in the atmospheric oxidation of isoprene, the most abundant non-methane volatile organic compound (VOC) in the atmosphere.^1,2 MACR is also an important VOC in the atmosphere and its global emission can approach ∼45 Tg year⁻¹.³ In particular, due to its essential role involved in the oxidation of isoprene, the atmospheric fate of MACR has attracted a great deal of attention in the past decades.^1,4

The atmospheric fate of MACR is dominated by its reaction with the hydroxyl radical (OH), and mainly proceeds via two pathways with approximately equal rates: (i) the addition of OH to the C=C double bond of MACR to generate the OH–MACR adduct radical, and (ii) the abstraction of the aldehyde H-atom to produce the acyl radical, C₄H₅O.^5,6 In the atmosphere, these radicals can react with O₂ and lead to the production of the hydroxyl peroxy radical and the acyl peroxy radical. In addition, these transient peroxy radicals can perform bimolecular reactions with NO_x (NO and NO₂), with themselves or other peroxy radicals (RO₂), with HO₂ or OH radicals depending on the site and time, and contribute to air quality and regional climate.^7–12

In recent years, researchers found that halogen radicals, particularly chlorine (Cl) atoms formed from photolysis of chlorine (Cl₂), nitrosyl chloride (ClNO), nitryl chloride (ClNO₂) and bromine chloride (BrCl), can also have important impacts on the atmospheric degradation of VOCs.^13–16 Among them, Cl atoms can efficiently oxidize VOCs, and the reaction rate constants of Cl atoms with VOCs can be ten times larger than those with OH.^15–18 For example, the reaction rate constant of MACR with Cl atoms was measured at 3.3 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, much larger than that of MACR with OH (3.4 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹).^19,20 Therefore, considering the concentrations of OH radicals (∼10⁶ molecules cm⁻³) and Cl atoms (∼3 × 10⁵ molecules cm⁻³ during daytime and up to ∼8 × 10⁶ molecules cm⁻³ in the marine boundary layer),^21,22 the Cl-initiated oxidation of MACR can play a significant role in the atmosphere next to that initiated with OH, at least in the marine boundary layer, and has been the subject of experimental and theoretical studies.^20,23

Similar to the case with the OH radical, the reaction of MACR with Cl atoms can proceed via two pathways too: the addition of Cl atom to the C=C bond of MACR to get the C₄H₆OCl adduct radical, and the hydrogen abstraction to produce the C₄H₅O radical.¹⁹ Previous theoretical calculations show that the Cl-addition to the external carbon of the C=C double bond is the main pathway with a predicted branching ratio of 86%, whereas the branching ratio of the aldehyde-H abstraction is 12% and that of the methyl-H abstraction is 2%.²³

In the atmosphere, the C₄H₆OCl and C₄H₅O radicals will react with O₂ and the transient peroxy radicals, C₄H₆OClO₂ and C₄H₅OO₂, can perform various reactions.¹¹ For example, the peroxy radical C₄H₆OClO₂ reacts with itself to form the C₄H₆OClO radical, and its decomposition product, chloroacetone (C₃H₅OCl), was detected with the technique of gas chromatography flame ionization detection (GC-FID).²⁴ The stable secondary products 2,3-dichloro-2-methylpropanal (C₄H₆OCl₂) and methacryloyl chloride (C₄H₅OCl) formed from the reactions of C₄H₆OCl and C₄H₅O radicals with the remaining precursor Cl₂ were also detected in experiments with GC-FID.²⁵ But, due to the use of online probe techniques to study these reactions, the available information of the Cl-initiated oxidation of MACR is still scarce and the detailed reaction mechanism as well as the formation of the above theoretically proposed key radicals and intermediates needs to be clarified or confirmed.

In this work, the Cl-initiated oxidation reaction of MACR under NO_x-free conditions has been investigated in a fast flow tube reactor by using the online analytical method of vacuum ultraviolet (VUV) photoionization time of flight mass spectrometry complemented by high-level theoretical computations of the structures of radicals and the reaction potential energy surfaces. VUV photoionization mass spectrometry is a powerful analytical method and key species such as radicals and intermediates as well as products involved in the oxidation reactions are directly observed in the experiments.^26,27 The adiabatic ionization energies (AIEs) of these key species are also predicted and utilized to assign the photoionization mass spectra. In addition, to confirm the origins of the products and then to reveal the embedded reaction mechanism, kinetic experiments have also been performed and the time behaviors of products have been obtained and compared with modeling results.

2. Methods

A fast flow tube has been used as a chemical reactor to study the Cl-initiated oxidation reaction of MACR under NO_x-free conditions and a home-made VUV photoionization orthogonal acceleration reflectron time-of-flight (TOF) mass spectrometer is employed to probe and analyze reaction intermediates, radicals and products. The configuration of the fast flow tube and the photoionization mass spectrometer has already been introduced in detail before and so only a brief description is presented here.^28,29

Briefly, the photoionization mass spectrometer is composed of three vacuum chambers: a source chamber, a photoionization chamber and a TOF chamber. The fast flow tube reactor is installed inside the source chamber and is composed of a 45 cm long Pyrex main tube with a 16/12 mm outer/inner diameter and a 60 cm long coaxial movable injector with a 6/4 mm outer/inner diameter. Cl atoms were generated by microwave discharge (GMS-200W, Sairem) of 1% diluted Cl₂ gas in helium and introduced into the fast flow tube to initiate the oxidation reactions. MACR was introduced through bubbling its liquid at room temperature, and O₂ and helium bath gas were injected via the arms of the main tube or the injector. The total pressure inside the flow tube was monitored by a capacity gauge and fixed at 6 Torr by a closed-loop feedback throttle valve. The initial concentrations of MACR, O₂ and Cl atoms in the flow tube were 8 × 10¹³, 1 × 10¹⁶ and ∼1 × 10¹³ molecule cm⁻³, respectively. Note that the microwave discharge efficiency is not precisely known and so only an estimated concentration of Cl atoms is provided here. The inner surface of the main tube and the outer surface of the injector were coated with halocarbon wax to reduce radical loss on the walls. The reaction time was adjusted by changing the distance between the injector and the sampling skimmer.

After passing through the skimmer (1 mm diameter), the gas mixture from the fast flow tube entered into the photoionization chamber, which was equipped with a commercial Kr discharge lamp (PKS 106, Heraeus) inside. Then the molecules and radicals absorbed the photons of the lamp with energies of hν = 10.0 and 10.6 eV, and photoionization processes were induced. A TOF mass analyzer with an orthogonal acceleration and reflectron structure was employed to analyze the masses of ions. The total ion flight length of the TOF mass spectrometer is ∼1 m, and its mass resolving power has been measured to be M/ΔM ∼ 2000 (FWHM, the full width at half maximum). During the experiments the pressures of the source chamber, the photoionization chamber and the TOF chamber were 800, 1 × 10⁻² and 1 × 10⁻⁴ Pa, respectively.

High-level theoretical calculations have also been carried out to get the reaction potential energy surfaces, the structures and the ionization energies of radicals and products. Concretely, the potential energy surfaces were calculated for the reactions of C₄H₆OCl and C₄H₅O radicals with O₂ to examine the formation channels leading to the peroxy radicals and other products. The potential energy surfaces were computed at the CCSD(T)-F12a/aug-cc-pVTZ//PBE0/aug-cc-pVDZ level of theory. The structural optimizations and the vibrational frequency calculations were performed with the Gaussian 16 program package.³⁰ Then the energies of the reactants, the transition states and the products were calculated with the Molpro package.³¹ The AIEs of the products were determined from the energy difference of cations and molecules at the PBE0/aug-cc-pVDZ level of theory.

3. Results and discussion

To get detailed information on the reaction system inside the fast flow tube, experiments were performed in sequence and can be divided into two parts.^32,33 Firstly, the experiment was performed without adding oxygen into the fast flow tube, and C₄H₆OCl and C₄H₅O radicals were formed from the Cl-addition reaction and the hydrogen abstraction reaction of MACR with Cl atoms, respectively. Then, with the addition of oxygen, the transient C₄H₆OCl and C₄H₅O radicals reacted with oxygen to produce peroxy radicals and these peroxy radicals could perform self-reactions or react with HO₂ radials in the flow tube under NO_x-free conditions.¹¹

3.1 The reaction of MACR with Cl

Without adding oxygen into the fast flow tube, a typical photoionization mass spectrum is presented in Fig. 1(a). Many peaks can be observed in the mass spectrum and the major species have been assigned. The most intense peak at m/z = 70 is ascribed to the reactant MACR itself, whose ionization energy locates at 9.92 eV,³⁴ below the photon energy of the Kr lamp. The mass peaks at m/z = 69 and m/z = 105/107 should be assigned as C₄H₅O and C₄H₆OCl radicals, respectively, formed from the hydrogen abstraction reaction and the addition reaction of MACR with Cl atoms.^19,23 As far as we known, the transient C₄H₅O and C₄H₆OCl radicals are directly detected here for the first time. The ratio of the mass peaks at m/z = 105 and 107 corresponding to the C₄H₆O³⁵Cl and C₄H₆O³⁷Cl isotopes is measured at ∼3, agreeing well with the ratio of the natural abundances of the ³⁵Cl and ³⁷Cl isotopes (100 : 32).³⁵

Fig. 1 Photoionization mass spectra acquired (a) without O₂ and (b) adding O₂ into the fast flow tube.

The transient C₄H₅O and C₄H₆OCl radicals can react with the residual Cl₂ molecules in the fast flow tube and the corresponding products have been observed and identified in the mass spectrum, partially with the aid of the isotopic ratio of ³⁵Cl and ³⁷Cl. For example, the mass peaks at m/z = 104 and 106 are assigned as methacryloyl chloride (C₄H₅O³⁵Cl and C₄H₅O³⁷Cl), the product of the reaction of C₄H₅O radicals with Cl₂.²⁵ The small peaks at m/z = 140, 142 and 144 are assigned as 2,3-dichloro-2-methylpropanal (C₄H₆OCl₂) from the reaction of C₄H₆OCl radicals with Cl₂.²⁵ Some products are fragile in the photoionization and their fragment ions have also contributed to the mass spectrum, i.e., at m/z = 76, 78 and 111. The detailed assignment of these peaks and their origins can be found in Table S1 (ESI†).

3.2 The reactions of C₄H₅O and C₄H₆OCl with O₂

After adding abundant O₂ (∼10¹⁶ molecule cm⁻³) into the fast flow tube, the transient C₄H₅O and C₄H₆OCl radicals will mainly react with O₂ and the resulting photoionization mass spectrum is presented in Fig. 1(b). To get a detailed understanding of these processes, the potential energy surfaces (PES) of the reactions of C₄H₅O and C₄H₆OCl radicals with O₂ have been calculated at the CCSD(T)-F12a/aug-cc-pVTZ//PBE0/aug-cc-pVDZ level of theory and are presented in Fig. 2 and 3, respectively, as well as the optimized structures of the species involved in the reactions.

Fig. 2 Potential energy surface of the reaction of C₄H₅O with O₂.

Fig. 3 Potential energy surface of the reaction of C₄H₆OCl with O₂.

As shown in Fig. 2, the reaction of C₄H₅O with O₂ is an exothermic reaction and the production of the C₄H₅OO₂ peroxy radical is energetically favorable. In addition, the theoretical calculations show that the C₄H₅OO₂ peroxy radical can perform an H-transfer reaction via a transition state (TS) with a submerged barrier of −6.34 kcal mol⁻¹ and then dissociate to C₄H₄O₂ and OH. The formation of C₄H₄O₂ and OH is exothermic with an energy of −39.27 kcal mol⁻¹, indicating that their formation is energetically feasible. As shown in Fig. 1(b), the product C₄H₄O₂ has been observed and ascribes to the peak of m/z = 84 in the photoionization mass spectrum.

In Fig. 3, the potential energy surface of the reaction of C₄H₆OCl with O₂ shows that the formation of the C₄H₆OClO₂ peroxy radical is also energetically favorable. But, unlike the above case of C₄H₅OO₂, the unimolecular reaction of the C₄H₆OClO₂ peroxy radical to produce C₄H₅ClO + HO₂ and C₄H₅ClO₂ + OH has an energy barrier with a height of 30.09 kcal mol⁻¹, 9.55 kcal mol⁻¹ higher than the total energy of C₄H₆OCl with O₂, which is too high to be overcome at room temperature. Thus the C₄H₆OClO₂ peroxy radical is the only product in the reaction of C₄H₆OCl with O₂.

As discussed above, the C₄H₅OO₂ and C₄H₆OClO₂ peroxy radicals are the main products of the reactions of C₄H₅O and C₄H₆OCl radicals with O₂ and their corresponding molecular ions should locate at m/z = 101 and m/z = 137/139, respectively, in the photoionization mass spectrum. However, no such mass peaks can be observed in Fig. 1(b).

Previous studies have shown that except the smallest peroxy radical, CH₃O₂, the cations of most alkyl peroxy radicals are not stable and dissociate to the fragments of alkyl cations and oxygen following the photoionization.^32,33,36 Here, to help to assign the photoionization mass spectrum and to explain the dissociative photoionization processes of the C₄H₅OO₂ and C₄H₆OClO₂ peroxy radicals, theoretical calculations have also been performed. Fig. 4 presents the optimized structures of the C₄H₅OO₂ and C₄H₆OClO₂ peroxy radicals and their cations in the ground electronic state. The ground electronic state of these cations is a triplet state after removing an electron from the σ(Cα-OO) orbital of the C₄H₅OO₂ and C₄H₆OClO₂ peroxy radicals. To be specific, the C–OO equilibrium bond length of the neutral C₄H₆OClO₂ is calculated at 1.450 Å, whereas it is substantially elongated to 2.781 Å for the cationic ground electronic state of C₄H₆OClO₂⁺. Similarly, the C–OO bond length of C₄H₅OO₂⁺ at the ground electronic state is also elongated from 1.426 to 3.263 Å during photoionization.^27,33

Fig. 4 The structures of the C₄H₆OClO₂ and C₄H₅OO₂ peroxy radicals and their cations optimized at the PBE0/aug-cc-pVDZ level of theory. The bond lengths are in Å.

These apparent elongations of the C–OO bond length will reduce the stability of the peroxy radicals’ cations, also making C₄H₅OO₂⁺ and C₄H₆OClO₂⁺ only slightly bound and out of the Franck–Condon transition during photoionization.^27,33 In addition, the AIEs of the C₄H₅OO₂ and C₄H₆OClO₂ peroxy radicals are calculated and take the values of 7.93 and 9.23 eV, respectively, far below the present photon energy of 10.6 eV. The appearance energies (AEs) of the fragment ions C₄H₅O⁺ (m/z = 69) and C₄H₆OCl⁺ (m/z = 105/107) in the dissociation of C₄H₅OO₂⁺ and C₄H₆OClO₂⁺ are also calculated and locate at 8.00 and 9.08 eV. Therefore, in the photoionization the C₄H₅OO₂⁺ and C₄H₆OClO₂⁺ cations will be produced with a high internal energy and dissociate to the C₄H₅O⁺ + O₂ and C₄H₆OCl⁺ + O₂ fragments, respectively. Thus, the mass peak at m/z = 69 (C₄H₅O⁺) in Fig. 1(b) is attributed to C₄H₅OO₂, and the mass peaks at m/z = 105/107 (C₄H₆OCl⁺) is ascribed to the C₄H₆OClO₂ peroxy radical.

3.3 The reactions of C₄H₅OO₂ and C₄H₆OClO₂ under NO_x free conditions

Under NO_x free conditions in the fast flow tube, the C₄H₅OO₂ and C₄H₆OClO₂ peroxy radicals can perform self-reaction or react with HO₂ radicals.¹¹ Normally, the self-reaction of peroxy radicals (RO₂) has two channels, (i) 2RO₂ → 2RO + O₂ and (ii) 2RO₂ → R_–HO + ROH + O₂.³⁷ But, based on the specific structures of C₄H₅OO₂ and C₄H₆OClO₂, as shown in Fig. 4, there are no H atoms on the tertiary α-oxyl carbon to be abstracted and thus the self-reactions of C₄H₅OO₂ and C₄H₆OClO₂ do not proceed via channel (ii).³⁸

Normally the reaction of the alkoxy radical RO with O₂ is one of the major sources to produce HO₂ radicals. But, the reaction of C₄H₅OO with O₂ will not produce HO₂, as still there is no H atoms on the tertiary α-oxyl carbon to be abstracted. A previous study shows that the reaction of C₄H₆OClO with O₂ can generate HO₂, via more than one elementary reaction step,³⁹ and presently this reaction should be the main source of HO₂ in the fast flow tube. Note that the AIE of HO₂ locates at 11.359 eV, above the present photon energy, and thus no HO₂ radicals can be observed in the mass spectra.⁴⁰

A previous study predicts that the reaction of the C₄H₅OO₂ peroxy radical with HO₂ has three channels, (R1a) C₄H₅OO₂ + HO₂ → C₄H₅OO₂H + O₂, (R1b) C₄H₅OO₂ + HO₂ → C₄H₅OOH + O₃, and (R1c) C₄H₅OO₂ + HO₂ → C₄H₅OO + OH + O₂.³⁹ The reaction of C₄H₆OClO₂ with HO₂ has two channels, (R2a) C₄H₆OClO₂ + HO₂ → C₄H₆OClOOH + O₂, (R2b) C₄H₆OClO₂ + HO₂ → C₄H₆OClO + OH + O₂, and the branching ratio was predicted at Y_R2a = 0.2 ± 0.2 and Y_R2b = 0.8 ± 0.2.³⁹

The alkoxy radical C₄H₆OClO formed from the self-reaction of C₄H₆OClO₂ or its bimolecular reaction with HO₂ can decompose to the stable product of chloroacetone (C₃H₅O³⁵Cl and C₃H₅O³⁷Cl) with a fast rate of ∼10⁷ s⁻¹ and has been completely consumed in the fast flow tube within the reaction time of ∼1 ms under the present experimental conditions.³⁹ Therefore, the signal of C₄H₆OClO (m/z = 121) is not observed in the photoionization mass spectrum of Fig. 1(b), and its decomposition product of chloroacetone (C₃H₅OCl) contributes to the mass peaks of m/z = 92 and 94 with a ratio of 3 : 1.

The reaction of C₄H₆OClO₂ with HO₂ can produce the hydroperoxide C₄H₆OClOOH (m/z = 138, 140). The AIE of C₄H₆OClOOH is calculated at 9.30 eV at the PBE0/aug-cc-pVDZ level of theory, and the appearance energy of its fragment ion C₃H₅ClO₂H⁺ is predicted at 9.97 eV. Thus in the photoionization the hydroperoxide cation C₄H₆OClOOH⁺ is produced with a high internal energy and dissociates into the fragment ion of C₃H₅ClOOH⁺, plus the neutral fragment of CHO, contributing to the peaks of m/z = 109 and 111 in the photoionization mass spectrum of Fig. 1(b).

The peroxy radical of C₄H₅OO₂ performs self-reaction and reacts with HO₂ to generate the radical C₄H₅OO, contributing to the mass peak of m/z = 85 in Fig. 1(b). The reaction of C₄H₅OO₂ with HO₂ can also produce C₄H₅OOH (m/z = 86) and C₄H₅OO₂H (m/z = 102). The AIE of C₄H₅OO₂H is calculated at 9.44 eV at the PBE0/aug-cc-pVDZ level of theory. In the photoionization, the C₄H₅OO₂H⁺ cation was produced with a high internal energy and then dissociated into the C₄H₅O⁺ (m/z = 69) and HO₂ fragments.

3.4 Reaction mechanism

Kinetic experiments have been performed for the Cl-initiated oxidation reaction of MACR by changing the distance between the injector and the sampling skimmer, to get information inside the fast flow tube and to confirm the above species’ assignments.^28,29 The time evolution of ion signals has been measured and some of them, i.e. m/z = 84, 85 and 92 corresponding to C₄H₄O₂, C₄H₅OO and C₃H₅OCl, are presented in Fig. 5. In addition, the time evolutions of the species’ concentrations have been theoretically calculated based on the oxidation reactions listed in Table 1 and are presented as solid lines, also seen in Fig. S1 and S2 (ESI†).^41,42 Note that the experimental concentration equivalents are less well-defined, as some unknown and conversion factors such as photoionization cross-sections add to the measurement uncertainty. The theoretical results also rely on a number of assumptions, for instance, the initial concentration of Cl atoms and the unknown or estimated rate constants. As shown in Fig. 5, although with some differences, the overall shape of the theoretical modeling results can compare with the time evolution of the ion signals.

Fig. 5 The experimental (dots) and theoretical (lines) time behavior of products in the Cl-initiated oxidation of MACR. (a) m/z = 84, (b) m/z = 85 and (c) m/z = 92.

Table 1 Reaction mechanism used to model experiments in this work

	Reaction	Rate coefficient/cm^3 molecule^−1 s^−1 or s^−1	Ref.
a Analogic result from ref. 45. b The reaction consists of more than one elementary reaction step, seen the detail in ref. 39.
1	C4H6O + Cl → C4H6OCl	1.98 × 10^−10	23
2	C4H6O + Cl → C4H5O + HCl	2.76 × 10^−11	23
3	C4H6OCl + O2 → C4H6OClO2	2.87 × 10^−12	43
4	C4H5O + O2 → C4H5OO2	2.0 × 10^−12	44
5	C4H5OO2 → C4H4O2 + OH	12.9^a	45
6	2C4H6OClO2 → 2C4H6OClO + O2	2.4 × 10^−12	39
7	2C4H5OO2 → 2C4H5OO + O2	1.0 × 10^−11	39
8(a)	C4H6OClO2 + HO2 → C4H6OClO2H + O2	2.0 × 10^−12	39
8(b)	C4H6OClO2 + HO2 → C4H6OClO + OH + O2	8.0 × 10^−12	39
9(a)	C4H5OO2 + HO2 → C4H5OO2H + O2	7.7 × 10^−12	39
9(b)	C4H5OO2 + HO2 → C4H5OOH + O3	5.5 × 10^−12	39
9(c)	C4H5OO2 + HO2 → C4H5OO + OH + O2	8.8 × 10^−12	39
10	C4H6OClO (+O2) → C3H5OCl + HO2 + CO	1 × 10^7^b	39
11	C4H5OO → C3H5 + CO2	7 × 10^2	46
12	2HO2 → H2O2 + O2	1.7 × 10^−12	47
13	HO2 → diffusion	3	41

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We can see that the ion signal of m/z = 84 increases with time within the experimentally covered time region, which is consistent with the theoretically modeled time behavior of C₄H₄O₂ in Fig. 5(a). The signal intensity at m/z = 85 firstly increases and then decreases with time, which is a typical radical behavior of C₄H₅OO, as shown in Fig. 5(b). The signal of m/z = 92 firstly increases and then stabilizes, which is in agreement with the expected time behavior of chloroacetone. The detailed reaction mechanism of the Cl-initiated oxidation of MACR under NO_x free conditions has been summarized in Fig. 6. Note that Fig. 6 only presents the two dominant entrance channels for the reaction of MACR with Cl atoms, the terminal addition of the C=C double bond and the aldehyde-H abstraction with their branching ratios of about 86% and 12%, respectively.³⁹ The branching ratio of the minor methyl-H abstraction channel is predicted at only 2%, not shown in Fig. 6, and this isomeric production channel is difficult to be separated only with the fixed-photon-energy photoionization mass spectrometry.

Fig. 6 Reaction mechanisms of Cl-initiated oxidation of MACR under NO_x free conditions.

4. Conclusions

In summary, we present here a combined experimental and theoretical study on the Cl-initiated oxidation of MACR under NO_x-free conditions. A microwave discharge fast flow tube was used to investigate the oxidation reaction of MACR. The reaction products and the key intermediates as well as radicals have been observed online by using a home-made VUV photoionization mass spectrometer. It is shown that the reaction of MACR with Cl atoms can proceed via two pathways, the addition of Cl atom to the C=C double bond of MACR to produce the C₄H₆OCl adduct radical and the abstraction of the aldehyde hydrogen atom to produce the C₄H₅O radical. These radicals can react with oxygen to produce their corresponding peroxy radicals, whose photoionization processes have also been discussed and utilized to assign the photoionization mass spectra. The transient C₄H₅O and C₄H₆OCl radicals, as well as the peroxy radicals C₄H₅OO₂ and C₄H₆OClO₂, are experimentally detected here for the first time, directly confirming the above reaction mechanisms. Under NO_x-free conditions, these peroxy radicals will perform self-reaction and react with HO₂ radicals. The resulting specific products are clearly identified in the photoionization mass spectra too, with the aid of theoretical calculations. In addition, the time evolutions of products have been measured in the kinetic experiments and compared with theoretically modeled results. The present work provides a detailed insight into the reaction mechanisms of the Cl-initiated oxidation of MACR and will be helpful to understand the atmospheric fate of MACR.

Author contributions

Xiaoxiao Lin: conceptualization, formal analysis, investigation, writing – original draft. Rongrong Hu: investigation, writing – original draft. Ziji Ma: investigation. Hao Yue: investigation. Zuoying Wen: investigation. Cuihong Zhang: investigation. Christa Fittschen: writing – review & editing. Weijun Zhang: investigation. Xiaofeng Tang: conceptualization, methodology, investigation, funding acquisition, project administration, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 42075113, 91961123, 42120104007), the International Partnership Program of Chinese Academy of Sciences (No. 116134KYSB20170048) and the Key Program of Research and Development of Hefei Science Center, CAS (No. 2020HSCKPRD001).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cp02101c

‡ These authors contributed equally to this work.

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