Saturated aldehydes, e.g. 2-methylbutanal (2 MB,
CH3CH2CH(CH3)C(O)H), are emitted into the atmosphere by
several biogenic sources. The first step in the daytime atmospheric
degradation of 2 MB involves gas-phase reactions initiated by hydroxyl (OH)
radicals, chlorine (Cl) atoms, and/or sunlight. In this work, we report the
rate coefficients for the gas-phase reaction of 2 MB with OH (kOH) and Cl
(kCl), together with the photolysis rate coefficient (J), in the
ultraviolet solar actinic region in Valencia (Spain) at different times of
the day. The temperature dependence of kOH was described in the 263–353 K range by the following Arrhenius expression:
kOH(T)=(8.88±0.41)×10-12 exp[(331±14)/T] cm3 molec.-1 s-1. At 298 K, the reported kOH and
kCl are (2.68±0.07)×10-11 and (2.16±0.32)×10-10 cm3 molec.-1 s-1, respectively. Identification and quantification of
the gaseous products of the Cl reaction and those from the photodissociation
of 2 MB were carried out in a smog chamber by different techniques (Fourier
transform infrared spectroscopy, proton transfer time-of-flight mass
spectrometry, and gas chromatography coupled to mass spectrometry). The
formation and size distribution of secondary organic aerosols formed in the
Cl reaction were monitored by a fast mobility particle sizer spectrometer. A
discussion on the relative importance of the first step in the daytime
atmospheric degradation of 2 MB is presented together with the impact of the
degradation products in marine atmospheres.
Introduction
The saturated aldehyde 2-methylbutanal (2 MB,
CH3CH2CH(CH3)C(O)H) is emitted into the low atmosphere from
several sources. It is known that 2 MB is formed during the fermentation and
drying processes of cocoa beans (Utrilla-Vázquez et al.,
2020), the manufacturing process of tea leaves (e.g. Camellia Sinensis, Flaig et al., 2020), and as a consequence of the stress
suffered by grapevine leaves due to drought (Griesser et al.,
2015). Besides, this aldehyde is emitted into the atmosphere from wildland
fires (Urbanski et al., 2008) and many industrial activities,
such as poultry rendering operations during which animal by-products are processed
(Kolar and Kastner, 2010). As a secondary pollutant, 2 MB can be
formed in situ in the atmosphere by oxidation of 2-methyl-1-butanol
(CH3CH2CH(CH3)CH2OH), which is used as a biochemical
pesticide, commercially used as a solvent in paints and oils, and used as
flavorant in many processed foods.
Once in the atmosphere, 2 MB can react with daytime tropospheric oxidants,
such as hydroxyl (OH) radicals or chlorine (Cl) atoms in coastal or marine
regions. During daytime, 2 MB can also be photolysed by the solar actinic
radiation (λ>290 nm). Other possible sinks of 2 MB are
dry and wet depositions that have been previously reported to be minor sinks
for other aldehydes such as acetaldehyde
(Millet et al., 2010). Since the
gas-phase chemistry of 2 MB can lead to the formation of secondary
pollutants, its emission may have a significant impact on tropospheric
chemistry and air pollution at a local/regional scale. For that reason, in
this work we evaluate the potential impact of the diurnal degradation of
2 MB. Firstly, the photodissociation of 2 MB (Reaction R1) has been
investigated at room temperature by determining its absorption cross
sections (σλ) between 220 and 360 nm, the photolysis
rate coefficient (J) under the irradiation conditions of this work (λ≥290 nm) and the corresponding effective quantum yield
(φeff).
CH3CH2CH(CH3)C(O)H+hν(λ≥290nm)→ProductsJ
Secondly, the gas-phase kinetics with OH (Reaction R2) and Cl (Reaction R3)
under tropospheric conditions of temperature and pressure was investigated
to assess the tropospheric lifetime (τ) of 2 MB due to both removal
routes.
OH+CH3CH2CH(CH3)C(O)H→ProductskOHCl+CH3CH2CH(CH3)C(O)H→ProductskCl
The rate coefficient kOH was determined between 263 and 353 K as a
function of total pressure (50–600 Torr of He; 1 Torr = 133.32 Pa), while kCl was measured
at 298 K and (760±5) Torr of air. Finally, the gaseous products of
Reactions (R1) and (R3) have been identified under NOx-free conditions
using several detection techniques, and a reaction mechanism is proposed for
NOx-free atmosphere. Furthermore, the formation yield of secondary organic
aerosols (SOAs) formed in Reaction (R3) has been measured to assess the impact
of 2 MB on the formation of ultrafine particles. With all this information,
we discuss the potential impact of atmospheric 2 MB on local or regional air
quality in terms of the estimated tropospheric lifetime and the reaction
products formed.
Experimental methods
In this section, a brief description of the experimental techniques and the
methodology employed in this work is given. More details can be found in the
Supplement.
Photodissociation of 2-methylbutanalGas-phase ultraviolet (UV) absorption spectroscopy (220–360 nm)
Ultraviolet absorption spectroscopy was used to determine the absorption
cross sections of 2 MB as a function of wavelength (σλ in
base e) between 220 and 360 nm. The experimental set-up employed in this work
has been described in detail elsewhere (Blázquez et al.,
2020). This system consists of a deuterium-tungsten light source (DT-200,
StellarNet) placed at the entrance of a 107.15 cm jacketed
Pyrex® cell, connected by an optical fibre to a f/2
spectrometer that possesses a concave holographic grating (590 grooves mm-1)
and a 2048-pixel charge-coupled device (CCD) camera (BLACK-Comet model C, StellarNet). The
absorbance is recorded in a computer with the data acquisition software
SpectraWiz v5.33. The experiments were carried out by introducing pure
gaseous 2 MB (1.085–6.642 Torr) into the UV cell in static mode. Applying
the Beer–Lambert law, σλ was determined from the slope
of the absorbance (in base e) versus 2 MB concentration ([2 MB]).
Figure S1 shows some examples of the absorbance versus [2 MB].
Even though the spectral resolution of the recorded UV spectra was higher
than 0.11 nm, in Table S1 absolute values of σλ
are only given at 1 nm intervals for ease of presentation. Note that the
tabulated values are not the integrated σλ in 1 nm. All
absolute σλ values are available in the Excel file included in the
Supplement.
Schematics of the photolysis set-up using a solar
simulator as a continuous irradiation source.
Continuous irradiation with a solar simulator (λ≥290 nm)
A schematic of the set-up used in this work to investigate the
photochemistry of 2 MB under atmospheric conditions is shown in Fig. 1. A Pyrex (l=20 cm and i.d. = 4 cm) cell sealed with quartz windows
was filled with (760±3) Torr of diluted 2 MB (interval of dilution
factor from 6.96×10-4 to 1.92×10-3 in
synthetic air) from a 10 L Pyrex storage bulb at (298±2) K. The
partial pressure of 2 MB, the total pressure inside the storage bulb, and the
pressure in the photolysis cell were measured by capacitance pressure
transducers (Leybold, model CERAVAC, 10 and 1000 Torr full scale). The
initial concentration of 2 MB in the photolysis cell ranged from 1.2 to
6.9×1016 molec. cm-3. These concentrations were
determined by FTIR (Fourier transform infrared) spectroscopy using the absorption cross sections (see
Supplement). Before irradiating the sample and after each irradiation time, the FTIR spectrum of the mixture was recorded in a
16 L White-type cell with an optical path length of 96 m by a FTIR
spectrometer (Thermo Fisher Scientific, model Nicolet Nexus 870) with a
liquid-N2-cooled MCT (mercury cadmium telluride) detector. IR spectra
were recorded between 650 and 4000 cm-1 at a resolution of 2 cm-1,
after the accumulation of 32 interferograms. The selected IR bands for
monitoring 2 MB were the ones centred at 2700 and 2800 cm-1.
The sample of 2 MB in air was irradiated by an ABA class solar simulator
(SunLite™ solar simulator, model 11002-2) during 30, 60, 90, 120, and
150 min. The solar simulator, equipped with a Xe arc lamp and an air mass
(AM) 1.5G filter, emits radiation over 290 nm with a spectrum that simulates
the solar reference spectrum AM 1.5G (see Fig. S2). This
reference spectrum corresponds to the terrestrial solar spectral irradiance
at the surface when the air mass factor is 1.5 and the sun is about
41∘ above the horizon and under specific atmospheric
conditions defined by the U.S. Standard Atmosphere (Gueymard
et al., 2002). Irradiance was determined in each experiment with a
1 cm2 solar reference cell for which a 100 mV output corresponds to 1
Sun, defined as the irradiance of the AM 1.5G reference solar spectrum
described above. In this work, the measured irradiance was 2.225±0.145 Suns.
As shown in Fig. 1, during the irradiation of the diluted sample of
2 MB, control valves 2 and 3 were closed. After irradiation, the gas sample
was expanded to the 16 L FTIR cell to measure the 2 MB concentration over
time ([2 MB]t) and, thus, to determine the photolysis rate coefficient,
J. In that case, the total pressure decreases to 7 Torr and control valves 1
and 4 were closed.
Possible losses due to the heterogeneous reaction of 2 MB onto the walls were
evaluated in experiments without light at different times (45, 90, and 135 min). As the photolysis cell used has a large surface / volume ratio (276.46 cm2/ 251.33 cm3=1.1 cm-1), the heterogeneous loss of 2 MB
contributes significantly to the total decay of 2 MB. The heterogeneous loss
rate, kheterog (in s-1), has been measured to contribute around
24 % to the total loss. Therefore, the photolysis rate coefficient of
2-methylbutanal at λ>290 nm, J (in s-1), can be
obtained from the slope of the plot of ln([2 MB]0/ [2 MB]t) versus time
according to Eq. (1).
ln([2MB]0/[2MB]t)=(kheterog+J)t
For the identification of the photolysis products, three complementary
detection techniques were used: FTIR spectroscopy, gas chromatography–mass
spectrometry (GC-MS), and proton transfer reaction time-of-flight mass
spectrometry (PTR-ToF-MS). The gas chromatography–mass spectrometer (Thermo
Electron, models Trace GC Ultra and DSQ II) was equipped with a BPX35 column
(30 m × 0.25 mm i.d. × 0.25 µm, SGE Analytical
Science) working at a temperature ramp that ranged between 40 and 250 ∘C (Ballesteros et al., 2017). The
solid-phase micro-extraction technique was used as a sampling method in these
experiments; thus, a 50/30µm
Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS) fibre (Supelco)
was exposed for 10 min to the gas mixture of the photolysis cell, and
then the fibre was injected into the GC-MS for 5 min. Independently, in
some experiments a PTR-ToF-MS (IONICON, model PTR-TOF 4000), working with a
field density ratio (E/N) of 137 Td, a time resolution of 20 s, and a detected
mass range between 29 and 390.86 amu (Antiñolo et al.,
2020), was coupled to the FTIR cell, in which the content of the photolysis
cell was diluted in 760 Torr of air, to detect the photolysis products at
the end of the experiment. However, quantification of the photolysis
products could only be carried out by FTIR spectroscopy. The PTR-ToF-MS
could not be used to quantify them as the sampling flow rate was too high,
making the total pressure in the FTIR cell drop rapidly, and it did not allow
for a stable signal. The quantification by GC-MS was not performed due to
the absence of an internal standard that would have been needed due to the
instability of the GC-MS signal.
Pulsed laser photolysis coupled to laser-induced fluorescence (PLP-LIF)
technique
The absolute kinetic study of the reaction of OH radicals with 2 MB (Reaction R2) was performed as a function of temperature (T=263–353 K) and total
pressure (PT=50–600 Torr of He) in a Pyrex doubled-wall reactor of
ca. 200 mL (Martínez et al., 1999). The gas temperature
was measured by a type K (chromel–alumel) thermocouple inserted several
millimetres above the reaction, and T was kept constant (±0.2 K) during
each experiment. The total pressure inside the reactor was controlled by a
needle valve placed at the exit of the reactor and prior to the pump, and it
was measured by pressure transducers (Oerlikon Leybold Vacuum, model CERAVAC
CTR 100N, 0–100 and 0–1000 Torr). All gases were introduced in the
reactor by means of mass flow controllers. The total flow through the
reactor, FTotal, was kept constant in the 485–496 sccm (standard cubic centimetre) range, while the
flow rate of diluted 2 MB and OH precursor (H2O2 or HNO3) were
varied between 3.1 and 29 sccm) and between 14.4
and 96.8 sccm, respectively. The residence time in the cell ranged between
0.8 and 13 s, depending on the total flow, pressure, and temperature
conditions of the experiment. More details are given in the Supplement.
The OH radicals were generated in situ from the pulsed laser photolysis (PLP) of
H2O2 or HNO3 at 248 nm, the radiation coming from a KrF excimer
laser (Coherent, ExciStar 200). The laser fluence was 23 mJ pulse-1 cm-2 at 10 Hz. The OH radicals generated in the electronic ground state
were excited at ca. 282 nm (laser energy between 0.4 and 1.0 mJ pulse-1 at
10 Hz) to the first excited electronic state by a tuneable pulsed laser
(Continuum, ND60; pumped by Continuum, NY 81 CA-10). The photolysis and
excitation lasers traverse the reactor perpendicularly. From the
intersection of both lasers, in which the reaction takes place, the excited OH
radicals emit off-resonance laser-induced fluorescence (LIF) (at ca. 310 nm),
which was collected by a photomultiplier tube (Thorn EMI, 9813B model) set
perpendicular to both lasers.
Under pseudo-first-order conditions (i.e. when the initial concentration of
OH precursor and 2 MB are in large excess with respect to that of OH
radicals), the time evolution of the LIF signal follows a single exponential
decay (see some examples in Fig. S3). From the analysis of these
decays, the pseudo-first-order rate coefficient, k′, was determined at the given
[OH precursor]0 and [2 MB]0 for each temperature and pressure. In
the absence of 2 MB, the measured pseudo-first-order rate coefficient is
k0′. Examples of the k′-k0′ versus [2 MB]0 plots at 263 and 353 K, from
which the rate coefficient kOH was obtained, are depicted in
Fig. S4. [OH precursor]0 was measured by UV absorption
spectroscopy between 200 and 230 nm using the same experimental system
described in Sect. 2.1.1. In this case, the absorption measurements were
carried out under flowing conditions. From [OH precursor]0 and the
laser fluence at 248 nm, the initial amount of OH radicals inside the
reactor cell was estimated. The ranges of [OH precursor]0, [2 MB]0,
dilution factor f of 2 MB in the storage bulb, and k′ are also listed for each
temperature and pressure in Table S2 of the Supplement.
Simulation (smog) chambers coupled to a variety of detection techniques
For the kinetic and mechanistic study of the Cl reaction with 2 MB, several
experimental systems were used (Ballesteros et al., 2017; Antiñolo et
al., 2019, 2020). Two cylindrical chambers made of
Pyrex were available to perform the different experiments: a 16 L cell and a
264 L reactor. Both chambers were surrounded by actinic lamps (Philips
Actinic BL TL 40W/10 1SL/25, λ=340–400 nm): four for the 16 L
gas cell and eight for the 264 L. These lamps were used to continuously
generate Cl atoms in situ by UV photolysis of Cl2. The gas-phase species (2 MB
and Cl2) were introduced in the reactor from a gas line, and they were
diluted with synthetic air at (298±2) K and (760±5) Torr of
total pressure. The total pressure in the gas line and in the reactors was
measured with two capacitor pressure transducers (Leybold, model CERAVAC,
10 and 1000 Torr full scale). Three different detection methods were
employed: FTIR spectroscopy, GC-MS, and PTR-ToF-MS, previously described in
Sect. 2.1.2.
Relative rate kinetic studies (298 K and 760 Torr of air)
The kinetic experiments were carried out by mixing 2 MB, a reference compound
(ethanol or isoprene), Cl2, and synthetic air in the 16 L cell and
using FTIR to monitor 2 MB and the reference compound as a function of
reaction time. The IR bands selected for monitoring the loss of 2 MB and the
reference compounds were centred at 2700 cm-1 for 2 MB, 1070 cm-1 for ethanol, and 3095 cm-1 for isoprene. The disappearance of 2 MB and
the reference compound is mainly due to reaction with Cl, although they can
also be lost by heterogeneous reaction onto the reactor walls, UV photolysis,
and/or reaction with the oxidant precursor. These loss processes were
evaluated in preliminary and independent experiments in the absence of
Cl2 and UV light (wall loss, kw), in the absence of UV light
(reaction with Cl2,kCl2′=kCl2[Cl2]0), and by
irradiating in the absence of Cl2 (UV photolysis, khν) as
described in previous studies (Antiñolo et al., 2019, 2020). Table 1 summarizes the loss rate coefficients for these
processes and the overall loss rate coefficients, kTotal_loss (kloss for 2 MB and kRef,loss for the reference compound).
Measured loss rate coefficients of 2 MB and the reference
compounds.
Therefore, taking into account the overall losses for both 2 MB and the
reference compound, the integrated rate equation is given by the following
expression:
ln[2MB]0[2MB]t-klosst=kClkrefln[Ref]0[Ref]t-kRef,losst,
where kref is the rate coefficient for the Cl reaction with the
reference compound at 298 K and 760 Torr. [2 MB]0, [2 MB]t,
[Ref]0, and [Ref]t are the concentrations of 2 MB and the reference
compound at the beginning of the reaction and at a reaction time t,
respectively. Initial concentrations in the cell were [2 MB]0=(4.9-6.6)×1014 molec. cm-3,
[Cl2]0=(3.3-5.9)×1014 molec. cm-3,
[ethanol]0=(4.6-3.6)×1014 molec. cm-3,
and [isoprene]0=(5.1-5.7)×1014 molec. cm-3. In Fig. S5, an example of the plots of Eq. (2) is
presented for both reference compounds.
Product study in the Cl reaction
The identification of the gaseous products generated in the reaction of
2-methylbutanal with Cl atoms was performed using complementary detection
techniques: FTIR spectroscopy (using the 16 L reactor) (Ballesteros et
al., 2017; Antiñolo et al., 2019), GC-MS, and PTR-ToF-MS (using the
264 L simulation chamber) (Antiñolo et al., 2020) in separate
experiments. In all cases, preliminary tests were carried out to check if
products were generated during the dark reaction of 2 MB with Cl2 and/or
during the UV light exposure of 2 MB. When GC-MS was used, no products due to
these processes were observed. Nevertheless, with FTIR and PTR-ToF-MS the
formation of some products was observed during UV light exposure of 2 MB (see
Sect. 4.3.1). In all the experiments, synthetic air was used as diluent gas
at 298 K and 760 Torr, and the reaction mixture was irradiated for 60 min.
IR spectra, chromatograms, and mass spectra were recorded every 2 min, 10 min, and 20 s, respectively. In Table 2, the initial concentrations of
2 MB, Cl2, and air mixtures are summarized. Because of the high sensitivity of
the PTR-ToF-MS, the initial concentrations of 2 MB and Cl2 were reduced
with respect to those employed in the FTIR and GC-MS experiments.
Furthermore, at the inlet of the PTR-ToF-MS the reaction mixture was diluted
(a factor of 1/5) with an air flow by means of a dynamic inlet dilution
system to avoid signal saturation. Quantified products were corrected to
account for their loss due to Cl reaction according to the method described
by Ceacero-Vega et al. (2012).
In addition to the gaseous reaction products, the secondary organic aerosols
produced in the Cl+2 MB reaction were detected, and the size distribution of
these SOAs was measured by a fast mobility particle sizer (FMPS)
spectrometer (TSI 3091). The detailed experimental procedure can be found in
the Supplement.
Initial concentrations of 2 MB and Cl precursor in the
reactor.
For gases, synthetic air (99.999 %, Air Liquide), Cl2 (Sigma Aldrich,
99.8 %), and He (Nippon Gases, 99.999%) were used as supplied. Liquids
from Sigma Aldrich, with purities in brackets, were used after
freeze–pump–thaw cycles: 2-methylbutanal (95 %), ethanol (99.8 %),
isoprene (99 %), butanone (99 %), and acetaldehyde (99.5 %). The aqueous
solution of H2O2 (Sharlab, >50 % v/v) was
preconcentrated as described by Albaladejo et al. (2002).
Results and discussionUV photochemistry of 2-methylbutanalPhotolysis frequency
(J) and effective
quantum yield (Φeff) at λ≥290 nm
As shown in Fig. 2, 2 MB absorbs in the ultraviolet range,
exhibiting a weak absorption band in the 220–360 nm range due to the
forbidden n-π∗ transition of the C=O chromophore, with a
maximum at 296 nm. The maximum σλ was determined to be
(6.25±0.08)×10-20 cm2 per molecule (stated
uncertainty in our results throughout the document is ±2σ
statistical). Therefore, in the troposphere the actinic radiation (λ≥290 nm) can initiate photochemical processes for 2 MB. Four photolysis
experiments were performed under the conditions described in Sect. 2.1.2. In
Fig. 3 the average values of individual
ln([2 MB]0/ [2 MB]t) obtained in the four experiments are plotted against
t, showing a good linearity. From the slope of such a plot and correcting it
with the wall losses of 2 MB, as described in Sect. 2.1.2, J=(1.96±0.32)×10-5 s-1 was obtained.
UV absorption cross sections of 2 MB at 298 K. The grey
shadow represents the statistical uncertainty.
The photolysis quantum yield at a single wavelength (Φλ)
is related to the photolysis rate coefficient as follows:
J=∫λ1λ2ΦλσλIλdλ,
where Iλ is the irradiance in photons per square centimetre per nanometre per second (cm-2 nm-1 s-1) at λ. However, J can be approximated to
J≅Φeff∑λ1λ2IλσλΔλ,
where, in this work, Φeff is the effective quantum yield of 2 MB,
Iλ>290nm is the measured irradiance at each wavelength,
σλ is the experimentally determined UV absorption cross
sections of 2 MB, and Δλ=1 nm. Taking into account all
these parameters, Φeff=(0.30±0.05). An experiment
was done in the presence of cyclohexane ([cyclohexane] / [2 MB] = 8.2), which
is widely used as the OH scavenger in this kind of experiment to evaluate the
impact of oxidants formed in this reaction. No difference was observed with
the experiments with no cyclohexane, indicating that the chemistry of the OH
radical is negligible. This value is ca. half of that previously reported
(Wenger, 2006) measured in the atmospheric simulation chamber
EUPHORE (Valencia) under natural irradiation conditions: Φeff=(0.72±0.03). The reason for this difference is unknown. It is
worth to note though that in the same study, for structurally similar
aldehydes like pentanal or 3-methybutanal, the reported Φeff
were closer (0.30±0.02 and 0.27±0.01, respectively) to
that determined in this work.
Plot of Eq. (1) in which the average
ln([2 MB]0/ [2 MB]t) over time has been represented
UV photolysis products of 2 MB
Photodissociation of 2 MB by UV radiation can proceed through the following
channels.
R1aCH3CH2CH(CH3)C(O)H+hν(λ≥290nm)→CH3CH2CH(CH3)C(O)+HR1b→CH3CH2CH(CH3)+HC(O)R1c→CH3CH2CH2CH3+CO
Reactions (R1a) and (R1b) are radical-forming channels, while Reaction (R1c)
yields close-shell molecules, i.e. butane
(CH3CH2CH2CH3) and carbon monoxide (CO). Figure 4a shows the recorded IR spectra of the 2 MB and air sample before irradiation.
After 150 min of irradiation, the IR features from 2 MB were subtracted to
identify the formed products, as shown in Fig. 4b. Products from
Reaction (R1c), butane and CO, were clearly identified at 2800–3000 cm-1 and 2000–2300 cm-1, respectively, indicating that this
channel is open in the investigated wavelength range. The product yield
(Yproduct) is obtained from the slope of the concentration of the product versus the consumed
2 MB, Δ[2 MB]. To determine the effective quantum yield of Reaction (R1c), φR1c, the yield of formation of CO (YCO) or butane
(Ybutane) could be used if no secondary chemistry was present, which is
not the case for CO since it is a very end degradation product and can be
formed in many oxidation reactions. Therefore, CO is not a good marker for
the importance of the photolysis Reaction (R1c). In addition, YCO was
observed to increase when the initial concentration of 2 MB decreased. Butane
was quantified using the reference spectrum shown in Fig. S6 and
the 2800–3000 cm-1 IR band, yielding Ybutane=(9.80±0.31) % (see Fig. S10). As presented in Fig. 4b,
butanone (CH3CH2C(O)CH3) was also identified as a product of
2 MB photolysis in the presence of air, and its product yield was
Ybutanone=(14.8±0.5) % (see Fig. S10).
Quantification of butanone was done using the reference spectra shown in
Fig. S6 for the IR bands: 2850–3100 cm-1 and 1650–1800 cm-1. After subtracting the IR features of CO, butane, and butanone,
some IR features were left in the residual spectrum (Fig. 4c).
The band centred around 1040 cm-1 can be assigned to the C-O
stretching mode of an alcohol (probably 2-butanol or methanol according to
the proposed mechanism in Sect. 3.4.2), and the one centred around 1740 cm-1 can be assigned to the C=O stretching mode of a carbonyl
group, alluding to aldehydes or ketones formation. The bands centred around
3000–2860 cm-1 can be assigned to the Csp3-H stretching mode, and
finally, the bands located around 1300–1400 cm-1 can correspond to
the C-H bending mode. However, the remaining IR bands in the residual
spectrum could not be assigned to other gaseous oxidation products.
FTIR spectra of a 2-methylbutanal and air mixture (a) before
irradiation and (b) after 150 min of photolysis with the features of 2 MB
subtracted (major products are labelled in the spectrum, exact positions of
the bands are given in the main text), as well as (c) residual spectrum after the
subtraction of the reference spectra of the identified products shown in
Fig. S6.
Butanone was also identified by GC-MS and PTR-ToF-MS. Figure S7
shows an example of the chromatogram of a mixture of 2 MB in synthetic air
before and after photolysis (150 min). Although the PTR-ToF-MS technique
allows the measurement of volatile organic compounds in gaseous samples as a function of time by
taking some flow from the sample, the volume of the photolysis cell was too
small to maintain the pressure during the photolysis experiments. For that
reason, after 150 min of photolysis, the maximum photolysis time used in
this work, the content of the photolysis cell was transferred to the 16 L
chamber and diluted in synthetic air. This diluted sample was flowed through
the PTR-ToF-MS for 5 min, in which mass spectra were measured. From the
average of these mass spectra, only products could be detected, but a
temporal evolution could not be obtained. Under these conditions, butanone
(C4H8OH+, m/z=73.06) was detected.
Individual rate coefficients of the 2 MB + OH reaction as
a function of temperature and total pressure.
∗ Values obtained from the k′-k0′ versus [2 MB]0 plot combining all
kinetic data at different total pressures.
Arrhenius plot for the rate coefficient of the 2 MB + OH
reaction between 263 and 353 K.
Kinetics of 2 MB with OH at T and P conditions of the troposphere
The individual rate coefficients for the 2 MB + OH reaction obtained at a
certain temperature and total cell pressure (PT) are listed in
Table 3. No pressure dependence of kOH(T) in the studied
temperature range was observed, within the experimental uncertainties. For
that reason, all k′-k0′ values obtained at different total pressures were
combined and plotted versus [2 MB]0, according to Eq. (3)
of the Supplement,
as shown in Fig. S4 for 263 and 353 K. The resulting
kOH(T) are listed in the last column of Table 3. We observe a
slight negative T dependence of kOH(T), increasing 40 % from 353 to
263 K. In Fig. 5, kOH(T) as a function of temperature is
depicted, together with the previous reported data by D'Anna et
al. (2001), at room temperature. The observed T dependence is well-described
by the following Arrhenius equation (solid line in Fig. 5):
kOH(263-353K)=(8.88±0.81)×10-12exp[(331±27)/T]cm3molec.-1s-1,
where the activation energy is (-2.75±0.23) kJ mol-1. The
reported kOH by D'Anna et al. (2001) at room temperature was
(3.28±0.09)×10-11 cm3 molec.-1 s-1,
which is 22 % higher than the one obtained in this work: kOH(298 K) =(2.68±0.07)×10-11 cm3 molec.-1 s-1. The reason for this discrepancy may be found in the different
methods used: D'Anna et al. (2001) used a relative rate kinetic
method with FTIR spectroscopy as the detection technique in which they used air
as bath gas at (298±2) K and (760±7) Torr. They only used
one reference compound for their analysis, 1-butene, and that may result in
an important source of error. In addition, the selected IR band for 1-butene
(3140–3070 cm-1) is too weak to be accurately monitored. Another
possibility is that this band, or the selected one for 2 MB (2740–2670 cm-1), might interfere with IR features from reaction products.
Chromatograms obtained for a 2 MB and Cl2 mixture before
irradiation (a) and after 60 min of irradiation (b).
Results obtained in the kinetic experiments for the
gas-phase reaction of Cl with 2-methylbutanal at 298±2 K and 760±5 Torr of air.
a Atkinson et al. (2006), b Orlando et al. (2003).
Cl reaction of 2 MB at T and P conditions of the marine boundary layerKinetics with Cl atoms
The plots of Eq. (2) for the two references used in this work can be seen in
Fig. S5, in which a good linearity is shown, indicating that no
secondary reactions were interfering. From the slope of these plots,
kCl/kRef was obtained according to Eq. (2). Thus, from the rate
coefficients of the reference compounds previously reported for the
Cl reaction of ethanol and isoprene (Atkinson et al., 2006; Orlando et
al., 2003), kCl was determined. Table 4 shows the rate
coefficients obtained in this work with each reference compound and the
averaged value: (2.16±0.32)×10-10 cm3 molec.-1 s-1. The uncertainty in kCl includes the
propagation of the reported errors in kRef, the uncertainties in
kloss, and the statistical errors from the slope of the plots shown in
Fig. S5. The contribution of kloss to the total loss of 2 MB was
found to be 4 %.
FTIR spectra used in the identification of the products
in the Cl reaction of 2 MB. The top panel shows the initial spectrum, the
central panel shows the spectrum obtained after 50 min of Cl reaction, with
the features of 2 MB subtracted, and the bottom panel shows the residual
spectrum after the subtraction of reaction products shown in Fig. S6.
Identification and quantification of the gaseous products of the Cl + 2 MB reactionIdentification by GC-MS
Figure 6 shows the
obtained chromatograms before and after 60 min reaction time, and
Fig. S8 shows the mass spectra of the detected products
corresponding to those chromatographic peaks. The peak corresponding to 2 MB
was observed at a retention time (RT) of 3.76 min. The rest of the peaks
that appear in the chromatogram were assigned, according to their mass
spectrum, to the following products: acetaldehyde (RT = 2.15 min),
2-butanol (RT = 3.08 min), butanone (RT = 3.21 min), methylglyoxal (RT = 4.44 min), and 2-methylbutanoic acid (RT = 5.55 min).
Identification and quantification by FTIR Spectroscopy
Figure 7 shows the FTIR spectrum recorded after 50 min of
reaction time. The features of 2-methylbutanal have been subtracted for
clarity purposes. By comparison with the reference spectra (shown in
Fig. S6), the most abundant products observed were HCl, butanone,
and CO. The yield for butanone, Ybutanone, obtained from the plots shown
in Fig. S9a, was (53.1±1.6) %. Formation of acetaldehyde
and formaldehyde was also observed, but their quantification was very
imprecise due to the low signal of the residual spectrum, the similarity
between both IR spectra, and the possible presence of other minor products
such as propanal and 2-butanol. After subtracting HCl, butanone, and CO,
ketene (CH2C(O)) could be identified in the residual spectrum (shown in
the bottom panel of Fig. 7) by comparison with the IR features
reported by Wallington et al. (1996). The
remaining bands could come from methylglyoxal, which was observed by GC-MS.
It is worth noting that small amounts of acetaldehyde, formaldehyde,
butanone, and CO (<9×1012 molec. cm-3) were
observed during UV light exposure of 2-methylbutanal, but their amount
was negligible compared with the observed ones during the Cl reaction
(>1×1014 molec. cm-3).
Temporal evolution of 2-methylbutanal (sum of the two
identified ions) and the most abundant products measured by PTR-ToF-MS
during the 2-methylbutanal + Cl reaction. Vertical dashed line indicates
when UV lamps are switched on.
Identification by PTR-ToF-MS
The identified reaction
products, with an average ion concentration greater than or equal to 0.2 ppb, were acetaldehyde (C2H4OH+, m/z=45.03), butanone
(C4H8OH+, m/z=73.06), methylglyoxal
(C3H4O2H+, m/z=73.03), 2-butanol
(C4H10OH+m/z=75.08), formaldehyde (CH2OH+m/z=31.02), ketene (C2H2OH+, m/z=43.02), methanol
(CH4OH+, m/z=33.03), 2-methylbutanoic acid
(C5H10O2H+, m/z=103.07), and propanal
(C3H6OH+, m/z=59.05). All the products observed by GC-MS and
FTIR, except CO and HCl, were also observed by PTR-ToF-MS. In the PTR-ToF-MS
analysis, it was observed that 2 MB yields two different ions when ionized:
C5H10OH+ (39 %) and C2H4OH+ (61 %). As
the most abundant fragment from 2 MB, C2H4OH+, overlaps with
the molecular ion from acetaldehyde, in order to quantify this
product the contribution of 2 MB to the C2H4OH+ signal was
eliminated taking into account its correlation with the
C5H10OH+ signal when only 2 MB was present in the chamber. In
Fig. 8 the time-evolution of 2 MB (plotted as the sum of
the two identified ions) and the main products, acetaldehyde, butanone, and
methylglyoxal, are shown. The molar product yields, obtained from the plots
shown in Fig. S9b, were (67.9±0.8) %, (34.9±0.6) %, and (14.8±0.2) %, respectively. In addition, very low
concentrations of 2-butanol were detected with a product yield of
(0.11±0.03) %. It must be noted that formaldehyde and ketene are
formed during UV light exposure of 2-methylbutanal in the test prior to the Cl
reaction but at very low concentrations (<7.38×1010 molec. cm-3) compared with the observed levels after the Cl
reaction.
Relative contribution of the three removal routes studied
in this work to the total diurnal loss.
Secondary organic aerosols (SOAs)
The yield of SOAs formed in the Cl + 2 MB reaction, YSOA, was determined
under different conditions (see Table S3) from plots such as the one
presented in Fig. S11. As summarized in Table S3, YSOA ranged from 0.16 % to 0.76 % and shows a negative dependence
on [2 MB]0/ [Cl2]0, i.e. it decreased when that ratio increased.
The size distribution of the particles formed in the Cl + 2 MB reaction with
diameters (Dp) between 6 and 523 nm is shown in Fig. S12 in
terms of the normalized particle number, dN /dlogDp, and mass,
dM /dlogDp. The maximum dM /dlogDp was observed at diameters near 500 nm, close to the maximum Dp that the FMPS apparatus can measure. After
12–30 min of reaction, SOAs coagulated (Dp>500 nm) and were not
able to be detected by the FMPS.
Photodegradation mechanismsMechanism of the UV photodissociation of 2 MB
It is clear that the molecular elimination of CO (Reaction R1c) occurs
since butane (Ybutane=9.80 %) and CO were detected. Although CO is
directly produced in Reaction (R1c), it can also be rapidly produced by the
reaction of formyl radical, HCO, formed in Reaction (R1b) with O2
(Fig. S13b). Butanone (Ybutanone=14.8 %) is plausible to
be formed from the radical-forming Reactions (R1a) and (R1b). The
CH3CH2CH(CH3)CO and CH3CH2CH(CH3) radicals
formed in those reactions, respectively, react rapidly with O2 to
produce the corresponding peroxy (RO2) radical that is involved in a
sequence of reactions to generate butanone, among other species. Other
molecular photolysis pathways are the following (Gruver and Calvert, 1956; Wenger,
2006).
R1dCH3CH2CH(CH3)C(O)H+hυ→CH4+CH3CH=CHC(O)HR1e→CH2=CH2+CH3CH=CHOH
None of these products were detected by PTR-ToF-MS or GC-MS. However, it is
possible that CH3CH=CHC(O)H and/or CH3CH=CHOH were
responsible of the remaining bands observed in the residual IR spectrum,
although this could not be justified due to the lack of reference spectra.
Neither CH4 nor CH2=CH2 could be clearly detected.
Mechanism of the 2 MB + Cl reaction
After evaluating the gas-phase products formed in the Cl reaction with
2-methylbutanal, some information can be inferred concerning the reaction
mechanism. The presence of HCl as a primary product and the fact that no
other chlorinated products were observed indicate that the reaction proceeds
via the H abstraction from different sites in 2 MB. There are five susceptible
reaction sites in 2 MB.
R3aCl+CH3CH2CH(CH3)C(O)H→CH3CH2CH(CH3)C(O)+HClR3b→CH3CH2C(CH3)C(O)H+HClR3c→CH3CHCH(CH3)C(O)H+HClR3d→CH2CH2CH(CH3)C(O)H+HClR3e→CH3CH2CH(CH2)C(O)H+HCl
The mechanisms for the H abstraction from the -C(O)H group and from the
hydrocarbon chain are depicted in Figs. S13a and S14,
respectively. As shown in the figures, acetaldehyde (CH3C(O)H),
formaldehyde (HC(O)H), or methanol (CH3OH) formation can be explained by
any of the five possibilities, CH3CH2C(O)CH3 can be only formed
if the aldehydic H (Fig. S13a) or the tertiary H in C-2 are
abstracted (Fig. S14a), and methylglyoxal (CH3C(O)C(O)H) is
a product only when a hydrogen atom from C-2 and C-3 is abstracted
(Fig. S14a–b). Although abstraction at the -C(O)H and tertiary
C-2 site is likely dominant, methylglyoxal may not be a major product of
C-3, and thus our product data are not conclusive on this issue.
Atmospheric implications
Considering the most important diurnal degradation pathways (UV photolysis
and reactions with OH radicals and Cl atoms), the relative importance of
these three degradation routes on the total atmospheric loss of 2 MB,
katmos_loss (2 MB), can be estimated, according to Eq. (6).
katmos_loss(2MB)=J(z,θ)+kOH[OH]+kCl[Cl],
where J(z,θ) is the photolysis rate of 2 MB at a certain altitude z and
zenith solar angle (θ); kOH and kCl are those determined in
this work at 298 K; and [OH] and [Cl] are the tropospheric concentrations of
OH radicals and Cl atoms which depend on the considered scenario. Each term
was estimated for a coastal city (Valencia, Spain) at sea level (z=0 km)
for two scenarios: (i) as a function of time from 06:00 to 18:00 LT (local
time, corresponding to GMT + 2) and (ii) for 24 h average conditions.
J(z,θ) is defined as follows (Jiménez et al., 2007):
J(z,θ)≅Φeff∑λ>290nmF(λ,z,θ)σλΔλ,
where σλ values used in the calculation are those listed in
Table S1, and Φeff=0.30, determined in this work. F(λ,z,θ) (in photons cm-2 nm-1 s-1) is the solar
spectral actinic flux at 0 km for a specific θ in the troposphere,
obtained using the TUV radiative transfer model (5.3 version) developed by
Madronich and Flocke (1999), and Δλ=1 nm. In
scenario (i), F(λ,z,θ) was set for θ between
16∘ at 14:00 LT and 96∘ at 06:00 LT, while in
scenario (ii) F(λ,z,θ) is the 24 h average solar spectral
actinic flux. The estimated J (z, θ) values at sea level as a function
of the zenith angle are provided in Table 5. As expected, it is
observed that photolysis is faster at 14:00 (J(z,θ)=2.90×10-5 s-1) when the solar actinic flux is maximum,
whereas it is negligible at the beginning and the end of the day. In
scenario (ii), the calculated J(z,θ) was 9.84×10-6 s-1.
Estimated photolysis rate coefficients of 2-methylbutanal
at sea level (z=0 km) in the summer solstice day in Valencia (Spain).
Local timeθJ(z,θ)(GMT + 2)(∘)(10-5 s-1)06:0096∼008:00760.35610:00531.5112:00312.5114:00162.9016:00292.5518:00521.57
To calculate the contribution of the OH reaction to
katmos_loss (2 MB) in scenario (i), the temporal values of
[OH] are considered to be similar to those modelled by
Forberich et al. (1999) for a day at the end of June at
Weybourne (UK), a location with similar θ as Valencia in June
except for the middle time of the day. In scenario (ii) a 24 h average of
1×106 radicals cm-3 (Krol et al., 1998)
was accepted for [OH]. In contrast, [Cl] was considered as a
time-independent value in both scenarios. In scenario (i) an upper limit of
1.3×105 atoms cm-3, taken as the peak Cl
concentration predicted by Spicer et al. (1998) in
marine environments, was considered, while a 24 h average value of
1×103 atoms cm-3 (Singh et al.,
1996) was assumed in scenario (ii).
Taking into account all these calculations, the relative contribution of
each degradation route evaluated in this study to the diurnal loss of 2 MB in
scenario (i) is depicted in Fig. 9. At dawn, the Cl reaction
dominates the loss of 2 MB with a relative contribution of 84 %, followed
by the OH reaction (16 %). On the other hand, in the middle times of the
day, OH reaction is clearly the main removal route for 2 MB, with a relative
contribution of 79 % at 14:00, followed by a competition between
photolysis (11 %) and Cl reaction (10 %). Note that the relative
contribution of Cl reaction is an upper limit as the considered [Cl] is a
peak value. In scenario (ii), the OH reaction is the main removal route for
2 MB with a relative contribution of 72.7 %, followed by UV photolysis
(26.7 %). The relative contribution of the Cl reaction is very small
(0.6 %), which seems to be more realistic than using the peak Cl
concentration.
The tropospheric lifetime of 2 MB, τ, can be estimated according to Eq. (6) using the conditions of scenario (ii).
1τ=1J(z,θ)+1kOH[OH]+1kCl[Cl]
In the atmosphere of Valencia (Spain) in June, the overall τ for 2 MB
was estimated to be 7.5 h. Once emitted, 2 MB is degraded in few hours during
daytime, so it will not be transported over long distances. Its degradation
products include CO and carbonyl compounds such as butanone, acetaldehyde,
or methylglyoxal that can be further oxidized in the troposphere and
contribute to photochemical smog, impacting on human health. The evidence
found in this work shows that ultrafine particles are formed in the Cl + 2 MB
reaction but with yields of less than 1 %. Therefore, little impact on
human health is expected by inhalation at those concentration levels.
Finally, HCl, detected in the oxidation of 2 MB by Cl, can contribute to acid
rain. However, it must be noted that the impact of the observed
products on air quality and human health has a strong dependence on the
amounts of 2 MB emitted to the troposphere.
Conclusions
This work presents a comprehensive study on the relative importance of the
most important diurnal atmospheric degradation routes of 2-methylbutanal
(2 MB) and on the formation of secondary pollutants (particulate matter and
gaseous products). This study describes, for the first time, the kinetics of
the UV photolysis of 2 MB at (298±2) K and (760±3) Torr,
the temperature dependence of the rate coefficient for the OH + 2 MB reaction
between 263 and 353 K at PT=50–600 Torr of He, and the rate
coefficient of the Cl + 2 MB reaction at 298 K and 760 Torr. For the summer
solstice day in Valencia (Spain), it was found that, for the 24 h average, the
OH reaction is the main removal route for 2 MB, with a relative contribution
of ca. 73 %. UV photolysis of 2 MB is also important, corresponding to a
ca. 27 % of its total diurnal loss. Under these conditions, the overall
tropospheric lifetime for 2 MB was estimated to be 7.5 h. In terms of the
degradation products, it must be noted that the conditions used in this work
([RO2] > [HO2]) cannot be directly extrapolated to a
real clean atmosphere, where the HO2 reactions are more important than
those for RO2 radicals. However, the end-products identified in this
work (CO, HCl, butanone, acetaldehyde, and methylglyoxal) are expected to be
the same. Major products of the Cl + 2 MB reaction are butanone,
acetaldehyde, CO, and HCl, but methylglyoxal was also detected. UV photolysis
of 2 MB in the actinic region produces butane, butanone, and CO as major
products. The carbonyl products formed can contribute to photochemical smog,
whereas HCl can contribute to acid rain; however the real impact will
depend on the amounts of 2 MB emitted to the troposphere. In addition, the
yield of secondary organic aerosols formed in the Cl + 2 MB reaction is too
small (<1 %) to affect human health.
Data availability
Data are presented either in the paper or the Supplement.
The electronic Supplement includes additional tables,
figures, and description of the methods. In addition, the Supplement also includes
an Excel file with the determined UV and IR absorption cross sections as a
function of wavelength. The supplement related to this article is available online at: https://doi.org/10.5194/acp-22-2689-2022-supplement.
Author contributions
MAs, MAn, and SB designed and conducted the experiments and
analysed the experimental data. EJ and JA designed and supervised the
experiments and managed the project. All the co-authors have contributed to
preparing the manuscript and discussing the obtained results.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
This work has been supported by the Regional government of Castilla-La
Mancha through CINEMOL project (Ref.: SBPLY/19/180501/000052) and by the
University of Castilla-La Mancha – UCLM (Ayudas para la financiación de actividades de investigación dirigidas a grupos (REF: 2019-GRIN-27175, 2021-GRIN-31279). María Antiñolo and Sergio Blázquez also acknowledge UCLM (Plan Propio de Investigación) for funding their
contracts during the performance of this investigation.
Financial support
This research has been supported by the Junta de Comunidades de Castilla-La Mancha (grant no. SBPLY/19/180501/000052) and the Universidad de Castilla-La Mancha (grant no. 2019-GRIN-27175, 2021-GRIN-31279).
Review statement
This paper was edited by John Orlando and reviewed by two anonymous referees.
ReferencesAlbaladejo, J., Ballesteros, B., Jiménez, E., Martín, P., and Martínez, E.: A PLP–LIF kinetic study of the atmospheric reactivity of a series
of C4–C7 saturated and unsaturated aliphatic aldehydes with OH, Atmos. Environ., 36, 3231–3239,
10.1016/S1352-2310(02)00323-0, 2002.Antiñolo, M., Jiménez, E., Notario, A., Martínez, E., and Albaladejo, J.: Tropospheric photooxidation of CF3CH2CHO and
CF3(CH2)2CHO initiated by Cl atoms and OH radicals, Atmos. Chem. Phys., 10, 1911–1922, 10.5194/acp-10-1911-2010, 2010.Antiñolo, M., Olmo, R. D., Bravo, I., Albaladejo, J., and Jiménez,
E.: Tropospheric fate of allyl cyanide (CH2=CHCH2CN): Kinetics,
reaction products and secondary organic aerosol formation, Atmos. Environ., 219, 117041,
10.1016/j.atmosenv.2019.117041, 2019.Antiñolo, M., Asensio, M., Albaladejo, J., and Jiménez, E.:
Gas-Phase Reaction of trans-2-Methyl-2-butenal with Cl: Kinetics, Gaseous
Products, and SOA Formation, Atmosphere, 11, 715,
10.3390/atmos11070715, 2020.Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J., Troe, J., and IUPAC Subcommittee: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II – gas phase reactions of organic species, Atmos. Chem. Phys., 6, 3625–4055, 10.5194/acp-6-3625-2006, 2006.Ballesteros, B., Jiménez, E., Moreno, A., Soto, A., Antiñolo, M.,
and Albaladejo, J.: Atmospheric fate of hydrofluoroolefins,
CxF2x+1CH=CH2 (x=1,2,3,4 and 6): Kinetics with Cl atoms
and products, Chemosphere, 167, 330–343,
10.1016/j.chemosphere.2016.09.156, 2017.Blázquez, S., González, D., Neeman, E. M., Ballesteros, B.,
Agúndez, M., Canosa, A., Albaladejo, J., Cernicharo, J., and
Jiménez, E.: Gas-phase kinetics of CH3CHO with OH radicals between
11.7 and 177.5 K, Phys. Chem. Chem. Phys., 22, 20562–20572,
10.1039/D0CP03203D, 2020.Ceacero-Vega, A. A., Ballesteros, B., Bejan, I., Barnes, I., Jiménez,
E., and Albaladejo, J.: Kinetics and Mechanisms of the Tropospheric
Reactions of Menthol, Borneol, Fenchol, Camphor, and Fenchone with Hydroxyl
Radicals (OH) and Chlorine Atoms (Cl), J. Phys. Chem. A,
116, 4097–4107, 10.1021/jp212076g, 2012.D'Anna, B., Andresen, Ø., Gefen, Z., and Nielsen, C. J.: Kinetic study of
OH and NO3 radical reactions with 14 aliphatic aldehydes, Phys. Chem. Chem. Phys., 3, 3057–3063,
10.1039/B103623H, 2001.Flaig, M., Qi, S., Wei, G., Yang, X., and Schieberle, P.: Characterization
of the Key Odorants in a High-Grade Chinese Green Tea Beverage (Camellia sinensis; Jingshan cha) by Means
of the Sensomics Approach and Elucidation of Odorant Changes in Tea Leaves
Caused by the Tea Manufacturing Process, J. Agr. Food
Chem., 68, 5168–5179,
10.1021/acs.jafc.0c01300, 2020.
Forberich, O., Pfeiffer, T., Spiekermann, M., Walter, J., Comes, F. J.,
Grigonis, R., Clemitshaw, K. C., and Burgess, R. A.: Measurement of the
Diurnal Variation of the OH Radical Concentration and Analysis of the Data
by Modelling, J. Atmos. Chemi., 33, 155–181,
10.1023/A:1005973130335, 1999.Griesser, M., Weingart, G., Schoedl-Hummel, K., Neumann, N., Becker, M.,
Varmuza, K., Liebner, F., Schuhmacher, R., and Forneck, A.: Severe drought
stress is affecting selected primary metabolites, polyphenols, and volatile
metabolites in grapevine leaves (Vitis vinifera cv. Pinot noir), Plant Phys. Biochem., 88, 17–26,
10.1016/j.plaphy.2015.01.004, 2015.Gruver, J. T. and Calvert, J. G.: The Vapor Phase Photolysis of
2-Methylbutanal at Wave Length 3130 Å.1, J. Am. Chem. Soc., 78, 5208–5213,
10.1021/ja01601a021, 1956.Gueymard, C. A., Myers, D., and Emery, K.: Proposed reference irradiance
spectra for solar energy systems testing, Solar Energ., 73, 443–467,
10.1016/S0038-092X(03)00005-7, 2002.Jiménez, E., Lanza, B., Martínez, E., and Albaladejo, J.: Daytime tropospheric loss of hexanal and trans-2-hexenal: OH kinetics and UV photolysis, Atmos. Chem. Phys., 7, 1565–1574, 10.5194/acp-7-1565-2007, 2007.Kolar, P. and Kastner, J. R.: Low-temperature catalytic oxidation of
aldehyde mixtures using wood fly ash: Kinetics, mechanism, and effect of
ozone, Chemosphere, 78, 1110–1115,
10.1016/j.chemosphere.2009.12.033, 2010.Krol, M., van Leeuwen, P. J., and Lelieveld, J.: Global OH trend inferred
from methylchloroform measurements, J. Geophys. Res.-Atmos., 103,
10697–10711, 10.1029/98JD00459, 1998.Madronich, S. and Flocke, S.: The Role of Solar Radiation in Atmospheric
Chemistry, in: Environmental Photochemistry, edited by: Boule, P., Springer
Berlin Heidelberg, Berlin, Heidelberg, 1–26,
10.1007/978-3-540-69044-3_1, 1999.Martínez, E., Albaladejo, J., Jiménez, E., Notario, A., and Aranda,
A.: Kinetics of the reaction of CH3S with NO2 as a function of
temperature, Chem. Phys. Lett., 308, 37–44,
10.1016/S0009-2614(99)00579-5, 1999.
Millet, D. B., Guenther, A., Siegel, D. A., Nelson, N. B., Singh, H. B., de Gouw, J. A., Warneke, C., Williams, J., Eerdekens, G., Sinha, V., Karl, T., Flocke, F., Apel, E., Riemer, D. D., Palmer, P. I., and Barkley, M.: Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations, Atmos. Chem. Phys., 10, 3405–3425, 10.5194/acp-10-3405-2010, 2010.Orlando, J. J., Tyndall, G. S., Apel, E. C., Riemer, D. D., and Paulson, S.
E.: Rate coefficients and mechanisms of the reaction of cl-atoms with a
series of unsaturated hydrocarbons under atmospheric conditions,
Int. J. Chem. Kinet., 35, 334–353,
10.1002/kin.10135, 2003.Singh, H. B., Thakur, A. N., Chen, Y. E., and Kanakidou, M.:
Tetrachloroethylene as an indicator of low Cl atom concentrations in the
troposphere, Geophys. Res. Lett., 23, 1529–1532,
10.1029/96GL01368, 1996.Spicer, C. W., Chapman, E. G., Finlayson-Pitts, B. J., Plastridge, R. A.,
Hubbe, J. M., Fast, J. D., and Berkowitz, C. M.: Unexpectedly high
concentrations of molecular chlorine in coastal air, Nature, 394, 353–356,
10.1038/28584, 1998.Urbanski, S. P., Hao, W. M., and Baker, S.: Chapter 4 Chemical Composition
of Wildland Fire Emissions, in: Developments in Environmental Science,
edited by: Bytnerowicz, A., Arbaugh, M. J., Riebau, A. R., and Andersen, C.,
Elsevier, 8, 79–107,
10.1016/S1474-8177(08)00004-1, 2008.Utrilla-Vázquez, M., Rodríguez-Campos, J., Avendaño-Arazate, C.
H., Gschaedler, A., and Lugo-Cervantes, E.: Analysis of volatile compounds
of five varieties of Maya cocoa during fermentation and drying processes by
Venn diagram and PCA, Food Res. Int., 129, 108834,
10.1016/j.foodres.2019.108834, 2020.Wallington, T. J., Ball, J. C., Straccia, A. M., Hurley, M. D., Kaiser, E.
W., Dill, M., Schneider, W. F., and Bilde, M.: Kinetics and mechanism of the
reaction of Cl atoms with CH2 CO (Ketene), Int. J. Chem. Kinet., 28, 627–635, 1996.Wenger, J. C.: Chamber Studies on the Photolysis of Aldehydes Environmental,
Dordrecht, 111–119,
10.1007/1-4020-4232-9_8, 2006.