Journal topic
Atmos. Chem. Phys., 18, 8001–8016, 2018
https://doi.org/10.5194/acp-18-8001-2018
Atmos. Chem. Phys., 18, 8001–8016, 2018
https://doi.org/10.5194/acp-18-8001-2018

Research article 07 Jun 2018

Research article | 07 Jun 2018

# Investigation of the oxidation of methyl vinyl ketone (MVK) by OH radicals in the atmospheric simulation chamber SAPHIR

Investigation of the oxidation of methyl vinyl ketone (MVK) by OH radicals in the atmospheric simulation chamber SAPHIR
Hendrik Fuchs1, Sascha Albrecht1, Ismail–Hakki Acir1,a, Birger Bohn1, Martin Breitenlechner2, Hans-Peter Dorn1, Georgios I. Gkatzelis1, Andreas Hofzumahaus1, Frank Holland1, Martin Kaminski1,b, Frank N. Keutsch2, Anna Novelli1, David Reimer1, Franz Rohrer1, Ralf Tillmann1, Luc Vereecken1, Robert Wegener1, Alexander Zaytsev2, Astrid Kiendler-Scharr1, and Andreas Wahner1 Hendrik Fuchs et al.
• 1Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich GmbH, Jülich, Germany
• 2School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA
• anow at: Institute of Nutrition and Food Sciences, Food Chemistry, University of Bonn, Germany
• bnow at: Bundesamt für Verbraucherschutz, Abteilung 5 – Methodenstandardisierung, Berlin, Germany

Correspondence: Hendrik Fuchs (h.fuchs@fz-juelich.de)

Abstract

1 Introduction

Isoprene (C5H8) emitted by plants has the highest emission rate among non-methane organic compounds. Isoprene is mainly oxidised by the photochemically generated hydroxyl radical (OH) forming the first-generation organic compounds methacrolein (MACR), methyl vinyl ketone (MVK), formaldehyde (HCHO and isoprene hydroxyperoxides (ISOPOOH) (e.g. Karl et al.2006). The latter ones are formed without the involvement of nitric oxide (NO) so that ISOPOOH becomes increasingly important with decreasing concentrations of nitrogen oxides which are mainly released by anthropogenic activities. The formation of MVK and MACR is accompanied by the production of HO2, which can further recycle OH, whereas ISOPOOH formation is a radical termination reaction. However, field studies have shown that also in environments where NO concentrations are less than a few 100 pptv a high OH regeneration rate can be maintained which is not explained by chemical models . The gap between measured and modelled OH is correlated with the abundance of isoprene . Since then, it has been recognised that organic peroxy radical (RO2) pathways which do not require NO as reaction partner can also significantly recycle OH . These reactions include

• hydrogen-shift reactions of RO2 radicals forming OH or HO2,

• reaction of peroxy radicals with HO2.

These pathways have both been identified in the oxidation chain of isoprene . A 1,6 H-shift of RO2 formed in the reaction of isoprene with OH leads to the formation of hydroxyperoxy aldehydes (HPALD) and HO2. The photolysis of HPALD gives even additional OH radicals. Its relevance for the atmosphere has been shown in laboratory experiments and chamber experiments . In the supplement of the first publication of the LIM the authors also suggested that a 1,5-H-shift reaction could be relevant for MVK and MACR. However, for MACR a 1,4 H-shift reaction for RO2 was found, which efficiently recycles OH . Its impact on the radical budget has been shown in chamber experiments . H-shift reactions have also been proposed to be important for RO2 radicals which are formed in the oxidation chain of ISOPOOH also potentially enhancing the OH regeneration rate.

The reaction of peroxy radicals with HO2 forms not only hydroxyperoxides, but also OH together with an alkoxy radical. This was shown for the acetylperoxy radical with an OH yield of 50 % . In a recent study by , product yields from the reaction of MVK derived peroxy radicals with HO2 were investigated. Similar to the acetylperoxy radical, product yields demonstrated that only one third of the reaction yields hydroxyperoxides and that two additional reaction channels exist, both of which could lead to the reformation of OH.

In this study, the oxidation of MVK by OH was investigated in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric Photochemistry In a Large Reaction Chamber) at Forschungszentrum Jülich. Experiments were performed under controlled conditions with atmospheric trace gas and radical concentrations. In these experiments, not only organic compounds like in previous studies were measured, but also radical species (OH, HO2, and RO2) allowing for an analysis of the OH budget. In the low NO case, NO mixing ratios were kept below 100 pptv so that different RO2 radical reactions, i.e. reaction with HO2 and unimolecular isomerisation reactions, can compete. Measured time series of radical concentrations are compared to model calculations applying the Master Chemical Mechanism version 3.1.1 (MCM2017) and modifications suggested in literature.

2 Methods

## 2.1 Simulation experiment in SAPHIR

Experiments were performed in the outdoor atmospheric simulation chamber SAPHIR. Details of the chamber can be found in previous publications (e.g. Rohrer et al.2005). SAPHIR has a cylindrical shape (length 18 m, diameter 5 m, volume 270 m3 and consists of a double-wall FEP (fluorethylene-propylene) film. A small overpressure (45 Pa) prevents ambient air entering the chamber. The replenishment flow that is required to maintain this pressure leads to a dilution of all trace gases by approximately 3 to 5 % per hour. A shutter system shades the chamber before the photooxidation experiment is started. Natural sunlight is used to irradiate the mixture. Small sources of nitrous acid (HONO) and formaldehyde (HCHO) are present in the sunlit chamber (100 to 200 pptv h−1). The photolysis of HONO is typically the primary source for OH radicals and nitrogen oxides.

In total, four experiments were conducted in this study, two of them at low NO (23 June 2016: NO< 70 pptv and 23 May 2017: NO< 40 pptv) and two of them at high NO conditions (20 August 2014: 0.7 to 6 ppbv NO and 17 May 2017: approximately 0.1 to 0.4 ppbv NO). Results from experiments performed at similar conditions gave consistent results. The discussion of results focuses on the two experiments for which the number of trace gas measurements was highest and results for the other experiments are shown in the Supplement.

The experiments started with cleaning the chamber air by flushing out impurities from previous experiments until trace gas concentrations were below the detection limit of the instruments. The chamber air was first humidified by flushing water vapour from boiling water into the dark chamber (relative humidity approximately 70 %). In the low NO experiments, approximately 140 ppbv ozone produced from a silent discharge ozoniser (O3onia) was injected in the dark in order to suppress NO concentrations. In contrast, 6 to 10 ppbv of NO2 or NO were injected from a gas mixture in case of the high NO experiments. In one of the two low NO experiments (23 May 2017), 20 ppbv MVK (Sigma-Aldrich, purity 99 %) in water was injected in the dark chamber from a Liquid Calibration Unit (LCU, Ionicon). MVK (1.5 ppbv) was reinjected after 3.5 h of photooxidation. In the other experiments, MVK was injected several times during the experiment after an initial phase of illumination of the chamber air without additional OH reactants by injecting liquid MVK into a heated inlet line that is flushed by synthetic air. This procedure was similar to that applied in previous studies (e.g. Fuchs et al.2013, 2014; Kaminski et al.2017). The photooxidation of MVK was then observed for several hours. No significant particle formation was observed in the experiments so that only gas-phase chemistry needs to be considered in the evaluation.

## 2.2 Instrumentation

Trace gas concentrations were measured with a comprehensive set of instruments. Nitric oxide (NO) was detected by chemiluminescence (Eco Physics) and nitrogen dioxide (NO2) by the same instrument but with a blue-light converter in the inlet. In one of the experiments (17 May 2017), no NOx measurements were available. A cavity ring-down instrument (Picarro) monitored water vapour and carbon monoxide and a UV photometer (Ansyco) detected ozone.

The total OH reactivity (inverse lifetime of OH) was measured by a pump-probe method , in which the decay of OH radicals produced by laser flash photolysis of ozone is observed by laser-induced fluorescence (LP-LIF). OH reactivity gives a measure of all OH reactant concentrations, so that potential gaps in the detection of e.g. organic compounds that are relevant for the radical chemistry can be identified (e.g. Nölscher et al.2012). Unfortunately, the instrument failed in 2014, thus OH reactivity was only measured in one of the two high NO experiments.

Organic compounds were measured by a proton-transfer time-of-flight mass spectrometer (PTR-TOF-MS, Ionicon), which was calibrated to quantify MVK. Methylglyoxal (CHOCOCH3, MGLYOX), and glycolaldehyde (HOCH2CHO) were quantified in one low and one high NO experiment. In the other two experiments, performed in different years, the PTR-TOF-MS was calibrated for MVK, but not for methylglyoxal and glycolaldehyde for all experiments so that these species could not be quantified in all experiments. Acetic acid was detected on the same mass as glycolaldehyde in the PTR-TOF-MS instrument. However, model calculations suggest that the contribution of acetic acid was less than 10 % of the total signal. Therefore, measurements represent glycolaldehyde concentrations reasonably well.

A second PTR-TOF-MS instrument (PTR-3, Ionicon) quantified MVK concentrations in the experiment on 23 May 2017. Measurements of both instruments agreed within 20 %. In addition to direct measurements of MVK concentrations, measurements of the OH reactivity can be used to calculate the MVK concentration that was injected in the experiments because the increase in OH reactivity at that point in time can be attributed to the MVK concentration increase. The comparison with the increase in MVK measurements by the PTR-TOF-MS instrument shows good agreement.

In the experiments in 2017, formaldehyde was measured by the same differential optical absorption spectroscopy (DOAS) instrument that also detects OH radicals in the chamber . In the other years, HCHO was measured by a Hantzsch monitor. The 1σ-precision of the formaldehyde measurement of 230 pptv is less than that of the Hantzsch monitor (20 pptv), but it is sufficiently high for the detection of HCHO in the experiments here.

Figure 1Simplified OH oxidation scheme for MVK. Names of compounds are assigned similar to MCM. Modifications to the MCM mechanism (M2) applied in model sensitivity runs M1 and M2 (Table 1) are shown as grey arrows. Reaction yields are calculated for conditions of the experiment with low NO (high ozone concentrations) on 23 May 2017. Grey numbers refer to model run M2.

OH was detected by DOAS in all experiments except for the high NO experiment in 2014. In addition, OH, HO2 and RO2 radicals were measured by laser-induced fluorescence (LIF). The instrument has been described in detail elsewhere . OH concentrations measured by LIF in the SAPHIR chamber have been shown to agree with measurements by DOAS in several comparison exercises (e.g. Schlosser et al.2009; Fuchs et al.2012). Good agreement was also observed in this work so that significant potential artefacts in the LIF detection scheme as reported for some instruments in the field can be excluded.

HO2 and RO2 are chemically converted to OH by the reaction with NO prior to OH detection by laser-induced fluorescence in the LIF instrument. The conversion of RO2 requires at least two subsequent reactions with NO. First, RO2 is converted through adding NO to HOx in a flow reactor upstream of the fluorescence cell. Added CO in the reactor ensures that the HOx consists predominantly of HO2, which has a small wall loss compared to OH. The reactor is operated at higher pressure (25 hPa) compared to the low-pressure (4 hPa) LIF detection cell . The HOx is sampled from the reactor into the LIF detection cell where HO2 is converted by a large excess of added NO to OH. Operational parameters of the RO2 system are optimized for the efficient detection of RO2 radicals that have a similar reaction rate with NO as methylperoxy radicals. As a consequence, RO2 radicals are not efficiently detected, if their reaction with NO does not directly and quantitatively result in the production of HO2. This is for example the case for the peroxy radical HMVKBO2 (as named in the MCM) that is formed from the reaction of MVK with OH (see below for details). This has to be taken into account, if measured RO2 radicals are compared with model calculations.

The HO2 detection cell consists of a fluorescence cell, in which HO2 reacts with excess NO that is injected behind the inlet nozzle. As shown for several LIF instruments, the HO2 signal can also contain contributions from RO2 radicals that rapidly form HO2 in the reaction with NO . This applies for those RO2 radicals which form an alkoxy radical (RO) in the reaction with NO that rapidly produces HO2 and other products. The interferences from RO2 can be minimised; however, if the instrument is operated with an NO concentration, for which the HO2 to OH conversion efficiency is only approximately less than 10 %. In this case, the RO2 to OH conversion efficiency becomes much smaller for all RO2 species, because the two reactions with NO needed to produce OH limit the overall conversion efficiency. In this study, the HO2 channel of the LIF instrument was operated such that RO2 interferences can be assumed to be negligible.

In one of the experiments (23 May 2017), HO2 was additionally detected by a newly developed chemical ionisation mass spectrometry (CIMS) instrument using Br as ionisation reagent. HO2 is detected as cluster ion similar to the approaches reported by using an I and Br CIMS, respectively. Details of this new instrument will be presented in a separate publication. HO2 measurements of the CIMS instrument agreed with [HO2] detected by the LIF instrument within 15 %.

Solar radiation was measured outside the chamber using a spectroradiometer. Photolysis frequencies are then calculated by applying a model to transfer outside conditions to conditions inside the chamber . Latest recommendations for absorption spectra and photolysis yields are used.

Table 1Changes of reactions and additional reactions applied to the MCM.

a value from MCM: 0.625 KRO2HO2 $=\mathrm{2.91}×{\mathrm{10}}^{-\mathrm{13}}\mathrm{exp}\left(\mathrm{1300}\phantom{\rule{0.125em}{0ex}}\mathrm{K}\phantom{\rule{0.125em}{0ex}}{\mathrm{T}}^{-\mathrm{1}}\right)$cm3 s−1 (MCM2017) b from theoretical calculation (see Table 2). c A reaction partner could not be determined from these experiments.

## 2.3 Model calculations

Model calculations were performed using the Master Chemical Mechanism in its latest version 3.3.1 (MCM2017). A simplified reaction scheme is shown in Fig. 1. The MCM mechanism was modified (MCM*) to take results reported in literature into account and findings in this work. Details are listed in Table 1.

Chamber specific properties were added such as dilution of traces gases due to the replenishment flow. Sources for HONO and HCHO production from the chamber were parameterised as described in previous publications (e.g. Fuchs et al.2014; Kaminski et al.2017).

Model calculations were constrained to physical parameters (pressure, temperature, photolysis frequencies and dilution rate of trace gases). A small, constant background OH reactivity of unknown OH reactants that was measured by the OH reactivity instrument after humidification of the chamber air was modelled as an OH reactant that converts OH to HO2. However, the magnitude of this background reactivity was small (<1s−1) compared to the OH reactivity from MVK during the experiment (>15s−1) so that it did not affect the chemistry.

Injections of trace gases were modelled as sources during the time of injection, but injected trace gases were not constrained to measured values at later times. [NO], [NO2] and [O3] were only constrained to measurements for the high NOx experiment, because differences between modelled and measured values would have led to significant differences in other observables. No modelling could be performed for one of the high NO experiments (17 May 2017) due to the lack of NOx measurements. No measurements for the reaction rate constants of RO2 species from MVK exist. The sensitivity of model results to a change of the RO2 reaction rate constants, however, is rather small so that their uncertainties could not explain observed model–measurements discrepancies.

Table 2H-migration and HO2 elimination in $\mathrm{hydroxy}-\mathrm{MVK}-\mathrm{peroxy}$ radicals. Barrier height Eb, reaction energy Ereact and the rate coefficient k at a temperature of 300 K are listed. Arrhenius expressions for a temperature range between 200 and 400 K are available in the Supplement.

a estimated at 0.01s−1 by .

## 2.4 Quantum-chemical calculations

A set of H-migration reactions for the main MVK-derived peroxy radicals was investigated by quantum chemical and theoretical kinetic methodologies. The reactions studied included migration of hydroxyl, α−OH, and methyl H-atoms; direct HO2 elimination forming an enol was also investigated (Table 2).

Several methodologies were applied, as detailed in the supporting information. From these data, the M06-2X/cc-pVTZ rovibrational data , with CCSD(T)/aug-schwartz4(DT) single point energy calculations extrapolated to the basis set limit were selected. All quantum chemical calculations were performed using the Gaussian-09 program suite . The high-pressure rate coefficients for each of the elementary processes was then calculated using multi-conformer canonical transition state theory, MC-CTST based on a rigid rotor harmonic oscillator paradigm, an exhaustive search of the reactants and TS conformers, and asymmetric Eckart tunnelling and WKB zero-curvature (ZCT) tunnelling. For the 1,4- and 1,6-H-shift in HMVKAO2, a large difference between Eckart and ZCT tunnelling was found; the geometric average is reported here (see Supplement).

Figure 2Time series of radicals, inorganic and organic species during the MVK photooxidation for the high NO experiment (20 August 2014) together with results from model calculations applying MCM. Dark shaded areas indicate the time before opening the chamber roof and vertical dashed line times when trace gases were injected into the chamber. OH reactivity was not measured during this experiment. NO, NO2 and O3 are constrained to measurements in the model. RO2 loss rates (most lowest right panel) are calculated from modelled HO2, RO2 and NO concentrations. However, contributions from the reactions with RO2 and HO2 or RO2 are too small to be visible. Modelled acetic acid concentrations are small compared to modelled glycolaldehyde concentrations (measured together in the PTR-TOF-MS).

Figure 3Time series of radicals, inorganic and organic species during the MVK photooxidation at low NO (23 May 2017) together with results from model calculations applying MCM. Dark shaded areas indicate the time before opening the chamber roof and vertical dashed line times when trace gases were injected into the chamber. RO2 loss rates (most lowest right panel) are calculated from modelled HO2, RO2 and NO concentrations.

3 Results

## 3.1 Product yields

The reaction of OH with MVK leads to the addition of OH to either one of the double-bonded C-atoms so that two different RO2 radical species can be formed (Fig. 1):

$\begin{array}{}\text{(R1)}& \mathrm{MVK}+\mathrm{OH}& \to & {\mathrm{HMVKAO}}_{\mathrm{2}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\left(\mathrm{yield}:\mathrm{0.3}\right),\text{(R2)}& \mathrm{MVK}+\mathrm{OH}& \to & {\mathrm{HMVKBO}}_{\mathrm{2}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\left(\mathrm{yield}:\mathrm{0.7}\right).\end{array}$

Yields are from current recommendations that are also used in the MCM. The further reaction chain with NO gives glycolaldehyde and an acetylperoxy radical for HMVKBO2 and methylglyoxal, formaldehyde and HO2 for HMVKAO2. In the low NO experiment (23 May 2017), approximately 30 % of the RO2 reacted with NO assuming that the reaction with HO2 was the only competing reaction. In contrast, more than 90 % of RO2 reacted with NO in the high NO experiment. Mixing ratios of the major products formaldehyde, glycolaldeyde and methylglyoxal increased to 6, 8, and 1.3 ppbv, respectively, in the high NO experiment when 13 ppbv MVK was oxidised (Fig. 2). Less of these products was observed in the low NO experiment with 4.5 ppbv formaldehyde, 5 ppbv glycolaldehyde and 1 ppbv methylglyoxal (Fig. 3) when 17 ppbv MVK is oxidised. The smaller concentrations of these products in the low NO case might be expected, because other products can be formed in the competing RO2 reaction channels (Fig. 1).

Table 3Yields of organic products from photooxidation of MVK by OH from this work and from the literature. Errors of values from this work take into account the accuracy of measurements and precision of the calculation. HCHO is not only produced in the first oxidation step of MVK but also produced in the subsequent oxidation of glycolaldehyde and methylglyoxal. Therefore, the yield can increase over the course of the experiment.

a approximately after 1 h of oxidation. b after 3 h of oxidation. c RO2+RO2 contribution to the total RO2 loss: <20 %.

Because part of the products are oxidised during the experiment, a correction procedure described in detail by and is applied, in order to calculate products yields originating from the oxidation of MVK. This correction takes loss of products due to the reaction with OH and photolysis into account and also small production from chamber sources. The relationship between consumed MVK and corrected product concentrations gives the yield of the product species (Table 3, Fig. S3).

For the high NO experiment, when RO2 nearly exclusively reacted with NO, the ratio of product yields for glycolaldehyde (0.65±0.14) and methylglyoxal (0.19±0.03) can be related to the branching ratio of the MVK reaction with OH (Reactions 1 and 1), because these products are formed in either one of the two reaction channels. The values derived from this experiment agree within their uncertainties with studies by , , and all reporting lower methylglyoxal yields than suggested by current recommendations . This indicates that the branching ratio assumed in the MCM of 0.7 : 0.3 needs to be shifted towards HMVKBO2. suggests a branching ratio of 0.76 : 0.24. The formaldehyde yield of 0.73±0.15 is higher than the methylglyoxal yield, because formaldehyde is not only a co-product of methylglyoxal in the reaction chain of HMVKAO2 (Fig. 1), but can also be formed from secondary products in the oxidation scheme of MVK. This might also explain why the HCHO yield in this chamber experiment is higher than that in the study by . Product yields reported by differ for all three species (largest difference for formaldehyde) from the other studies for unclear reasons.

In the low NO case, other RO2 reaction channels than reaction with NO gain in importance. Lower yields of methylglyoxal (0.05±0.002) and glycolaldehyde (0.37±0.09) compared to the high NO experiment are therefore found in this case. However, the relative decrease of glycolaldehyde is much smaller than that of methylglyoxal. This indicates that glycolaldehyde is also formed from other reaction channels than the reaction of HMVKBO2 with NO. This agrees with results reported by . In that study, a glycolaldehyde yield of 0.38±0.05 was determined in experiments, in which RO2 exclusively reacted with HO2.

## 3.2 Modelled and measured time series of radical concentrations

Time series of trace gas and radical concentrations are shown together with model calculations using the MCM without modifications for one of the high NO experiments (Fig. 2, 20 August 2014) and for one of the low NO experiments (Fig. 3, 23 May 2017). Although the instrumentation that performed measurements in the experiments were partly different specifically for experiments done in different years, consistent results are obtained. In addition, parameters that are measured by two instruments agree within their combined uncertainties.

MVK (12 ppbv) was injected twice into the sunlit chamber in the high NO experiment (Fig. 2). Approximately half of the MVK reacted away before the second MVK addition was done. The NO mixing ratio decreased over the course of the experiment from nearly 6 to 0.5 ppbv, but was sufficiently high that 90 % of HO2 reacted with NO for most of the time. OH concentrations ranged from (2–4)×106cm−3 modulated by changes of the OH reactivity and radiation. Model calculations of [OH] agree with measurements within 20 % at all times during the oxidation of MVK. This corresponds to a good description of the measured MVK concentration by the model (deviations less than 5 %) demonstrating that the OH concentration fits the observed oxidation rate of MVK.

HO2 and RO2 radical concentrations were rather small ($<\mathrm{2}×{\mathrm{10}}^{\mathrm{8}}$cm−3) due to their fast loss in the presence of high NO. Measured and modelled HO2 concentrations show good agreement until the NO mixing ratio decreased below 0.5ppbv (at 11:00, Fig 2). In the case of RO2, the model yields significantly larger concentrations than measurements. This discrepancy is plausible due to the incomplete conversion of the peroxy radical HMVKBO2 in the pre-reactor. The transformation to HO2 requires more than one NO reaction step and therefore remains incomplete during the transit through the reactor. If the modelled concentration of [HMVKBO2] is subtracted from the total modelled [RO2], good model–measurement agreement is obtained.

Overall, the good agreement between modelled and measured radical concentrations demonstrates that radical chemistry during the oxidation of MVK is well described by state-of-the-art chemical models, if RO2 radicals are mainly lost in the reaction with NO.

In the low NO experiment (Fig. 3), approximately 20 ppbv of MVK was injected, before photooxidation started. As a consequence of the low NO (<40pptv), RO2 radicals formed in the reaction of MVK with OH not only reacted with NO, but reaction with HO2 and RO2 were competitive. Model calculations using the MCM (Fig. 3) suggest that at least half of the RO2 reacted with HO2 and a smaller fraction (10 to 20 %) with other RO2 radicals.

After nearly 6 h of oxidation, only 4 ppbv MVK was left in the presence of (2–4)×106cm−3 OH. The amount of MVK that is injected in the model is adjusted to the increase in OH reactivity during the time of injection. The 10 % discrepancy to measured MVK mixing ratios is within the uncertainty of the PTR-TOF-MS calibration. However, the decay of the measured [MVK] is slightly faster than the decay in the model specifically during the first 2 h of oxidation. This corresponds to modelled OH concentrations, which are up to a factor of 2 smaller than measured OH concentrations during this time. At later times of the experiment, measured and modelled [OH] as well as the relative change in [MVK] are matched. Differences between measured and modelled OH concentrations in the first phase of the experiment are accompanied by HO2 concentrations, which are approximately 2×108cm−3 lower in the model compared to measured values. Measured [HO2] increased from (4–8)×108cm−3 during the first 2 h. Modelled values match measurements at later times of the experiment like observed for [OH]. The concurrent underestimation of [OH] and [HO2] suggests that a radical source is missing in the model.

In a sensitivity run (MCM*), modifications of reactions that are reported in literature, but do not directly affect the fate of RO2 are implemented:

• The reaction rate constant of glycolaldehyde with OH is lowered by 20 % following measurements by .

• Following the results of the product analysis (see above), the branching ratio of Reaction (1) and 1 is changed from 0.3:0.7 to 0.24:0.76 as suggested by .

• OH yield from ozonolysis of MVK is lowered to 16 % as determined by and compared to 36 % assumed in the MCM.

Details are listed in Table 1. Differences with results with the current version of the MCM are rather small (not shown here) so that these modifications do not significantly affect the model–measurement agreement. They are included in the sensitivities model runs shown below.

Figure 4Time series of radicals, inorganic and organic species during the MVK photooxidation at low NO (experiment on 23 May 2017). Dark shaded areas indicate the time before opening the chamber roof and vertical dashed line times when trace gases were injected into the chamber. Model sensitivity runs M1 and M2 include modifications listed in Table 1.

4 Discussion

## 4.1 Additional RO2+HO2 reactions

Product yields indicate that an additional source for glycolaldehyde that is not included in the MCM is required to explain observations in the low NO experiment. This is consistent with chamber experiments by , which were performed under conditions that RO2 exclusively reacted with HO2. In this case, the glycolaldehyde yield is expected to be small, because glycolaldehyde is mainly formed in the subsequent reaction of RO2 with NO (Fig. 1). In that study, also a C4α−diketone (CH3COCOCH2OH, MCM name: BIACETOH) was measured with a yield of 0.14. From these product yields, the authors suggest that there are reaction channels of the HMVKBO2 plus HO2 reaction in addition to the formation of hydroxyperoxides :

$\begin{array}{ccc}{\mathrm{HMVKBO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}\to & \mathrm{HMVKBOOH}& \\ \text{(R3)}& & \left(\mathrm{yield}:\mathrm{0.34}\right),& {\mathrm{HMVKBO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}\to & \mathrm{HMVKBO}+\mathrm{OH}& \\ \text{(R4)}& & \left(\mathrm{yield}:\mathrm{0.48}\right),& {\mathrm{HMVKBO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}\to & \mathrm{BIACETOH}+\mathrm{OH}+{\mathrm{HO}}_{\mathrm{2}}& \\ \text{(R5)}& & \left(\mathrm{yield}:\mathrm{0.18}\right),& \end{array}$

Including Reactions (3)–(3) in the MCM* mechanism (M1 Table 1) results in an improved description of observations for the low NO experiment (Fig. 4). Modelled [OH] agrees with measurements within 20 %. The largest increase of modelled [OH] is obtained 1 to 2 h after starting the oxidation due to the additional radical regeneration from Reactions (3) and (3). The model–measurement agreement is worsened for [RO2] compared to the MCM model run, but still within the uncertainty of the measurement. The additional production of glycolaldehyde from the subsequent chemistry of Reaction (3) brings modelled organic product species into agreement with measurements within 20 % so that all observations except for [HO2] can be explained by the new reactions of HMVKBO2 with HO2. HO2 concentrations, however, are still significantly lower in the model compared to measurements. Results are consistent with experiments by , but a potential underestimation of [HO2] could not be recognised, because HO2 radicals were not measured in their experiments.

## 4.2 RO2 isomerisation reactions

RO2 isomerisation reactions have been shown to be of importance for the atmospheric fate of RO2 from isoprene . also suggested from quantum-chemical calculations that HMVKBO2 undergoes a fast 1,5-H shift with subsequent decomposition (Fig. 1 and Table 1):

$\begin{array}{}\text{(R6)}& {\mathrm{HMVKBO}}_{\mathrm{2}}\to \mathrm{MGLYOX}+\mathrm{HCHO}+\mathrm{OH}.\end{array}$

Here, the possibility of RO2 isomerisation reactions for both major RO2 species formed from the reaction of MVK with OH were investigated in detail by means of quantum-chemical calculations.

Table 2 shows the reaction pathways that were examined for the HMVKAO2 and HMVKBO2 peroxy radicals. More information can be found in the Supplement. For both radicals, 1,5-H-migration of the hydroxyl H-atom is energetically most favourable with a barrier of 22 kcalmol−1. The predicted reaction rate constant remains below $\mathrm{5}×{\mathrm{10}}^{-\mathrm{4}}$s−1, mostly due to limited tunnelling owing to the large reaction endothermicity, and the broad energy barrier protruding less than 2 kcalmol−1 above the reaction products. The concomitant low energy barrier for the reverse H-migration in the product implies that the reaction might be competitive against the alkoxy product decomposition with estimated barriers 4 kcalmol−1 . Most of the other reactions considered are several orders of magnitude slower and can be neglected. The predicted rates for all processes considered remain over an order of magnitude below that required to fit the measured OH, HO2 and glycolaldehyde concentrations so that these H-migration reactions do not have a discernible impact on the MVK oxidation chemistry radical budget or product yields at room temperature. As such, the subsequent chemistry after these reactions was not investigated.

The isomerisation rate constant estimated by for Reaction (R1) of 0.01 s−1 is about two orders of magnitude faster than calculated here. This is mainly related to the higher energy barriers found at the levels of theory applied in this work. The small reaction rate constant for this reaction is consistent with the small product yield for methylglyoxal found in the low NO experiment, which would need to be significantly higher than calculated, if Reaction (R1) was competitive with the other RO2 reaction channels. A similar conclusion was drawn from product yields obtained in the study by .

The fastest reaction rate coefficient of RO2 isomerisation reactions calculated here is found to be the HMVKAO2 1,4-H-migration of the hydrogen atoms adjacent to the −OH group, followed by H abstraction at the −OH site by O2 forming HO2 together with a bi-ketone (named BIACETOOH in the MCM):

$\begin{array}{}\text{(R7)}& {\mathrm{HMVKAO}}_{\mathrm{2}}+\left({\mathrm{O}}_{\mathrm{2}}\right)\to {\mathrm{HO}}_{\mathrm{2}}+\mathrm{BIACETOOH}.\end{array}$

A reaction rate constant of 0.003 s−1 is calculated making this reaction competitive with the reaction of HMVKAO2 with HO2 and NO in the low NO experiments. Approximately 20 to 30 % of the HVMKAO2 undergoes the 1,4-H shift reaction in this experiment. However, the resulting increase of the HO2 concentration is rather small ($<\mathrm{0.5}×{\mathrm{10}}^{\mathrm{8}}$cm−3) because of the small HMVKAO2 yield in the reaction of MVK with OH. This reaction is included in the model sensitivity runs M2.

## 4.3 Potential additional RO2 reaction channel

A fast conversion of HMVKBO2 to HO2 would be required to fit HO2 measurements. Glycolaldehyde would need to be a co-product to match measured values:

$\begin{array}{}\text{(R8)}& {\mathrm{HMVKBO}}_{\mathrm{2}}+\left(\mathrm{X}\right)\to {\mathrm{HO}}_{\mathrm{2}}+\text{products}\end{array}$

X represents an unknown reaction partner not needed in case of a unimolecular reaction. M2 in Fig. 4 gives the model result, if Reaction (R3) is included in the MCM* in addition to the 1,4-H shift of HMVKAO2 (Reaction R2) and the additional channels of the reaction of HMVKBO2 with HO2 (Reactions 33).

In order to fit HO2 and glycolaldehyde concentration measurements, a reaction rate constant of 0.006±0.004s−1 is required. This reaction rate makes the Reaction (R3) competitive with the reaction of RO2 with NO (reaction rate approximately 0.004s−1) and HO2 (reaction rate approximately 0.008s−1). In M2, 40 % of HMVKBO2 reacts with HO2 and 30 % of HMVKBO2 forms directly HO2 and glycolaldehyde in Reaction (R3) for conditions of this experiment. In comparison, 60 % of HMVKBO2 reacts with HO2 in the model run M1. However, the overall effect on the [OH] is similar in both model runs so that modelled [OH] becomes consistent with measurements. This is due to the conversion of HO2 produced in Reaction (R3) to OH. Overall, however, the major difference in the results of M1 and M2 is in the improved model–measurement agreement of [HO2].

Unfortunately, experiments here do not give hints about the exact nature of Reaction (R3). Quantum-chemical calculations (see above) shows that Reaction (R3) cannot be a unimolecular reaction such as H-atom migration, because they are not fast enough to compete with other RO2 reaction channels. Photolysis of RO2 that could results in OH∕HO2 have been observed for acetylperoxy radicals and isoprene derived RO2 . However, the reaction rate constant of 0.006±0.004s−1 needed here to explain observations would require an unrealistically high absorption cross section. A reaction partner in Reaction (R3) could also be a RO2 radical. However, in this case products of the HMVKBO2 plus RO2 reaction that are assumed in the MCM would need to be changed according to Reaction (R3) and the reaction rate constant would need to be increased by a large factor of 20 to 50 compared to recommendations for RO2 self-reaction rate constants in order to make this reaction competitive with the other RO2 reaction channels.

## 4.4 Model–measurement agreement of nitrogen oxide species

So far, only radicals and organic products have been discussed. However, there is also disagreement between measured and modelled NO2 mixing ratios. The NO2 concentration produced by the model is 30 % smaller in the low NO experiment (23 May 2017)) compared to measured values. This discrepancy increases to 40 %, if the OH concentration and therefore the MVK oxidation rate is increased. This is due to the increased production of peroxy radicals, which form peroxy acyl nitrate (PAN) or PAN like species, which act as nitrogen oxide reservoirs. Acetylperoxy radicals forming PAN are mainly produced from HVMKBO2 as a co-product of glycolaldehyde, but another PAN-like species (MCM name PHAN) is additionally produced by the oxidation of glycolaldehyde. If no production of PHAN is assumed, measured and modelled NO2 mixing ratios agree within 100 pptv (M2 in Fig. 4), less than the accuracy of the NOx formation in the chamber. However, also reduction of the production of acetylperoxy radicals could improve the model–measurement agreement. The change in NO and peroxy radical concentrations is rather small, because of the suppression of NO by O3 and the overall small turnover rate of HO2 and NO. More specific experiments concerning the NOx budget would be required to decide, which NOx reservoir species is overestimated by the model.

Figure 5OH budget during the experiment at low NO concentration (23 May 2017) for the period, when MVK photooxidation took place. The vertical dashed line indicates when MVK was reinjected into the chamber. Red and blue coloured areas add contributions to the OH production calculated from the model results (M2 sensitivity run in Fig. 4). The contribution RO2+HO2 refers to the OH production from Reaction (3). In addition to modelled OH production contributions, total OH production calculated from the product of OH concentration and OH reactivity (kOH) is shown. These quantities are either taken from the model results (M2) or from measurements. The coloured grey area gives the uncertainty of the total OH production calculation, if measured OH concentrations and measured OH reactivity are used.

## 4.5 OH budget analysis

OH is in steady state because of its short lifetime, so that its rates of production and destruction are equal. Therefore, OH reactivity together with OH concentration measurements allows determination of the total OH production rate from only two measured quantities. Under conditions with high NO concentrations, only few chemical reactions are typically controlling the OH production. The dominating process is usually the recycling of HO2 by its reaction with NO. In addition, photolysis of ozone and HONO make significant contributions. For low NO conditions and in the presence of high VOC concentrations, field and chamber experiments often show larger total OH productions rates (derived from measured OH and OH reactivity) than can be explained by the above processes. Under these conditions, other OH sources linked to the degradation of VOCs become relevant.

Figure 5 shows the OH budget for the low NO experiment (23 May 2017) using model results (M2, Table 1) to calculate contributions to the OH production. In addition, total OH production is calculated from measured and modelled OH concentrations and OH reactivity. Results from calculations using either modelled or measured values give similar numbers that agree within the uncertainty of the calculation of 20 %.

The OH production rate is dominated by HO2 recycling reactions and primary OH production (HONO and O3 photolysis). These contributions explain 70 to 80 % of the total OH production during the MVK oxidation. The model would give lower OH production compared to calculations using measurements if Reaction (R3) was not included because HO2 concentrations and therefore OH recycling by HO2 would be underpredicted in this case. This reaction is responsible for approximately half of the HO2 concentration during the first 2 h of MVK oxidation. This demonstrates the importance of including all HO2 sources in models.

Another 10 to 15 % of the total OH production rate is due to the OH formation from the additional HO2 plus RO2 reaction channel suggested by (Reaction 3). A large number of other OH forming reactions included in the model such as photolysis of hydroxyperoxides fills the remaining gap between these major contributions and the total calculated OH production.

5 Summary and conclusions

The photooxidation of MVK, one of the major oxidation products of isoprene, was investigated at atmospheric conditions in the simulation chamber SAPHIR. NO was varied from high to low concentrations. For high NO, RO2 is mainly lost in the reaction with NO and current chemical models can describe radical concentrations within 20 %. Product yields of the major oxidation products glycolaldehyde (0.65±0.14) and methylglyoxal (0.19±0.03) are consistent with previous measurements .

OH radical concentrations are underestimated (maximum factor 2) by the MCM at low NO concentrations (<100pptv), when other RO2 reaction channels can compete with the reaction of RO2 with NO. At the same time, also HO2 and glycolaldehyde concentrations are smaller in the model compared to measurements. Only part of the model–measurements discrepancies can be explained by findings in recent studies investigating the MVK photooxidation. The higher glycolaldehyde yield is consistent with a study by . The additional channels for the RO2 plus HO2 reaction suggested by these authors can reproduce glycolaldehyde and OH concentrations, but do not explain the model–measurement discrepancy for [HO2].

The possibility of RO2 isomerisation reactions for both major RO2 species formed from the reaction of MVK with OH were investigated in detail by means of quantum-chemical calculations. Additional HO2 can be produced from the 1,4-H shift reaction of HVMKAO2. The reaction rate constant of 0.003s−1 is competitive with other RO2 reaction channels at low NO conditions. The overall impact, however, is small due to the small HMVKAO2 yield. Other reactions considered here can be neglected for atmospheric conditions. This also includes the isomerisation reaction suggested by (Reaction R1). The rate constant for this reaction is about two orders of magnitude smaller than calculated by due to the higher energy barriers found at the higher levels of theory applied in this work.

Because HO2 and glycolaldehyde concentrations are underestimated at the same time, a reaction that converts RO2 to HO2 and glycolaldehyde (Reaction R3) would explain observations in these experiments. A reaction rate constant of 0.006±0.004s−1 is required to bring measured and modelled values into agreement. Unimolecular H-shift reactions are found to be too slow. Alternatively, reaction of HMVKBO2 with RO2 that would produce directly HO2 and glycolaldehyde would explain [HO2] observations and would give similar OH and glycolaldehyde concentrations as the mechanism by . However, more than the product species would need to be different from what is described in the MCM, but also the reaction rate constants would need to be increased by a large factor of 20 to 50 for the HMVKBO2 plus RO2 reaction. More studies will be needed to explore the exact role of HO2 in the MVK oxidation scheme. In addition, open questions remain concerning the fate of nitrogen oxides in the MVK oxidation scheme. The MCM suggests the built-up of nitrogen oxide reservoirs by the formation of PAN and PAN-like species. Experiments here indicate that these reservoirs are less important.

The need for an additional HO2 source was also found in the oxidation of monoterpenes. Field studies, in which OH reactivity was dominated by monoterpenes, showed that models underestimate HO2 and OH concentrations . A chamber study investigating the photochemistry of β−pinene found that an additional HO2 source is required to explain observed HO2 and OH and suggested a rearrangement of radical intermediates as explanation .

The impact on the OH recycling efficiency and observed organic products in the MVK oxidation are the same regardless whether OH is directly produced from HO2 plus RO2 like in the mechanism or if OH is produced from enhanced HO2 as suggested by experiments here. The enhanced OH recycling is demonstrated in this study by the direct quantification of the OH radical concentration during the photochemical oxidation of MVK. Similar as for isoprene and the second major organic product from isoprene oxidation, methacrolein, , HOx radicals are faster recycled in the MVK oxidation scheme than previously assumed. For all three species, OH concentrations are found to be a factor of 2 to 3 higher than calculated by models for atmospheric conditions with low NO concentrations. Current state-of-the-art models increased already OH production for isoprene and methacrolein oxidation by including additional reaction pathways. The study here shows that this is also needed for the MVK oxidation scheme.

Data availability
Data availability.

Data of the experiments in the SAPHIR chamber used in this work is available on the EUROCHAMP data homepage (https://data.eurochamp.org/, last access: April 2018).

Supplement
Supplement.

Competing interests
Competing interests.

The authors declare to have no competing interests.

Acknowledgements
Acknowledgements.

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (SARLEP grant agreement No. 681529) and from the European Commission (EC) under the European Union's Horizon 2020 research and innovation programme (Eurochamp 2020 grant agreement No. 730997). The authors thank the Forschungszentrum Jülich for travel support under the project “Seed Money”. Frank N. Keutsch and Alex Zaytsev were supported by the National Science Foundation (AGS 1628491 and 1628530). Martin Breitenlechner was supported by the Austrian Science Fund (FWF), Erwin Schrödinger Stipendium, Grant No. J-3900. The authors thank Thomas Mentel, Yare Baker and Sungah Kang from Forschungszentrum Jülich for supporting the HO2 CIMS measurements.

The article processing charges for this open-access
publication were covered by a Research
Centre of the Helmholtz Association.

Edited by: Sergey A. Nizkorodov
Reviewed by: two anonymous referees

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