In-cloud chemistry has important ramifications for atmospheric particulate matter formation and gas-phase chemistry. Recent work has shown that, like hydrogen peroxide (H2O2), the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH) oxidize sulfur dioxide dissolved in cloud droplets (SO2,aq) to sulfate. The work revealed that the pathway of SO2,aq oxidation with ISOPOOH differs from that of H2O2. We investigate the chemical mechanisms of oxidation of SO2,aq with ISOPOOH in the cloud-relevant pH range of 3–6 and compare them with the previously reported mechanisms of oxidation of SO2,aq with H2O2, methyl hydroperoxide and peroxyacetic acid. The organic products of the reaction are identified, and two pathways are proposed. For 1,2-ISOPOOH, a higher yield pathway via proposed radical intermediates yields methyl vinyl ketone (MVK) and formaldehyde, which can react to hydroxymethanesulfonate (HMS) when SO2,aq is present. A lower yield non-fragmentation oxygen addition pathway is proposed that results in the formation of isoprene-derived diols (ISOPOH). Based on global simulations, this mechanism is not a significant pathway for formation of MVK and formaldehyde relative to their gas-phase formation but, as previously reported, it can be regionally important for sulfate production. The study adds to previous work that highlights similarities and differences between gas-phase and cloud-droplet processing of reactive organic carbon.
Introduction
Isoprene (C5H8) is the main non-methane biogenic volatile organic compound emitted to the atmosphere with global emission estimates of
∼ 470 TgC (Guenther et al., 2006, 2012; St. Clair et al., 2016). In the atmosphere, C5H8 primarily reacts with hydroxyl
radicals (OH) forming peroxy radicals (RO2) after oxygen addition (Wennberg et al., 2018). Under pristine, HO2-dominated (low-NO)
conditions, isoprene RO2 reacts with hydroperoxyl radicals (HO2) to form multifunctional organic hydroperoxides, isoprene hydroxyl
hydroperoxides (ISOPOOH, C5H10O3), of which 1-hydroxyl-2-hydroperoxyl- and 4-hydroxyl-3-hydroperoxyl-ISOPOOH (1,2-ISOPOOH and
4,3-ISOPOOH, respectively) are the most abundant isomers (Rivera-Rios et al., 2014; Krechmer et al., 2015; St. Clair et al., 2016). ISOPOOH mixing
ratios of up to 1 ppb and up to ∼ 2 ppb have been reported in the Amazon rainforest and the Blodgett Forest Research Station in
California, respectively (Worton et al., 2013; Liu et al., 2016).
Sulfate production rate constants and yields in the aqueous phase due to oxidation of SO2,aq via 1,2-ISOPOOH and 4,3-ISOPOOH (Dovrou et al., 2019b).
In the gas phase, ISOPOOH is oxidized by OH to primarily produce isoprene epoxydiols (IEPOX), which contribute to secondary organic aerosol (SOA) mass
(Paulot et al., 2009a; Surratt et al., 2010; Lin et al., 2012; Nguyen et al., 2014; McNeill, 2015; Zhang et al., 2018). St Clair et al. (2016) investigated the gas-phase oxidation mechanism of ISOPOOH and elucidated some non-IEPOX oxidation pathways, and Krechmer et al. (2015) observed the formation of non-IEPOX low volatile organic compounds that contribute to SOA. These pathways are summarized and compiled into a complete gas phase mechanism in the work of Wennberg et al. (2018).
Having Henry's law constants on the order of 105Matm-1, ISOPOOH isomers can partition into cloud and fog water and participate in
condensed-phase reactions (Rivera-Rios, 2018). Dovrou et al. (2019b) showed that ISOPOOH oxidizes sulfur dioxide (SO2,aq) in cloud and fog
water, producing sulfate (SO42-). SO2,aq reacts with water to form sulfurous acid (SO2⋅H2O), which
dissociates to bisulfite (HSO3-) when pH > 2. At higher pH (pH > 6), HSO3- subsequently dissociates to form sulfite
(SO32-). In cloud pH range of 3–6 the dominant form of SO2,aq is HSO3-, and our study investigated pH values of 3,
4.5 and 5.5 (Fig. S1 in the Supplement). Aqueous hydrogen peroxide (H2O2) oxidizes SO2,aq to sulfate with a 100 % yield
with a rate constant that increases with decreasing pH (Lind et al., 1987). Dovrou et al. (2019b) reported that at pH = 5.5 the rate constant for
oxidation of SO2,aq by 1,2-ISOPOOH is equal, within uncertainty, to that of oxidation of SO2,aq by H2O2 at
pH = 5.5, while the rate constant of the oxidation for SO2,aq by 4,3-ISOPOOH is an order of magnitude lower. The rate constants for the
reactions of both ISOPOOH isomers with SO2,aq have a much smaller pH dependence than that of
H2O2+SO2,aq. Similar to H2O2, the sulfate yield at pH = 4.5 and 3 is 100 % for both ISOPOOH
isomers. However, Dovrou et al. (2019b) showed that for pH = 5.5 the sulfate yield is only 67 % and 83 % of the reacted SO2,aq
for 1,2-ISOPOOH and 4,3-ISOPOOH, respectively (Table 1). Due to its higher abundance, solubility and rate of reaction with SO2,aq,
1,2-ISOPOOH contributes much more to atmospheric sulfate production than the 4,3 isomer (Dovrou et al., 2019b).
The differences between both the sulfate yield and the pH-dependence of rate constants suggest that the chemical mechanisms of SO2,aq
oxidation via H2O2 and ISOPOOH are different. Hoffmann and Edwards (1975) proposed a mechanism for the oxidation of HSO3- by H2O2:
R1H2O2+HSO3-⇋HOOSO2-+H2O,R2HOOSO2-+H+→HSO4-+H+,
with Reaction (R2), rearrangement of peroxysulfite, being the rate-limiting step. To compare by H2O2 and understand the mechanism of
SO2,aq oxidation with ISOPOOH, time-dependent proton nuclear resonance (1H-NMR) spectra of the reaction were obtained.
Materials and methodsChemical and sample preparation
Sodium metabisulfite was purchased from Sigma Aldrich (purity ≥ 99 %) and used as the source of bisulfite (HSO3-) in the
solutions (Dovrou et al., 2019b). Formaldehyde solution (37 wt.% in H2O, 10 %–15 % methanol as stabilizer), methyl
vinyl ketone (MVK, purity 99 %), formaldehyde-sodium bisulfite adduct (hydroxymethanesulfonate, HMS, purity 95 %), acetic acid (purity
≥ 99.7 %), dimethyl sulfoxide (DMSO, purity ≥ 99.7 %), 2-methyl-2-vinyloxirane (purity 95 %) and
3-(trimethylsilyl)-1-propanesulfonic acid sodium salt (purity 97 %) were purchased from Sigma Aldrich. The two main ISOPOOH isomers, 1,2-ISOPOOH
and 4,3-ISOPOOH, were synthesized in the lab according to the procedures described by Rivera-Rios (2018). Hydrogen peroxide (H2O2) was
purchased from Sigma Aldrich (30 wt.% in H2O), and filtered Milli-Q water and deuterium oxide (D2O) were used as
solvents.
2-Methyl-3-butene-1,2-diol (1,2-ISOPOH) was synthesized, following the procedure in Zhang et al. (2012) and Bates et al. (2014), by hydrolysing 2-methyl-2-vinyloxirane. Approximately 5400 mg of 2-methyl-2-vinyloxirane was dissolved in 54 mL of 0.1 MHCl at 50 ∘C for 30 min. The procedure was modified by saturating the reaction mixture with sodium chloride (NaCl), instead of lyophilizing it. Subsequently, the diol was isolated via extraction with diethyl ether. The diethyl ether solution was then evaporated under reduced pressure to isolate the diol. The purity of the diol was determined to be 70 % using NMR analysis, and the identified impurities were diethyl ether, water, and the 1,2-ISOPOH dimer.
Samples for kinetics experiments were prepared with a ratio of [HSO3-][hydroperoxide]=21 and
[HSO3-][MVK]=21, as discussed in Dovrou et al. (2019b). The concentrations of the ion chromatography (IC) and NMR spectrometry experiments were 10 s of µM and a few mM, respectively, based on the
sensitivity of the methods. For the NMR analysis of standards, 1 mM concentrations were used. After completion of reactions, 0.1–2 mM
standards were added to identify proposed products by observing increases in the intensity of the corresponding peaks in the spectra.
Carbon budget of the 1,2-ISOPOOH+SO2,aq reaction and molar yields. Carbon concentrations are in mM. Hydrated formaldehyde could not be quantified as it is under the large D2O signal. At lower pH, acetic acid may be lost to the gas phase.
A Dionex ICS-5000+ IC system and an Agilent I500B NMR were used to analyse the samples and quantify sulfate produced. The AG12A
guard column and the AS12A analytical column (Dionex Ionpac) were selected in order to separate sulfur-containing species (Dovrou et al., 2019a). The
mobile phase was 4.5mM:1.4mM sodium carbonate/sodium bicarbonate with a flow rate of 1.5 mLmin-1 (Dovrou et al.,
2019b). 1H-NMR was used to identify the organic reaction products. For the conducted experiments, 16 and 32 scan averages were chosen, with a relaxation delay of 45 s and at
proton operating frequency of 400 MHz.
All experiments were repeated at least 4 times, each sample was prepared separately prior to analysis, and temperature and pH were
monitored. DMSO, methanol and 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt ((CH3)3Si(CH2)3SO3Na) were used as internal
standards for quantitative yields.
Proton NMR spectra (1H-NMR, 400 MHz, D2O) of (A) 1,2-ISOPOOH (d) and the products of the reaction of SO2,aq+ 1,2-ISOPOOH at (a) pH = 3, (b) pH = 4.5 and (c) pH = 5.5 and (B) 4,3-ISOPOOH (d) and the products of the reactions of SO2,aq+ 4,3-ISOPOOH at (a) pH = 3, (b) pH = 4.5 and (c) pH = 5.5. The concentration of the ISOPOOH isomers was 1 mM, and the concentration of SO2,aq was 2 mM. The spectra are focused on the areas of change. Chemical structures of (a,d) 1,2-ISOPOOH and (b,d) 4,3-ISOPOOH are presented. The labels at each peak represent the hydrogens of the compounds and the spectra are shifted for D2O at 4.67 ppm.
Results and discussionProduct identification
Figure 1 shows the NMR spectra of 1,2- and 4,3-ISOPOOH standards in D2O (broad peak at 4.7 ppm) together with the NMR spectra after
reaction of ISOPOOH with a two-fold excess of SO2,aq at three different pH values. After completion of the SO2,aq+ ISOPOOH
reaction at pH = 5.5 the ISOPOOH precursors are not observable and a number of high-intensity 1H-NMR signals are observed. Signals with
the same shifts are observed under more acidic conditions but with slightly lower intensity, while some of the smaller peaks from the pH = 5.5
spectra of both isomers are not observed in the pH = 4.5 and 3 spectra. This is consistent with the hypothesis that the reaction and product
distribution have a pH dependence and suggests competition between different reaction paths. These different paths may also explain the pH dependence
of the observed sulfate yields.
Proton NMR spectra (1H-NMR, 400 MHz, D2O) for 1,2-ISOPOOH (a), and products of the reaction of SO2,aq+ 1,2-ISOPOOH (b). The marked peaks represent the position of the hydrogens for each molecule. The pH of the samples was 5.5 and the concentrations used were [1,2-ISOPOOH] = 1 mM and [SO2,aq] = 2 mM. D2O shift at 4.67 ppm.
Products were identified via comparison of 1H-NMR spectra of standards as well as spiking SO2,aq+ ISOPOOH mixtures after
completion of reaction with standards of the proposed products. Methyl vinyl ketone (MVK), methacrolein (MACR) and (hydrated) formaldehyde (HCHO), all
major products of gas-phase oxidation, were identified by their 1H-NMR chemical shifts (St. Clair et al., 2016). Due to its higher
importance, the 1,2-ISOPOOH reaction and products were examined in more detail. For this reaction at pH = 5.5 MVK (30 % molar yield),
hydrated formaldehyde (CH2(OH)2), HMS (30 % molar yield), 1,2-ISOPOH (22 % molar yield) and acetic acid (5 % molar yield)
were the main products (Fig. 2a and Table 2). The products of the SO2,aq+ 4,3-ISOPOOH reaction included CH2(OH)2,
4,3-ISOPOH and HMS (Fig. 2b). In both reactions, CH2(OH)2 is under the large D2O peak, δ≈ 4.74 ppm,
which prevents quantification (Sect. 3.2). 2-Methyl-2-vinyloxirane was also used as a standard, but it was not observed as a product (Fig. S2 in the
Supplement). The standard spectrum showed that 2-methyl-2-vinyloxirane hydrolyzed to 1,2-ISOPOH under the experimental conditions, although not very
rapidly. Thus, we cannot exclude that 2-methyl-2-vinyloxirane is formed and rapidly hydrolyzed to 1,2-ISOPOH, although the timescale of the experiments
should have allowed its observation. Some minor product signals could not be identified; for example, a signal in the
SO2,aq+ 1,2-ISOPOOH pH = 5.5 spectrum at δ= 1.43 ppm (Fig. 1) is likely a methyl group not adjacent to a
carbonyl group based on its chemical shift; however, we were not able to identify the corresponding compound.
Proposed reaction pathways
It is instructive to consider the previously studied oxidation of SO2,aq with H2O2 (Reactions R1–R2) and methyl hydroperoxide
(CH3OOH; Reaction R3), and to lesser degree peroxyacetic acid (CH3C(O)OOH; Reaction R4) (Lind et al., 1987).
HOSO2-+CH3OOH→0.73(SO42-+H++CH3OH)+0.27(CH3OSO3-+H2O)HOSO2-+CH3C(O)OOH→SO42-+H++CH3COOH
Reaction (3) has two pathways oxidizing S(IV) to S(VI): a higher yield pathway (Reaction R3a), forming sulfate and methanol and a lower
yield pathway (Reaction R3b), forming methyl bisulfite.
R3aHOSO2-+CH3OOH→SO42-+H++CH3OHR3bHOSO2-+CH3OOH→CH3OSO3-+H2O
Lind et al.'s (1987) results for CH3OOH and CH3(O)OOH are consistent with a fast initial equilibrium, like Reaction (R1), followed
by rearrangement to the products. In addition, Lind et al. (1987) stated that at pH values above 4.8 the CH3(O)OOH reaction had another mechanism, but the authors did not elaborate, only stated that the mechanism was first-order in both reactants. Reactions (R3a) and (R4) are effectively oxygen atom transfer reactions, analogous to the net (R1) and (R2) Reactions of H2O2. Reaction (R3b) forms an organosulfate as the stable reaction product (Lind et al., 1987). In our discussion of ISOPOOH reaction pathways we focus on the atmospherically more important 1,2-ISOPOOH. Based on the product distribution we propose a non-fragmentation pathway for 1,2-ISOPOOH forming 1,2-ISOPOH (22 % molar yield at pH = 5.5) and two fragmentation pathways producing MVK (30 % molar yield at pH = 5.5) together with formaldehyde/HMS (> 30 % molar yield at pH = 5.5) and acetic acid (5 % molar yield at pH = 5.5) with other small species such as CO. These pathways, accounting for 57 % of 1,2-ISOPOOH reaction at pH = 5.5, will be discussed below.
Non-fragmentation pathway
Analogous to Reaction (R1), we also consider an initial fast equilibrium of ISOPOOH with SO2,aq. However, because of the lower symmetry than
H2O2, there are two possibilities, depending on which peroxy O is involved in the nucleophilic attack on sulfur.
R5aR5b
Peroxysulfite from Reaction (R5a) will rearrange to sulfate following Reaction (R2). The organo-peroxysulfite from Reaction (R5b) could potentially
rearrange to form an organosulfate (Reaction R6a) which is known to rapidly hydrolyze, forming sulfate and ISOPOH (Darer et al., 2011; Hu et al.,
2011). This rapid hydrolysis of tertiary sulfate likely also applies to the organo-peroxysulfite. The hydrolysis product (Reaction R6b) is again
ISOPOH and peroxysulfite, which will rearrange to sulfate following Reaction (R2).
R6aR6b
Proposed chemical mechanisms of the oxidation of SO2,aq by (a) 1,2-ISOPOOH with carbon yields of identified products at pH = 5.5 (Table 2) presented in purple and (b) 4,3-ISOPOOH. There are two competing mechanisms: after ISOPOOH reacts with SO2,aq, displacing water, a hydrolysis reaction [1] or an O-O bond breakage [2] is taking place. In mechanism [1], the product hydrolysis results in the same intermediate that the reaction of SO2,aq with H2O2 is forming, and either a formation of a diol or an epoxide is being generated (Fig. S2). In mechanism [2], an alkoxy radical and sulfite radical are formed leading to the production of MVK, MACR, HCHO and other products. Considering the reaction of SO2,aq with 1,2-ISOPOOH (a), the sum of the products' carbon yields is 57 % and the remaining 43 % is attributed to hydrated formaldehyde, repartition of MVK to the gas phase, and possible formation of CO or other small molecules that have repartitioned to the gas phase, as discussed in Sect. 3.2.2.
The net outcome of all pathways is effectively an oxygen atom transfer from 1,2-ISOPOOH to sulfite to form sulfate and 1,2-ISOPOH. This net mechanism
is consistent with the NMR spectra and IC analysis, which show clear evidence for ISOPOH and sulfate production. This effective reaction path is shown
in the left-hand side of Fig. 3a[1]. This pathway accounts for 22 % of 1,2-ISOPOOH reaction at pH = 5.5.
Fragmentation pathways
1,2-ISOPOH yield is only about two-thirds of the yield of MVK at all pH values, which is strong evidence that an additional pathway, likely not present for
CH3OOH, exists. MVK and formaldehyde, either as its hydrate or as the adduct with the excess SO2,aq, are in fact the highest yield
products. These products have to involve breaking of the carbon backbone, which strongly suggests a radical mechanism. The products are identical to
those resulting from oxidation of isoprene by OH under high-NO conditions, which involves an alkoxy radical (1,2-ISOPO, Fig. 3). In addition, the
products are identical to those observed for gas-phase ISOPOOH reacting on metal surfaces (Rivera-Rios et al., 2014). For the latter process,
Rivera-Rios et al. (2014) proposed cleavage of the 1,2-ISOPOOH peroxy bond producing 1,2-ISOPO, which rapidly forms MVK and formaldehyde. The high
sulfate yield at all pH values implies that both the ISOPOH and the fragmentation channel have to produce sulfate. In combination with the previous results
mentioned above, we propose that the second pathway involves cleavage of the peroxy bond of the organo-peroxysulfite resulting in 1,2-ISOPO, which
rapidly forms MVK/formaldehyde, and a sulfite radical. The sulfite radical rapidly reacts with O2 producing a peroxysulfate radical, which
can undergo further reaction, e.g., with SO2,aq or organic molecules to form sulfate and oxidized organics (Neta and Huie, 1985; Yang
et al., 2015). The subsequent (secondary) reaction of HCHO with SO2,aq explains the formation of HMS, and thus the pathway forms MVK, HCHO and
the sulfite radical as first-generation products with the sulfite radical likely forming sulfate subsequently (Fig. 3) (Dovrou et al., 2019a; Munger
et al., 1984, 1986). This pathway accounts for 30 % of 1,2-ISOPOOH reaction at pH = 5.5.
The observation of acetic acid also requires a fragmentation of the backbone. This pathway likely involves formation of additional formaldehyde and
CO, which are both not quantifiable in our experiments. For this reason, this is the most uncertain pathway, although it is clear that it accounts for
5 % of 1,2-ISOPOOH reaction at pH = 5.5, which makes the sum of the non-fragmentation and the two fragmentation pathways 57 % at
pH = 5.5 (Rivera-Rios et al., 2014; Paulot et al., 2009b).
There likely are three contributors to the carbon yield of less than 100 %. Hydrated formaldehyde could not be quantified due to interference with
D2O although this likely is already accounted for in the MVK and acetic acid pathways. However, MVK could also repartition to the gas phase,
as discussed in Sect. 4; thus, its aqueous contribution might be underestimated. It is likely that formation of CO or other small molecules that
potentially partition to the gas phase are not detected by NMR. Lastly, it is also possible that numerous small-yield products are formed that are
below the NMR detection limit.
pH dependence
The sulfate yield increases while the observed carbon decreases with decreasing pH, and the ratio of ISOPOH to MVK is pH independent. Moreover, at
lower pH, ISOPOH and MVK molar yields are reduced by no more than 10 %, and the acetic acid path is greatly reduced. We propose two explanations
for the fact that the observed carbon balance decreases with decreasing pH. Kok et al. (1986) showed that the formation of HMS is a factor of 10 slower at pH = 4 than pH = 5, which could explain that at pH≤4.5 the sulfate yield observed in the oxidation of SO2,aq with ISOPOOH is 100 %, indicating that SO2,aq is oxidized faster to sulfate than when reacting with formaldehyde to form HMS, while at pH = 5.5 the sulfate yield is lower. Thus, the decreased HMS yield is attributed to slow HMS formation so that more formaldehyde stays in the hydrated form which could not be quantified. Second, at lower pH acetic acid may partition to the gas phase, and thus it cannot be observed. The results do not provide a clear explanation for the lower sulfate yield at pH = 5.5. The non-fragmentation (ISOPOH) pathway, which is more important at pH = 5.5, cannot explain this as it should have a 100 % sulfate yield. Thus, the cause has to result from one of the fragmentation pathways. It is possible that at higher pH the sulfite radical has a lower yield of forming sulfate; however, studying this goes beyond the scope of this work.
In addition, S(IV) speciation might affect the sulfate yield at high pH. While the dominant form of SO2,aq in our experiments is
bisulfite, at the highest pH value of our experiments, pH = 5.5, sulfite is likely formed and could potentially react with ISOPOOH (Fig. S1). The
ratio of bisulfite to sulfite at pH = 5.5 is approximately 60. However, if the rate constant of ISOPOOH with sulfite is orders of magnitude faster
than the reaction of ISOPOOH with bisulfite, and the reaction mechanism is different for sulfite than bisulfite, this could contribute to changes in
product distributions. Our experiments do not allow unambiguous and quantitative analysis of this aspect. Performing experiments at higher pH,
exceeding the cloud-relevant pH, to shift the equilibrium to sulfite has the significant complication that formaldehyde will react rapidly with it to
form HMS.
In summary, we observe clear evidence for a non-fragmentation pathway that likely is similar to the mechanism of H2O2 and
CH3OOH. A difference to CH3OOH is that in contrast to methyl bisulfate the organo-sulfates from ISOPOOH hydrolyze easily,
especially the tertiary one from 1,2-ISOPOOH. The non-fragmentation pathway effectively corresponds to an oxygen atom transfer, and in contrast to
H2O2 and CH3OOH this is not the major pathway. We observe fragmentation pathways that we hypothesize involve cleavage of the
peroxy bond based on previous work on ISOPOOH. Such a pathway was not observed for CH3OOH as the product of this channel would be neither
methanol nor methyl bisulfate. There are at least two fragmentation pathways, with the major observed one producing MVK and HCHO, which subsequently
forms HMS, and a minor one forming acetic acid and probably small molecules including CO. It is likely that additional smaller molecules that
partition to the gas phase are formed as well as other molecules at low concentration that cannot be detected via NMR, which could include
organo-sulfur compounds from the reaction of the sulfite radical with organics.
Atmospheric implications
In the atmosphere, MVK and MACR will partition to the gas phase as they have high vapour pressures of 11 and 16 kPa and low Henry's law
constants of 41 and 6.5 Matm-1, respectively (Iraci et al., 1999). The diffusion time of MVK in a cloud droplet is less than 25 s. The
rate of the reaction of MVK with SO2,aq was estimated using NMR analysis, from which we estimate a lower limit for the rate constant of
k≥ 104M-1s-1. However, considering the diffusion time, SO2,aq and expected MVK concentrations, the formation
of the organosulfur compound by reaction of MVK with SO2,aq is unlikely in the atmosphere.
The annual production of the products formed via the reaction of ISOPOOH with SO2,aq in the atmosphere was calculated using the estimated
ISOPOOH amount participating in this reaction from GEOS-Chem (Bey et al., 2001; Park et al., 2004; Alexander et al., 2012; Dovrou et al., 2019b). The results from the simulations performed in the work of Dovrou et al. (2019b) were re-analysed with respect to the amount of ISOPOOH reacting in each pathway. Dovrou et al. (2019b) did not consider the organic products of the ISOPOOH+SO2,aq reaction as they focused on an estimate of the globally and regionally produced sulfate (see Dovrou et al., 2019b, for all
details on model configurations). This work focused on the organic reaction products, primarily to provide the first insight into this for multifunctional organic hydroperoxides, and to evaluate whether in addition to the clear regional importance for sulfate, the reactions are important
for the organic carbon budget. To be specific, the amount of ISOPOOH reacting in each pathway was calculated from the simulations and was adjusted for
each organic product using the product yields. The results were scaled by the relative molecular weights in order to calculate the total production of
the compounds of interest.
Globally, it is estimated that, under current conditions, 4.2 Tgyr-1 of gas-phase 1,2-ISOPOOH reacts with SO2,aq, representing
3.2 % of the global loss of the ISOPOOH isomer and 1.7 % of the global production of sulfate (Dovrou et al., 2019b). The 1,2-ISOPOOH pathway
contributes to the production of 0.40–0.53, 0.02–0.06, 0.28–0.36, 0.95–1.31 and 1.40–1.52 Tgyr-1 of MVK, acetic acid, HCHO, HMS and
1,2-ISOPOH, respectively (ranges are for the differences in yields across the experimental pH range). The production of MVK from these
ISOPOOH+SO2,aq oxidation pathways is equivalent to 0.5 % of the total global production under present conditions. Under pre-industrial
conditions, the total atmospheric burden of SO2 decreases; thus, the amount of ISOPOOH reacting with SO2,aq decreases somewhat:
2.4 Tgyr-1 of gas-phase 1,2-ISOPOOH reacts with SO2,aq, representing 1.6 % of the global respective losses of the isomer and
1.0 % of the global production of sulfate. The reaction of 1,2-ISOPOOH+SO2,aq results in 0.23–0.30, 0.01–0.04, 0.16–0.21,
0.55–0.76 and 0.80–0.88 Tgyr-1 of MVK, acetic acid, HCHO, HMS and 1,2-ISOPOH, respectively. The production of MVK from these ISOPOOH+SO2,aq oxidation pathways is equivalent to 3.7 % of the total global production under pre-industrial conditions. These results show that
the importance of these pathways lies in the production of sulfate on regional scales, especially for isoprene-dominated regions, rather than the
formation of the organic reaction products. The conversion of bisulfite to sulfate via the 1,2-ISOPOOH pathway can contribute up to ∼ 50 %
of total sulfate production in pristine regions such as the Amazon, and ∼ 3 % in more polluted regions such as the southeastern United States
under current atmospheric conditions (Dovrou et al., 2019b). The southeastern United States is an area of interest as NOx levels are
decreasing, creating an atmosphere where hydroperoxide formation can be favoured, which may increase their contribution to sulfate formation; in a
simulation without anthropogenic emissions, the contribution of ISOPOOH to sulfate formation increases from ∼ 3 % to ∼ 20 %
(Dovrou et al., 2019b).
Conclusions
This work is primarily useful as a foundation for the mechanistic understanding of the reactivity of multifunctional organic hydroperoxides with SO2,aq, for which there is no previous data in the literature. The results show that multifunctional organic hydroperoxides have a
significantly more complex behaviour with channels analogous to H2O2 and CH3OOH but also fragmentation channels. Dovrou et al. (2019b) demonstrated the importance of these pathways in the production of sulfate under isoprene-dominated regions compared to the formation of the organic reaction products. Although the reactions do not play an important role for the organic carbon budget, our work advances
understanding of the atmospheric chemistry of multifunctional organic hydroperoxides and adds new pathways to the isoprene oxidation mechanism. Furthermore, the results can help clarify the composition of cloud droplet residual aerosols.
Data availability
The data used in this work are available upon request. Please email Eleni Dovrou (dovrouel@gmail.com).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-8999-2021-supplement.
Author contributions
ED conducted the ion chromatography experiments and ED and JCRR conducted the NMR experiments. ED, KHB and FNK analysed the data. JCRR synthesized the two ISOPOOH isomers. JLC and JDS synthesized the 1,2-ISOPOH to provide the 1H-NMR standard, and JLC provided the 2-methyl-2-vinyloxirane and acetic acid 1H-NMR standards. ED prepared the paper with contributions from KHB, JCRR, JLC, JDS and FNK.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by the Harvard Global Institute. Eleni Dovrou is grateful for the Onassis Foundation scholarship for Hellenes.
Financial support
This research has been supported by the Harvard Global Institute. The article processing charges for this open-access publication were covered by the Max Planck Society.
Review statement
This paper was edited by Ryan Sullivan and reviewed by Becky Alexander and one anonymous referee.
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