ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-20-8939-2020Atmospheric fate of two relevant unsaturated ketoethers: kinetics, products
and mechanisms for the reaction of hydroxyl radicals with
(E)-4-methoxy-3-buten-2-one and (1E)-1-methoxy-2-methyl-1-penten-3-oneAtmospheric fate of two relevant unsaturated ketoethersGibiliscoRodrigo Gastóngibilisco@uni-wuppertal.deBarnesIanBejanIustinian Gabrieliustinian.bejan@uaic.rohttps://orcid.org/0000-0003-0399-2837WiesenPeterInstitute for Atmospheric and Environmental Research, Bergische Universität Wuppertal, 42097 Wuppertal, GermanyFaculty of Chemistry, Department of Chemistry, Alexandru Ioan Cuza University of Iasi, 11 Carol I, Iasi 700506, RomaniaIntegrated Centre of Environmental Science Studies in the North Eastern Region, Alexandru Ioan Cuza University of Iasi, 11 Carol I, Iasi 700506, Romania
The kinetics of the gas phase reactions of hydroxyl radicals with two
unsaturated ketoethers (UKEs) at (298±3) K and 1 atm of synthetic air
have been studied for the first time using the relative-rate technique in an
environmental reaction chamber by in situ Fourier-transform infrared spectroscopy (FTIR). The rate
coefficients obtained using propene and isobutene as reference compounds
were (in units of 10-10 cm3 molecule-1 s-1) as follows:
kTMBO (OH + (E)-4-methoxy-3-buten-2-one) = (1.41±0.11)
and kMMPO (OH + (1E)-1-methoxy-2-methyl-1-penten-3-one) = (3.34±0.43). In addition, quantification of the main oxidation
products in the presence of NOx has been performed, and degradation
mechanisms for these reactions were developed. Methyl formate, methyl
glyoxal, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) were identified as main reaction products and
quantified for both reactions. The results of the present study provide new
insights regarding the contribution of these multifunctional volatile organic compounds (VOCs) in the
generation of secondary organic aerosols (SOAs) and long-lived nitrogen
containing compounds in the atmosphere. Atmospheric lifetimes and
implications are discussed in light of the obtained results.
Introduction
Oxygenated volatile organic compounds (OVOCs) are ubiquitous atmospheric
constituents of anthropogenic and natural origin. From those OVOCs,
carbonyls have both direct and indirect sources, as a result of biogenic and
anthropogenic activities, and because they are formed during chemical
degradation processes, which occur in the atmosphere. Unsaturated carbonyls
present high reactivity and are easily decomposed throughout chemical
reactions into various OVOCs products.
Ketones are one of the dominant groups of carbonyls found in the lower
troposphere. They can be emitted into the atmosphere by anthropogenic
activities from industry and combustion engine vehicle exhaust and in a large
extent are formed as reaction products of other volatile organic compounds (VOCs) in the troposphere
(Calvert et al., 2011;
Jiménez et al., 2014; Mellouki et al., 2015).
More complex unsaturated carbonyls, namely the α, β-unsaturated ketones and α, β-unsaturated ethers are
either emitted by plants or are produced as a result of atmospheric
oxidation of conjugated dienes
(Lv
et al., 2018; Mellouki et al., 2015; Zhou et al., 2006). These compounds
have been considered as precursors for secondary organic aerosols (SOAs) (Calvert et
al., 2011).
α, β-unsaturated ketoethers are compounds with high structural
complexity found in the atmosphere. They were detected as reaction products
of the atmospheric degradation of furans and unsaturated ethers, compounds
which received substantial interest in the last decade, since they are
considered promising alternative fuels
(Cilek
et al., 2011; Li et al., 2018; Villanueva et al., 2009; Zhou et al., 2006).
UKEs are also produced during combustion and more specifically in biomass
burning (Hatch et al., 2015).
They are also of great interest in the pharmaceutical industry, since they
are often used as precursors and/or intermediates in the production of new
anticancer drugs (Gøgsig et al.,
2012; Kumar et al., 2016).
α, β-unsaturated ketoethers are a special type of olefins,
with an electron-rich π system, which makes them more susceptible to
rapid oxidation by addition of the OH radical to the double bond. Secondary
pollutants, which are formed in such a reaction sequence, could be even more
harmful than primary pollutants emitted into the atmosphere. Examples of
such secondary harmful pollutants are organic peroxynitrates, highly
oxidized molecules and SOAs
(Atkinson,
2000; Calvert et al., 2015).
Accordingly, it is important to study in detail how the OH radical initiated
oxidation of these compounds can affect the chemical composition and
reactivity of the troposphere and, furthermore, the impact of the secondary
pollutants formed during their gas phase chemical degradation.
In the present work the OH radical initiated reactions of
(1E)-1-methoxy-2-methyl-1-penten-3-one and (E)-4-methoxy-3-buten-2-one have
been investigated.
R1R2
In addition to the kinetic information, the gaseous reaction products of
Reactions (R1) and (R2) have been quantified, and reaction mechanisms have been
derived for both compounds.
The present study represents the first experimental determination of the
rate coefficients (kTMBO and kMMPO) and the reaction products
formed from the gas phase reactions in the presence of NOx. The
obtained results could be used to generate more complete atmospheric
chemical degradation mechanisms, i.e. the master chemical mechanism, which
is necessary for a better estimation of the contribution of such compounds
to photooxidant and SOAs formation.
Experimental
All experiments were performed in a 1080 L quartz-glass reaction chamber at
(298±3) K and a total pressure of (760±10) Torr of synthetic
air. A pumping system consisting of a turbo-molecular pump backed by a
double-stage rotary fore pump was used to evacuate the reactor to 10-3
Torr. Three magnetically coupled Teflon mixing fans are mounted inside the
chamber to ensure homogeneous mixing of the reactants. The photolysis system
consists of 32 superactinic fluorescent lamps (Philips TL05 40W: 290–480 nm, λmax=360 nm) and 32 low-pressure mercury vapour lamps
(Philips TUV 40W; λmax=254 nm), which are spaced evenly
around the reaction vessel. The lamps are wired in parallel and can be
switched individually, which allows for variation of the light intensity and
thus also the photolysis frequency and radical production rate, within the
chamber. The chamber is equipped with a White type multiple-reflection
mirror system with a base length of (5.91±0.01) m for sensitive in
situ long-path infrared absorption monitoring of reactants and products in
the spectral range 4000–700 cm-1. The White system was operated at
82 traverses, giving a total optical path length of (484.7±0.8) m.
Infrared spectra were recorded with a spectral resolution of 1 cm-1
using a Nicolet Nexus FTIR (Fourier-transform infrared spectroscopy) spectrometer equipped with a liquid-nitrogen-cooled mercury-cadmium-telluride (MCT) detector.
OH radicals were generated by photolysis of CH3ONO/air mixtures at 360 nm using fluorescent lamps.
R3CH3ONO+hν→CH3O+NOR4CH3O+O2→CH2O+HO2R5HO2+NO→OH+NO2
Quantification of TMBO and MMPO and gas phase products was performed by
comparison with calibrated reference spectra contained in the infrared (IR) spectral
databases of the Wuppertal laboratory.
To investigate the mechanism of the OH-radical-initiated oxidation of the
α, β-unsaturated ketoethers, the mixtures of the compound,
CH3ONO and air were irradiated for periods of 10–30 min during
which infrared spectra were recorded with the FTIR spectrometer. Typically,
up to 128 interferograms were co-added per spectrum over a period of
approximately 40 s, and 15–20 of such spectra were collected. Prior to the
reaction initiated by OH radicals, five spectra had been collected in the dark to
check the homogeneity and unexpected dark decay of the compounds under
investigation (e.g. wall losses and dark reactions).
The TMBO, MMPO and reference compounds were monitored at the following
infrared absorption frequencies (in cm-1): TMBO at 958, 1253 and 3020;
MMPO at 1245, 1653 and 2850; isobutene at 3085; and propene at 3091.
Rate coefficients for the reactions of OH radicals with MMPO and TMBO were
determined by comparing their decay rate with that of the corresponding
decay of the two reference compounds, isobutene and propene.
R6OH+UKE→ProductsR7OH+Reference→Products.
Provided that the reference compound and TMBO and MMPO are lost only by
Reactions (R6) and (R7), it can be shown that
lnUKE0UKEt=kUKEkreferencelnreference0referencet,
where [UKE]0, [reference]0, [UKE]t and [reference]t are
the concentrations of the α, β-unsaturated ketoethers and the
reference compound at times t=0 and t, respectively, and kUKE and
kreference are the rate coefficients of Reactions (R6) and (R7),
respectively.
The initial mixing ratios of the reactants in ppmv (1 ppmv =2.46×1013 molecule cm-3 at 298 K and 1 atm) were TMBO (1–3),
MMPO (2–4), isobutene (3–5) and propene (3–5). Methyl nitrite (6 ppmv)
photolysis has been used for OH radical formation. No additional NO has been
introduced in the reaction chamber.
Possible additional losses due to interferences and/or interactions with the
reactor walls could be neglected or corrected. To verify this assumption,
mixtures of CH3ONO/air with the α, β-unsaturated
ketoethers and the reference compound were prepared and allowed to stand in
the dark for 2 h. In all cases, the decay of the organic species in
the presence of the OH radical precursor and in the absence of ultraviolet (UV) light was
negligible. Furthermore, to test for a possible photolysis of the compounds,
the reactant mixtures without OH radical precursor were irradiated for 30 min, using all lamps surrounding the chamber. No significant photolysis
of any of the reactants was observed, and no additional decay has been
monitored due to a possible reaction with interfering radicals.
Materials
The following chemicals, with purities as stated by the supplier, were used
without further purification: synthetic air (Air Liquide, 99.999 %),
propene (Messer Schweiz AG, 99.5 %), isobutene (Messer, 99 %),
(E)-4-methoxy-3-buten-2-one technical grade (Aldrich, 90 %) and
(1E)-1-methoxy-2-methyl-1-penten-3-one (Aldrich, >89.5 %).
Methyl nitrite was prepared by the dropwise addition of 50 % sulfuric
acid to a saturated solution of sodium nitrite in water and methanol
(Taylor et al., 1980). The products
were carried by a stream of nitrogen gas through a saturated solution of
sodium hydroxide followed by calcium chloride to remove the excess of acid,
water and methanol, respectively. Methyl nitrite was collected and stored at
193 K in dry ice.
Relative-rate data for the reaction of OH radicals with
(E)-4-methoxy-3-buten-2-one using propene () and isobutene () as reference
compounds at 298 K and the atmospheric pressure of air.
Relative-rate data for the reaction of OH radicals with
(1E)-1-methoxy-2-methyl-1-penten-3-one using propene () and isobutene () as
reference compounds at 298 K and the atmospheric pressure of air.
Results and discussionRate coefficients for the reaction with OH radicals
Plots of the kinetic data obtained from the experiments of the reaction of
OH radicals with TMBO and MMPO using two different reference compounds are
shown in Figs. 1 and 2, respectively. At least two experiments have been
performed for each reference compound, and linear plots were obtained in all
cases. For better representation, data for all experiments have been plotted
against both references. The rate coefficient ratio of kUKE/kreference (±2σ) obtained by combining the experiments results in Figs. 1 and 2 were for TMBO, kTMBO/kisobutene= (2.56±0.13) and kTMBO/kpropene= (5.08±0.16) and for MMPO,
kMMPO/kisobutene= (6.40±0.31) and
kMMPO/kpropene= (11.64±0.82).
The linearity of the plots with near-zero intercepts confirms that no
interferences have affected the rate coefficient determination.
Additionally, the very good agreement of the rate coefficients using the two
reference compounds proved the correctness of the investigations.
Rate coefficient ratio of kUKE/kreference and rate
coefficients for the reaction of OH radicals with
(E)-4-methoxy-3-buten-2-one and (1E)-1-methoxy-2-methyl-1-penten-3-one at (298±3) K in 1 atm of air.
Table 1 lists the values of the rate coefficient ratio of
kUKE/kreference obtained in the individual experiments at 298 K
and 1 atm for each α, β-unsaturated ketoether. The errors
given for the kUKE/kreference ratios are the 2σ
statistical errors from the linear regression. The rate coefficients
kUKE for Reactions R1 and R2 were calculated using the recommended values
kpropene= (2.90±0.10) ×10-11cm3 molecule-1 s-1 (Atkinson et al., 2006) (OH + propene) and kisobutene= (5.23±0.24) ×10-11 cm3 molecule-1 s-1 (OH + isobutene)
(Atkinson and Aschmann,
1984).
In addition, Table 1 shows the rate coefficients for individual experiments
of each reference compound employed in this study as well as the final
quoted rate coefficients for the reactions of OH with UKE compounds as an
average from all experimental values obtained for the corresponding
compound. The error quoted for those final UKE rate coefficients is
obtained by using an error propagation approach.
To the best of our knowledge rate coefficients for the reactions of OH
radicals with (E)-4-methoxy-3-buten-2-one and
(1E)-1-methoxy-2-methyl-1-penten-3-one have not been reported previously in
the literature.
Reactivity trends
There is a general lack of studies on the reactivity of poly-substituted
oxygenated unsaturated compounds, such as the unsaturated ketoethers studied
in this work.
Only the reactivity of (E)-4-methoxy-3-buten-2-one towards ozone was
investigated by Grosjean and Grosjean (1999), who reported a rate coefficient
kO3 of 1.3×10-16 cm3 molecule-1 s-1 (Grosjean
and Grosjean, 1999). The authors identified and quantified two main products
from the ozonolysis of (E)-4-methoxy-3-buten-2-one, namely methyl glyoxal
(31.2±1.9 %) and methyl formate (>15.7 %). These two
species are potential products of the OH-initiated oxidation of
(E)-4-methoxy-3-buten-2-one as well.
It is well known that OH-initiated atmospheric degradation of unsaturated
VOCs proceeds mainly through the addition of the OH radical to the double
bond (Calvert et al., 2015). Some
studies also suggested that the presence of oxygenated functional groups in
unsaturated VOCs leads to an increase of kOH, perhaps due to the
possibility of hydrogen-bonding transition complexes stabilizing the
transition states involved in these reactions
(Blanco
et al., 2012; Gaona-Colmán et al., 2017; Mellouki et al., 2003).
Considering the findings mentioned above, it is interesting to analyse the
possible effect on kOH when the ether group (R-O) is directly
attached to the C=C bond of the unsaturated ketones and the presence of
different substituents in the molecule. For this purpose, Table 2 presents
two basic structures of unsaturated ketones (I and II) and the OH rate
coefficients for different unsaturated ketones obtained experimentally
and/or estimated using a structure–activity relationship (SAR) method (US EPA, 2018).
OH rate coefficients for different unsaturated ketones obtained
experimentally and predicted using an SAR method.
a Holloway
et al. (2005),
b Blanco et
al. (2012),
c Gaona-Colmán
et al. (2017), d This work,
e Blanco and Teruel (2011).
f Structure–-activity relationship (SAR) method (US EPA, 2018).
Starting with the less substituted compound, when the substituents
R1,R2 and R3 are all hydrogen atoms (3-buten-2-one), a value of
kOH=2×10-11 cm3 molecule-1 s-1 was
experimentally observed
(Holloway
et al., 2005). Successive replacement of H atoms with methyl groups, for the
positions R2(3-penten-2-one) and R3(4-methyl-3-penten-2-one), leads to
a considerable increment on the reactivity as shown in Table 2
(Blanco
et al., 2012; Gaona-Colmán et al., 2017). Considering the experimental
errors of the measurements, it is reasonable to conclude that the addition of
each methyl group leads to an increase of approximately 4×10-11 cm3 molecule-1 s-1 in the rate coefficient
relative to those of basic structure (I).
The methyl group added in positions R2 and R3 would stabilize the
radical formed after the addition of the OH at the Cα for two
different effects: (i) the positive inductive effect (I+) by the methyl
group, which stabilizes the positive charge in the Cβ atom, and
(ii) the stabilization due to the hyperconjugation of the carbocation formed
at Cβ.
Comparing the experimental value kTMBO=1.41×10-10 cm3 molecule-1 s-1obtained in the present work for
(E)-4-methoxy-3-buten-2-one with its methylated analogue 3-penten-2-one, one
can easily realize the increase by a factor of 2 in the rate coefficient
when the R3 substituent is a methoxy group. This can be explained by
the oxygen's lone pair of electrons, which delocalizes and increases the
electron density within the C=C bond. On the other hand, the methoxy group
is electron withdrawing through a negative inductive effect (I-) via the
σ bonds. However, the mesomeric effect is stronger than the
inductive one, which is reflected by an increase of the
(E)-4-methoxy-3-buten-2-one + OH reaction rate coefficient compared to its
mono- and bi-methylated analogues that can stabilize the corresponding
radical structures only by the inductive effect and hyperconjugation but
not by a mesomeric effect.
A similar assessment can be performed considering the basic structure (II).
The increasing trend in the reactivity towards OH radicals is quite similar
when methyl groups replace H atoms in the structure of 1-pentene-3-one. The
experimental rate coefficient kMMPO=3.34×10-10 cm3 molecule-1 s-1 obtained in the present work for the reaction
of OH radicals with (1E)-1-methoxy-2-methyl-1-penten-3-one is quite high, but
considering the approximate individual contribution of the substituents on
the C=C bond as was assumed previously for the basic structure (I)
reflects entirely the system reactivity.
In the present work, the AOPWIN (Atmospheric Oxidation Program) software included in the EPI Suite 4.1 (Estimation Programs Interface) was
used to estimate the rate coefficients of the structures listed in Table 2 (US EPA. Estimation Programs Interface Suite™ for
Microsoft® Windows, 2018).
It is worth mentioning that calculated kOH with AOPWIN fit quite well
with the experimental values of the simplest structures of the unsaturated
ketones shown in Table 2, namely 3-buten-2-one and 1-penten-3-one. However,
when the hydrogen atoms are replaced by methyl groups in the C=C system
for structures (I) and (II), differences between experimental values and
those estimated using the SAR method become evident by a factor of 1.2 and 1.5,
respectively. For structure (I) with two methyl substituents
(4-methyl-3-penten-2-one) the difference remains approximately the same
(factor of 1.3).
Comparing the kinetic results obtained in this work for MMPO and TMBO with
those predicted by AOPWIN, the differences become substantially larger. In
Table 2 it can be seen that for kTMBO the results differ by a factor
of 2 and for kMMPO by a factor of 3.
This fact highlights the limitations of the AOPWIN–SAR method for predicting
the specific site for the addition of the OH radical to each carbon atom of
an asymmetrical alkene, ignoring a possible stabilization of the reaction
intermediate. The stabilization could generate transition states involving
the formation of hydrogen-bonding complexes between the OH radical and the
oxygenated substituents as was suggested in previous publications
(Blanco
et al., 2012; Gaona-Colmán et al., 2017; Mellouki et al., 2003).
In conclusion, the AOPWIN–SAR estimation of reaction rate coefficients is a
useful tool for simple molecules. However, the OH rate coefficients of the
unsaturated ketoethers reported in this work showed significant
discrepancies compared with the predicted ones. Probably, as suggested
recently by Vereecken et al. (2018), it is not clear if the SAR method can be
easily expanded to multifunctional compounds, especially given the small
training set available from which to derive cross-substituent parameters or
base rate coefficients
(Vereecken et al., 2018).
Infrared spectral data. Trace A: infrared spectrum of a
TMBO/CH3ONO/air reaction mixture before irradiation. Trace B: mixture
after 10 min irradiation. Trace C: reference spectrum of TMBO. Trace D:
product spectrum. Trace E: reference spectrum of methyl formate. Trace F:
reference spectrum of peroxyacetyl nitrate. Trace G: reference spectrum of
methyl glyoxal. Trace H: residual spectrum after subtraction of the
identified reaction products in trace D.
Reaction product distribution and mechanism(E)-4-methoxy-3-buten-2-one + OH radicals
Figure 3 shows an IR spectrum recorded before (trace A) UV irradiation
applied for a mixture of TMBO and CH3ONO in air. Trace B shows the
spectrum recorded after 10 min of UV irradiation of the reaction mixture.
Trace D exhibits the product spectrum after subtraction of non-reacted TMBO
(from the reference spectra of trace C), NO, NO2, CH3ONO and
H2O. Traces E, F and G show reference spectra of methyl formate,
peroxyacetyl nitrate (PAN) and methyl glyoxal, respectively. Trace H
exhibits the residual product spectrum that is obtained after subtraction of
known products from the product spectrum in trace D. The absorption from
CO2 has been removed in all traces for clarity, since the band was
saturated and no information could be obtained from it accordingly. Methyl
formate, peroxyacetyl nitrate, and methyl glyoxal were readily identifiable
as reaction products. Concentration–time profiles of TMBO and the
identified products, methyl formate, PAN and methyl glyoxal are shown in
Fig. 4. The concentration–time distribution supports that methyl formate,
methyl glyoxal and PAN are primary reaction products. It is also easy
to observe the constant concentrations of TMBO prior to reaction begin. This
is accountable for homogeneity of the reaction mixture. Five spectra have
been collected before switching on the light which corresponds to 240 s. No
decay of TMBO is present in this time suggesting missing dark interference
in the reaction system. From Fig. 4 a conversion of up to
80 % of TMBO in 10 min of reaction time could be observed.
Simplified reaction mechanism for the addition channel in the
OH-radical-initiated oxidation of (E)-4-methoxy-3-buten-2-one. Quantified
products appear in the boxes, and the identified products are rounded by a
dashed rectangle.
Simplified reaction mechanism for the addition channel in the
OH-radical-initiated oxidation of
(1E)-1-methoxy-2-methyl-1-penten-3-one. Quantified products appear in the
boxes, and the identified products are rounded by a dashed rectangle.
Depending on the side addition of the OH radical leading to the Cα or Cβ, hydroxyalkoxy radicals, A1 and B1 will be
formed, respectively (Scheme 1). Decomposition of the A1 radical will
lead to the formation of methyl formate and methyl glyoxal as primary
products. On the other hand, the C3–C4 bond scission in the B1
radical will lead to the formation of methyl formate and methyl glyoxal.
Additionally, the radical B1 could decompose through a
C2–C3 scission generating 2-hydroxy-2-methoxyacetaldehyde and
the acetyl radical. This route would, beside the formation of
2-hydroxy-2-methoxyacetaldehyde, be responsible for the primary generation
of PAN through the further reaction of the acetyl radical with O2/NO2.
In addition, PAN is known to be formed due to the oxidation of methyl
glyoxal
(Fischer
et al., 2014). The reaction of OH radicals with methyl glyoxal occurs
exclusively by abstraction of the aldehydic H atom to form CH3C(O)CO
radicals, which have a very short lifetime, dissociating to form CH3CO+CO (Green et al., 1990). Finally, it is expected that
acetyl radicals react, in the presence of O2, with NO2 to form PAN
(Fischer
et al., 2014). Acetyl radicals are of particular importance in atmospheric
chemistry, as they are key contributors to important pollutants in the
atmosphere. PAN (peroxyacetyl nitrate), in high NOx environments, is formed
exclusively from acetyl peroxy radicals. However, in low NOx environments,
acetyl radicals, in the presence of oxygen, generate acetyl peroxy
radicals, which further react with HO2 radicals producing
CH3C(O)OOH, CH3C(O)OH, O3 and OH radicals. These secondary
products could have a high impact on the atmospheric chemistry on the global
scale
(Winiberg
et al., 2016).
Concentration–time dependencies for the reaction of TMBO ()
+OH radicals and the quantified products, methyl formate (MF), peroxyacetyl nitrate (PAN) and methyl glyoxal (MG).
After subtraction of the identified products, the most prominent absorption
feature in the IR residual spectra (Fig. 3, trace H) is a carbonyl band at
1730 cm-1, which is more characteristic for an aldehydic than a ketone
absorption. This feature suggests the formation of
2-hydroxy-2-methoxyacetaldehyde, which is unfortunately not commercially
available. Therefore, direct identification in the residual product spectrum
is not possible by using a recorded IR reference spectrum.
Carbonyl absorptions in the IR spectra are present in the 1600–1800 cm-1 range and 2-hydroxy-2-methoxyacetaldehyde identification in this
region of the IR spectrum is not possible. Beside the parent compound, which
is presenting features in this carbonyl absorption specific region, many
other products formed during the reaction have absorptions in this range.
All the products formed in the reaction system have a specific absorption in
the carbonyl range (methyl glyoxal, methyl formate, PAN and
2-hydroxy-2-methoxyacetaldehyde). However, the later one must have an
important pronounced peak in the O-H absorption area; therefore, we may
assume the unique absorption at 3550 cm-1 as being attributed to the
O-H absorption of 2-hydroxy-2-methoxyacetaldehyde (Fig. S1 in the Supplement). This is a
strong indication of the 2-hydroxy-2-methoxyacetaldehyde formation, which is
in agreement with the proposed mechanism in Scheme 1.
Plots of the concentrations of the carbonyls formed vs. reacted TMBO give
molar formation yields of (65±12) % for methyl formate, (56±16) % for PAN and (69±14) % for methyl glyoxal. The
yields have been corrected for secondary reactions with OH radicals as well
as for the photolysis and wall deposition processes where necessary
(Tuazon et al.,
1986). Exemplary plots for the product formation yields are shown in
Fig. S2.
Infrared spectral data. Trace A: infrared spectrum of an
MMPO/CH3ONO/air reaction mixture before irradiation. Trace B: mixture
after 10 min irradiation. Trace C: reference spectrum of MMPO. Trace D: product
spectrum. Trace E: reference spectrum of methyl formate. Trace F: reference
spectrum of 2, 3-pentanedione. Trace G: reference spectrum of PPN. Trace H:
residual spectrum after subtraction of the identified reaction products in
trace D.
Trace A in Fig. 5 shows the infrared spectrum for an initial reaction
mixture of an MMPO/CH3ONO/air mixture prior to irradiation; trace B exhibits the spectrum recorded after 10 min of irradiation and hence the
occurring reaction; trace C shows a reference spectrum of MMPO recorded in a
separate experiment in air at 1 atm and 298 K; trace D shows the product
spectrum recorded after 10 min of irradiation and after subtraction of non-reacted MMPO as well as subtraction of CH3ONO, NO, H2O and
NO2 absorption bands; trace E shows a reference spectrum of methyl
formate; trace F shows a reference spectrum of 2,3-pentanedione; and trace G shows a
reference spectrum of peroxypropionyl nitrate (PPN). Trace H shows the
residual product spectrum after subtraction of the identified reaction
products in trace D.
Concentration–time profiles for the reaction of MMPO () +OH
radicals and the quantified product of methyl formate (MF) and
peroxypropionyl nitrate (PPN).
The absorption from CO2 has been removed in all traces for clarity,
since the band was saturated and no additional information could be
obtained, accordingly. Methyl formate and peroxypropionyl nitrate were
identified as reaction products. Concentration–time profiles of MMPO,
methyl formate and peroxypropionyl nitrate are shown in Fig. 6. Figure 6
supports that methyl formate and peroxypropionyl nitrate are primary
reaction products. MMPO concentration is constant during five spectra recorded
in the dark, which consist of 120 s mixing time. Perfect homogeneity and no dark
interferences could be observed. From Fig. 6 a total
conversion of MMPO in 10 min of reaction time could be observed.
After the addition of the OH radical to the double bond of MMPO and
subsequent addition of an oxygen molecule followed by reaction with NO, two
different hydroxyalkoxy radicals, A2 and B2 (Scheme 2), could be
generated. Unlike for TMBO, the reaction of MMPO with OH radicals at the
Cβ position could lead to the formation of the more stable
tertiary radical A2 due to the presence of a methyl group in the
α position to the carbonyl group.
Scheme 2 shows that both addition channels would lead to the formation of
methyl formate and 2,3-pentanedione if the hydroxyalkoxy radical would
follow the dissociation of bond I in the A2 radical intermediate and the
dissociation of bond II in the B2 radical intermediate.
The hydroxyalkoxy radical B2 could lead, beside the formation of
2,3-pentanedione and methyl formate by following the scission of bond II, to the
formation of 2-hydroxy-2-methyl-3-oxopentanal as a product and formaldehyde as a
reaction co-product as a result of the decomposition of the B2 radical from the
scission of bond I. Formaldehyde could not be identified as a reaction product,
since it is formed from CH3ONO photolysis and is present in the
reaction spectra. 2-Hydroxy-2-methyl-3-oxopentanal is not commercially
available, and in the absence of a mass spectrometry technique, which could
at least identify the mass of this product there, its formation is only an
assumption.
Decomposition channel for the A2 radical could follow route I, leading to the
formation of 2,3-pentanedione and the radical CH3OCHOH, which could
further, in the presence of oxygen, form methyl formate as a co-product.
Trace E in Fig. 5 shows a reference spectrum of methyl formate. The
absorption bands at 1210 and 1755 cm-1 were used to identify
and quantify the formation of methyl formate.
The formation of 2,3-pentanedione is confirmed qualitatively by comparison of
the product spectrum (Fig. 5, trace D) with the existing reference spectrum
(Fig. 5, trace F). Although there is no doubt in the formation of
2,3-pentanedione, the partial or total overlap of the low-intensity
absorption bands did not allow us to perform reliable subtraction results to
proceed for its quantification. 2,3-Pentanedione exists predominantly in the
keto form with the enol form being present to a few percent, at the most, in
the gas phase at room temperature
(Kung, 1974;
Szabó et al., 2011). The predominance of the keto form for this compound
makes its reactivity toward OH radicals much lower. Furthermore, in
comparison with 2,4-pentanedione, a dicarbonyl compound having the enolic
form predominantly and thus being more reactive toward OH radicals
(9.05×10-11 cm3 molecule-1 s-1)
(Zhou et al., 2008),
2,3-pentanedione, with a rate coefficient for the reaction with OH radicals
of 2.25×10-12 cm3 molecule-1 s-1 (Szabó et al.,
2011) is 40 times less reactive, and consequently the secondary
reaction with OH radicals could be of less importance
(Messaadia et al., 2015).
On the other hand, photolysis quantum yields for 2,3-pentanedione using XeF (xenon flouride)
laser radiation and UV lamps at room temperature in 1000 mbar of air were
studied by Szabó et al. (2011). The results obtained in their work
suggest that 2,3-pentanedione would suffer significant photochemical changes
even at relatively long wavelengths involving short photolysis lifetime in
the troposphere. If we consider these facts, it would be possible to expect
a non-negligible photolysis of the compound in our experimental system under
the conditions used for this study.
Decomposition of the A2 hydroxyalkoxy radical could follow the scission on
route II leading to 1-hydroxy-1-methoxypropan-2-one and a propionyl radical.
1-Hydroxy-1-methoxypropan-2-one is not commercially available, and thus it is not
possible to identify this compound by comparison with an infrared reference
spectrum. However, the absorption band with the maximum at 3512 cm-1
could be assumed to the O-H stretching band of
1-hydroxy-1-methoxypropan-2-one (see Fig. S3). The infrared spectrum in
Fig. S3 presents one main absorption feature that could be attributed to the
O-H stretching of 1-hydroxy-1-methoxypropan-2-one produced by the more
stable tertiary radical (A2). The propionyl radical could further form
peroxypropionyl nitrate (PPN) (Fig. 5, trace G) in the presence of O2
and NO2.
Plots of the concentrations of methyl formate and PPN formed against reacted
MMPO in the OH radical reaction give molar formation yields of (40±12) % and (17±6) %, respectively. The yields have been corrected
for secondary reactions with OH radicals using the method outlined by
Tuazón et al. (1986). Exemplary plots of the product formation yields are
shown in the Fig. S4.
Atmospheric implications and conclusions
Once emitted into the atmosphere, it is expected that unsaturated ketoethers
such as TMBO and MMPO will follow gas phase degradation processes initiated
by the main tropospheric oxidants (OH radicals, ozone, chlorine atoms and
NO3 radicals). Rate coefficients obtained in this work for the reaction
of TMBO and MMPO with OH radicals were used to calculate their tropospheric
lifetimes using the expression τx=1/kOx[Ox], where [Ox] is
the typical atmospheric concentration of the oxidant in the troposphere and
kOx is the rate coefficient for the reaction of TMBO and MMPO
towards the oxidants. Considering a 12 h daytime average OH radical
concentration of 2×106 molecule cm-3 (global
weighted-average concentration) (Bloss et
al., 2005), average lifetimes of 0.98 and 0.42 h were estimated for
TMBO and MMPO, respectively. As mentioned before, in the literature there is
only one experimental determination for the TMBO reaction rate coefficient
with O3 performed by Grosjean et al. (1999). By using
kO3=1.3×10-16 cm3 molecule-1 s-1 and a
24 h average O3 concentration of 7×1011 molecule cm-3 (Logan, 1985), an estimated
tropospheric residence time of 3.1 h was calculated. A similar
tropospheric lifetime is expected for MMPO towards ozone, but due to the lack
of kinetic data, no exact value could be calculated for MMPO. Unfortunately,
no kinetic data are available for the reactions of TMBO and MMPO with Cl
atoms and NO3 radicals. However, it is reasonable to conclude that
a reaction with OH radicals is the main tropospheric removal pathway during
daytime for the two ketoethers studied due to the short lifetimes calculated
in this work. For ethers it is known that photodissociation quantum yields
are relatively low, and the photolysis of ketones becomes important only at
high altitudes (Mellouki et al., 2015). Thus, it
is reasonable to assume that photolysis of the studied compounds is only of
minor importance for their atmospheric removal.
The reaction products of the OH radical initiated degradation of MMPO and
TMBO confirm that the main degradation mechanisms follow the addition
pathways to the double bonds. Products identified and quantified from these
reactions are carbonyls like methyl formate, methyl glyoxal and
2,3-pentanedione and long-lived nitrogen containing compounds such as PAN
and PPN. Both types of these oxygenated products could have a further impact on
atmospheric processes. The present study proposes new gas phase contributors
to the total budget of methyl glyoxal in the atmosphere, a well-known
precursor for SOAs formation (Fu et
al., 2008). Even more this study becomes important, since MMPO and TMBO are
VOCs possibly released from open biomass-burning events whose emissions
factors for methyl glyoxal are not well established (Zarzana et al., 2018).
PAN and PPN, quantified also as reaction products, are phytotoxic air
pollutants, which act as an NOx reservoir in remote areas
(Taylor, 1969). Beside a large number of PAN
measurement campaigns, most recent chemical transport models still unsolved
the PAN global distributions due to the lack of understanding of the PAN
source attribution in the atmosphere
(Fischer
et al., 2014). Although the acetyl radical is intermediary in the formation
of PAN in this study, by its acetyl peroxy radical formed in the presence of
oxygen, this radical could play an important role in the HOx balance
over the low-NOx environment. The acetyl peroxy radical is a well-known
precursor of OH radicals as a result of the reaction with HO2 in the
remote atmosphere
(Winiberg
et al., 2016). Therefore, the gas phase mechanism proposed in this study
could be of importance for understanding atmospheric processes at the global
scale, either in the atmosphere with low NOx levels or in the
atmosphere with increased NOx. The results of the present study provide
improved insights regarding the important contribution of multifunctional
VOCs in the chemistry of the atmosphere.
Data availability
Data can be provided upon request to the corresponding authors: Rodrigo Gaston Gibilisco (gibilisco@uni-wuppertal.de) and Iustinian Gabriel Bejan (iustinian.bejan@uaic.ro).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-8939-2020-supplement.
Author contributions
RGG, IB, IGB and PW designed the experimental setup. RGG conducted the measurements.
RG and IGB processed the data. RGG, IGB and PW prepared the paper with
contributions from all the co-authors at different stages of the writing process.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Rodrigo Gastón Gibilisco acknowledges the Alexander von Humboldt Foundation for
providing a Georg Forster Research Fellowship. All the authors thank the EUROCHAMP-2020 European project and the Deutsche Forschungsgemeinschaft (DFG). Iustinian Bejan thanks the UEFISCDI IGAC-CYCLO project.
Financial support
This research has been supported by the Alexander von Humboldt Foundation (grant no. ARG 1187109 GFP), the EUROCHAMP2020 project (grant no. 730997), and the UEFISCDI (grant no. PN-III-P4-ID-PCE-2016-0807).
Review statement
This paper was edited by James B. Burkholder and reviewed by two anonymous referees.
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