A perspective on the reactions of organic peroxy radicals with HO₂

Abstract

The chemistry of organic peroxy radicals (RO₂) is crucial for ozone and secondary organic aerosol formation in the troposphere. The level of nitrogen monoxide (NO) exerts a major control on further reactions of peroxy radicals. The research on these reactions in the absence of NO has been receiving increasing attention recently. The current studies under these conditions, typically associated with pristine environments, are focused on understanding the formation of highly oxygenated organic molecules (HOMs) via autoxidation and generation of accretion products, which supposedly result from peroxy radical permutation reactions (RO₂ + RO₂). Apart from the potential OH production from some oxygenated peroxy radicals, there is less research activity on the reactions of peroxy radicals with HO₂. This article reviews the existing literature data available on RO₂ + HO₂ reactions and highlights the gaps where future research is required. To date, limited information has been provided on the reactions of HO₂ with functionalized RO₂, particularly for β-hydroxyalkyl peroxy radicals, carbonyl-substituted peroxy radicals other than acyl peroxy, and peroxy radicals containing at least two functionalities. In addition, the temperature dependence of product branching ratios is not well established. Future studies targeting the influence of RO₂ + HO₂ on the tropospheric HO_x (=OH + HO₂) budget should ideally enlarge the dataset of OH yields from various peroxy radical structures. This also highlights the need to broaden the investigations on the formed hydroperoxides, whose gas-phase chemistry is not well known.

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Environmental significance

Organic peroxy radicals (RO₂) are key species involved in chain reactions that yield tropospheric ozone and secondary organic aerosols. Emission control strategies, upon international policy agreements, lead to a continuous reduction in nitrogen oxides. Chemical conditions commonly associated with pristine regions are thus expected to become more frequent. Peroxy radical reactions favoured under these conditions, such as unimolecular isomerization, permutation reactions or reactions of RO₂ with HO₂, are complex, and experiments targeting these chemical regimes are challenging. The present work offers an overview of the current state of knowledge and uncovers gaps that need to be addressed to draw a comprehensive picture of the importance of peroxy radical reactions with HO₂ in atmospheric chemistry.

Introduction

Organic peroxy radicals (RO₂) are essential intermediates in the atmospheric oxidation of volatile organic compounds (VOCs). These are emitted into the troposphere predominantly from biogenic¹ but also from anthropogenic sources. Besides its central role in the formation of tropospheric ozone and secondary organic aerosol (SOA), RO₂ chemistry controls the recycling of reactive radicals such as OH (e.g. ref. 2 and 3). Once formed through VOC oxidation, e.g. in the OH-initiated oxidation either via abstraction

RH + OH → R + H₂O (R1)

R + O₂ + M → RO₂ + M (R2)

or addition (R3) pathways,

R₂C=CR₂ + OH + M → R₂C(OH)CR₂ + M (R3)

R₂C(OH)CR₂ + O₂ + M → R₂C(OH)C(O₂)R₂ + M, (R4)

the further fate of a specific organic peroxy radical includes unimolecular isomerization (e.g. H shift reactions) and bimolecular reactions, such as with nitrogen oxides (NO and NO₂), halogen oxides (ClO, BrO, and IO) or radicals (OH, HO₂, RO₂, and NO₃). The competitiveness of a single process depends heavily on the ambient conditions and the structure of the individual RO₂ species. For instance, NO_x (=NO + NO₂) plays a central role in efficient radical-recycling, including the regeneration of OH and causing the formation of tropospheric ozone (Fig. 1). The reaction with NO typically represents the major RO₂ loss in air masses influenced by anthropogenic activities.

Fig. 1 Simplified, generic scheme for the major paths of RO₂ chemistry under NO_x-dominated conditions (blue) and NO_x-poor conditions, where other pathways become competitive (red).

The knowledge of peroxy radical chemistry, which is incorporated into chemistry-transport models, is crucial for both forecasting local air pollution episodes on regional scale and predicting global earth-climate interactions. Although our understanding of RO₂ chemistry is continuously evolving and has led to improvements in chemistry models, there is always a need to critically review the available chemical mechanisms and experimental set-ups. One of the famous turn-around examples is the unimolecular isomerization of peroxy radicals, which is found to be competitive with bimolecular processes in the troposphere,^4,5 but, to be observed, requires simulating atmospheric conditions (e. g. concentrations) in the experiment as closely as possible.

Apart from elucidating the HO_x (=OH + HO₂) budget in forested regions (e.g. ref. 4 and 6), research on the atmospheric oxidation of VOC during the last 15 years has considerably focused on understanding the mechanisms yielding SOA. The detection of highly oxygenated organic molecules (HOMs) formed through a chain of unimolecular isomerization steps called autoxidation initiated the allocation of tremendous research effort in a rapidly growing scientific community (see Bianchi et al.⁷ and references therein). However, the formation of low-volatility vapours, which act as aerosol precursors, is not limited to unimolecular processes. Several observations suggest the relevance of accretion products (organic peroxides, ROOR) formation in the permutation reactions (self- and cross-reactions) of (HOM) peroxy radicals, which terminate autoxidation (e.g. ref. 8–10).

In contrast to the increasing number of reports on RO₂ isomerization and permutation reactions, there are very few recent studies on the reactions of peroxy radicals with HO₂ except for some short, renewed interest following the detection of discrepancies in the HO_x budget in forested regions dominated by isoprene emissions (e.g. ref. 6 and 11). The present perspective article reviews the available information on this particular class of organic peroxy radical reactions. In addition, it aims to discuss the challenges that occur when RO₂ + HO₂ are being investigated experimentally and, finally, gaps that require further research are identified.

Experimental and analytical approaches

Retrieving information on RO₂ + HO₂ reactions requires experimental approaches and analytical instrumentation that allow the time-resolved detection of either radicals or closed-shell species or both. Although the sections below are spiked with both experimental and analytical information, a brief overview is provided in this section. However, a detailed evaluation and a complete representation of the available methods are beyond the scope of the present work. In addition, there exist a number of developments in radical detection that have not yet been applied to investigate RO₂ + HO₂ chemistry.

Traditionally, product yields and branching ratios have been determined in static systems, such as simulation chambers, while flow tube applications are commonly used for absolute rate coefficient determination. The first investigations on RO₂ + HO₂ reactions date back to the work by Cox and Tyndall,^12,13 who investigated the kinetics of CH₃O₂ + HO₂ using molecular modulation spectrometry. In these studies, methyl peroxy radicals were generated by the photolysis of CH₄/Cl₂ mixtures in the presence of oxygen and a relative increase in HO₂ was achieved by the addition of H₂. This pioneering experimental approach has been adapted ever since in many of the later studies targeting RO₂ + HO₂ reactions, namely the generation of a specific peroxy radical by the reaction of an organic precursor with Cl atoms and the increase in HO₂ relative to RO₂ by the addition of an HO₂ precursor, such as H₂, methanol, and formaldehyde. In simulation chamber studies, this approach has been applied to determine product branching ratios by systematically varying the HO₂/RO₂ ratio. This ranges from a system usually dominated by RO₂ permutation reactions, at zero HO₂ precursor, to a system that favours RO₂ + HO₂ reactions when the concentrations of the HO₂ precursor are sufficiently high. In the ideal case, the product branching ratios for RO₂ + HO₂ reactions are then obtained from the observed product yields.

In chamber studies, branching ratios were derived from stable end-product analysis of continuously irradiated reaction mixtures mostly by means of Fourier transform infrared spectroscopy (FTIR).^14–33 In some instances, organic hydroperoxides were measured by the selective reaction of hydroperoxides with peroxidase.^27,32 The organic hydroperoxides are first trapped in the liquid phase inside a stripping coil attached to the simulation chamber and collected for high performance liquid chromatography (HPLC) analysis. After separation on the column, a reagent consisting of horseradish peroxidase (HRP) and p-hydroxyphenylacetic acid is added. The enzyme catalyses a reaction between the hydroperoxide and p-hydroxyphenylacetic acid, yielding a dimer whose fluorescence is measured following excitation.³⁴ It is worth noting that the HRP-fluorescence method coupled to HPLC has been applied more regularly to investigations of ozonolysis systems, particularly for detecting H₂O₂ and hydroxyalkyl hydroperoxides (e.g. Hasson et al.³⁵ and references therein). The Caltech group has developed a chemical ionisation mass spectrometry (CIMS) instrument based on clustering with CF₃O⁻ for detecting hydroperoxides.³⁶ The technique was successfully tested against the HPLC-fluorescence method in the field and subsequently used in numerous investigations, including RO₂ + HO₂ chemistry.^36–39 Apart from stable end-products, Winiberg et al. were the first to directly quantify OH formation from RO₂ + HO₂ chemistry in a chamber study through detection with the Fluorescence Assay by Gas Expansion (FAGE) method based on laser-induced fluorescence (LIF).³³

In flow tube applications, peroxy radical generation was commonly initiated by flash^{18,19,40–52} or laser photolysis^24,53–61 of VOC/Cl₂/HO₂-precursor or VOC/H₂O₂ mixtures or, less often, by molecular modulation methods^12,13,62,63 or discharge flow.^64,65 Time-resolved ultraviolet (UV) absorption spectroscopy has often been used to monitor peroxy radical species. Rate coefficients are finally obtained by fitting kinetic parameters to the decay rates recorded for the respective species. The UV spectra of small peroxy radicals have been critically reviewed in the literature.⁶⁶ A larger set of absorption cross sections can be found in the Mainz database.⁶⁷

Information on branching ratios was obtained from flow tube experiments in some cases for OH- and O₃-forming pathways by means of either UV (O₃),^42,58,60 transient absorption (O₃),⁵⁹ or IR-wavelength-modulated spectroscopy (OH),^60,61 CIMS (OH, O₃),^64,65 and LIF (OH).^56,59 In contrast, there are a few examples of absolute rate coefficient determinations by simulation chamber experiments. Moortgat et al.⁶⁸ obtained the rate coefficient for CH₃O₂ + HO₂ by the modulated photolysis of acetaldehyde/air mixtures in a 44 L quartz cell. Winiberg et al.³³ obtained the absolute rate coefficient for CH₃C(O)O₂ + HO₂ by global modelling of the reaction system. Rate coefficients for all pathways of the CH₃C(O)O₂ + HO₂ reaction were optimized individually to match the experimental data over a range of conditions. More recently, Østerstrøm et al.⁶⁹ redetermined the rate coefficient for the reaction of methyl peroxy with HO₂. Both CH₃O₂ and HO₂ were detected by FAGE-LIF, and rate coefficients were obtained by applying a numerical model used to fit the temporal decay of both radical species. In contrast to common flash photolysis applications, the authors used a simulation chamber (HIRAC, University of Leeds) to build up steady-state radical concentrations by irradiating CH₄/CH₃OH/Cl₂/air mixtures for about 5 min before monitoring the temporal decay of CH₃O₂ and HO₂ after switching off the lamps. This approach has led to substantially lower peroxy radical concentrations than those typically found for kinetic studies in flow tube systems.

Although most kinetic studies relied on UV absorption, a number of methods for peroxy radical detection, including matrix isolation and electron spin resonance spectroscopy (MIESR),^70,71 laser-induced fluorescence,^72–78 peroxy radical chemical amplification (PERCA),^79–91 CIMS^92–116 and spectroscopic approaches such as cavity ring-down (CRD)^117–125 or wavelength modulation techniques,^{57,60,61,126–129} were deployed in published works, increasing in the last two decades. These methods differ primarily in the level of chemical speciation of the detected radical species.

The MIESR method is an offline technique that relies on cryofocusing of radicals at 77 K prior to quantifying using electron spin resonance spectroscopy.^70,71 MIESR can differentiate HO₂ and CH₃C(O)O₂ from other RO₂. The method was developed and predominantly used by the research groups at Forschungszentrum Jülich.

The original FAGE-LIF method is selective for OH but also represents an established method for HO₂ quantification. In essence, HO₂ is detected as OH after titrating the HO₂ with NO. The approach has been expanded for the detection of RO₂ in a second channel (RO_xLIF) by applying a two-stage process that consists of chemical conversion of RO_x (=RO₂ + RO + HO₂ + OH) into OH.⁷⁶ Discrimination between HO₂ and RO₂ is achieved in this set-up by modulating the chemical conversion.⁷⁶ The fundamentals of the FAGE technique and possible interferences have been described in detail in several reviews.^130–132 More recently, Onel et al.⁷⁸ presented an adaptation of FAGE for the sensitive and selective quantification of CH₃O₂ by detecting the off-resonant LIF of CH₃O formed upon the addition of NO.

Chemical amplification techniques rely on efficient radical recycling in the presence of NO. In the original PERCA approach, pioneered by Cantrell and Stedman,^79,80 amplified NO₂ formed upon the addition of NO and CO is used for peroxy radical quantification. Several techniques have been applied to quantify the resultant amplified NO₂, including luminol-chemiluminescence,^{79–83,85,87} laser-induced fluorescence,⁸⁶ cavity-enhanced absorption spectroscopy (CEAS)⁹⁰ and cavity ring-down spectroscopy (CRDS).^88,89 PERCA instruments consequently yield a sum parameter RO_x. Wood et al.⁹¹ presented a chemical amplification technique (ECHAMP) based on ethane instead of carbon monoxide. In the approach presented by Reiner et al.,⁸⁴ carbon monoxide is replaced by SO₂ and the resultant H₂SO₄ is detected by CIMS. A separation between HO₂ and RO₂ is achieved by modulating the NO/O₂ ratio.¹³³

CIMS is applied to detect peroxy radicals using various reagent ions, such as H₃O⁺(H₂O)_n,^{97,99,101–103,106–108,113,115} NH₄⁺,¹¹⁴ O₂⁺,⁹² NO₃⁻,^109,110 I⁻,^94,111 Br⁻,^112,116 O₂^−95,96 or SF₆⁻.^{64,65,93,94,98,104,106,107} These ionisation schemes differ primarily in the resulting ions and their ability to detect specific classes of peroxy radicals. For instance, although H₃O⁺(H₂O)_n, NH₄⁺ and NO₃⁻ result in cluster ions, RO₂ were detected as parent ions in O₂⁻ and SF₆⁻ mode.^{93–96,98,100} Exceptions to the SF₆⁻ mode are HO₂ and CH₃O₂, which are detected as SF₄O₂⁻ and FO₂⁻, respectively.^{64,65,106,107}

A number of studies have provided evidence that H₃O⁺(H₂O)_n can detect different alkyl peroxy^{97,99,101–103,106–108,113,115} and bicyclic peroxy radicals.^106–108 Both I⁻ and SF₆⁻ were found to be suitable for measuring acyl peroxy radicals, particularly acetyl peroxy.^93,94,98 HO₂ detection has been successfully proven by clustering with I⁻ (ref. 111) or Br⁻,^112,116 while the detection of highly oxidized RO₂ has been reported by clustering with NO₃⁻.^109,110 CRD approaches have been developed in the near-infrared and used for the detection of CH₃O₂,^{117,121,124,125} C₂H₅O₂,^117,123 hydroxyalkyl peroxy,¹¹⁹ acetyl peroxy,^124,125 acetonyl peroxy¹²⁵ and particularly HO₂.^{118,120,122,123,125}

The applicability of these techniques to pulsed photolysis/flow tube applications and chamber experiments depends on the respective detection limits, time resolution and the chemical speciation of radical species. For example, flow tube experiments require a high time resolution to enable monitoring of the temporal decay of radicals but result in radical concentrations that are typically orders of magnitude larger than those in simulation chambers.

An overview of RO₂ + HO₂

Based on the body of kinetic data published since those of Cox and Tyndall^12,13 (Table 1), it is established that rate coefficients for the title reactions should lie in the range of (0.5–2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at room temperature. They tend to be larger in the case of β-hydroxy-substituted peroxy radicals, such as those resulting from the addition of OH to an unsaturated organic compound, while substitution with halogen atoms appears to slow down the reactivity towards HO₂ for specific substitution patterns (Table 1). Overall, RO₂ + HO₂ reactions are thus rather rapid. The variation in the rate coefficient values spans a factor of roughly only 4–5 and is obviously significantly less pronounced than that of RO₂ permutation reactions, whose rate coefficients span several orders of magnitude (e.g. ref. 135 and 136). The reader should note that in all instances, the values recommended by the International Union of Pure and Applied Chemistry (IUPAC) for room temperature rate coefficients are uncertain by a factor of 1.6–2.¹³⁴ These recommended uncertainty ranges reflect either a limited number of determinations or significant variation in the available data. In the case of methyl and acetonyl peroxy, the IUPAC recommendation is identical to the values of the Jet Propulsion Laboratory (JPL).¹³⁷ In contrast, although within the large error limits, the IUPAC and JPL recommendations differ by about 30% for the room temperature rate coefficient for acetyl peroxy + HO₂. Nevertheless, it is well established that room temperature rate coefficients appear to increase with size in the case of the reaction of alkyl peroxy radicals with HO₂. For example, the rate coefficients for both neopentyl and cyclopentyl peroxy (C₅) are almost identical while being a factor of about three larger than that for methyl peroxy (Table 1).

Table 1 Rate coefficients for RO₂ + HO₂ reactions. Temperature-dependence follows the Arrhenius expression k(T) = A × exp(−E_a/RT)

Peroxy radical	k298 K × 10^12 (cm^3 s^−1)	A × 10^13 (cm^3 s^−1)	Ea/R (K)	Ref.
a Represents the mixture of peroxy radicals generated from decane + OH.b Represents the mixture of peroxy radicals generated from tetradecane + OH.c Peroxy radicals generated from the reaction of the parent VOC with OH. The β-hydroxyalkyl peroxy radical isomers are the predominant RO2 in the system.d Represents the mixture of HOC5H8O2 isomers formed from isoprene + OH.
Alkyl peroxy
CH3O2	5.2	3.8	−780	134
CH3CH2O2	6.9	6.4	−710	134
neo-C5H11O2	15	1.4	−1380	18 and 55
cyclo-C5H9O2	18	2.1	−1320	19
cyclo-C6H11O2	17	2.6	−1250	19
C10H21O2^a	20			55
C14H29O2^b	21			55
Haloalkyl peroxy
CH2ClO2	5.0	3.2	−820	48
CHCl2O2	5.9	5.6	−700	22
CCl3O2	5.1	4.8	−706	22
CF2ClO2	3.4			50
CF2ClCH2O2	6.8			50
CFCl2CH2O2	9.2			50
CF3CCl2O2	1.9			49
CF3CHFO2	4.3	2.0	−910	134
CF3CF2O2	1.2			50
CF2ClCF2O2	1.2			50
β-Aryl peroxy
C6H5CH2O2	12	1.5	−1310	134
Alkoxyalkyl peroxy
CH3OCH2O2	12			134
Acyl peroxy
CH3C(O)O2	20	1.73	−730	134
C6H5C(O)O2	38	110	−364	58
Carbonyl-containing peroxy
CH3C(O)CH2O2	9.0			134
α-Hydroxyalkyl peroxy
HOCH2O2	12	0.056	−2300	134
β-Hydroxyalkyl peroxy
HOCH2CH2O2	13			134
HO–(1,2-C2H2Cl2)–O2	6			55
(CH3)2C(OH)CH2O2	14	0.56	−1650	134
CH3CH(OH)CH(O2)CH3	15			54
(CH3)2C(OH)C(O2)(CH3)2	15			55
HO–(cyclohexene)–O2^c	22			55
HO–(α-pinene)–O2^c	21			55
HO–(γ-terpinene)–O2^c	20			55
HO–(D-limonene)–O2^c	21			55
Hydroxyalkenyl peroxy
HO–(isoprene)–O2^d	17			55

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This observation led to the construction of expressions relating the rate coefficient k to the number of carbon atoms.^55,138,139 For example, Boyd et al.⁵⁵ presented a relationship of the form k = 2.2 × 10⁻¹¹ × [1 − exp(−0.26 × n)] for the room temperature rate coefficients, where n is the number of carbon atoms. Subsequently, this equation was updated by Calvert et al.¹⁴⁰ The more recent estimation method by Jenkin et al.¹³⁶ introduced a modified expression by relating the rate coefficient to a newly developed parameter n_CON, which is the number of carbon, oxygen and nitrogen atoms of the organic group R without counting the peroxy radical oxygen atoms. This approach resulted in two parameterisations that differentiate between acyl peroxy and non-acyl peroxy radicals, considering the fact that the rate coefficients of acyl peroxy radicals appear to be larger than their alkyl analogues. For completeness, it is worth mentioning that two other correlation-type structure–activity relationships have been reported for the prediction of RO₂ + HO₂ rate coefficients. The concept by King et al.¹⁴¹ relies on perturbation frontier molecular orbital (PFMO) theory, and RO₂ + HO₂ rate coefficients are correlated with the single occupied molecular orbital (SOMO) energy of the peroxy radical. Johnson et al.¹⁴² proposed a correlation between the logarithm of k and the (calculated) ionisation potential of a structural analogue R–CH=CH₂, where R corresponds to the same organic substituent present in the peroxy radical.

As discussed in several studies, RO₂ + HO₂ reactions exhibit a negative temperature-dependence (with rather large negative activation energies, Table 1), suggesting that these reactions proceed via an intermediate complex. In contrast to RO₂ + NO reactions, with NO being an electrophile, the HO₂ radical acts rather as a nucleophile in RO₂ + HO₂ reactions according to frontier molecular orbital theory.¹⁴¹ In a simplified manner, these reactions are often described as occurring through a single channel yielding quantitatively a hydroperoxide although this turned out to be merely true for alkyl peroxy radicals (e.g. ref. 3 and 135). Meanwhile, product studies have shown the occurrence of other reaction channels, where the accessibility of some of the pathways depends on the peroxy radical structure.

RO₂ + HO₂ → ROOH + O₂ (R5a)

RO₂ + HO₂ → ROH + O₃ (R5b)

RO₂ + HO₂ → RO + OH + O₂ (R5c)

RO₂ + HO₂ → R_–H=O + OH + HO₂ (R5d)

RO₂ + HO₂ → R_–H=O + H₂O + O₂ (R5e)

Interestingly, the first product study focusing on RO₂ + HO₂ investigated the acetyl peroxy radical and not the simpler CH₃O₂, as might be intuitively expected.¹⁴ In the early 1980s, information on CH₃O₂ + HO₂ was obtained as an offshoot of investigations targeting the CH₃O₂ self-reaction.^143,144 In fact, Calvert and co-workers had already observed methyl hydroperoxide formation from CH₃O₂ radicals generated by the photolysis of azomethane in the early 1960s.^145,146 However, at that time, the occurrence and participation of HO₂ in the reaction system were not yet known, and methyl hydroperoxide was postulated to occur through either the CH₃O₂ self-reaction, the reaction of CH₃O₂ with CH₃O, or H atom abstraction from an organic species by excited CH₃O₂.^145,146

In 1985, Niki et al. investigated the oxidation of acetaldehyde by irradiating CH₃CHO/Cl₂/air mixtures at atmospheric pressure and room temperature.¹⁴ In these experiments, the HO₂ radical concentrations were increased by the addition of formaldehyde, and the reaction mixtures were monitored by Fourier-transform infrared (FTIR) spectroscopy. Based on the product analysis, the authors showed that the reaction of acetyl peroxy with HO₂ must proceed via at least two channels yielding either peracetic acid (≈75%) or O₃ (≈25%).

CH₃C(O)O₂ + HO₂ → CH₃C(O)OOH + O₂ (R6a)

CH₃C(O)O₂ + HO₂ → CH₃C(O)OH + O₃ (R6b)

As pointed out by Calvert et al.,³ channel (R6b) is intriguing in the sense that it represents one of the few known chemical reactions yielding ozone directly, even though its contribution to tropospheric ozone is, however, rather small. In essence, this early investigation has already drawn a more complex picture of RO₂ + HO₂ reactions.

Subsequently, these results were confirmed by Moortgat et al.⁴² who generated the target radicals by the flash photolysis of Cl₂/CH₃CHO/CH₃OH/N₂/O₂ mixtures and detected the species by time-resolved UV spectroscopy. Based on this analysis, they obtained the rate coefficient for the target reaction (over the temperature range 253–368 K) and the branching ratio for channel (R6b). They suggested that an H-bond in the tetroxide intermediate (ROOOOH), possible only due to the carbonyl group, allows for decomposition into CH₃C(O)OH + O₃.⁴²

The occurrence of a third reaction channel, producing OH, was first reported for a perfluorinated acyl peroxy radical.²⁶ The branching ratio obtained in this study has been corrected in follow-up work.²⁸ This observation, together with a revised infrared cross section for peracetic acid¹⁴⁷ and thermochemical considerations, has motivated Hasson et al.²⁷ to undertake a reinvestigation of the CH₃C(O)O₂ + HO₂ system. Based essentially on two observations, they did prove the existence of channel (R6c):

CH₃C(O)O₂ + HO₂ → CH₃ + CO₂ + OH + O₂. (R6c)

First, using the updated infrared cross section reduced the peracetic yield and led to a carbon balance well below 100% when considering only the well-established reaction products (peracetic acid + acetic acid). Second, the change observed for both the methyl hydroperoxide and the CO₂ yield while transitioning from an RO₂-dominated to an HO₂-dominated system was inconsistent with a reaction scheme that considered only channels (R6a) and (R6b) for CH₃C(O)O₂ + HO₂.

In these first investigations, the OH-forming channel was identified by closed-shell co-products. Meanwhile, OH formation is quantified by both direct and indirect measurements and, as presented in the section below, is also reported for other RO₂ + HO₂ reactions. One of the established methods to scavenge OH in systems where organic precursor/Cl₂ mixtures are irradiated is the addition of excess benzene. The reaction of benzene with Cl atoms is extremely slow (k_Cl < 2 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹),²⁰ while benzene is reasonably reactive towards OH radicals (k_OH ≈ 1 × 10⁻¹² cm³ molecule⁻¹ s⁻¹)¹⁴⁸ and consequently allows scavenging of OH without representing a significant sink for Cl atoms. Jenkin et al.²⁹ adapted this technique for investigations on CH₃C(O)O₂ + HO₂ and used phenol formation (from benzene + OH) to diagnose OH. It was previously found that, under these conditions, the phenol yield is about 53% ± 7%.¹⁴⁹ The authors performed a detailed analysis showing that benzene chemistry does not affect the removal of acetyl peroxy radicals under HO₂-dominated conditions. In later studies, direct measurements of OH were performed by laser-induced fluorescence or mid-infrared wavelength modulation spectroscopy, resulting in branching ratios consistent with the former indirect measurements within the assigned uncertainties.^33,56,59,60

The apparent differences between the reactions of alkyl peroxy radicals and functionalized peroxy radicals with HO₂ were addressed in several theoretical studies.^52,150–154 Although it was initially believed that RO₂ + HO₂ reactions proceed via a tetroxide intermediate (ROOOOH) similar to RO₂ permutation reactions, it was found that hydroperoxide formation occurs via a hydrogen-bonded pre-reactive complex (ROO··HOO) on a triplet potential energy surface (PES).^150,151,153 Other reaction channels, such as the formation of OH or O₃, proceed through the tetroxide intermediate on a singlet PES (Fig. 2).^52,150,152 As pointed out by Vereecken and Francisco,¹⁵⁵ the hydrogen bonding results in a submerged barrier for H migration (on the triplet surface); consequently, the accessibility of pathways other than hydroperoxide formation depends on the barriers for the decomposition of the tetroxide intermediate (formed on the singlet surface). In the case of simple alkyl peroxy radicals, these barriers cause all pathways proceeding via the ROOOOH intermediate to appear uncompetitive. However, for oxygenated peroxy radicals, such as those containing a carbonyl group, several theoretical calculations suggest lower energy barriers owing to strong hydrogen bonding in the tetroxide intermediate, hence facilitating decomposition towards other reaction products.^52,150,154

Fig. 2 Pathways of RO₂ + HO₂ reactions illustrated by CH₃C(O)O₂ + HO₂. The pathway yielding the hydroperoxide (peracid) proceeds over a triplet potential energy surface, while tetroxide formation proceeds over a singlet surface. The acetoxy radical formed in the OH channel immediately decomposes and yields CO₂ + CH₃O₂ under atmospheric conditions.

Interestingly, the number of alkyl peroxy radicals investigated in experimental product studies is rather small; so far, such studies have mainly addressed methyl^15,17,156 and ethyl peroxy,^16,25,27,65 followed by single investigations on neopentyl,¹⁸ cyclopentyl¹⁹ and cyclohexyl¹⁹ peroxy, and at least for the methyl peroxy radical, the occurrence of a second channel, yielding a carbonyl + H₂O (R5e), has been postulated. Jenkin et al.⁶² reported significant HDO formation from the photolysis of Cl₂/CD₄/CH₃OH/O₂ mixtures analysed by UV and infrared diode laser spectroscopy. Although the authors presented a detailed error analysis and identified potentially interfering processes yielding HDO, they were able to explain their results only by HDO formation (about 40%) from CD₃O₂ + HO₂. Subsequently, Wallington and Japar¹⁵ investigated CH₃O₂ + HO₂ by FTIR spectroscopy and concluded, based on the quantification of methyl hydroperoxide, that this reaction produces only a hydroperoxide. In their preceding work, the authors presented similar results on ethyl peroxy.¹⁶ In contrast, Elrod et al.¹⁵⁶ reported an 11% contribution of the carbonyl + H₂O channel from CH₃O₂ + HO₂ at 298 K, increasing to about 31% at 218 K.

Theoretical work on CH₃O₂ + HO₂ has been performed by applying different computational methods.^{151,153,157,158} Although most of these studies found methyl hydroperoxide formation to be the only reaction channel,^151,157 Drougas¹⁵⁸ concluded that formaldehyde formation is also accessible, particularly at lower temperatures. At present, IUPAC recommends a branching ratio for HCHO formation of 0.1 ± 0.1 at 298 K, while JPL continues to recommend a 100% yield for methyl hydroperoxide.^134,137 Owing to the ubiquity of methyl peroxy radicals throughout the troposphere, further studies on this subject, particularly on the temperature-dependence, would be very valuable.

Similarly, carbonyl formation of 71–100% has been reported for the reactions of halogenated alkyl peroxy radicals, such as CH_3−nCl_nO₂ (n = 1–3) or CH₂FO₂ with HO₂.^21–23 The co-product of this pathway was originally proposed to be H₂O. Meanwhile, theoretical work has shown that the channel proceeds over a singlet PES via a five-membered ring intermediate with the halogen atom exhibiting a stabilizing effect that lowers energy barriers.^151,152 The expected co-products are then OH + HO₂. These calculations reproduce the experimental data for CH₂FO₂ and CH₂ClO₂ radicals. Interestingly, no hydroperoxide was detected in the experiments on CHCl₂O₂ and CCl₃O₂ + HO₂.²² However, as pointed out by Hou et al.,¹⁵² these observations might rely on chemical activation in the nascent hydroperoxide product. Subsequent decomposition of the hydroperoxide yields carbonyl products, which will add to the carbonyl yield of the carbonyl + OH + HO₂ channel (R5d).¹⁵² Clearly, halogenated alkyl peroxy radicals behave differently from alkyl peroxy in their reaction with hydroperoxy radicals.

How relevant are RO₂ + HO₂ reactions to tropospheric chemistry?

Based on the reports of various field campaigns, it has been established that tropospheric HO₂ is of the order of 10⁸ cm⁻³ with peak daytime concentrations of up to 1 × 10⁹ cm⁻³ (e.g. Stone et al.¹³² and references therein). Since the rate coefficients for the reactions of organic peroxy radicals with HO₂ are close to those with NO (≈9 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ at 298 K), RO₂ + HO₂ becomes thus competitive to RO₂ + NO once NO concentrations are close to HO₂. Before it was discovered that unimolecular isomerization of peroxy radicals is competitive under atmospheric conditions, RO₂ + HO₂ reactions were generally considered to be one of the major loss processes in so-called “low NO” environments. One should note that, although the term “low NO” is insufficient to define a chemical regime, it is used here for simplicity and represents the conditions under which the reaction of peroxy radicals with NO no longer represents the major peroxy radical loss process.

H migration was found to be particularly rapid, e.g. for allylic peroxy radicals or migration of aldehydic H atoms.⁵ For example, unimolecular isomerisation was shown to account for about 50% of the loss of isoprene peroxy radicals at NO levels below 200 pptv.¹⁵⁹ Overall, rate coefficients for H migration depend heavily on the peroxy radical structure and thus span several orders of magnitude in contrast to rate coefficients for reactions of organic peroxy radicals with HO₂.⁵ As a rule of thumb, RO₂ + HO₂ appears competitive to unimolecular isomerisation at room temperature once H migration is <0.01 s⁻¹.

Low concentrations of NO, which are essential for the occurrence of RO₂ + HO₂ reactions, are commonly found in air masses that are not impacted by anthropogenic activities; for example, those found over tropical and boreal forests. However, this traditional perspective appears to change, e.g. as a consequence of changing emission profiles, and examples of “low NO” chemistry are also reported for urban areas. Praske et al. concluded that autoxidation becomes increasingly competitive to RO₂ + NO chemistry in urban and suburban areas of the United States owing to the overall reduction in NO_x emissions.¹⁶⁰ Although the authors have shown this effect mainly for peroxy radicals exhibiting large H-shift rate coefficients, the competitiveness of RO₂ + HO₂ reactions is also supposed to increase if NO_x emissions are further declining. Newland et al. provided evidence for traditional “low NO” chemistry in the polluted air masses of central Beijing in the afternoon when high levels of photochemically produced O₃ limit the availability of NO.¹⁶¹ Products of peroxy radical reactions with HO₂, namely hydrogen peroxide (HO₂ + HO₂) and peracetic acid (acetyl peroxy + HO₂), have also been identified in biomass burning plumes. Yokelson et al. observed initial fire emissions of both these peroxide species from biomass burning in the Yucatán; however, the authors highlighted that fast initial photochemistry may account partly for the significant amounts present in the young smoke.¹⁶² Indeed, according to current emission inventories, peracetic acid is not emitted directly from biomass burning,¹⁶³ yet elevated levels of both hydrogen peroxide and peracetic acid are observed when air masses are influenced by biomass burning plumes.¹⁶⁴ In essence, the occurrence of chemical conditions allowing for RO₂ + HO₂ appears to be less limited to typical pristine conditions and becomes more frequently encountered.

Role of RO₂ + HO₂ in the tropospheric HO_x budget

The reactions of peroxy radicals with HO₂ are primarily considered a sink for HO_x owing to the formation of an organic hydroperoxide, which (supposedly) terminates radical-chain reactions. This perspective appears simplified since OH formation was discovered for some RO₂ + HO₂ reactions. After model-to-measurement discrepancies indicated that significant unidentified HO_x recycling must occur in regions dominated by isoprene emissions (e.g. ref. 6, 11 and 165–167), it has been initially hypothesized that this might be the effect of peroxy radical reactions with HO₂.^6,11 However, following tremendous research activities during the last 15 years, there seems now to be a consensus that the efficient OH recycling emerges primarily from unimolecular isomerization of isoprene peroxy radicals (e.g. ref. 4, 159 and 168–170). Simultaneously, these radicals were shown to yield predominantly isoprene hydroperoxides (ISOPOOHs) when reacting with HO₂.¹⁷¹ The reader should note that this is different for peroxy radicals resulting from the major isoprene oxidation products, methacrolein and methyl vinyl ketone. Both methacrolein and methyl vinyl ketone peroxy radicals synthesized by the photolysis of VOC/alcohol/Cl₂/O₂/N₂ mixtures were shown to generate OH with yields of about 80% ± 20%.³² Praske et al.³⁹ confirmed significant OH formation from methyl vinyl ketone + OH under HO₂-dominated conditions, which consequently adds to the OH recycling by unimolecular isomerization.

Table 2 summarizes OH yields reported for reactions of HO₂ with different organic peroxy radicals. These were mostly determined for carbonyl-substituted, particularly acyl peroxy radicals. However, in addition to these data, some outstanding observations should be considered. Significant OH formation was reported to occur from the reaction of HO₂ with the β-hydroxy peroxy radicals of α-pinene (Fig. 3), following the observation of substantial pinonaldehyde formation under HO₂-dominated conditions, which is in contrast to the very low OH yield for simple β-hydroxy peroxy radicals, such as HOCH₂CH₂O₂.^37,56 One should note that, although a computational study showed that OH production appears thermodynamically accessible, at least for some of the α-pinene-derived peroxy radical isomers, a more recent experimental study performed under “low NO” conditions determined a substantially lower pinonaldehyde yield.^172,173 This stresses the need to extend the studies on OH recycling to other monoterpenes. Rollins et al. argued that the carbonyl/hydroperoxide ratio observed in their experiments suggests pathways other than hydroperoxide formation for the reaction of HO₂ with peroxy radicals formed in the isoprene + NO₃ system; subsequently, OH formation was reported for different nitrooxy alkylperoxy radicals derived from isoprene.^38,174 In addition, Birdsall and co-workers found OH regeneration in the oxidation of aromatics under HO₂-dominated conditions and proposed the reaction of bicyclic peroxy radicals with HO₂ to produce OH.^106,107

Table 2 OH yields reported for different RO₂ + HO₂ reactions at room temperature

Peroxy radical	OH yield (%)	Reference
a Benzoyl peroxy.b Surrogate for OH–(methacrolein)–O2.c Surrogate for OH–(methyl vinyl ketone)–O2.
CH3C(O)O2	40 ± 16	27
	<10	52
	43 ± 10	29
	50 ± 20	56
	61 ± 9	59
	51 ± 12	33
	48 ± 9	60
CH3CH2C(O)O2	40 ± 10	32
CH3CH2CH2C(O)O2	47 ± 15	32
CF3C(O)O2	52 ± 5	28
CF3CF2C(O)O2	50 ± 8	28
CF3CF2CF2C(O)O2	47 ± 11	28
CF3CF2CF2CF2C(O)O2	27 ± 18	28
C6H5C(O)O2^a	≈20	56
	20 ± 5	58
CH3C(O)CH2O2	67 ± 20	27
	≈15	56
	15 ± 8	30
	25 ± 13	32
	30 ± 4	61
CH3C(O)CH(O2)CH3	≈20	56
	58 ± 10	32
CH2ClC(CH3)(O2)CHO^b	80 ± 20	32
CH2ClCH(O2)C(O)CH3^c	80 ± 20	32
CH2OHCH(O2)C(O)CH3	48	39
HOCH2O2	20 ± 5	29
HOCH2CH2O2	<4	56
CH3OCH2O2	19 ± 8	31
O2NO–(isoprene)–O2	53	38

---

Fig. 3 OH production via the reaction of α-pinene-derived peroxy radicals with HO₂, as suggested by Eddingsaas et al.³⁷

It appears that two pathways define the role of the reactions of organic peroxy radicals with HO₂ in the HO_x budget. One is obviously OH production in the radical-propagating channel (R5c), which seems accessible for various oxygenated peroxy radicals and hence reduces the strength of acting as a HO_x sink. However, it is worth mentioning that, apart from the reaction of acyl peroxy radicals with HO₂, the further chemistry of the alkoxy co-product also recycles HO₂ as long as alkoxy isomerization is unimportant. The second major control for the HO_x budget is organic hydroperoxide formation, as mentioned earlier. Classifying the reactions of organic peroxy radicals with HO₂ as a HO_x sink relies purely on the fact that organic hydroperoxides supposedly act as temporary HO_x reservoirs on a significant timescale before possibly regenerating OH by photolysis. In addition, when gas-phase photochemical and photophysical processes are slow, the removal of organic hydroperoxides is likely dominated by heterogeneous losses. For example, the atmospheric lifetime of the most abundant organic hydroperoxide, methyl hydroperoxide, is on the order of 1 day; although its solubility is comparatively low, it was found in water droplets.^175–177 It was already demonstrated that dissolved organic hydroperoxides are still able to act as a source of HO_x and initiate photochemistry in the liquid phase.^178,179 The overall picture might change when considering more complex or multi-functional hydroperoxides. It is intriguing that, although formed through unimolecular processes, rapid photolysis of hydroperoxy aldehydes (HPALDs) is one of the key routes of OH recycling in isoprene chemistry.^159,180,181 In addition, Praske et al.³⁹ observed rapid photolysis of a multifunctional hydroperoxide formed in the reaction of HO₂ with a peroxy radical resulting from OH addition to methyl vinyl ketone. These results indicate that HO_x recycling from the reactions of organic peroxy radicals with HO₂ may also depend strongly on the identity of the organic hydroperoxide, and further studies on a larger set of species can help refine the role of RO₂ + HO₂ reactions in the tropospheric HO_x budget.

What are the challenges?

The (mainly historical) overview presented above, in particular on the reaction of the acetyl peroxy radical with HO₂, highlights that the quantification of organic hydroperoxides is not only key for understanding RO₂ + HO₂ reactions but certainly represents one of the major experimental challenges. Quantifying organic hydroperoxides is demanding for three reasons: the behaviour of organic hydroperoxides under experimental conditions, the accessibility of standards, and the rather limited number of analytical methods for gaseous organic hydroperoxides.

It is well known that hydroperoxides decompose readily; for example, they are thermally sensitive in general. However, two observations illustrate another feature besides thermal decomposition. Conversion processes of hydroperoxides into the corresponding carbonyl were reported to occur inside analytical instrumentation, such as GC systems or PTR-MS, leading to bias in both atmospheric and experimental observations if they remain unrecognized.¹⁸² Similar observations have been made in atmospheric simulation chamber experiments. Bernhammer et al.¹⁸³ investigated the formation and loss processes of isoprene oxidation products under low NO_x conditions. The authors showed that isoprene hydroperoxides (ISOPOOHs) are partly converted into methyl vinyl ketone and methacrolein on the stainless steel surface of the simulation chamber. Hence, a quantitative description of isoprene hydroperoxides was achieved indirectly by considering the larger-than-expected methyl vinyl ketone and methacrolein yields observed in the oxidation experiments.

Both observations rely on the same chemical principle: metal ions, such as copper, iron, and manganese, can initiate hydroperoxide decomposition by electron-transfer (e.g. Sanchez and Myers¹⁸⁴ and references therein):

ROOH + Mⁿ⁺ → RO + OH⁻ + M⁽ⁿ⁺¹⁾⁺ (R7)

ROOH + M⁽ⁿ⁺¹⁾⁺ → RO₂ + H⁺ + Mⁿ⁺ (R8)

If both reactions are accessible, that is both valence states can react as is the case for most transition metals, this results in the catalytic destruction of the hydroperoxide, which affects the experimental results.

The accessibility and usage of hydroperoxide standards are twofold challenging. First, it is obvious that special care must be taken in the preparation and handling of pure hydroperoxide samples, owing to the explosive nature of organic hydroperoxides, at least for hydroperoxides with a high peroxide oxygen content, such as methyl and ethyl hydroperoxide. An option to overcome this issue is the handling of diluted solutions, which, to a certain extent, limits their application to analytical methods quantifying hydroperoxides in the liquid phase. Although various synthesis routes exist, e.g. methylation of H₂O₂ by dimethyl sulphate for the preparation of methyl hydroperoxide,¹⁸⁵ the reaction of alkyl methanesulfonates (mesylates) with H₂O₂ for larger and branched alkyl hydroperoxides,^186,187 and the reaction of tertiary alcohols with H₂O₂ for tertiary alkyl hydroperoxides, the preparation of higher functionalized hydroperoxides is demanding. However, it should be mentioned that research efforts in the last decade have made accessible, for example, the preparation of isoprene hydroperoxides.^182,188,189

Additionally, achieving a quantitative conversion of a precursor compound into the desired hydroperoxide is not always possible. For example, peracids are easily synthesized by mixing concentrated H₂O₂ solutions with the corresponding acid often in the presence of an acidic catalyst (commonly sulphuric acid).¹⁹⁰ In the case of peracetic acid, this results in equilibrium mixtures containing a peracid fraction on the order of 40%. The determination of infrared absorption cross sections based on the vaporization of these samples requires scrupulously accounting for other volatilized components, such as H₂O, acetic acid and the acetic acid dimer, which hence represents a potential source of error. As pointed out by Orlando et al.,¹⁴⁷ this might have been one of the reasons for the low absorption cross section reported for peracetic acid by Crawford et al.²⁴ FTIR spectroscopy is one of the workhorses for analysing gaseous reaction systems in atmospheric simulation chamber experiments and has consequently been used regularly for the quantification of organic hydroperoxides. However, as discussed above, the reliability of the retrieved information depends on well-established absorption cross sections and hence on well-characterized procedures for vaporizing samples with known organic hydroperoxide content.

As presented in the overview section, the product study of Hasson et al.²⁷ was the first to use the revised absorption cross section for peracetic acid. However, the authors validated the FTIR data against the HPLC-fluorescence method. In separate experiments, by preparing gas phase mixtures of peracetic acid in the chamber, Hasson et al.²⁷ showed that FTIR and HPLC analyses agree within a factor of about 1.2. Based on the reported product yields in different experiments, both methods agreed on average within a factor of 1.2, 1.1 and 1.5 for methyl hydroperoxide, ethyl hydroperoxide and peracetic acid, respectively, in the oxidation experiments. There is no doubt about these experimental results, yet these data highlight that the quantification of organic hydroperoxides remains a challenging task. Hasson et al.³⁵ pointed out that hydroperoxide yields obtained from different ozonolysis systems (where RO₂ + HO₂ reactions occur) in several studies vary considerably although both experimental methodologies and analytical instrumentation were similar. The authors presumed that this might be due to the complexity of applying the HPLC-fluorescence technique to simulation chamber experiments. In addition, the selectivity of horseradish peroxidase (HRP) limits the detectability of organic hydroperoxides. Although linear n-alkyl hydroperoxides up to C₁₈ were found to react with HRP with similar efficiencies, secondary and tertiary hydroperoxides exhibit either significantly lower responses or are undetectable.^191,192

Overall, there is a need for additional analytical methods preferably preserving the chemical integrity of the sample. Chemical ionisation mass spectrometry (CIMS), which is increasingly used in atmospheric science, is certainly one of these methods. Generally, the number of ionisation schemes used in CIMS instruments is increasing; particularly, reagent ions, such as NH₄⁺ or I⁻, are promising. It is realistic to estimate that a large body of work will be necessary to reduce overall uncertainties and that the lack of hydroperoxide standards might remain a limiting factor.

Another major experimental challenge relies on the procedures used for peroxy radical generation and the subsequent radical chemistry. Most of the data on reaction products and branching ratios were obtained from atmospheric simulation chamber experiments by either Cl atom reactions of an organic precursor or VOC + OH oxidation experiments performed under HO₂-dominated conditions. It is obvious that the generation of (almost exclusively) a single peroxy radical in the initial step is limited in these approaches to small organics such as methane (for methyl peroxy), ethane (for ethyl peroxy), acetaldehyde (for acetyl peroxy), acetone (for acetonyl peroxy) or unsubstituted cycloalkanes (for cycloalkyl peroxy). For larger molecules, the reaction with Cl, and even OH, concurrently generates several different peroxy radicals, initiating a series of RO₂ permutation reactions. As illustrated by the reaction sequence of the acetyl peroxy radical (Fig. 4), even the generation of a single RO₂ species results in a cascade of reactions producing additional RO₂ species. Moreover, this is complicated by the fact that further oxidation of closed-shell first-generation products adds to the radical pool by generating peroxy radicals and possibly OH. Advances in the understanding of site-specific chemistry (e.g. ref. 193) and refined structure–activity relationships (e.g. ref. 194) will help at least to determine the relative ratio of peroxy radicals initially formed in oxidation systems more accurately; however, as pointed out by Ervens et al.,¹⁹⁵ further experimental work on this subject is highly required. In addition, there is a growing number of reports on the generation of specific peroxy radicals from photolytic precursors, such as iodine-containing organic compounds.^63,196–200

Fig. 4 Reaction sequence for CH₃C(O)O₂ in the absence of NO. The co-product O₂ is omitted for readability reasons. Branching ratios (in bold) represent the current IUPAC recommendations.¹³⁴

The addition of an HO₂ precursor aims at shifting the reaction system towards conditions where the reaction of RO₂ with HO₂ is favoured, and in the limiting case, RO₂ reacts almost exclusively with HO₂. However, both the experimental set-up and the chemistry often impede the attainment of this rather ideal case. The amount of HO₂ precursor is often limited by the deployed analytical instrumentation; for example, high concentrations of methanol complicate the analysis of FTIR spectra or might deplete the reagent ions in CIMS instruments. In addition, when increasing the HO₂ concentration, the self-reaction and the reaction with aldehydes, particularly with formaldehyde, become significant loss processes for HO₂.

The overall consequence is that the kinetic information on RO₂ + HO₂ reactions is mostly obtained from the modelling of a complex chemical reaction system. This not only requires accurate analytics data but also information on rate coefficients and branching ratios of all involved reactions. Missing or incomplete data add uncertainty to the branching ratio determination for the target reaction. For instance, acetic acid is formed from CH₃C(O)O₂ + HO₂, as discussed above, but it also appears to arise from the cross-reaction of acetyl peroxy with methyl peroxy (Fig. 4). According to the currently recommended values, this channel accounts for about 10% of the CH₃C(O)O₂ + CH₃O₂ reaction.¹³⁴ However, the relative error is 100%. Recently, Assali and Fittschen¹²⁴ reported a rate coefficient for the cross-reaction, which is a factor of two larger than the current recommendations. Simultaneously, they obtained a lower contribution from the radical-propagating channel.

The above-discussed challenges, partly or entirely, also apply to the absolute rate coefficient determination work. Most of the kinetic data presented in Table 1 were obtained from flash photolysis or pulsed laser photolysis experiments combined with UV absorption spectroscopy in the 1990s and early 2000s. Accordingly, accurate absorption cross sections of the radical species present in the system and a successful separation of overlapping absorption bands are crucial for the determination of rate coefficients. Since rate coefficients are finally obtained by fitting kinetic parameters to the decay rates recorded for the respective species, the uncertainty of the rate coefficient depends on the accuracy of the available kinetic mechanism. For example, by trend smaller rate coefficients and stronger temperature dependence were reported for CH₃C(O)O₂ + HO₂ before the radical-propagating channel was discovered and included in the analysis.^{24,33,42,52,56,59,60}

Some of these issues were minimized by Boyd et al.⁵⁵ by determining the rate coefficients under a large excess of HO₂. The authors employed nearly pseudo-first order conditions using an excess of H₂O₂, hence suppressing RO₂ self-reactions and subsequent chemistry. However, this approach is limited to reaction systems without significant radical recycling. Østerstrøm et al.⁶⁹ reduced the uncertainties in the CH₃O₂ + HO₂ rate coefficient significantly by detecting methyl peroxy and HO₂ using the FAGE-LIF method and applying an updated kinetic model, which uses a revised rate coefficient for the CH₃O₂ self-reaction they have obtained in preceding work.²⁰¹ The authors determined temperature-dependent rate coefficients of about 15% lower than current recommendations.

The progress made in the detection of peroxy radicals and possibly the rethinking of experimental designs will help reduce uncertainties. For example, Zuraski et al.⁶¹ investigated the kinetics and OH production from the acetonyl peroxy + HO₂ reaction by monitoring the three radical species independently using a combination of IR and UV measurements. However, further advances in the detection of radicals, particularly with respect to chemical speciation, are still desirable.

Gaps in knowledge

Substantial progress has been made over the last approximately 25 years in understanding the reactions of organic peroxy radicals with HO₂, particularly with respect to the occurrence of pathways other than organic hydroperoxide formation. However, there remain several areas that require further research as a consequence of either a lack of data or significant uncertainties and divergence in existing data:

(1) Rate coefficients

Currently, there are few recommended values (assigned with large uncertainty ranges) for the rate coefficients of the title reactions, which are insufficient to cover the variety of peroxy radicals that are potentially produced in the troposphere.

(2) OH formation

As evident from Table 2, significant scattering is found for the OH yield of some RO₂ + HO₂ reactions, and additional measurements are required to reduce overall uncertainties. Thus far, most data on acyl peroxy radicals have been reported. Systematic investigations of a broader range of peroxy radicals would help draw a more comprehensive picture of OH formation from RO₂ + HO₂ and, more specifically, assess the influence of functional groups and structures on OH yield.

(3) β-Hydroxy peroxy radicals

The dataset on β-hydroxy peroxy radicals is scarce. Given the importance of this class of peroxy radicals in the atmosphere (they are formed following the addition of OH to nearly all unsaturated VOC) and the apparent discrepancies reported in the literature (e.g. ethene-derived vs. α-pinene-derived peroxy radicals), there is a need for systematic investigations on β-hydroxy peroxy radicals with respect to structural variation.

(4) Multifunctional peroxy radicals

Investigations reported in the literature are limited to peroxy radicals containing very few different functional groups. Given the partial focus on acyl peroxy radicals, additional systematic studies on the different functional groups of oxygenated peroxy radicals would be valuable for refining our understanding of RO₂ + HO₂ reactions. In particular, it is entirely unknown at present how reactivity and product branching ratios are affected by a combination of different functionalities.

(5) Temperature-dependence

Although the temperature-dependence was reported for the absolute rate coefficients of a range of peroxy radicals (Table 1), the temperature-dependence of product branching ratios, in particular for oxygenated peroxy radicals, is not well established. Dedicated temperature-dependent product studies were, to the best of the author's knowledge, reported only for ethyl peroxy²⁵ and acetyl peroxy.⁶⁰ There is a clear need for further research to assess the relative change in branching ratios with temperature.

(6) Hydroperoxides

As discussed above, the atmospheric fate of an organic hydroperoxide is crucial for the HO_x budget related to a specific RO₂ + HO₂ reaction. Accordingly, organic hydroperoxides may be regarded as an additional area of research, particularly in the case of multifunctional hydroperoxides, which are potentially more sensitive to photolysis.

(7) Theoretical investigations

Few studies have applied computational methods to RO₂ + HO₂ reactions. As with experimental data, further theoretical studies, particularly on the accessibility of pathways other than hydroperoxide formation for different peroxy radical structures, are desirable.

Conclusions

The present work highlights that despite the progress made in understanding RO₂ + HO₂ reactions, there are several areas requiring further research activities. Although H-shift isomerization is now accepted to represent a significant peroxy radical loss process for certain peroxy radical classes, it might be less competitive for others. Thus, under pristine-like conditions, the reaction with HO₂ still represents one of the major loss processes for a large range of peroxy radicals formed in the atmosphere. This, together with the observed increased incidence of low NO_x tropospheric conditions, indicates that the systematic scrutiny of the title reaction is essential to draw a comprehensive image of the peroxy radical chemistry.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this perspective.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

NI gratefully acknowledges Iulia Patroescu-Klotz for proofreading this manuscript.

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