Carbonyl compounds are ubiquitous in atmospheric multiphase system
participating in gas, particle, and aqueous-phase chemistry. One important
compound is methyl ethyl ketone (MEK), as it is detected in significant
amounts in the gas phase as well as in cloud water, ice, and rain.
Consequently, it can be expected that MEK influences the liquid-phase
chemistry. Therefore, the oxidation of MEK and the formation of
corresponding oxidation products were investigated in the aqueous phase.
Several oxidation products were identified from the oxidation with OH
radicals, including 2,3-butanedione, hydroxyacetone, and methylglyoxal. The
molar yields were 29.5 % for 2,3-butanedione, 3.0 % for
hydroxyacetone, and 9.5 % for methylglyoxal. Since methylglyoxal is often
related to the formation of organics in the aqueous phase, MEK should be
considered for the formation of aqueous secondary organic aerosol (aqSOA).
Based on the experimentally obtained data, a reaction mechanism for the
formation of methylglyoxal has been developed and evaluated with a model
study. Besides known rate constants, the model contains measured photolysis
rate constants for MEK (
In the last decades, carbonyl compounds have been a subject of intense research due to their ubiquitous abundance and their effect on atmospheric chemistry and human health. They are emitted directly from biogenic and anthropogenic sources or formed through the oxidation of hydrocarbons (e.g., Atkinson, 1997; Matthews and Howell, 1981; Lipari et al., 1984; Ciccioli et al., 1993; Mopper and Stahovec, 1986; Carlier et al., 1986; Hallquist et al., 2009). One carbonyl compound that is emitted from numerous and mainly biological sources is methyl ethyl ketone (MEK). It is released from grass, clover (Kirstine et al., 1998; de Gouw et al., 1999), different types of forests, and biomass burning processes (Khalil and Rasmussen, 1992; Warneke et al., 1999; Isidorov et al., 1985). Anthropogenic emissions are also important MEK sources, such as artificial biomass burning (Andreae and Merlet, 2001; Akagi et al., 2011; Yokelson et al., 2013; Brilli et al., 2014) and tobacco smoke (Buyske et al., 1956; Yokelson et al., 2013). In addition, MEK is emitted into the atmosphere through the application as solvent for the production of glue, resins, cellulose, rubber, paraffin wax, and lacquer (Ware, 1988).
Tropospheric MEK gas-phase concentration was found to be in the range of 0.02–15 ppbv, depending on the region (Grosjean et al., 2002; Riemer et al., 1998; Singh et al., 2004; Snider and Dawson, 1985; Goldan et al., 1995; Grosjean et al., 1983; Grosjean, 1982; Müller et al., 2005; Feng et al., 2004). Singh et al. (2004) measured a concentration in a remote region of 0.02 ppbv, whereas Grosjean et al. (1983) observed a MEK concentration of 11.3 ppbv in Los Angeles. Brown et al. (1994) concluded that MEK is one of the major volatile organic compounds (VOCs) in indoor air.
In addition to the gas-phase measurements, the concentrations measured in
bulk water samples collected at an open station near The Bahamas reached a
concentration of < 0.5 nmol L
The Henry constants at a temperature of 25
In the present study, the reaction of MEK with OH radicals in water was investigated. Based on the experimentally obtained data, a reaction mechanism was developed to explain methylglyoxal formation. The mechanism was included in a COPASI (Complex Pathway Simulator) model and evaluated by comparing the experimentally obtained data and the model results.
Cyclohexanone-2,2,6,6-d4 (98 %), hydrochloric acid, and catalase from
bovine liver (40 000–60 000 units mg
The aqueous-phase oxidation of MEK was conducted in a 300 mL batch reactor
using the photolysis of hydrogen peroxide (H
Conducted experiments in the bulk reactor.
MEK: methyl ethyl ketone.
Two types of samples were taken over a period of 4 h. For the first type
of samples, 60
Samples of all sets were derivatized with 300
Derivatized carbonyl compounds were analyzed using a GC system (6890 series, Agilent Technologies, Frankfurt, Germany) coupled with an electron ionization
quadrupole mass spectrometer in splitless mode at a temperature of
250
MEK was oxidized with OH radicals, and the decay of MEK was monitored by GC/MS. Figure 1a shows the consumption of the precursor compound MEK. As can be seen, MEK was almost consumed after 180 min of reaction time. From the analysis of the collected samples, 2,3-butanedione, hydroxyacetone, and methylglyoxal were observed as the most dominant oxidation products. The formation of methylglyoxal from the oxidation of MEK was unexpected as it has not been reported in the literature before. Due to the relevance of methylglyoxal for aqSOA formation, its formation was comprehensively characterized in the present study.
Nevertheless, in Fig. 1b, 2,3-butanedione was found as the main oxidation
product of MEK, reaching a maximum yield of
A similar trend was observed for hydroxyacetone and methylglyoxal, as the
concentration of methylglyoxal was the highest after 15 min
(
Methylglyoxal and hydroxyacetone were completely consumed at the end of the
experiment. The strong decrease in the concentrations of the detected
carbonyl compounds might result from the reaction with OH radicals and/or
from photolysis. Both mechanisms are most likely, as it has been demonstrated
that the detected carbonyl compounds react quickly with OH radicals (Lilie et
al., 1968; Gligorovski and Herrmann, 2004; Doussin and Monod, 2013; Monod et
al., 2005; Ervens et al., 2003; Herrmann et al., 2005; Tan et al., 2010;
Stefan and Bolton, 1999) and they are prone to photolysis (Faust et al.,
1997; Tan et al., 2010). The photolysis rate constants of the detected
carbonyl compounds were determined in the present study because of the
dependency on the setup used (Set 7; see Supplement Sect. S2 for more details).
Methylglyoxal and hydroxyacetone showed higher photolysis rate constants of
For 2,3-butanedione, a huge discrepancy of the rate constants for the OH
radical oxidation can be found between the different literature studies. They
vary by 1 order of magnitude in a range of
Since 2,3-butanedione is the main oxidation product, it was necessary to investigate the contribution of 2,3-butanedione to the product distribution, especially for the formation of methylglyoxal. In the oxidation of 2,3-butanedione (Set 5), no methylglyoxal was detected in the GC/MS chromatogram over a reaction period of 240 min (Fig. 2). Consequently, a contribution of 2,3-butanedione to the methylglyoxal formation could be excluded.
Despite the low molar yield of hydroxyacetone during MEK oxidation, the oxidation of hydroxyacetone was investigated for methylglyoxal formation as well (Herrmann et al., 2005; Schaefer et al., 2012; Set 6). During the oxidation, a molar yield of 100 % was found after a reaction time of 60 min (see Sect. S3 in the Supplement, Fig. S3). After 60 min of reaction time, the molar yield of methylglyoxal decreases through further reactions, as was observed during MEK oxidation. However, due to the low molar yield of hydroxyacetone (3.0 %), the oxidation has only minor importance for the observed molar yield of methylglyoxal.
GC/MS chromatogram of oxidation of 2,3-butanedione
To ensure methylglyoxal was only formed during the oxidation of MEK, an
experiment was conducted to investigate the non-radical reaction of MEK with
H
In summary, the oxidation of MEK constitutes a source for methylglyoxal, and
due to the high concentration of MEK in cloud water
(70–650 nmol L
Photolysis of MEK and time-resolved formation of 2,3-butanedione.
Recommendation of an oxidation mechanism of MEK for the formation of 2,3-butanedione, hydroxyacetone, and methylglyoxal.
According to the structure of MEK, the OH radical attack can proceed at three different positions (Fig. 4; H atoms at carbons 1, 3, and 4). For the present study, only the attack at carbons 3 and 4 is considered because these processes lead to the formation of the observed products (Fig. 4). Note that the abstraction of a hydrogen atom at carbon 3 leads to a secondary alkyl radical (A), whereas at the terminal carbon, a primary alkyl radical is formed (B). The branching ratios for the formation of the primary and secondary alkyl radicals will be discussed in detail in the next section (Sect. 3.2.1).
Maximal molar yields of the oxidation products.
Reaction mechanism and rate constants for the modeling of the experiment with COPASI.
The primary and secondary alkyl radicals react rapidly with oxygen to form
alkylperoxy radicals. The alkylperoxy radical recombines to a tetroxide and
reacts further in three different ways, i.e., the formation of a
carbonyl compound and an alcohol (i), the formation of two carbonyl compounds
and H
Extracted ion chromatogram (EIC) of
Comparison of the molar yields of 2,3-butanedione
The relevant reactions for HO
The results are shown in Fig. 6 and discussed based on the molar yields of
the products. As can be seen for 2,3-butanedione (Fig. 6a), a branching ratio
of 60 % for the primary H-atom abstraction and 40 % for the secondary
H-atom abstraction leads to lower molar yields, whereas the molar yields
start to increase with an increasing fraction of secondary H-atom
abstraction. According to the mechanism (Fig. 4), 2,3-butanedione is only
formed via secondary H-atom abstraction, and thus it is feasible to reach
higher molar yields with a higher fraction of secondary H-atom abstraction.
However, with an increasing secondary H-atom abstraction, the experimentally
determined concentration was increasingly overestimated, especially at the
beginning of the experiment. After 60 min of reaction time, the highest
experimentally determined molar yield (29.5
In contrast, methylglyoxal molar yields were increasingly underestimated with
an increasing fraction of secondary H-atom abstraction (Fig. 6b). Thus, after
15 min of reaction time, the experimental molar yield (
The primary and secondary alkyl radicals react further with oxygen
(Reactions R9/R21) with a rate constant of
Only the formation of the alkoxy radical (iii; Reaction R10) leads to the formation of
methylglyoxal. The alkoxy radical further reacts rapidly with oxygen into an
acetonylperoxy radical under elimination of formaldehyde. The acetonylperoxy
radicals can recombine again to form a tetroxide (Schaefer et al., 2012). The
latter is able to decompose through pathways (i)–(iii), which are illustrated in
Reactions (R11)–(R13). Consequently, the decomposition of the tetroxide can
explain the formation of hydroxyacetone and methylglyoxal (Reaction R11;
In addition to the discussed pathway, the primary alkylperoxy radical has the
opportunity to react with HO
Comparison of the model and experimental results for MEK
As described for the primary alkylperoxy radical, the secondary alkylperoxy
radical recombines and forms a tetroxide. This reacts to form either
(i) 2,3-butanedione and acetoin (Reaction R22) or (ii) 2,3-butanedione and
hydrogen peroxide (Reaction R23) and is considered with rate constants of
The products 2,3-butanedione, methylglyoxal, and hydroxyacetone positively
identified by GC/MS analysis might also react further, forming a variety of
oxidation products. The rate constant of methylglyoxal with OH radicals is
given in the range of
2,3-Butanedione is also prone to OH radical oxidation and photolysis. As
discussed, a huge discrepancy exists in the rate constants for the reaction
of OH radicals with 2,3-butanedione (Lilie et al., 1968; Doussin and Monod
2013; Gligorovski and Herrmann, 2004). In the present study, the value of
The oxidation of hydroxyacetone with OH radicals was also considered in the
model study with a rate constant of
The described reactions are included in a model, and the decomposition of MEK
and molar yields of formed products were compared to the experimentally
obtained data (Fig. 7). The model was not validated with the time course of
hydroxyacetone due to the high standard deviation of the experimental
results. The comparison of the model study and the experiment showed very
good agreement for the consumption of MEK (Fig. 7a). There is also good
agreement for the formation of methylglyoxal and limited agreement for the
molar yields of 2,3-butanedione. The initial high molar yield of
2,3-butanedione is reflected well (Fig. 7b). Thus, after 60 min of reaction
time, molar yields of 23.7 % in the model study and
29.5
The determined molar yields up to a reaction time of 120 min showed very
good conformity with the experiment (18.2 % and 22.1
The sources of methylglyoxal in the aqueous phase are thus far not fully
elucidated. Methylglyoxal can originate in the atmospheric aqueous phase
through (i) uptake from the gas phase and/or (ii) formation in the aqueous
phase. The importance of the uptake from the gas into the aqueous phase is
discussed in the literature, but large discrepancies can be found. Kroll et
al. (2005) investigated the uptake of methylglyoxal on inorganic seed
particles under varying relative humidity. It was found that the uptake was
not relevant for methylglyoxal even under high relative humidity. Conversely,
Zhao et al. (2006) measured an uptake coefficient of
The in situ formation of methylglyoxal in the aqueous phase could be an
important source as well (Blando and Turpin, 2000; Sempere and Kawamura
1994). Within the present study, MEK was found as a new precursor compound
for methylglyoxal in the aqueous phase yielding methylglyoxal with a molar
yield of 9.5 %. Although the Henry constant of MEK (up to
As the oxidation of MEK yielding methylglyoxal has not been studied much before, it should be considered as a formation process of methylglyoxal.
In the present study, MEK was identified as a new source for methylglyoxal
in the aqueous phase. It was demonstrated that methylglyoxal originates
directly from MEK oxidation and not from side reactions such as photolysis
or non-radical reactions. A molar yield of
Further carbonyl compounds could be identified and quantified.
2,3-Butanedione was found as the main oxidation product (molar yield
The oxidation mechanism of MEK in aqueous solution was elucidated, and MEK was demonstrated to be a precursor compound for methylglyoxal in the aqueous phase. Regarding the important role of methylglyoxal for the aqSOA formation, MEK has to be considered for aqSOA as well, which could be a next step in reducing the underestimation of the SOA burden by model studies.
This study was supported by the Scholarship program of the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt, DBU; grant number 20013/244).Edited by: M. Ammann