The atmospheric fate of a series of saturated alcohols (SAs) was
evaluated through kinetic and reaction product studies with the
main atmospheric oxidants. These SAs are alcohols that could be used as fuel
additives. Rate coefficients (in cm3 molecule-1 s-1)
measured at ∼298 K and atmospheric pressure (720±20 Torr) were
as follows: k1 ((E)-4-methylcyclohexanol + Cl) = (3.70±0.16)
×10-10, k2 ((E)-4-methylcyclohexanol + OH) = (1.87±0.14) ×10-11, k3 ((E)-4-methylcyclohexanol +NO3) = (2.69±0.37) ×10-15, k4
(3,3-dimethyl-1-butanol + Cl) = (2.69±0.16) ×10-10, k5 (3,3-dimethyl-1-butanol + OH) = (5.33±0.16)
×10-12, k6 (3,3-dimethyl-2-butanol + Cl) = (1.21±0.07) ×10-10, and k7 (3,3-dimethyl-2-butanol +
OH) = (10.50±0.25) ×10-12. The main products
detected in the reaction of SAs with Cl atoms (in the absence/presence of
NOx), OH radicals, and NO3 radicals were (E)-4-methylcyclohexanone for the
reactions of (E)-4-methylcyclohexanol, 3,3-dimethylbutanal for the reactions
of 3,3-dimethyl-1-butanol, and 3,3-dimethyl-2-butanone for the reactions of
3,3-dimethyl-2-butanol. Other products such as formaldehyde,
2,2-dimethylpropanal, and acetone have also been identified in the reactions
of Cl atoms and OH radicals with 3,3-dimethyl-1-butanol and
3,3-dimethyl-2-butanol. In addition, the molar yields of the reaction
products were estimated. The products detected indicate a hydrogen atom
abstraction mechanism at different sites on the carbon chain of alcohol in
the case of Cl reactions and a predominant site in the case of OH and
NO3 reactions, confirming the predictions of structure–activity
relationship (SAR) methods. Tropospheric lifetimes (τ) of these SAs
have been calculated using the experimental rate coefficients. Lifetimes are
in the range of 0.6–2 d for OH reactions, 7–13 d for NO3 radical
reactions, and 1–3 months for Cl atoms. In coastal areas, the lifetime due to
the reaction with Cl decreases to hours. The calculated global tropospheric
lifetimes, and the polyfunctional compounds detected as reaction products in
this work, imply that SAs could contribute to the formation of ozone and
nitrated compounds at local, regional, and even global scales. Therefore,
the use of saturated alcohols as additives in diesel blends should be
considered with caution.
Introduction
A multitude of scientific studies on combustion emissions confirm that
fossil fuels, especially diesel fuel, are the substances mainly responsible
for air pollution. The loss of air quality and its consequences for health
as well as for global warming are some of the most important problems caused
by air pollution (https://www.iea.org, last access: 3 June 2019). These consequences have led
governments to set restrictive limits on the presence of certain pollutants
in the atmosphere, such as in the case of particulate matter (PM; the Euro 6 emissions standard) and
to develop biofuels (Sikarwara et al., 2017) as alternatives to conventional
fuels. Biodiesel is obtained from the transesterification of oils of animal or
vegetable origin. Moreover, the fermentation of vegetal biomass gives methanol
and or ethanol (bioethanol). These lower alcohols have been used as fuels
and show advantages, such as a reduction in smoke, due to the presence of
the OH group that increases the oxygen content during the combustion process
(Ren et al., 2008; Lapuerta et al., 2010; Sarathy et al., 2014; Sikarwara et
al., 2017). However, several studies have shown certain complications in the
use of lower alcohols due to their low cetane number, the high latent heat
of vaporization, and the high resistance to autoignition (Karabektas and Hosoz,
2009). In order to avoid or to minimize these limitations, alcohol–diesel
blends and alcohol–diesel emulsions have been used in diesel engines
(Ozsezen et al., 2011). Another alternative is the use of longer-chain
alcohols (propanol, n-butanol, isobutanol, and n-pentanol) that have superior fuel
properties compared with lower alcohols when mixed with diesel fuel (Cheung et al.,
2014; Kumar and Saravanan, 2016; Li et al., 2017).
Alcohols are present in the atmosphere from a wide variety of anthropogenic
and biogenic sources (Calvert et al., 2011). Methanol, ethanol, and
isopropanol are some of the main alcohols detected in urban areas such as
Osaka and Sao Paulo cities (Nguyen et al., 2001) with concentrations of
5.8–8.2 and 34.1–176.3 ppbv, respectively. Other alcohols, such as
(E)-4-methylcyclohexanol, have been identified in exhaust gas emissions
resulting from burning fuel blends containing 7 % v/v (B7) and 20 % v/v
(B20) of soy bean/palm biodiesel (84%/16%) (Lopes et al., 2014).
3,3-Dimethyl-1-butanol is a glass-forming material used as a chemical
intermediate in organic syntheses (https://www.capotchem.com, last access: 3 June 2019).
3,3-Dimethyl-2-butanol is a potential precursor for prohibited chemical
weapons such as soman, which is a nerve agent (Murty et al., 2010). It is also used
in the conversion of ribose-and glucose-binding proteins into receptors for
pinacolyl methylphosphonic acid (Allert et al., 2004). The use of long-chain
alcohols as biofuels could present an additional important source of these
alcohols in the atmosphere. Therefore, prior to use, it is necessary to
study the reactivity of long-chain alcohols under atmospheric conditions in
order to establish and evaluate their atmospheric impact.
In the case of small alcohols, the knowledge of their reactivity is well
established and indicates that the main degradation process of saturated
alcohols in the atmosphere is the reaction with hydroxyl radicals (OH)
during the daytime (Grosjean, 1997; Atkinson and Arey, 2003; Atkinson
et al., 2006; Calvert et al., 2011; Caravan et al., 2015; Mellouki et al,
2015). Kinetics with chlorine (Cl) atoms are expected to be high; therefore,
reactions with Cl could also be an important degradation route, especially
in coastal areas where concentration peaks of Cl atoms can be found.
Reactions with molecular ozone (O3) (k≤10-20 cm3 molecule-1 s-1) and the nitrate radical (NO3) (∼10-15 cm3 molecule-1 s-1) are too low to make a
significant contribution to their degradation. However, the determination of
the rate coefficients and the reaction products of alcohols with the nitrate
radical are also necessary to better understand the general reactivity of
alcohols in the atmosphere, as the reactions with this radical are a
source of OH during nighttime (Finlayson-Pitts and Pitts, 2000).
Although some studies into the reactivity of higher alcohols
have been carried out over the last few years (Ballesteros et al., 2007; Hurley et al., 2009; Andersen et
al., 2010; Calvert et al., 2011; Moreno et al., 2012, 2014; Mellouki et al.,
2015), the kinetic and mechanistic database is still scarce. In the case of
3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol (derivatives of 1-butanol
and 2-butanol), there is a lack of information with respect to the diurnal
reactivity (Moreno et al., 2014; Mellouki et al., 2015). Regarding cyclic
alcohols, only data concerning the reactivity of Cl atoms and OH
radicals with cyclohexanol (Bradley et al., 2001; Ceacero-Vega et al., 2012)
and OH with cyclopentanol (Wallington et al., 1988) have been reported.
Therefore, in the present work, studies into the kinetics and the products of
gas-phase reactions of some SAs, such as (E)-4-methylcyclohexanol (4MCHexOH),
3,3-dimethyl-1-butanol (3,3DM1ButOH), and 3,3-dimethyl-2-butanol
(3,3DM2ButOH) with Cl atoms and with OH and NO3 radicals have been carried
out. These SAs have been chosen for their potential use as biofuels and
because their reactivity has not yet been evaluated. Therefore, our work will
permit us to complete the kinetic and mechanism database, to improve our
knowledge of the atmospheric chemistry of higher alcohols, and to assess
their environmental chemical impact in the case of their future use as
biofuels.
Experimental sectionKinetic experiments
The reactions of a series of SAs with the main atmospheric oxidants were
studied:
Rate coefficients were determined using a relative rate method. This method
relies on the assumption that the organic compound (the SA) and the reference
compound (R), are removed solely by their reactions with the oxidants (Ox:
OH and NO3 radicals and Cl atoms):
R8Ox+SA→Products(kSA)R9Ox+R→Products(kR)
Here, kSA and kR are the rate coefficients of the SA and the
reference compound, respectively.
On the assumption that the SA and the reference compound are only consumed
by reaction with the oxidants, the kinetic treatment for the reactions
expressed by Reactions (R8) and (R9) gives the following relationship:
ln[SA]0[SA]t=kSAkRln[R]0[R]t,
where [SA]0, [R]0, [SA]t, and [R]t are the initial
concentrations and those at time t for the SA and the reference compound,
respectively. Two reference compounds were used with each oxidant to ensure
that the reference compound had no influence on the overall rate coefficient.
According to Eq. (1), a plot of ln([SA]0/[SA]t) versus
ln([R]0/[R]t) should be a straight line that passes through the
origin. The slope of this plot gives the ratio of the rate coefficients
kSA/kR. Therefore, the value of kSA can be obtained if the rate
coefficient kR of the reference compound is known.
The experimental systems have been described in previous works (Tapia et
al., 2011; Martin et al., 2013); thus, only a brief description is given here.
Kinetic measurements were performed at room temperature (∼298 K) and atmospheric pressure (720±20 Torr) by employing two separate
experimental setups. In the first experimental framework, a 50 L Pyrex® glass reactor was
coupled to an Fourier transform infrared spectrometer as a detection
technique (”online” analysis). Inside of the Pyrex® glass
reactor there was a multi-reflection system with three mirrors that allowed for an
infrared radiation path of 2.8–200 m. This reactor is known as white cell
(Saturn Series Multi-Pass cell). The FTIR spectrometer (Thermo Nicolet 6700)
was equipped with a KBr beam splitter and liquid-nitrogen-cooled MCT detector.
Typically, for each spectrum, 60 interferograms were co-added over 98 s and
approximately 30-40 spectra were recorded per experiment with a spectral
resolution of 1 cm-1. In the second experimental setup, a Teflon ® 500 L gas bag reactor
with solid-phase microextraction (SPME) for sampling and gas
chromatography with a time-of-flight mass spectrometer (SPME/GC-TOFMS; AccuTOF GCv, JEOL) was utilized (“off-line” analysis). Samples were collected by
exposing a 50/30 mm DVB/CAR/PDMS SPME (divinylbenzene/carboxen/polydimethylsiloxane; SUPELCO) for 5 min during the
reaction and then thermally desorbing it for 15 min at 250 ∘C
in the heated GC injection port. A capillary column (30m×0.3mm
i.d. ×1.0 mm film thickness, TRACSIL TRB-1701, Teknokroma) was used
to separate the compounds. The chromatographic conditions used for the
analysis were as follows: the injector was 250 ∘C; the interface was 250 ∘C; the initial oven temperature was 40 ∘C for 4 min; the ramp was 30 ∘C min-1 to 120 ∘C and this temperature was
held for 6 min; the second ramp was 30 ∘C min-1 to 200 ∘C and this temperature was held for 3 min.
In each independent experiment, the reactants were injected into the
reactors from a vacuum line by dragging them with a stream of carrier gas that was used in
the reaction. Both reactors were inside a metallic housing that had a rack of actinic lamps (Philips, TL 40 W, Actinic BL,
λmax=360 nm) in the walls. A scheme of the experimental systems is
shown in Fig. S1 in the Supplement.
The kinetic experiments for the Cl and OH reactions were performed in the
50 L Pyrex® glass reactor coupled to a FTIR spectrometer. A
spectral subtraction procedure was used to derive the concentrations of the
reactant and reference compounds at time t=0 and time t. The reaction of
NO3 with 4MCHexOH was studied using a 500 L Teflon®
reactor in order to minimize the wall deposition and dilution effects of
consecutive additions of N2O5. Chlorine atoms and OH radicals were
obtained via the photolysis of Cl2 in N2 and methyl nitrite
(CH3ONO) in the presence of NO in air. Methyl nitrite was synthesized in
the laboratory as described elsewhere (Taylor et al., 1980).
Nitrate radicals were generated in situ in the dark by the thermal
decomposition of N2O5 (Atkinson et al., 1984, 1988).
N2O5 was obtained by mixing O3 with an excess of NO2 (Schott
and Davidson, 1958). Prior to the kinetic experiments, a series of tests under dark and photolytic conditions were carried out to evaluate secondary
reactions, such as wall depositions and the photodegradation of all reactants.
The range of the concentrations of reactants employed were as follows: 2–16 ppm
of 4MCHexOH, 3–9 ppm of 3,3DM1ButOH and 3,3DM2ButOH, 9–30 ppm of
Cl2, 4–13 ppm of 1-butene and 2-methylpropene, 7–14 ppm of propene
and cyclohexene, 5–7 ppm of isopropanol, 3–5 ppm of 2-methyl-2-butanol,
26–55 ppm of CH3ONO, 20–60 ppm of NO, 3–4 ppm of
2-ethyl-1-hexanol, and 4–5 ppm of 1-butanol. For reactions of 4MCHexOH with
nitrate radicals, two to five additions of N2O5 with concentrations
between 8 and 36 ppm were made per experiment.
Product experiments
The product study was carried out at room temperature (∼298 K) and at a pressure of 720±20 Torr of synthetic air, employing the
two experimental setups previously described. In some experiments carried
out in the 50 L Pyrex® reactor, the simultaneous identification of
products was performed using both detection techniques. In the abovementioned experiments, one sample
of reaction mixture was taken from this reactor using the SPME and
subsequently analyzed by GC-TOFMS. In addition, independent experiments
using SPME/GC-TOFMS in a 150 L Teflon® reactor were developed.
Product analyses were carried out using the same procedure as for the
kinetic experiments, without the reference compound, and employing synthetic
air as the bath gas. On this occasion, the heating of the oven was changed
slightly in order to get a better separation and to detect the products
generated. The temperature ramps of the oven employed in the chromatograph
were as follows: 40 ∘C for 4 min; a ramp of 25 ∘C min-1 to 120 ∘C that was held for 10 min; and a second ramp of 25 ∘C min-1 to 200 ∘C that was held for 4 min.
The qualitative analysis in the FTIR experiments was carried out using the
FTIR library that provided the FTIR spectrophotometer (Aldrich vapor phase
sample library, https://www.thermofisher.com/search/browse/results?customGroup=Spectral+Libraries, last access: 3 June 2019) and/or the FTIR database of Eurochamp
(https://data.eurochamp.org/data-access/spectra/, last access: 3 June 2019).
For the SPME/GC-TOFMS experiments, the NIST WebBook (https://webbook.nist.gov/chemistry/, last access: 3 June 2019) and the mass spectra database of the
instrument were used to identify the products. Calibrated FTIR spectra and
SPME/GC-TOFMS chromatograms were used for quantification in cases
where the product was commercially available.
The molecular yields of the reaction products were estimated from the slopes
of plots of the concentration of product formed versus the amounts of SA
(Δ[SA]) consumed. In the cases where an important loss of the
reaction product by reaction of the oxidant and/or by photolytic process was
observed, the concentration of the product was corrected using the formulism
of Tuazon et al. (1986) (see Sect. S1 in the Supplement). The ranges of the
concentrations of the reactants employed were as follows: 2–14 ppm of SA,
8–31 ppm of Cl2, 12–57 ppm of NO, 19–66 ppm of CH3ONO, and
6–36 ppm of N2O5.
The chemicals used were as follows: 4MCHexOH (97 %, Aldrich), 3,3DM1ButOH, and
3,3DM2ButOH (98 %, Aldrich); 1-butene, propene, 2-methyl-2-butanol,
isopropanol, 2-methylpropene, 4-methylcyclohexanone, and cyclohexene (≥99 %, Aldrich); 2-ethyl-1-hexanol (≥99 %, Fluka), 1-butanol
(99.8 %, Aldrich), 3,3-dimethylbutanal (95 %, Aldrich), and
3,3-dimethyl-2-butanone (98 %, Aldrich); and NO (99 %, Praxair), Cl2
(> 99.8 %, Praxair), synthetic air (Praxair ultrahigh purity
99.999 %), and N2 (99.999 %, Praxair). For N2O5
synthesis, N2O4 was used (> 99.5 %, Fluka), and
P2O5 (98 %, such as desiccant, Fluka) and O3 were
synthesized using a generator (model TRCE-5000), 5 gO3 h-1 (OZOGAS).
Results and discussionKinetic study
Preliminary test experiments indicated that dark heterogeneous reactions and
photolytic losses of SAs and reference compounds could be considered
negligible under our experimental conditions (k≈10-6 s-1).
The kinetic study was carried out at room temperature (∼298 K) and at
720±20 Torr of N2 gas for Cl atom reactions and synthetic air
for OH radical reactions. Nitrate radical experiments were performed using
N2 gas in a 500 L Teflon reactor and employing the SPME/GC-TOFMS
system. A number of injections of the unreacted mixture were carried out in
order to determine the associated precision of the sampling method to be
used in the error analysis (Brauers and Finlayson-Pitts, 1997). The standard
deviations (σ) were as follows: 3.7 % for 4MCHexOH, 1.7 % for
1-butanol, and 3.5 % for 2-ethyl-1-hexanol.
Relative rate plots for the reaction of (a) SAs with chlorine atoms,
employing 1-butene as a reference compound; (b) 3,3-dimethylbutanol and the OH
radical with isopropanol as a reference compound; and (c) 4MCHexOH and
NO3 with two reference compounds. Data for 1-butanol have been
vertically displaced for clarity.
Figure 1 shows examples of the kinetic data plotted according to Eq. (1) for
the reactions of SAs with different atmospheric oxidants. A good correlation
was obtained with an intercept close to zero, which indicated the absence of
secondary reactions. From the slopes of the plots (kSA/kR) and using
known values of the rate coefficients for the reference compounds
employed (kR), the value of the absolute rate coefficient for each
saturated alcohol (kSA) was determined. Rate coefficients of the reactions
of reference compounds with Cl atom reactions (in 10-10 cm3 molecule-1 s-1) were 3.40±0.28 for 2-methylpropene,
3.38±0.48 for 1-butene (Ezzel et al., 2002), and 2.23±0.31 for propene (Ceacero-Vega et al., 2009); rate coefficients of the reactions of reference compounds with OH radicals (in 10-11 cm3 molecule-1 s-1) were 2.66±0.40 for propene (Atkinson and Aschmann, 1989), 6.77±1.69 for cyclohexene (Atkinson and Arey, 2003),
0.51±0.008 for isopropanol (IUPAC http://www.iupac-kinetic.ch.cam.ac.uk, last access: 19 December 2017), and
0.36±0.06 for 2-methyl-2-butanol (Jiménez et al., 2005); and rate coefficients of the reactions of reference compounds with
NO3 radicals (in 10-15 cm3 molecule-1 s-1) were 3.14±0.97 for 1-butanol and 2.93±0.92 2-ethyl-1-hexanol
(Gallego-Iniesta et al., 2010).
Rate coefficient ratios, absolute rate coefficients, and average
rate coefficients for the reactions of a series of SAs with Cl atoms and OH
and NO3 radicals at 298 K and 720±20 Torr of pressure. Rate
coefficients, k, are shown in cubed centimeters per molecule per second ( cm3 molecule-1 s-1).
a The uncertainties for rate coefficients of SAs (σSA) were
calculated from the uncertainty of the slope of plots (σslope) and
the uncertainty of the reference (σkR) using the propagation
of uncertainties. b The weighted average according to the equation
(w1k1+w2k2+…)/(w1+w2…); (wi=1/σι2). The uncertainty of the weighted
average (σ) was given by (1/w1+1/w2+…)-0.5.
The ratios of the rate coefficients, kSA/kR, the absolute rate
coefficients, and the weighted averages are shown in Table 1. The error of
kSA/kR was given by 2 times the statistical deviation calculated
from the least squares fit of the plot of Eq. (1). The uncertainties for
rate coefficients of SA (σkSA) were calculated from the
uncertainty of the slope of plots (σslope) and the uncertainty of
the reference (σkR) using the propagation of uncertainties.
The average value of the rate coefficients obtained with different reference
compounds and their associated errors were obtained by the weighted average (see the footnote to Table 1). The rate coefficients obtained in this work are the
first kinetic data reported for these SAs; therefore, the results obtained
cannot be compared with literature values.
It is well established that the gas-phase reaction mechanism of saturated
organic compounds (alkanes, alcohols, ethers, etc.) with atmospheric
oxidants (Cl atoms as well as OH and NO3 radicals) are initiated via hydrogen
atom abstraction from the organic compound to form a stable molecule and an
alkyl radical (Finlayson-Pitts and Pitts, 2000; Atkinson and Arey, 2003;
Calvert et al., 2011; Ziemann and Atkinson, 2012). The presence of the OH
group in saturated alcohols implies two types of hydrogens that can be
abstracted: hydrogen bonded to a carbon (C–H) of the main chain or of an
alkyl substituent, and hydrogen bonded to an oxygen of the OH group. Two literature
reviews of the reactivity of saturated alcohols (Calvert et al., 2011;
Mellouki et al., 2015) conclude that (1) the reactions of aliphatic
alcohols with atmospheric oxidants proceed mainly by H atom abstraction from
various C–H groups in the alkyl chain, with the abstraction of the H atom from the
O–H group being negligible; (2) rate coefficients for the reactions of
Cl, OH, and NO3 are higher for alcohols than those of the corresponding
alkanes due to the activating effect of the OH group, with this effect
extending over about four carbon atoms (Nelson et al., 1990); and (3) the
attack percentage of a radical to the different sites of the alcohol
(α, β, γ, and δ) depends on the oxidant, the
structure of saturated alcohol, the type and numbers of substituents, and
temperature (Moreno et al., 2012, 2014; McGillen et al., 2013, 2016).
In order to verify these remarks, the reactivity of the SAs studied in this
work were analyzed and discussed by comparing the rate coefficients of the
SAs obtained with different oxidants, by comparing the rate coefficients of the
SAs and the rate coefficients of their homologous alkanes available in
literature, and by comparing the rate coefficients obtained in the reaction of
the same oxidant but with different alcohols. The data used in the
comparison are summarized in Table S1 in the Supplement.
From the analysis of all of the data shown in Table S1, the following can be observed:
The trend in the reactivity of SAs in relation to the different oxidants
is the same that observed for other saturated alcohols: kCl (k ≈10-10) > kOH (k≈10-11)
≫kNO3 (k≈10-15) (k is shown in
cm3 molecule-1 s-1). This behavior could be explained
by considering the geometry and electronic density of each oxidant, along
with kinetic collision theory. As the Cl atom has a spherical electronic
density distribution, any collision orientation is suitable; in addition,
the Cl atoms present low steric hindrance. Therefore, the Cl reaction is
comparatively less selective and faster, with values for the rate
coefficient, k, in the collision limit. However, the OH radical presents an
asymmetrical electron density located mostly over its oxygen atom. Therefore,
for the OH reaction the oxygen of the OH radical must be specifically
oriented toward the hydrogen of the SA that will be abstracted. The
electronic density of the nitrate radical is distributed around the three
oxygens, which implies several appropriate orientations, but as the
nitrate radical has a nonlinear structure, the steric hindrance is much
greater than for the OH radical, which reduces the reactivity of NO3 in
relation to that of OH.
The rate coefficient for the reaction of 4MCHexOH with Cl atoms is
similar to the rate coefficient of its homologous alkane
((E)-1,4-dimethylcyclohexane): k4MCHexOH+Cl=37.0×10-11≈k(E)-1,4-dimethylcyclohexane+Cl=36.3×10-11. In the case of the reaction with OH radical, the rate
coefficient of 4MCHexOH is 1.5 times higher than with
(E)-1,4-dimethylcyclohexane: k4MCHexOH+OH=18.7×10-12>k(E)-1,4-dimethylcyclohexane+OH=12.1×10-12 (Table S1).
These results show that the activating effect of the OH group of the SA is
less important for the reaction with Cl atoms than with the OH radical, which is
behavior that agrees with that established by the structure–activity
relationship (SAR) methods (Kwok and Atkinson, 1995; Calvert et al., 2011).
There are no rate coefficient data for the reactions with the NO3 radical
of the alkanes homologous to the SAs studied in this work; therefore, it
was not possible to check the effect of the OH group in the reaction with
NO3. However, according to the SAR method developed by Kerdouci et al. (2010) for the reactions of alcohols with NO3, this effect is greater
than in the corresponding Cl and OH reactions.
The activating effect of the chain length in the reactivity of alcohols
is also different for the Cl and OH reactions (see Table S1). Furthermore,
if the rate coefficients of 3-methyl-1-butanol (3M1ButOH) and 3,3DM1ButOH
with Cl and OH are compared, a slight increase in the rate coefficient for
Cl reaction can be observed (k3M1BuOH+Cl=25.0×10-11; k3,3DM1ButOH+Cl=26.9×10-11)
along with an important decrease in the rate coefficient for the OH
reactions (k3M1BuOH+OH=14×10-12;
k3,3DM1ButOH+OH=5.33×10-12). This behavior could
be explained by the different order of reactivity of the two oxidants. Thus, for the
Cl atom, which is more reactive (k of the order of 10-10 cm3 molecule-1 s-1) but less selective than OH, an increase in the
chain length or in the number of methyl groups in the SA implies more
hydrogens available to be abstracted and, therefore, an increase in the rate
coefficient. However, for the OH radical, which is less reactive (k of the order of
10-11–10-12 cm3 molecule-1 s-1) and more selective
than Cl, the attack for H abstraction will be carried out at a specific
position in the SA, so an increase in the chain length of the alcohol does
not have a significant effect on the reactivity; the presence of a second
methyl group even disfavors the reaction, probably due to steric hindrance
near the attack position.
In addition, as can be seen in Table S1, the position of the OH group of SA
has a different effect on the reactivity depending on the oxidant. In the
case of the Cl atom reactions, the rate coefficients for primary alcohols
(1-propanol, 1-butanol, 1-pentanol, 3-methyl-1-butanol, and
3,3-dimethyl-1-butanol) are higher than the those of secondary alcohols
(2-propanol, 2-butanol, 2-pentanol, 3-methyl-2-butanol, and
3,3-dimethyl-2-butanol) contrary to the OH and NO3 radical reactions.
This indicates that the formation of the most
stable radical seems to have less importance with respect to the reactivity than the
number of hydrogens in the α-position available to be abstracted in the reaction of Cl atoms.
All of these observations could imply a different mechanism for the H-abstraction process for Cl atoms than for OH radicals. Nelson et al. (1990)
and Smith and Ravishankara (2002) indicate the possible formation of an
intermediate adduct between the OH radical and the oxygen of the OH group
that will imply a specific orientation. Theoretical studies found in the
literature show this different H-abstraction process in the reaction
of saturated alcohols with Cl atoms (Garzon et al., 2006) and OH radicals
(Moc and Simmie, 2010). There are insufficient rate coefficient data to establish conclusions about the
mechanism for the
reaction of SAs with NO3 radicals. Different mechanisms in the hydrogen atom abstraction process for
each oxidant will imply different product distributions and molecular
yields, as will be shown in Sect. 3.2.
Estimation of rate coefficients
In order to estimate the rate coefficients of the reactions of organic
compounds with the atmospheric oxidants, a multitude of methods have been
proposed (Vereecken et al., 2018). The most popular and widely used is the
SAR method that was initially developed by Kwok and Atkinson (1995) to estimate the
rate coefficients for gas-phase reactions with the OH
radical at room temperature. This method has been updated for OH reactions (Jenkin et al., 2018)
and extended to reaction with NO3 (Kerdouci et al., 2010, 2014) and Cl (Calvert et al., 2011; Poutsma 2013). The US EPA (United States Environmental Protection Agency) has developed the EPI Suite™-Estimation
Program Interface that allows for the estimation of the rate coefficients for the
reactions of the OH radical with organic compounds using the AOPWIN
program (v1.92).
Estimated and experimental rate coefficients (kSAR, klog, and
kexp) for the reaction of SAs with atmospheric oxidants and the ratio of
rate coefficients (kexp/kSAR and kexp/klog).
kCl/10-11, kOH/10-12, and kNO3/10-15 are shown in cubed centimeters per molecule per second ( cm3 molecule-1 s-1).
a Data obtained in this work.
b Data obtained by Moreno et al. (2014).
c See S2. d Estimated using the correlations logCl–logOH (Eq. 2) and
logNO3–logOH (Eq. 3) described by Calvert et al. (2011).
In our work, the rate coefficients of SAs with the three oxidants have been
estimated using the SAR method (see Sect. S2 in the Supplement). The
results are shown in Table 2. The estimated values of the rate coefficients
agree with experimental data, with kexp/kSAR ratios between
0.9 and 1.28, except for the case of 3,3DM1ButOH and the NO3 radical, which
showed a kexp/kSAR ratio of 3.24. In general, the SAR method
when applied to alcohols predicts better rate coefficients for the Cl atom
and the OH radical than for the NO3 radical, especially for primary
alcohols. It is important to note that the kinetic database for the NO3
reactions is more limited than for Cl and OH reactions, so the estimated
rate coefficient for the NO3 radical should be treated with caution
(Kerdouci et al. 2010, 2014; Calvert et al., 2011).
It is known that organic compounds that react in the same way with different
atmospheric oxidants present a correlation between their rate coefficients.
In this sense, over the years, different correlations have been proposed to
allow the estimation of the unknown rate coefficient when the other is known
(Wayne et al., 1991; Wayne, 2000; Atkinson, 1994; Calvert et al., 2011; Gallego-Iniesta et
al., 2014). The logkCl–logkOH and logkNO3–logkOH
correlations have been built for a set of ethers and saturated alcohols by
Calvert et al. (2011), obtaining the following relationships:
2logkCl/cm3molecule-1s-1=0.634×logkOH/cm3molecule-1s-1-2.71(r2=0.72);3logkNO3/cm3molecule-1s-1=1.11×logkOH/cm3molecule-1s-1-2.42(r2=0.66).
These equations have been used to estimate the rate coefficients of the
reactions of SAs with Cl and the NO3 radical using the experimental rate
coefficients measured in this work for OH reactions. The estimated rate
coefficients, klog, according to Eqs. (2) and (3), and the ratios
(kexp/klog), are also shown in Table 2. This estimation method
obtains a slightly better rate coefficient for the 3,3DM1ButOH +NO3
reaction (kexp/klog=1.53) than the SAR method
(kexp/kSAR=3.24). However, for Cl reactions, the
kexp/klog ratios are in the range of 0.6–1.97, indicating that Eq. (2) predicts rate coefficients worse than SAR method. Again, this could
be due to the different reaction mechanism in the H-abstraction process for
the Cl and OH reactions. It is important to indicate that in the case of Cl
reactions, other effects, such as thermochemistry and the polar effect, must
be considered to estimate the rate coefficients for H-abstraction
reactions (Poutsma, 2013).
Product and mechanistic study
A product study of the reaction of 4MCHexOH, 3,3DM1ButOH, and 3,3DM2ButOH
with Cl atoms in the absence/presence of NOx, hydroxyl, and
nitrate radicals has been performed. IR absorption bands of HCl, CO2,
CO, HNO3, N2O, NO2, HCOOH, HCOH, ClNO, ClNO2, and
CH3NO3 were observed in the FTIR experiments. Some of these
compounds are products from the reactions of the SAs with oxidants. They can
also be formed by the decomposition of the employed precursors (Cl2,
CH3ONO, and N2O5) and in some cases, by heterogeneous
reactions of these precursors with the Pyrex glass reactor walls. The
formation of O3 and N2O5 have also been observed after long
reaction times for the reactions of the SAs with Cl atoms in the presence of
NOx, due to the high concentration of NO2 in the reaction medium
and the presence of radiation. Quantitative analysis was carried out by
linear subtraction of a spectrum's absorption bands and the peak areas of GC
chromatograms using the calibrated spectra and reference chromatograms.
Experimental conditions and molecular yields of
(E)-4-methylcyclohexanone for the reaction of 4MCHexOH with atmospheric
oxidants.
a Rate coefficient of k=11.2×10-11 (in cm3 molecule-1 s-1) used
to correct the concentration of (E)-4-methylcyclohexanone by loss with the
reaction of Cl atoms (data of
2-methylcyclohexanone and Cl atoms Herath et al., 2018). Photolysis rate
coefficient estimated for (E)-4-methylcyclohexanone under our experimental
conditions, kp=5×10-5 s-1.
b Rate coefficient of k=13.7×10-12 (in cm3 molecule-1 s-1) used
to correct the concentration of (E)-4-methylcyclohexanone by loss with the
reaction of OH radicals (estimated using
AOPWIN, v1.92). Photolysis rate coefficient estimated for
(E)-4-methylcyclohexanone under our experimental conditions, kp=5×10-5 s-1.
c Rate coefficient of k=2.28×10-16 (in cm3 molecule-1 s-1) used
to correct the concentration of (E)-4-methylcyclohexanone by loss with the
reaction of NO3 radicals (estimated
using the SAR method, Kerdouci et al., 2014).
d Indicated errors are the errors associated with the slope of plots
obtained in the least squares analysis.
e Experiment using a 150 L Teflon gas bag.
f Standard deviations 1σ.
The experimental conditions and molecular yields of the main products formed
in the reactions of SAs and analyzed by FTIR and SPME/GC-TOFMS are given in
Tables 3–6. Molecular yields could be affected by large errors associated
with the SPME sampling method and due to the presence of interfering IR
absorption bands, mainly associated with precursors of the OH and NO3
radicals or by the nitrated compounds formed.
Product spectra for the reaction of 4MCHexOH with (a) chlorine atoms at
10 min (×2 for clarity), (b) Cl atoms in the presence of NOx at 7 min, (c) the OH
radical at 40 min, and (d) the NO3 radical at 32 min. (e) Spectrum of the
4-methylcyclohexanone commercial sample.
4MCHexOH
(E)-4-methylcyclohexanone was identified in the reactions with Cl
(in the absence/presence of NOx), OH, and NO3. An example of the product
spectra obtained by FTIR is shown in Fig. 2. Formation of
(E)-4-methylcyclohexanone was confirmed by introducing a sample of the
commercial product (spectrum (e)). A set of experiments using the SPME as
the sampling method and the GC-TOFMS as the detection technique were also carried
out for the reactions of 4MCHexOH with Cl atoms and with the respective OH and NO3
radicals. An example of the chromatogram obtained for the reaction of
4MCHexOH with Cl atoms is shown in Fig. 3. In all of the studied
reactions, the formation of a product peak at 10.35 min was observed. The peak
(b) shown in Fig. 3 was assigned to (E)-4-methylcyclohexanone and confirmed by
comparing it with the retention time and MS of a commercial sample. In the
reactions with Cl atoms (in the absence/presence of NOx) and the OH radical two
additional peaks at 19.80 (c) and 20.25 min (d) were observed.
SPME/GC-TOFMS chromatogram for the reaction of 4MCHexOH with Cl atoms
after 15 min of reaction. Peak (a) is 4MCHexOH, peak (b) is
(E)-4-methylcyclohexanone, and peaks (c) and (d) are reaction products.
Plots of the corrected concentration of (E)-4-methylcyclohexanone
([(E)-4-methylcyclohexanone]corr) against 4MCHexOH consumed (Δ[4MCHexOH]) for Cl atoms (in the absence of NOx) and OH and NO3
radical reactions.
The time–concentration profiles of 4MCHexOH and (E)-4-methylcyclohexanone
obtained by FTIR analysis for the reaction with Cl atoms in the presence of
NOx are shown in Fig. S2. The concentrations of
(E)-4-methylcyclohexanone, corrected according to Eqs. (S1), (S2), and (S3) in the Supplement were
plotted against the amounts of 4MCHexOH consumed in order to obtain the yield
of 4-methylcyclohexanone from the slope. An example of the plots obtained is
shown in Fig. 4. Molecular yields, Y (%), of (E)-4-methylcyclohexanone
obtained in all experiments are listed in Table 3. Based on the average
molecular yield of (E)-4-methylcyclohexanone, the carbon balance was below
50 % for reactions with Cl and the OH radical and ∼60 % for NO3.
Residual spectra after the subtraction of (E)-4-methylcyclohexanone show IR
absorption bands compatible with the presence of hydroxycarbonyl compounds
(∼1750, 1720, and 1060 cm-1) and nitrated organic compounds
(RONO2∼1660, 1264 and 862 cm-1, and/or ROONO2∼1720, 1300 and 760 cm-1); see the residual spectra in Fig. S3. The amount of nitrated compounds was estimated
using the average integrated absorption coefficient of 1.2×10-17 cm molecule-1 of similar compounds corresponding to the IR
range from 1260 to 1305 cm-1 (Tuazon and Atkinson, 1990). The calculated
yields of RONO2 were 20 % and 60 % for Cl (in the presence of
NOx) and NO3 reactions, respectively. A yield of 10 % of
nitrated compounds was estimated for the reaction with the OH radical. This
lower yield could be due to fact that the NOx present in the reaction
medium reacts faster with the CH3O radical, formed by photolysis of
CH3ONO, than other alkoxy radicals. Table 6 shows a summary of the
average yields of reaction products quantified for 4MCHexOH reactions.
Reaction mechanism for the degradation of 4MCHexOH with X (the Cl atom and the
OH and NO3 radicals). (a) The mechanism for the formation of carbonyl
compounds. (b) The mechanism for the formation of nitrated compounds. Compounds
marked with a solid line are positively identified. Compounds marked with a
dotted line are not positively identified.
Concentration–time profiles of SA (3,3DM1ButOH) and reaction products
formed in the reaction of 3,3DM1ButOH with Cl atoms in the absence (a) and
in the presence of NOx(b).
Considering the products detected here and those detected in the study
of Bradley et al. (2001) relative to cyclohexanol with OH radical
reactions, a degradation mechanism for 4MCHexOH with the atmospheric
oxidants has been proposed. Figure 5a shows the paths that explain the
formation of organic compounds (carbonyl, hydroxycarbonyl, etc.), and Fig. 5b shows an example of a path to explain the formation of nitrated organic
compounds (ROONO2 and RONO2). Similar nitrated compounds could be
formed by routes II–IV. The abstraction of the hydrogen atom at the α-position with respect to the OH group (route I) followed by the addition of
oxygen, the formation of a peroxy radical, and the fast decomposition of this
radical explain the formation of (E)-4-methylcyclohexanone. Based on the
molecular yield obtained for (E)-4-methylcyclohexanone for each oxidant (see
Table 6), this route represents ∼25%/30 %, ∼40 %, and ∼60 % of the reaction mechanism of 4MCHexOH with Cl (in the absence/presence of
NOx), OH and NO3, respectively. Percentages are 2-fold higher
than those predicted by the SAR method in the case of the Cl reactions and
1.3- and 1.5-fold lower for the OH and NO3 reactions, respectively.
These data should be taken with caution, as they could include many
sources of error.
Apart from (E)-4-methylcyclohexanone, other carbonyl and hydroxycarbonyl
compounds could be formed by routes II, III, and IV. The presence of these
types of compounds has been observed in the reactions with Cl and OH.
According to the electron ionization mass spectra (EIMS) (Fig. S4) an assignation of peaks (c) and (d) shown in Fig. 3,
to 2-hydroxy-5-methylcyclohexanone, 5-hydroxy-2-methylcyclohexanone, and/or
3-methyl-1,6-hexanedial has been proposed. However, according to the
atmospheric reactivity (Finlayson and Pitts, 2000; Calvert et al., 2011;
Ziemann and Atkinson, 2012) and the study of Bradley et al. (2001), the
compound that would be expected is 3-methyl-1,6-hexanedial, which arises
from the decomposition of the alkoxy radical formed in route II. However,
confirmation was not possible as these compounds are not commercially
available. The detection of about 9 % of HCOH in the reaction with Cl atoms
indicates that the elimination of the methyl group in route IV is minor.
In the case of nitrate radical, the only carbonyl compound detected was
(E)-4-methylcyclohexanone, suggesting that route I may be the dominant pathway
for this radical. The large difference between the yields of
(E)-4-methylcyclohexanone obtained using the SPME/GC-TOFMS system (∼75 %) and the FTIR (35 %) could be due to the different methods of
adding the precursor to the two reactors (small aliquots of N2O5
in the Teflon® reactor versus one large addition in the
Pyrex® glass reactor). This procedure causes a lower initial
concentration of nitrated inorganic species (NO3, NO2, and HNO3)
in the 150 L reactor than in the 50 L reactor, favoring the formation of
carbonyl compounds instead of nitrated organic compounds. Taking the yields of (E)-4-methylcyclohexanone and the nitrated compounds for
the NO3 reaction using FTIR analysis into
account, a total carbon balance of 100 %
is obtained (see Table 6).
3,3DM1ButOH
For the reaction of 3,3DM1ButOH with the three atmospheric oxidants,
3,3-dimethylbutanal was identified as the main reaction product. Figure S5a
shows the FTIR spectra obtained for the reactions of 3,3DM1ButOH with Cl
(in the absence/presence of NOx), OH, and NO3. Residual FTIR spectra after the
subtraction of 3,3-dimethylbutanal (Fig. S5b), the SPME/GC-TOFMS
chromatograms (Fig. S6), and the EIMS spectra (Fig. S7), show that other
reaction products such as carbonyl, hydroxycarbonyl, and nitrated compounds
are formed. These reaction products could be formaldehyde,
2,2-dimethylpropanal, glycolaldehyde, acetone, and
peroxy-3,3-dimethylbutyryl nitrate (P33DMBN)
(CH3)3CCH2C(O)OONO2. These compounds can be formed as
primary products (see Fig. 6) and/or secondary products from the degradation
of 3,3-dimethylbutanal (see Fig. S8). The SPME/GC-TOFMS chromatograms show
common peaks for the three oxidants, but the numbers of peaks and their
distribution are very different, especially for OH reactions. In the case of
the SPME/GC-TOFMS system, a set of experiments using field ionization was
carried out in order to help establish the identification of reaction
products.
Time–concentration profiles of 3,3DM1ButOH, 3,3-dimethylbutanal, and those
reaction products positively identified by FTIR analysis were made to test
whether the profiles correspond to primary or secondary reaction products.
An example of the reactions with Cl atoms in the absence/presence of
NOx is presented in Fig. 6, showing that in the absence of NOx the
profiles of acetone and formaldehyde have two trends. This indicates that
these compounds are formed as primary and secondary products. This profile
is clearly observed for nitrated compounds in the reaction with Cl atoms in
the presence of NOx (Fig. 6b).
Experimental conditions and molecular yields of 3,3-dimethylbutanal
for the reaction of 3,3DM1ButOH with atmospheric oxidants.
a Rate coefficient of k=1.7×10-10 (in cm3 molecule-1 s-1) used
to correct the concentration of 3,3-dimethylbutanal by loss with the
reaction of Cl atoms (data of
isobutyraldehyde and Cl atoms from Thevenet et al., 2000). Photolysis rate
coefficient estimated for 3,3-dimethylbutanal under our experimental
conditions, kp=1×10-4 s-1.
b Rate coefficient of k=2.73×10-11 (in cm3 molecule-1 s-1) used
to correct the concentration of 3,3-dimethylbutanal by loss with the
reaction of OH radical (Aschmann et al.,
2010). Photolysis rate coefficient estimated for 3,3-dimethylbutanal under
our experimental conditions, kp=1×10-4 s-1.
c Rate coefficient of k=1.27×10-14 (in cm3 molecule-1 s-1) used
to correct the concentration of 3,3-dimethylbutanal by loss with the
reaction of NO3 radical (D'Anna et al., 2001).
d Experiment using a 50 L FTIR gas cell.
e Experiment using a 150 L Teflon gas bag.
f Indicated errors are the errors associated with the slope of plots
obtained in the least squares analysis.
g Standard deviations 1σ.
h Yield estimated using only experimental data.
A commercial sample of 3,3-dimethylbutanal was used to estimate the
molecular yields in both experimental systems. These are shown in Table 4.
The yields of acetone and formaldehyde were calculated using a FTIR
reference spectrum of a commercial sample and a FTIR reference spectra from the Eurochamp database (https://data.eurochamp.org/data-access/spectra/, last access: 3 June 2019), respectively. A FTIR
reference spectrum of 2-methylpropanal (from this same database) was used to
estimate the molecular yield of 2,2-dimethylpropanal.
The amounts of 3,3-dimethylbutanal formed were corrected by their reactions
with Cl atoms and with OH and NO3 radicals using the rate coefficients
available in the literature or for reactions of structurally similar compounds
(see the footnote for Table 4). Estimated yields of formaldehyde, acetone,
2,2-dimethylpropanal, and nitrated compounds are summarized in Table 6 along
with an average yield of 3,3-dimethylbutanal. The higher yield of nitrated
compounds in the reaction of 3,3DM1ButOH with the nitrate radical could indicate
an extra formation of nitrated compounds from secondary reactions (see Fig. S8). Total carbon yields of 60 %, 81 %, and 36 % have been justified
for Cl, OH, and NO3 reactions, respectively. It must be noted that there
are reaction products that could not be quantified as dihydroxy or
hydroxycarbonyl compounds in the reactions with Cl atoms in the absence of
NOx and primary nitrated compounds in the reactions with Cl atoms in
the presence of NOx and with the NO3 radical.
Reaction mechanism for the degradation of 3,3DM1ButOH with X (the Cl
atom and the OH and NO3 radicals). The mechanism for the formation of carbonyl
compounds. Compounds marked with a solid line are positively identified.
Compounds marked with a dotted line are not positively identified.
Reaction mechanism for the degradation of 3,3DM2ButOH with X (the Cl
atom and the OH and NO3 radicals). The mechanism for the formation of carbonyl compounds.
Compounds marked with a solid line are positively identified. Compounds marked
with a dotted line are not positively identified.
This work is the first study of the products of the reaction of 3,3DM1ButOH with
atmospheric oxidants; therefore, there is no other study with which to compare the results.
Figure 7 shows the reaction mechanism proposed based on existing studies
of saturated alcohols' reactions with Cl atoms and the OH radical in the literature (Cavalli et
al., 2002; Hurley et al., 2009; McGillen et al., 2013; Welz et al., 2013)
and considering the reaction products identified in our work. Table S2 shows a summary of the reaction products, either
observed or tentatively identified, proposed in this mechanism of the
reaction of 3,3DM1ButOH with the atmospheric oxidants.
As can be seen from Table 4, the estimated molecular yields of
3,3-dimethylbutanal (formed by H atom abstraction at the α-position of 3,3DM1BuOH) are very similar to those predicted by the
SAR method for the Cl and OH reactions. In the case of the NO3 radical, a
large difference between yields was observed (36 % estimated in this work and
86 % predicted by SAR). This discrepancy could be explained by the fact
that the SAR method applied to the NO3 radical reaction with primary
alcohols (Kerdouci et al., 2010, 2014) underestimates the attack of NO3
at the β-position, as it does not consider the possible
activating effect of the -CH2OH group (see Sect. S2).
This could also explain the large difference observed between the estimated
and measured rate coefficients of the reaction of 3,3DM1ButOH with NO3,
as shown in Table 2. Moreover, the volume of the reactor could also
have an influence on formation of 3,3-dimethylbutanal.
According to the molecular yields of the products quantified and/or observed
in the SPME/GC-TOFMS chromatograms, it can be concluded that route I (attack at the α-position) seems to be
the main reaction route for the OH
radical reaction. For Cl atoms, the three routes can occur to a
significant extent. The major molecular yields of formaldehyde and acetone
(route III) in the reactions with Cl atoms in the presence of NOx
versus those of in the absence of NOx could indicate that the self reactions of peroxy radicals
(RO2⚫) via the molecular pathway are more favored than reactions via
the radical pathway in the
absence of NOx. For the NO3 radical, routes I and II (attack at
the β-position) with the formation of nitrated compounds seem to be
the major routes.
Experimental conditions and molecular yields of
3,3-dimethyl-2-butanone for the reaction of 3,3DM2ButOH with atmospheric
oxidants.
a Rate coefficient of k=4.8×10-11 (in cm3 molecule-1 s-1) used
to correct the concentration of 3,3-dimethyl-2-butanone by loss with the
reaction of Cl atoms (Farrugia et al., 2015).
Photolysis rate coefficient estimated for 3,3-dimethyl-2-butanone under our
experimental conditions, kp=7×10-5 s-1.
b Rate coefficient of k=1.21×10-12 (in cm3 molecule-1 s-1) used
to correct the concentration of 3,3-dimethyl-2-butanone by loss with the
reaction of the OH radical (Wallington and
Kurylo, 1987). Photolysis rate coefficient estimated for
3,3-dimethyl-2-butanone under our experimental conditions, kp=7×10-5 s-1.
c Not corrected.
d Experiment using a 50 L FTIR gas cell.
e Experiment using a 150 L Teflon gas bag.
f Indicated errors are the errors associated with the slope of plots
obtained in the least squares analysis.
g Standard deviations 1σ.
3,3DM2ButOH
Analysis of the FTIR spectra obtained for the reactions of 3,3DM2ButOH with
Cl atoms in the presence/absence of NOx, the OH radical, and the NO3
radical shows the formation of 3,3-dimethyl-2-butanone as a main product
(see Fig. S9). Other compounds, such as formaldehyde, acetone,
2,2-dimethylpropanal, and peroxyacetyl nitrate (PAN), were also observed. The
residual FTIR spectra after the subtraction of all known IR bands again shows
the presence of carbonyl compounds (IR absorption in the range from 1820 to 1700 cm-1), hydroxy compounds (1060–1040 cm-1) in the reaction with
Cl atoms in the absence of NOx, and also nitrated compounds
(RONO2; 1650, 1305–1260, 890 cm-1) in the reaction with Cl in
the presence of NOx and the NO3 radical (Fig. S9c). The presence of an IR absorption band around 1800 cm-1 for
the reaction with Cl atoms at long reaction times in
the residual FTIR spectra could be due to the
formation of Cl compounds by reaction of 3,3-dimethyl-2-butanone with
Cl2 (Ren et al., 2018) or the formation of cyclic compounds such as
hydrofurans. The SPME/GC-TOFMS chromatograms and MS spectra (Figs. S10 and S11) confirm the presence of other reaction products (apart from
3,3-dimethyl-2-butanone) in the case of Cl (in the absence/presence of NOx) and
NO3 reactions. Only one significant peak is observed in chromatograms
of the OH reactions.
The estimated molecular yields of 3,3-dimethyl-2-butanone for all individual
experiments are given in Table 5, where the measured concentrations have
been corrected for secondary reactions. Acetone, formaldehyde,
2,2-dimethylpropanal, nitrated compounds, and acetaldehyde were also
quantified. Plots of concentration versus time for formaldehyde, acetone
(Fig. S12a), and nitrated compounds in the reactions of Cl in the presence of
NOx (Fig. S12b) show profiles with two trends. This type of profile
indicates that formaldehyde and acetone could also be formed by the degradation
of 3,3-dimethyl-2-butanone (Fig. S13). Table 6 summarizes the molecular
yields of all quantified products.
Total carbon yields of ∼60%/100%, 90 %, and 60 % have been
reported for Cl (in the absence and presence of NOx), OH, and NO3
reactions, respectively (see Table 6). It is important to note that in the
case of the reaction of Cl atoms in the absence of NOx, where the total
carbon yield was lower than 100 %, there were many reaction products that
could not be quantified, such as dihydroxy and/or hydroxycarbonyl compounds.
In the reaction with the NO3 radical, due to our experimental conditions, a
significant number of primary nitrated compounds were expected to be formed
(Fig. S9c).
A mechanism of hydrogen atom abstraction at different positions on the
carbon chain has been proposed for the reaction of 3,3DM2ButOH with Cl, OH,
and NO3. The mechanism is shown in Fig. 8. Table S3 shows a summary of the reaction products, either observed or
tentatively identified, proposed by this mechanism of the reactions of
3,3DM2ButOH with the atmospheric oxidants.
Summary of molecular yields (%) of reaction products identified
in the reaction of SAs with atmospheric oxidants and the total carbon
balance (%).
1 Average Tables 3–5.
2 Molecular yield obtained in earlier step of the reaction.
3 Total carbon (%) =∑1ino. of
carbon of productino. of carbon of SA×molar yieldi(%).
4 Nitrate compounds were not accounted for.
5 From analysis of experiment number 3 for the reaction of Cl in the
presence of NOx.
6 From average of experiments number 2, 3, and 4 for the reaction with
OH.
7 From analysis of experiments number 1 and 2 for reaction with
NO3.
Lifetimes of 4MCHexOH, 3,3DM1ButOH, and 3,3DM2ButOH.
a Determined using the 24 h average of chlorine atoms.
b Determined using the peak concentration of chlorine atoms.
c Determined using the rate coefficient obtained by Moreno et al. (2014).
Molecular yields of 3,3-dimethyl-2-butanone obtained in this work imply
percentages of attack of the oxidant at the α-position (route II)
of 43%/44% in the case of Cl atom (in the absence/presence of NOx),
81 % for the OH radical, and 58 % for the NO3 radical. Percentages
are very similar to those predicted by SAR methods except for the NO3 radical
(see Table 5). The high NO2 concentration present in the reactions with the
NO3 radical would greatly favor the formation of nitrated compounds
over 3,3-dimethyl-2-butanone. It could also justify the low estimated molecular
yield for 3,3- dimethyl-2-butanone.
The main reaction products observed in the reaction with Cl atoms in the
presence of NOx (3,3-dimethyl-2-butanone, formaldehyde,
2,2-dimethylpropanal, acetone, and acetaldehyde) confirm the attack of Cl
atoms at other sites apart from the α-position. Based on the
estimated molecular yield of acetone, attack at the δ-position
with the abstraction of the hydrogen atom from methyl groups (route III) could be
∼58 %, and based on the estimated molecular yield of
2,2-dimethylpropanal, the attack at the β-position (route I) could
account for 10 %. These data agree with the SAR predictions for Cl atom
reactions. Moreover, the major molecular yields of acetone,
formaldehyde, and acetaldehyde (route III) in the reaction with Cl atoms in
the presence of NOx rather than in its absence could indicate that the self-reactions of peroxy radicals via the
molecular pathway is negligible in
the presence of NOx. The lower yield of
acetaldehyde estimated (17 %) versus 58 % of its coproduct (acetone) is due to its fast
degradation by reaction with Cl atoms with the formation PAN, as observed in
the FTIR experiments (see Fig. S9b).
Atmospheric implications
Pollutants in the atmosphere can create serious environmental problems,
such photochemical smog, acid rain, and degradation of the ozone layer
(Finlayson-Pitts and Pitts, 2000). Therefore, it is important to evaluate the
parameters that help us to understand the impact of the presence of these
compounds in the atmosphere. These parameters are the time that such
compounds remain in the atmosphere, their global warming potential (GWP), and
their mechanisms of degradation that allow us to estimate the atmospheric effect
of the products formed.
The first important parameter with respect to the environmental impact of oxygenated
volatile organic compounds in the atmosphere is the global lifetime, τglobal, which considers all of the degradation processes that could
affect these compounds in the troposphere. This parameter can be obtained
from the sum of the individual sink processes, such as reactions initiated by
OH and NO3 radicals, Cl atoms, and O3 molecules as well as photolysis and
dry and wet deposition, Eq. (4):
τglobal=1τOH+1τCl+1τNO3+1τO3+1τphotolysis+1τother processes-1.
The tropospheric lifetimes (τ) of 4MCHexOH, 3,3DM1ButOH, and 3,3DM2ButOH
for each process have been estimated by considering Eqs. (4) and (5):
τ=1kOx[Ox],
where kOX and [Ox] are the rate coefficients obtained in this work for
each oxidant and the typical atmospheric concentrations of the oxidants Cl,
OH, and NO3, respectively. The concentrations employed were 1×103 atoms cm-3 for the
24 h average for Cl (Platt and Janssen,
1995), 1×106 radicals cm-3 for the 12 h average daytime concentration of OH (Prinn et al., 2001), 5×108 radicals cm-3 for the 12 h average daytime concentration of NO3 (Atkinson, 2000), and a peak concentration
of 1.3×105 atoms cm-3 for Cl in the coastal marine
boundary layer at dawn (Spicer et al., 1998). Reactions with O3 and
photolysis are negligible loss processes for this type of compound (Mellouki
et al., 2015). Other processes are referred to as dry and wet deposition. To
estimate the lifetime associated with wet deposition, Eq. (6) proposed by
(Chen et al., 2003) was used:
τwet=HatmvpmRTkH,
where kH is the Henry's law constant; Hatm is the height of the
troposphere, taking a value of 630 m; vpm is the average precipitation
rate for Ciudad Real (Spain) (402 mm yr-1) (http://www.aemet.es, last access: 10 May 2019); R is the gas
constant; and T is the temperature, considered to be constant and equal to
298 K. In the literature, there are only data of Henry's constant for
3,3DM2ButOH (5.6×10-1 mol m-3 Pa-1; Sander,
2015). To compare the available data for similar compounds, approximated
values of KH of 3 and 0.4 mol m-3 Pa-1 have been used for 4MCHexOH and 3,3DM1ButOH, respectively.
The calculated lifetimes of the three alcohols studied in this work are
shown in Table 7. It can be seen that the dominant tropospheric loss process
for the three alcohols is clearly their reaction with OH radicals followed
by their reaction with NO3 radicals at night. However, in places where
there is a peak concentration of Cl atoms (coastal areas), the reaction
of these alcohols with Cl atoms may compete with the reaction with OH radicals
as their main degradation process.
The global lifetime of the three alcohols is of the order of ∼1–2 d, indicating that these compounds will probably be degraded near their
sources. These global lifetimes also indicate that SAs do not make a
significant contribution to the radiative forcing of climate change
(Mellouki et al., 2015), which is supported by the estimation of their
GWP values. For a time horizon of 20 years, the estimated values are 8.33×10-4, 1.78×10-2, and 5.80×10-3 for 4MCHexOH, 3,3DM1ButOH, and 3,3DM2ButOH, respectively, which are
very low. Therefore, these compounds will only have an important impact in the
troposphere at a the local or regional level.
Their degradation products (mostly carbonyl-containing compounds and
nitro-compounds in polluted areas) must be considered. Thus, the nitrated
compounds generated can act as NOx reservoir species, especially during the
night (Altshuller, 1993), and could have an influence at the global scale.
Moreover, as 4MCHexOH, 3,3DM1ButOH, and 3,3DM2ButOH react quickly with Cl
atoms and OH radicals, their contribution to the formation of photochemical
smog might be important. For that reason, the contribution of these three
alcohols to the formation of smog was estimated by obtaining the average
ozone production during 99 % of their reactions with the OH radical, using the
equation indicated by Dash and Rajakumar (2013). The values obtained were
3.24, 0.90, and 1.69 ppm for 4MCHexOH, 3,3DM1ButOH, and 3,3DM2ButOH,
respectively. These values suggest that these compounds may be potential
generators of tropospheric ozone and could contribute significantly to the
formation of photochemical smog depending on their concentrations in the
atmosphere.
Conclusions
The main conclusions obtained from the present study are as follows:
The kinetic and product study support that (1) the atmospheric
degradation mechanism for SAs, and possibly for other unstudied saturated
alcohols, proceeds by the abstraction of a hydrogen atom bonded to a carbon
rather than a hydrogen atom bonded to the oxygen atom of the alcohol group;
and (2) the reaction mechanism in the H atom abstraction process depends
on the oxidant. Cl atoms abstract any type of hydrogen (α, β,
δ) from SAs with a high percentage compared to the OH and the
NO3 radicals. The OH and NO3 radicals mainly abstract the hydrogen
atom at the α-position if the saturated alcohols are secondary.
For primary alcohols, the abstraction of a hydrogen atom at the β-position could also be important in the reaction with the NO3 radical.
Therefore, more kinetic studies of the NO3 radical reaction with
primary alcohols are necessary to quantify the effect of the OH group at the
β-position (-CH2OH) and to update the SAR method developed
by Kerdouci et al. (2010, 2014).
Theoretical ab initio studies of the reactions of SAs with atmospheric
oxidants should be performed in order to obtain more information about their
reaction mechanisms in the H atom abstraction process.
The atmospheric conditions determine the reaction mechanism and, therefore,
the reaction products obtained in the degradation of SAs. Thus, in polluted
environments with high concentrations of NOx, the peroxy radicals mainly react with NO to form the alkoxy radical instead of molecular compounds. Under
these conditions, nitrated organic compounds (RONO2) are formed as well
as polyfunctional organic compounds. Moreover, when the concentration of
NO2 is higher than that of NO, ozone is formed. In a clean atmosphere,
as in the case for the experiments with Cl atoms in the absence of NOx,
the reaction products are different because peroxy radicals could react
mainly via a self-reaction molecular pathway instead of via a self-reaction
radical pathway with the formation of dihydroxy and hydroxycarbonyl compounds.
The unquantified polyfunctional organic compounds could explain the low
carbon balance obtained in the Cl or NO3 reactions. The carbon balance
must be taken with caution as the calculated molecular yields have a high
degree of uncertainty.
Calculated lifetimes for saturated alcohols (of the order of ∼1 d)
imply that these compounds are pollutants at a local–regional scale, but
it is also important to indicate that SAs are sources of stable nitrated
compounds (ROONO2), depending on environmental conditions, that can
travel large distances from their sources and contribute to ozone formation in
clean areas, such as forest or rural areas.
The main products of the degradation of the SAs, aldehydes and ketones,
develop a very important secondary chemistry with the formation of products
of special relevance, such as the PAN observed in the degradation of
3,3-dimethyl-2-butanol. More experiments should be carried out using other
detection techniques in order to evaluate the formation of secondary organic
aerosol (SOA), because it is well known that polyfunctional organic compounds
are important SOA precursors.
From the environmental point of view, this work shows that the degradation
of SAs is an important source of pollutants in the atmosphere of greater or
lesser impact depending on the environmental conditions and the quantities
of these alcohols present in the atmosphere. Therefore, the use of SAs as
additives in the production of biofuels should be controlled, as poor
handling could result in high concentrations of these alcohols in the
atmosphere.
The rate coefficients and reaction products reported in this work are the
first available data for the abovementioned species; therefore, this work contributes to a better understanding of
the atmospheric chemistry of oxygenated compounds, expands the kinetic and
mechanistic database, and contributes to developing or to improving
predictive models that help us to avoid or mitigate the effects of climate
change or air quality deterioration. However, kinetic experiments in the tropospheric
temperature range are necessary to obtain more information about the
reaction mechanism and to extrapolate the rate coefficient data to other
typical atmospheric conditions and, thus, be able to better establish the
atmospheric impact of these alcohols.
Data availability
The underlying research data are available upon email request from the contact author of this work.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-699-2020-supplement.
Author contributions
SS and MP designed the experiments. CB was
the leader of the research group, controlled the research, and obtained
the financial support for the project leading to this publication. CI carried out the 4MCHexOH experiments, TA carried out the 3,3DM1ButOH and 3,3DM2ButOH kinetic
experiments, and AI carried out the
3,3DM1ButOH and 3,3DM2ButOH product experiments. MP supervised the data analysis and prepared the paper with
contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors, Inmaculada Colmenar, Araceli Tapia, and Inmaculada Aranda, wish to thank the Ministerio de Ciencia e Innovación and the Consejería de Ciencia y Tecnología de la Junta de Comunidades de Castilla-La Mancha for their financial support contracts.
Financial support
This research has been supported by the Ministerio de Ciencia e Innovación (project no. CGL2014-57087-R) and the Consejería de Ciencia y Tecnología de la Junta de Comunidades de Castilla-La Mancha (project no. SB-PLY/17/180501/000522).
Review statement
This paper was edited by James B. Burkholder and reviewed by two anonymous referees.