Low-Pressure Photolysis of 2,3-Pentanedione in Air: Quantum Yields and Reaction Mechanism

Article abstract of DOI:10.1021/acs.jpca.5b09448

Dicarbonyls in the atmosphere mainly arise from secondary sources as reaction products in the degradation of a large number of volatile organic compounds (VOC). Because of their sensitivity to solar radiation, photodissociation of dicarbonyls can dominate the fate of these VOC and impact the atmospheric radical budget. The photolysis of 2,3-pentanedione (PTD) has been investigated for the first time as a function of pressure in a static reactor equipped with continuous wave cavity ring-down spectroscopy to measure the HO₂ radical photostationary concentrations along with stable species. We showed that (i) Stern-Volmer plots are consistent with low OH-radical formation yields in RCO + O₂ reactions, (ii) the decrease of the photodissociation rate due to pressure increase from 26 to 1000 mbar is of about 30%, (iii) similarly to other dicarbonyls, the Stern-Volmer analysis shows a curvature at the lower pressure investigated, which may be assigned to the existence of excited singlet and triplet PTD states, (iv) PTD photolysis at 66 mbar leads to CO₂, CH₂O and CO with yields of (1.16 ± 0.04), (0.33 ± 0.02) and (0.070 ± 0.005), respectively, with CH₂O yield independent of pressure up to 132 mbar and CO yield in agreement with that obtained at atmospheric pressure by Bouzidi et al. (2014), and (v) the PTD photolysis mechanism remains unchanged between atmospheric pressure and 66 mbar. As a part of this work, the O₂ broadening coefficient for the absorption line of HO₂ radicals at 6638.21 cm^-1 has been determined (γ_O2 = 0.0289 cm^-1 atm^-1).

Full text of DOI:10.1021/acs.jpca.5b09448

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Article
Low Pressure Photolysis of 2,3-Pentanedione in
Air: Quantum Yields and Reaction Mechanism
Hichem Bouzidi, Mokhtar Djehiche, Tomasz Gierczak, Pranay
Morajkar, Christa Fittschen, Patrice Coddeville, and Alexandre Tomas
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b09448 • Publication Date (Web): 26 Nov 2015
Downloaded from http://pubs.acs.org on November 29, 2015
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The Journal of Physical Chemistry A is published by the American Chemical Society.
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Low Pressure Photolysis of 2,3-Pentanedione in Air: Quantum Yields and
Reaction Mechanism
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Hichem BOUZIDI1,2, Mokhtar DJEHICHE1,2$, Tomasz GIERCZAK3, Pranay MORAJKAR4#,
Christa FITTSCHEN4, Patrice CODDEVILLE1,2, Alexandre TOMAS1,2*
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1 Mines Douai, SAGE, 59508 Douai, France
2 Université de Lille, 59000 Lille, France
3 Faculty of Chemistry, Warsaw University, ul. Pasteura 1, Poland
4 Université de Lille 1, PC2A, UMR 8522 CNRS/Lille 1, 59655 Villeneuve d'Ascq, France
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$
Current address: University M’Sila, Faculty of Sciences, M’Sila, Algeria
#
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Current address: Goa University, Department of Chemistry, Taleigao 403206, Goa, India
*: corresponding author:
Prof. Alexandre Tomas
Département Sciences de l’Atmosphère et Génie de l’Environnement, Ecole Nationale Supérieure
des Mines de Douai, 941 rue Charles Bourseul, CS 10838, Fꢀ59508 Douai Cedex, France
alexandre.tomas@minesꢀdouai.fr
Tel.: 00.33.3.27.71.26.51
Fax: 00.33.3.27.71.29.14
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23 Submitted to Journal of Physical Chemistry A
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Abstract
Dicarbonyls in the atmosphere mainly arise from secondary sources as reaction products in the
degradation of a large number of volatile organic compounds (VOC). Because of their sensitivity to
solar radiation, photodissociation of dicarbonyls can dominate the fate of these VOC and impact the
atmospheric radical budget. The photolysis of 2,3ꢀpentanedione (PTD) has been investigated for the
first time as a function of pressure in a static reactor equipped with continuous wave cavity ringꢀ
down spectroscopy to measure the HO2 radical photostationary concentrations along with stable
species. We showed that (i) SternꢀVolmer plots are consistent with low OHꢀradical formation yields
in RCO + O2 reactions (ii) the decrease of the photodissociation rate due to pressure increase from
26 to 1000 mbar is of about 30% (iii) similarly to other dicarbonyls, the SternꢀVolmer analysis
shows a curvature at the lower pressure investigated, which may be assigned to the existence of
excited singlet and triplet PTD states (iv) PTD photolysis at 66 mbar leads to CO2, CH2O and CO
with yields of (1.16±0.04), (0.33±0.02) and (0.070±0.005), respectively, with CH2O yield
independent of pressure up to 132 mbar and CO yield in agreement with that obtained at
atmospheric pressure by Bouzidi et al. (2014), (v) the PTD photolysis mechanism remains
unchanged between atmospheric pressure and 66 mbar. As a part of this work, the O2 broadening
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coefficient for the absorption line of HO2 radicals at 6638.21 cmꢀ1 has been determined (γO2
=
0.0289 cmꢀ1 atmꢀ1).
Keywords: dicarbonyl, photolysis, OH, acylperoxy, SternꢀVolmer
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1. INTRODUCTION
In the past decade, numerous field studies have encountered strong difficulties in their effort to
model the HOx concentration levels observed as much in forested regions1 as in anthropogenicꢀ
influenced areas2. More recently, these findings have been reinforced by total OH reactivity
measurement campaigns, where the calculated OH reactivities often underestimate the measured
ones3ꢀ4. The chemistry of multifunctional oxygenated volatile organic compounds (VOCs) has been
suggested as possibly responsible for the OH missing reactivity and the disagreement in the HOx
(OH + HO2) budget5ꢀ7.
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Dicarbonyls belong to the family of multifunctional VOCs, the main representatives being glyoxal
and methyl glyoxal. They are present in the atmosphere at pptꢀ to ppbꢀ concentration levels,
originating mainly from the secondary sources as reaction products in the degradation of several
different VOCs. As an example, Grosjean and Grosjean reported the formation of dicarbonyls in
alkene ozonolysis reactions following the rearrangement of the Criegee biradical8. Photochemistry
of aromatic compounds is also well known to generate dicarbonyl compounds, with increasing
yields as the degree of ring alkylation increases9ꢀ10. Recently, Crounse et al. proposed a new route
for organic compound oxidation in the atmosphere through the isomerization of peroxy radicals,
leading to dicarbonyl species11. The role of dicarbonyls may be crucial in atmospheric chemistry,
especially since they are very sensitive to photolysis in the nearꢀUV range and thus may
photodissociate by solar irradiation and directly produce radicals, influencing the oxidative capacity
of the atmosphere and the HOx budget. Yet, their gasꢀphase reactivity is still poorly understood and
they are generally not taken into account in atmospheric chemistry models. In addition, although the
lifetime of dicarbonyls in the atmosphere is rather short (of the order of a few hours)12, these
compounds could reach lowerꢀpressure tropospheric regions; there is thus a need to address also
lowꢀpressure photochemistry of dicarbonyls.
In this paper, we used the recently developed environmental simulation chamber coupled to in situ
continuous wave – cavity ringꢀdown spectrometry (cwꢀCRDS)13 to study the lowꢀpressure
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photolysis of 2,3ꢀpentanedione (PTD: CH3C(O)C(O)CH2CH3) as a model compound of αꢀ
dicarbonyls. PTD is used as flavouring agent in many perfume and food industries. Ozonolysis of
alkenes like 2ꢀethylꢀbutꢀ1ꢀene has also been shown to produce PTD8. PTD has been detected during
Indian festivals14; yet, its atmospheric abundance is not known. Jackson and Yarwood first reported
nonꢀdissociative emission processes following the UV irradiation of PTD with a total emission
quantum yield of 2.2% at 18 mbar and 30°C15ꢀ16. Szabo et al. investigated the OH radicalꢀinitiated
oxidation and UV photolysis of PTD at atmospheric pressure and suggested that atmospheric
photolysis could be a significant loss process for PTD17. The predominance of the photolysis
channel over the OH radical initiated reaction has been recently confirmed and a photolysis reaction
mechanism has been proposed12. Global primary quantum yields of about 20% were estimated for
2,3ꢀpentanedione and 2,3ꢀhexanedione over the wavelength range 330 – 470 nm at 1 bar. However,
the pressure effect was not evaluated, especially on the quantum yields where it could be significant
due to quenching effects.
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The main aim of this work was to investigate the photolysis of PTD as a function of pressure by
determining the product yields of CO, CO2 and CH2O and assessing the chemical mechanism
through measurements of the HO2 photostationary concentrations. The present study represents the
first lowꢀpressure investigation of 2,3ꢀpentanedione photolysis with direct measurements of both
HO2 radicals and stable reactant and products, and developing the chemical reaction mechanism.
2. EXPERIMENTAL SECTION
Experiments have been performed in a quartz photoreactor of cylindrical shape and 116 L volume.
A detailed description of the photoreactor could be found in Djehiche et al.13, 18. UV lamps emitting
in the near UV range (Philips Sylvania, 350 nm – 410 nm, emission spectrum given in Figure S1)
surround the reactor and initiated the reactions. Experiment duration lasted from 1h to 3h. The
reactants (either methanol and Cl2 or 2,3ꢀpentanedione) are introduced in the reactor at low pressure
(around 1 mbar) followed by addition of synthetic air to reach a total pressure of (6.6 – 1000) mbar.
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Initial concentrations of the reactants varied as [CH3OH]0 = (0.67 – 1.4) × 1015 molecules cmꢀ3,
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[Cl2]0 = (0.93 – 2.8) × 1014 molecules cmꢀ3, [PTD]0 = (2.0 – 3.0) × 1015 molecules cmꢀ3. Due to
strong absorption of the OHꢀscavengers in the near IR region, leading to decrease in the ringꢀdown
time and with this a decrease in sensitivity, no OH radical scavenger was used in the PTD
photolysis experiments.
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The first set of experiments employing chlorine photolysis in the presence of methanol aimed at i)
assessing the consistency of the formaldehyde absorption cross sections and ii) checking the
relevance of the present experimental setup to investigate reaction mechanisms of species of
atmospheric interest. For these experiments, a chlorine/methanol mixture in air was photolyzed at
low pressure and the absorption features of HO2 and CH2O have been determined simultaneously.
This wellꢀknown chemical system generates HO2 radicals as the main peroxy radical intermediate
and CH2O as the major stable product, both products being quantified by cwꢀCRDS. In subsequent
experiments, both species together with CO and CO2 have been monitored by cwꢀCRDS during the
photolysis of PTD: 2,3ꢀpentanedione was detected and quantified by onꢀline mass spectrometry at
m/z = 100 Da (Pfeiffer quadꢀMS) after calibration of the instrument.
CH2O, CO, CO2 and HO2 are measured by in situ cwꢀCRDS at 6639.33 cmꢀ1, 6355.8 cmꢀ1, 6355.92
cmꢀ1 and 6638.21 cmꢀ1, respectively. Because line broadening with increasing pressure leads to a
decreased sensitivity and selectivity, cwꢀCRDS measurements were only performed in the low
pressure part of this work (6.6 – 105 mbar).
CH2O concentration was determined at 6639.33 cmꢀ1. The pressureꢀdependent absorption cross
sections were calculated from the work of Morajkar et al.19 assuming a Voigt profile using a
broadening coefficient obtained by Barry et al.20 (air broadening coefficient γ = 1.4 × 10ꢀ4 cmꢀ
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1/Torrꢀ1) and the theoretical Doppler value (νDꢀCH2O = 7.44 × 10ꢀ3 cmꢀ1). As has been recently
reviewed by Ruth et al.21, several recent publications with different experimental setups18, 22,23ꢀ25
consistently indicated that the absorption cross sections reported earlier by Staak et al.26 are ca. a
factor of 2 larger than the values measured by Morajkar et al.
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The absorption cross sections of CO and CO2 were determined in the present work at 66 mbar of air
by using calibrated standards and absorption cross sections of 3.19 × 10ꢀ22 cm2 (6355.8 cmꢀ1) and
6.38 × 10ꢀ22 cm2 (6355.92 cmꢀ1) were found for CO and CO2, respectively.
HO2 radicals have been quantified at the strongest absorption line at 6638.21 cmꢀ1. O2 broadening
has never been determined for this line, and therefore the O2ꢀbroadening of this absorption line has
been measured in the frame of this work in independent experiments using laser photolysis coupled
to cwꢀCRDS. Details and results are described further down in the next section.
Methanol (99.8 %) and PTD (97%) were from SigmaꢀAldrich and used as received; Cl2 (10% in N2)
was bought from Air Products.
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3. RESULTS AND DISCUSSION
3.1 Determination of the O2-broadening coefficient for the HO2 absorption line at 6638.21
cm-1
The HO2 spectrum has been recorded by Thiébaud et al.27 between 6600 cmꢀ1 and 6700 cmꢀ1 and
exhibits the strongest line at 6638.21 cmꢀ1, twice as big as all other lines and thus very likely being
the convolution of two lines. The O2 / N2 pressure broadening for several absorption lines of HO2
radicals has been measured28, however the strongest line, used in this work, was not included at that
time. Only recently a pressure broadening of that line29 has been measured in helium. In that
experiment a very weak broadening coefficient (γHe = 0.0237 cmꢀ1 atmꢀ1) has been obtained
combined with a line width extrapolated to zero pressure larger than the theoretical Doppler width
(7.088×10ꢀ3 cmꢀ1 HWHM; represented as a black dot in Figure 2). This result was not unexpected
given that the line at 6638.21 cmꢀ1 is probably composed of two nearly perfectly superimposed
individual lines. In the frame of this work we have determined the O2 pressure broadening for this
line by determining the line shape at three different O2 pressures (20, 99 and 131 mbar), shown in
Figure 1.
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Laser photolysis coupled to time resolved cwꢀCRDS has been employed and details of the
experimental setꢀup have been published earlier30ꢀ31. Briefly, HO2 radicals have been generated by
248 nm photolysis of oxalyl chloride, (COCl)2, in the presence of excess CH3OH and O2. Under
these conditions, HO2 radicals are rapidly formed and decayed subsequently through selfꢀreaction
and diffusion. The two HO2 absorption lines at 6638.21 and 6638.11 cmꢀ1 have been measured with
a resolution of 0.002 cmꢀ1 (Figure 1). For this purpose, time resolved ringꢀdown events have been
accumulated over several photolysis pulses until enough ringꢀdown events had occurred to allow a
reliable description of the HO2 decay profile at that wavelength. Thereafter, the wavelength of the
DFB (Distributed FeedBack) laser has been automatically incremented by around 0.002 cmꢀ1 to
obtain the timeꢀresolved HO2 profile at the next wavelength. The absorption at time zero after the
photolysis pulse is then extracted by fitting the HO2 decay profiles to the second order kinetics and
extrapolating to time zero and with this, each time resolved HO2 decay leads to one data point in
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Figure 1. More details on the principle of this method can be found in earlier papers27, 32
.
Measurements have been carried out in pure O2 at three different pressures (20, 99 and 131 mbar).
The lines have been fitted to a Voigt profile using the free Fityk software33, and the obtained line
widths (HWHM) for both lines are plotted in Figure 2 as a function of the pressure.
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1.0
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98.7 mbar O2
131.6 mbar O2
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6638.10
6638.15
wavenumber / cmꢀ1
6638.20
6638.25
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Figure 1. Absorption lines at 6638.21 and 6638.11 cmꢀ1 at three different O2 pressures in arbitrary
units. The full lines represent the fit to a Voigt profile. Signals have been normalized for each
pressure to the surface area of the line at 6638.21 cmꢀ1 such as obtained by the fitting program (HO2
concentrations were not identical for all pressures) and have finally been divided by a constant
value so as to reach a maximum value of 1 at the lowest pressure.
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As it can be seen from Figure 2 the extrapolation to zero pressure for the weaker line (blue dots) is
in excellent agreement with the expected theoretical Doppler line width (see a black dot), while the
extrapolation for the stronger line results in a higher value (10.26×10ꢀ3 cmꢀ1). The pressure
broadening coefficient for the stronger line (red dots) γO2 = 3.8×10ꢀ5 cmꢀ1 Torrꢀ1 is very small, in
very good agreement with the result obtained for the Helium broadening29, while the broadening
coefficient for the weaker line (blue dots) equals 8.3×10ꢀ5 cmꢀ1 Torrꢀ1 and it is at the lower end of
the earlier published values28.
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0.020
0.015
0.010
0.005
0.000
Dopplerwidth
HWHM 6638.21 cmꢀ1
HWHM 6638.11 cmꢀ1
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p / atm
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Figure 2. Lineꢀwidth (HWHM) for both lines from Figure 1 as obtained by fitting to a Voigt profile
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for three pressures.
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Using the absolute absorption cross section determined for the line at 6638.21 cmꢀ1 by Thiébaud et
al. in 66 mbar of He (2.72 × 10ꢀ19 cm2,27), a line strength of S = 7.09 × 10ꢀ21 cmꢀ1 can be calculated
using the Heꢀbroadening coefficient (γHe = 0.024 cmꢀ1/atm) and the zeroꢀpressure line width
determined by Morajkar et al.29 (10.72×10ꢀ3 cmꢀ1 HWHM). Using this line strength as well as the
O2ꢀpressure broadening coefficient determined in this work enables calculating the peak absorption
cross sections of HO2 in air at the pressures of the experiments for the line at 6638.21 cmꢀ1 (Table 1).
Table 1. Pressure dependent HO2 (6638.21 cmꢀ1) and CH2O (6639.33 cmꢀ1) absorption cross
sections. σHO2 are from the present work while σCH2O are calculated using a line strength of 7.3 ×
10ꢀ24 cmꢀ1 and an airꢀbroadening coefficient of 1.4 × 10ꢀ4 cmꢀ1/Torrꢀ1.19ꢀ20
Pressure (mbar)
σ
HO2 (10ꢀ19 cm2)
σ
CH2O (10ꢀ22 cm2)
0
3.24
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3.2. Chlorine/methanol/air mixtures photolysis
The purpose of these experiments was to assess the consistency of the formaldehyde absorption
cross sections and to check the relevance of the present experimental setup to further investigate
reaction mechanisms of species of atmospheric interest. Experiments were done at low pressure (6.6
ꢀ 30 mbar) by monitoring CH2O and HO2 absorptions as a function of reaction time. The photolysis
of Cl2/CH3OH/air mixtures initiates the oxidation of methanol through CH3OH + Cl reaction
followed by the chemical mechanism reported in Table 2. Formaldehyde is the main stable organic
product formed in the reaction system while HO2 is the major peroxy radical intermediate. Note that
CH2O loss through photolysis was limited to 2% per hour and was thus neglected. As shown by
Djehiche et al.13, the diffusion and loss of HO2 to the reactor walls may be nonꢀnegligible at low
pressure and has thus been taken into account in the reaction mechanism, based on the recent results
of Ivanov et al.34. The CH2O and HO2 concentrations (noted [X]) were calculated using:


1
1 1
−




[X ] =
208
209
(1)
c×σ X ,P
(
λ
)
τ τ0


with c the speed of light, σX,P(λ) the absorption cross section of X at pressure P and wavelength λ,
210
211
212
213
214
215
and τ and τ0 the ringꢀdown times at the peak wavelength and at a wavelength just off the absorption
peak, respectively. Numerical simulations of the reaction mechanism were performed using CH3OH
and Cl2 concentrations as starting conditions and adjusting the Cl2 photolysis frequency to achieve
the best agreement with the experimental data. Figure 3 presents typical time profiles of CH2O and
HO2 following the irradiation of [CH3OH]0 = 1.4 × 1015 molecules cmꢀ3 and [Cl2]0 = 2.8 × 1014
molecules cmꢀ3 in 24 mbar of air. Both CH2O and HO2 plots displayed very nice agreement between
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experimental and simulated concentrations. Global uncertainties in CH2O and HO2 experimental
concentrations, including errors on cross sections and ringꢀdown times τ and τ0, range between 20%
and 30%.
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Table 2. Reaction mechanism used for the simulations of the CH2Oꢀ and HO2ꢀ experimental time
profiles
Rate coefficients (298 K, 24
Chemical reactions
Reference
mbar air) (cm3 molecule-1 s-1)
a
this work
Cl2 + hν → 2 Cl
CH3OH + Cl (+ O2) → HO2 + CH2O + HCl
HO2 + HO2 → H2O2 + O2
5.5
1.7
×
×
10ꢀ11
10ꢀ12
Atkinson et al, 2006 35
Atkinson et al, 2004 36
Ivanov et al., 2007 34
HO2 + wall → products
0.075 b
HO2 + CH2O → HOCH2O2
3.3
×
10ꢀ14
Morajkar et al., 2013 23
HOCH2O2 → HO2 + CH2O
55 b
HO2 + HOCH2O2 → HCOOH + O2 + H2O
HO2 + HOCH2O2 → HOCH2O2H + O2
CH2O + Cl (+ O2) → HO2 + CO + HCl
Cl + HCOOH (+ O2) → HO2 + HCl + CO2
4.8
7.2
7.3
1.9
×
×
×
×
10ꢀ12
10ꢀ12
10ꢀ11
10ꢀ13
Atkinson et al, 2006 35
222
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224
a : varies from 3.0 × 10ꢀ4 sꢀ1 to 8.5 × 10ꢀ4 sꢀ1 depending on the number of lamps switched on
b : units : sꢀ1
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Figure 3. Concentrationꢀtime profiles of CH2O (ꢀ, right scale) and HO2 (ꢀ, left scale). The lines
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represent the simulation fits using the reaction mechanism reported in Table 2.
3.3. 2,3-pentanedione photolysis
Photolysis experiments of 2,3ꢀpentanedione have been carried out between 26 and 1000 mbar of
synthetic air to evaluate the pressure effect on the photolysis frequencies and on the yields of the
endꢀproducts. Figure 4 presents the obtained PTD, CH2O and HO2 concentrations vs. reaction time
for two experiments performed at 66 mbar.
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Figure 4. Experimental and simulated temporal profiles for PTD photolysis. ꢀ: [PTD] (left scale);
ꢀ: [HO2] (right scale); ꢀ: [CH2O] (left scale). The lines correspond to the numerical simulations:
solid lines are for conditions 2 (YOH = 0.06) while dotted lines are for conditions 1 (YOH = 0.19).
Experimental conditions: [PTD]0 = 2.4 × 1015 molecule cmꢀ3; total pressure = 66 mbar.
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PTD absorption spectrum displays two broad bands after 200 nm, one centered at 270 nm, the other
at 415 nm17. With the irradiation system used in the present study, PTD has been shown to fragment
mainly into two acyl radicals: 12
CH3C(O)C(O)C2H5 + hν
→
CH3C(O) + C2H5C(O)
(R1)
The corresponding photolysis threshold for R1 is around 400 nm12, 17. At atmospheric pressure, only
a small contribution (~ 2%) was observed from channel R2 for which the thermodynamic threshold
is around 346 nm17:
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CH3C(O)C(O)C2H5 + hν
→
CH3C(O) + C2H5 + CO
(R2)
OH radicals are formed during the 2,3ꢀpentanedione photolysis in air mostly from RC(O)O2 + HO2
reactions where R = CH3 or C2H5 with branching ratios ca. 0.437ꢀ38. It is also well known that RCO
+ O2 reaction releases OH radicals from the excited RC(O)O2 adduct at low pressure. Yet, fairly
large discrepancies exist in the literature on the OH formation yields: studies of Carr et al. from the
Leeds group suggest high OH formation yields for both the CH3CO + O2 and C2H5CO + O2
reactions39ꢀ43 while other investigations propose much lower values44ꢀ47 including a very recent
one48 (see Table 3). No OHꢀscavenger has been used in these experiments, and therefore the OHꢀ
yield will influence the PTD photolysis constant returned by the model calculations (kPTD) through
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the reaction of OH with PTD: a higher OHꢀyield from reaction RCO + O2 will result in a lower kPTD
Therefore, in order to test the effect of the OH formation yield on the PTD photolysis and CH2O
and HO2 kinetic profiles, we performed two sets of numerical simulations based on the whole
reaction mechanism reported in Table 4: (i) assuming high OH formation yields (conditions 1 see
Table 3) and (ii) following the low OHꢀyield values (conditions 2 see Table 3). Because of absence
of data for the C2H5CO + O2 reaction in air and taking into account the only slightly higher values
for OHꢀyields obtained in He compared to N2 for CH3CO + O240, 42ꢀ43, we used the similar OH
formation yields for CH3CO + O2 and C2H5CO + O2 in our simulations.
.
Coꢀproducts of OH in RCO + O2 reactions are assumed to be lactones whose fate is presently
unclear; yet, no evidence has been found for lactone decomposition into CO and CH2O (resp. CO
and CH3CHO) in CH3CO + O2 (resp. C2H5CO + O2)43, 45, 49. Numerical simulation input data
included only the initial PTD concentration; the photolysis constant kPTD was the only parameter
adjusted to fit the PTD concentrations and to find the best agreement between modelled and
experimental PTD concentrations.
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Table 3. OH radical yields (YOH) from Carr et al.40 (conditions 1) and Gross et al.44 (conditions 2),
PTD photolysis constants (kPTD±1σ) and PTD primary quantum yields (ΦPTD±1σ) obtained in the
present work.
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8
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Conditions 1
kPTD (hꢀ1)
Conditions 2
kPTD (hꢀ1)
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P (mbar)
26
YOH (%)
ΦPTD
YOH (%)
ΦPTD
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10
3
0.122±0.011 0.17±0.02
0.137±0.007 0.19±0.01
0.133±0.011 0.19±0.02
0.162±0.011 0.23±0.02
0.153±0.007 0.21±0.01
0.142±0.007 0.20±0.01
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6
0.202±0.014 0.28±0.02
0.182±0.009 0.26±0.02
0.176±0.011 0.25±0.02
0.180±0.011 0.25±0.02
0.157±0.007 0.22±0.02
0.140±0.007 0.20±0.01
66
105
4
132
3
460
1
1000
1
0
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PTD photolysis constants were thus retrieved from numerical simulations undertaken for conditions
1 and 2. For conditions 1, kPTD varies between (0.122±0.011) hꢀ1 and (0.162±0.011) hꢀ1 in the
pressure range 26 ꢀ 1000 mbar; for conditions 2, kPTD decreases from (0.202±0.009) hꢀ1 at 26 mbar
to (0.140±0.007) hꢀ1 at 1000 mbar (see Table 3). The uncertainties on kPTD reflect the error (1σ) on
the simulation fit of [PTD] vs. time. Aiming at determining PTD primary quantum yields (ΦPTD) as
a function of pressure, NO2 was selected as actinometer and its photolysis constant (kNO2) was
retrieved in the same experimental conditions as PTD using the method developed by Holmes et al.
50. An NO2 photolysis rate of (9.8±0.5) hꢀ1 was determined, independent of pressure. Using the
known PTD and NO2 UV absorption cross sections17, 36 the photolysis rate constant ratio of
calc
PTD
calc
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k
/ k
was calculated according the following equation:
NO2
kicalc = Φi
(λ
)σi
(λ
)F
(
λ dλ
)
(2)
∫
λ
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F(λ) represents the relative actinic flux intensity and was recorded using a spectroradiometer
(SolaTell, spectral resolution of 0.5 nm). σi(λ) represent the wavelengthꢀdependent absorption cross
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sections of PTD or NO2 17, 36 and Φi(λ) represent the PTD or NO2 quantum yields: PTD photolysis
quantum yield was set to unity and ΦNO2(λ) were from Atkinson et al.36.
Then, ΦPTD were calculated as a function of total pressure for conditions 1 and 2 according to the
following equation12, 51
:
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kPTD
k
k
NO2
ΦPTD
=
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(3)
calc
PTD
calc
NO2
k
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The quantum yield data are summarized in Table 3 for all experiments carried out at total pressures
of 26, 66, 105, 132, 460 and 1000 mbar and are shown in the form of SternꢀVolmer (SV) plots in
Figure 5. Displayed uncertainties (±1σ) are calculated using the error propagation method and took
into account estimated uncertainties on kPTD and kNO2. The primary quantum yield obtained at 1 bar,
of 0.20, is in excellent agreement with that of 0.19 reported by Bouzidi et al.12 from a Teflon smog
chamber study. The difference between quantum yields for PTD photolysis obtained under
conditions 1 and 2 is striking: whereas no clear pressure effect is visible for high OH yield
conditions (conditions 1), a measurable effect is observed for low OH yield conditions (conditions
2). As observed for other carbonyls irradiated at wavelengths near 400 nm52, a nonꢀlinear SV
behaviour is observed in Figure 5 for conditions 2 with a large slope for the lower part of the
pressure range investigated and a strong reduction at high pressures. This behaviour has been
assigned to the existence of two excited PTD states (singlet and triplet) from which
photodissociation occurred, though internal conversion and roꢀvibrational relaxation from the
singlet state may also be invoked. Because of the relatively narrow emission band of the lamps
interacting with only the nearꢀUV broad PTD absorption band, it is expected that only one
electronic singlet state is concerned in the photochemical process. Therefore, we concluded that OH
formation from RCO + O2 reactions most likely follows data obtained by Gross et al.44 and recently
by Papadimitriou et al.48.
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Figure 5. SternꢀVolmer plot for 2,3ꢀpentanedione photolysis between 330 nm and 480 nm reporting
the reciprocal of the primary quantum yield as a function of total pressure. ꢁ: ΦPTD are calculated
from simulations under conditions 1 (high OH yield values). ꢁ: ΦPTD are calculated from
simulations under conditions 2 (low OH yield values); the values are fitted to a lognormal law.
Error bars represent 1σ.
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The zero pressure value (corresponding to the intercept of the SV plot) is difficult to extract from
the present data. Jackson and Yarwood reported nonꢀdissociative emission processes (fluorescence
and phosphorescence) following the UV irradiation of PTD15ꢀ16. Total emission (mainly
phosphorescence) quantum yields of only 0.022 at 18 mbar and 30°C upon excitation at 405 nm and
436 nm were obtained, dropping to 0.011 at 365 nm15ꢀ16. A pressureꢀdependent glyoxal photolysis
study at long wavelengths indicated the existence of multiple states52, which may be also the case
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for PTD, explaining the curved SV plot obtained. To the best of our knowledge, this is the first
study of pressure effect on the photodissociation kinetics of 2,3ꢀpentanedione.
Three reaction products CO2, CH2O, and CO have been detected and quantified by cwꢀCRDS for
the experiments carried out at 66 mbar. CH2O was also quantified in the experiments at 105 and 132
mbar. Product yields were obtained by plotting the product concentration vs. [PTD]reacted, where
[PTD]reacted represents the difference between initial [PTD] and [PTD] at time t. CH2O
concentrations have been corrected for wall loss and photolysis, representing a loss of about 15%
CH2O at the end of the experiment. As shown in Figure 6, CO2, CH2O and CO plots are linear
giving yields of 1.16±0.04, 0.33±0.02 and 0.070±0.005, respectively. No pressure effect on the
CH2O yield has been observed between 66 and 132 mbar. Note that these yields do not correspond
exactly to the PTD photolysis yields, as the unscavenged OH radicals reacting with PTD may also
in consequence produce CO2, CH2O, and CO. Quoted uncertainties represent precision on the linear
regression fits. Systematic errors can arise from uncertainties in CO2, CH2O and CO absorption
cross sections and in PTD concentrations. Using the propagation of errors enabled estimating global
uncertainties of 20%, 29% and 33% on CO2, CO, and CH2O yields.
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Figure 6. Product yield plots of CO2 (ꢀ: 66 mbar), CH2O (ꢁ: 66 mbar, ꢂ: 105 mbar, ꢀ: 132
mbar) and CO (ꢂ: 66 mbar). The CO concentrations have been multiplied by 2 for clarity. The
lines correspond to linear regression fits on the values at 66 mbar.
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The CO yield obtained at 66 mbar (0.070) is in excellent agreement with that resulting from the
numerical simulations (0.065) and with that obtained by Bouzidi et al. at 1 bar in the absence of OH
radical scavenger (0.076)12. One of the main sources of CO is probably the photolysis of
formaldehyde. In case of CH2O, the yield (0.33) determined from experiments at 66, 105 and 132
mbar is somewhat lower than that obtained at 1 bar (0.41)12. This may be due to the additional
CH2O loss by reaction with OH radicals coming from RCO + O2 at low pressure, as exemplified by
the lower CH2O yield obtained by Bouzidi et al. at 1 bar when the OH radicals were not scavenged.
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Carbon dioxide mainly comes from the rapid decomposition of acyloxy RC(O)O radicals produced
in the reactions of acylperoxy radicals, RC(O)O2 with alkylꢀ and acylꢀperoxy radicals.
In addition to stable products, HO2 photostationary concentrations were also recorded, starting with
levels of ~ 4 × 1010 radical cmꢀ3, and slightly decreasing with reaction time (about 5ꢀ10 %). Using
the same methodology as Bouzidi et al. to simulate the reaction mechanism displayed in Table 4,
CH2O and HO2 radical concentrations were simulated and compared to the measured ones (Figure
4). Overall, the better agreement appears for simulations performed using low OH formation yields,
especially on HO2 concentrations, as illustrated by Figure 4. This confirms the results obtained by
SternꢀVolmer analysis and also indicates that the reaction mechanism used for the numerical
simulations is satisfactory in terms of chemical reactions used and rate coefficients associated with
them.
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Table 4: Chemical mechanism for 2,3-pentanedione photolysis
Reaction
Rate
constant a
b
Reference
1
2
CH3CO + C2H5CO
CH3C(O)O2
OH + cꢀCH2C(O)O
C2H5C(O)O2
CH3C(O)C(O)C2H5 + hν
CH3CO + O2
→
→
→
→
Carr et al.40
See text
3.3 × 10ꢀ12
c
Carr et al.40
3.3 × 10ꢀ12
3
C2H5CO + O2
d
OH + cꢀCH2(CH3)C(O)O
2CH3O2 + 2CO2 + O2
2C2H5O2 + 2CO2 + O2
c
See text
→
→
→
1.5 × 10ꢀ11
1.7 × 10ꢀ11
4
5
CH3C(O)O2 + CH3C(O)O2
C2H5C(O)O2 + C2H5C(O)O2
Sander et al.53
Le Crâne et
al.54
1.5 × 10ꢀ11
6
CH3C(O)O2 + C2H5C(O)O2
CH3C(O)O2 + CH3O2
CH3O2 + C2H5O2 + 2CO2 + O2
Tomas
and
→
Lesclaux55
d
Sander et al.53
1.1 × 10ꢀ11
α7 = 0.9
7a
7b
8a
CH3O2 + CH3O + CO2 + O2
CH3C(O)OH + CH2O + O2
C2H5O2 + CH3O + CO2 + O2
→
→
→
1.1 × 10ꢀ11
C2H5C(O)O2 + CH3O2
d
α8 = 0.9 b
1 × 10ꢀ11
α9 = 0.82
1.2 × 10ꢀ11
α10 = 0.82
1.9 × 10ꢀ15
1 × 10ꢀ14
8b
9a
9b
C2H5C(O)OH + CH2O + O2
CH3O2 + C2H5O + CO2 + O2
CH3C(O)OH + CH3CHO + O2
C2H5O2 + C2H5O + CO2 + O2
C2H5C(O)OH + CH3CHO + O2
CH2O + HO2
→
→
→
→
→
→
→
CH3C(O)O2 + C2H5O2
Lesclaux56
10a C2H5C(O)O2 + C2H5O2
10b
Le Crâne et
al.54
Sander et al.53
11
12
CH3O + O2
C2H5O + O2
CH3CHO + HO2
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13a CH3C(O)O2 + HO2
13b
OH + CH3O2 + CO2
CH3C(O)OOH + O2
Tomas et al.57
1.4 × 10ꢀ11
α13 = 0.4
→
→
Dillon
and
Crowley38
13c
CH3C(O)OH + O3
OH + C2H5O2 + CO2
Tomas et al.57
Le Crâne et
al.54
β13 = 0.2
→
→
1.4 × 10ꢀ11
14a C2H5C(O)O2 + HO2
α14 = 0.4 d
β14 = 0.2
14b
14c
C2H5C(O)OOH + O2
Dillon
and
→
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C2H5C(O)OH + O3
Le Crâne et
→
al.54
15
16
17
CH3O2 + HO2
C2H5O2 + HO2
CH2O + HO2
CH3OOH + O2
C2H5OOH + O2
OHCH2O2
CH2O + HO2
Sander et al.53
5.2 × 10ꢀ12
8 × 10ꢀ12
3.3 × 10ꢀ14
55 e
→
→
→
→
Morajkar et
al.23
Szabo et al.17
ꢀ17 OHCH2O2
2.09 × 10ꢀ
18
OH + CH3C(O)C(O)C2H5
PTDO2 + H2O
→
12
19
20
21
22
23
24
25
OH + CH2O
OH + CH3CHO
CH2O + hν
HO2 + HO2 + M
CH2O + wall
HO2 + CO + H2O
CH3C(O)O2 + H2O
H2 + CO
H2O2 + O2 + M
Sander et al.53
8.5 × 10ꢀ12
1.5 × 10ꢀ11
6 × 10ꢀ6 e
→
→
→
→
→
→
→
this work
Sander et al.53
this work
1.5 × 10ꢀ12
2.4 × 10ꢀ6 e
8 × 10ꢀ12 d
PTDO2 + HO2
PTDO2 + CH3C(O)O2
PTDOOH + O2
CH3O2 + CO2 + CH3CHO +
CH3CO
C2H5O2 + CO2 + CH3CHO +
CH3CO
Sander et al.53
Sander et al.53
1.1 × 10ꢀ11
d, f
26
PTDO2 + C2H5C(O)O2
Sander et al.53
1.1 × 10ꢀ11
→
d, f
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
a: αi = kia/ki; βi = kic/ki; rate constants in cm3 moleculeꢀ1 sꢀ1
b: adjusted to fit the PTD concentrations (varies between 3.7 × 10ꢀ5 sꢀ1 to 5.1 × 10ꢀ5 sꢀ1)
c: see Table 3
d: rate constants unavailable in the literature, which were replaced by the rate constants of the corresponding acetylꢀ or
methylꢀperoxy radicals reactions. C2H5O2 was used as analogue for PTDO2.
e: in sꢀ1
f: the PTDO radical (CH3C(=O)C(=O)CH(O)CH3) is supposed to decompose instantaneously into CH3CHO and
CH3CO.
4. CONCLUSIONS
The photolysis of 2,3ꢀpentanedione (between 330 nm and 440 nm) has been investigated for the
first time as a function of pressure in a static reactor equipped with cwꢀCRDS. As at atmospheric
pressure, the lowꢀpressure photolysis mechanism initially proceeds by the carbonyl – carbonyl bond
scission. Analysis of the pressure effect on PTD photolysis primary quantum yields using Sternꢀ
Volmer analysis as well as CH2O and HO2 time profile numerical simulations revealed
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inconsistencies when using high OH formation yields from RCO + O2 reactions. Thus, the present
work clearly favours low OH formation yields determined by several groups.
The photolysis rate of PTD has been shown to be only slightly pressureꢀdependent with quantum
yields increasing from 0.20 to more than 0.28 from atmospheric to zero pressure and between 330
nm and 440 nm. The estimated photodecomposition lifetime of PTD is about 1 h in the low
troposphere and somewhat shorter at higher altitudes. Compared to the lifetime due to the reaction
with OH radicals, PTD atmospheric lifetime is of the order of 5 days (for an atmospheric OH
concentration of ca. 1 × 106 molecules cmꢀ3). It can be concluded that photolysis is by far the major
degradation pathway of 2,3ꢀpentanedione.
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Finally, the timeꢀresolved measurement of HO2 photostationary concentrations using cwꢀCRDS and
combined with the reaction mechanism modelling has allowed assessing the photolysis reaction
mechanism of 2,3ꢀpentanedione and demonstrate the interest of measuring both radicals and stable
compounds in atmospheric simulation experiments58.
SUPPORTING INFORMATION
PTD and NO2 absorption spectra as well as the lamp emission spectrum are given in Figure S1. The
material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGEMENTS
The present work takes place in the Labex CaPPA (Chemical and Physical Properties of the
Atmosphere) funded by the French National Research Agency (ANR) through the PIA (Programme
d'Investissements d'Avenir) under contract ANRꢀ11ꢀLABXꢀ005ꢀ01 and by the Regional Council
“NordꢀPas de Calais” and the European Funds for Regional Economic Development (FEDER). It is
also supported by the LefeꢀChat program from INSUꢀCNRS. IRENI program is also acknowledged
for financial support. H. Bouzidi is grateful for a PhD scholarship from the NordꢀPas de Calais
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Regional Council and Mines Douai. T. Gierczak is grateful for a 1ꢀmonth invitedꢀprofessor grant at
Mines Douai. The authors are grateful to D. Petitprez from PC2A for lending the mass spectrometer.
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atmospheric photolysis of αꢀdicarbonyls. Int. J. Chem. Kin. 2001, 33, 9ꢀ20.
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