Products from the gas-phase reaction of some unsaturated alcohols with nitrate radicals

Article abstract of DOI:10.1039/b000251h

Five structurally similar unsaturated alcohols, 2-propene-1-ol (allyl alcohol), 3-butene-2-ol, 2-methyl-3-butene-2-ol (MBO232), 2-butene-1-ol (crotyl alcohol) and 3-methyl-2-butene-1-ol (MBO321), were examined to clarify their atmospheric degradation pathways via oxidation initiated by NO₃ radicals. The reactions were investigated using a 0.153 m³ static glass reactor equipped with long-path FTIR spectroscopy. The experiments were performed at a pressure of 1020 ± 5 mbar and at a temperature of 297 ± 2 K in air or nitrogen as the bath gas. The identified and quantified gas phase products were small carbonyl compounds such as acetone, formaldehyde, acetaldehyde, glycolaldehyde and 2-nitrooxy acetaldehyde. The specific products and their yields varied for the five studied alcohols as follows: formaldehyde 37(±1)% and 2-nitrooxy acetaldehyde 41(±7)% from allyl alcohol; acetaldehyde 28(±6)%, formaldehyde 2(±1)% and 2-nitrooxy acetaldehyde 33(±4)% from 3-butene-2-ol; acetone 63(±6)% and 2-nitrooxy acetaldehyde 67(±8)% from MBO232; acetaldehyde 12(±2)%, formaldehyde 10(±3)% and glycolaldehyde 7(±2)% from 2-butene-1-ol; acetone 21(±6)%, formaldehyde 11(±3)% and glycolaldehyde 29(±10)% from MBO321. In addition, yields were estimated for total organic nitrates using an average integrated absorption cross section of unspecified organic nitrates. Tentative reaction schemes were proposed from the yielded products. The distribution between bond breakage and other processes such as abstraction of a hydrogen atom from the alkoxy radical, formed in the degradation process, was estimated. The small carbonyl compounds were produced by the bond breakage mechanisms. Large multi-functional organic compounds e.g. 1-hydroxy-3-nitrooxy-3-methyl-2- butanone from MBO321 were proposed to be formed by hydrogen abstraction. From the product distribution, the contribution of the number of methyl group substituents at the α and γ carbon atoms, influencing the bond breakage pattern, is discussed. The observed bond cleavage trends are correlated to a substitution pattern where electron donating methyl substituents increase the stability of the leaving radical groups.

Full text of DOI:10.1039/b000251h

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Products from the gas-phase reaction of some unsaturated alcohols
with nitrate radicals
J. Noda,a M. Hallquist,b S. Langera and E. Ljungstroma
a Department of Inorganic Chemistry, University of Goteborg, S-41296 Goteborg, Sweden.
E-mail: jun=inoc.chalmers.se; Fax: ]46 31 772 2853; T el: ]46 31 772 2865
b Centre for Atmospheric Science, University of Cambridge, Department of Chemistry,
L ensÐeld Road, Cambridge, UK CB2 1EW
Received 12th January 2000, Accepted 20th March 2000
Published on the Web 12th May 2000
Five structurally similar unsaturated alcohols, 2-propene-1-ol (allyl alcohol), 3-butene-2-ol, 2-methyl-3-butene-
2-ol (MBO232), 2-butene-1-ol (crotyl alcohol) and 3-methyl-2-butene-1-ol (MBO321), were examined to clarify
their atmospheric degradation pathways via oxidation initiated by NO radicals. The reactions were
3
investigated using a 0.153 m3 static glass reactor equipped with long-path FTIR spectroscopy. The
experiments were performed at a pressure of 1020 ^ 5 mbar and at a temperature of 297 ^ 2 K in air or
nitrogen as the bath gas. The identiÐed and quantiÐed gas phase products were small carbonyl compounds
such as acetone, formaldehyde, acetaldehyde, glycolaldehyde and 2-nitrooxy acetaldehyde. The speciÐc
products and their yields varied for the Ðve studied alcohols as follows: formaldehyde 37(^1)% and
2-nitrooxy acetaldehyde 41(^7)% from allyl alcohol; acetaldehyde 28(^6)%, formaldehyde 2(^1)% and
2-nitrooxy acetaldehyde 33(^4)% from 3-butene-2-ol; acetone 63(^6)% and 2-nitrooxy acetaldehyde
67(^8)% from MBO232; acetaldehyde 12(^2)%, formaldehyde 10(^3)% and glycolaldehyde 7(^2)% from
2-butene-1-ol; acetone 21(^6)%, formaldehyde 11(^3)% and glycolaldehyde 29(^10)% from MBO321. In
addition, yields were estimated for total organic nitrates using an average integrated absorption cross section
of unspeciÐed organic nitrates. Tentative reaction schemes were proposed from the yielded products. The
distribution between bond breakage and other processes such as abstraction of a hydrogen atom from the
alkoxy radical, formed in the degradation process, was estimated. The small carbonyl compounds were
produced by the bond breakage mechanisms. Large multi-functional organic compounds e.g. 1-hydroxy-3-
nitrooxy-3-methyl-2-butanone from MBO321 were proposed to be formed by hydrogen abstraction. From the
product distribution, the contribution of the number of methyl group substituents at the a and c carbon
atoms, inÑuencing the bond breakage pattern, is discussed. The observed bond cleavage trends are correlated
to a substitution pattern where electron donating methyl substituents increase the stability of the leaving
radical groups.
sured by Goldan et al. Guenther et al. observed the direct
Introduction
emission of MBO232 from P. teada (loblolly pine) trees.5 In
addition, emission of two structurally similar compounds, 3-
methyl-2-butene-1-ol (MBO321), and 3-methyl-3-butene-1-ol
(MBO331) has been observed from di†erent types of vegeta-
tion.6 Degradation processes and products from this group of
BVOC are of interest, considering the potentially high emis-
sion rates of these unsaturated alcohols to the atmosphere.
Degradation of unsaturated compounds in the atmosphere
is often initiated by addition of OH, NO , O or Cl atoms to
In recent years, the scientiÐc community has paid increasing
attention to biogenic volatile organic compounds (BVOC)
because such substances are thought to be important elements
of atmospheric chemistry. The emission of BVOC, including
compounds such as monoterpenes, isoprene and many other
reactive hydrocarbons, is estimated at greater than 1000 Tg
year~1 of C.1 BVOC often react with atmospheric oxidants as
fast, or faster than anthropogenically emitted organic sub-
stances.2 Thus, BVOC have a potential to contribute to the
adverse e†ects that are caused by anthropogenic organic air
pollutants, e.g. photochemical oxidant formation and haze.
3
3
the double bond. The substituent pattern of the carbon atoms
in the double bond a†ects the electron distribution of the
bond with consequences for addition rate and possibly also
for product distribution. As an example, an alkyl group exerts
a positive inductive e†ect and it makes the double bond more
susceptible to electrophilic attack.7 The strong connection
between the number of alkyl groups attached to the carbon
atoms and reaction rates is seen e.g. in the series ethene,
propene, 2-butene, 2-methyl-1-propene and 2,3-dimethyl
butene, with rate coefficients for nitrate radical reaction of
0.02, 0.94, 33, 940 and 5700 (]10~14 cm3 molecule~1 s~1),
respectively.8
Some unsaturated C alcohols have been identiÐed as com-
ponents of the complex BVOC mixture under certain condi-
5
tions and have been quantiÐed by several groups. In an early
study by Goldan et al.,3 2-methyl-3-butene-2-ol (MBO232)
was reported in mixing ratios of up to several ppb (nmol
mol~1) at a remote mountain site in Colorado, USA. This
study showed that the concentration of MBO232 could exceed
that of isoprene by a factor of between Ðve and eight. Recent-
ly, Harley et al.4 demonstrated signiÐcant emission of
MBO232 from P. contorta Dougl. var. latifolia Engelm.
(lodgepole pine) and P. ponderosa Laws. (ponderosa pine), that
were in agreement with the concentrations of MBO232 mea-
A number of mechanistic studies regarding C unsaturated
5
alcohols have been reported for the OH- and O -initiated deg-
3
radation,9h14 while fewer investigations have been conducted
DOI: 10.1039/b000251h
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2555

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for degradation by NO .9,11 In a recent mechanistic study of
40 m optical path length. Spectra of the reaction mixture were
3
the MBO232 reaction with NO radicals by Fantechi et al.,11
based on various numbers of scans (64, 84 or 180 at 1 scan
s~1) to obtain a balance between good spectral quality and
time resolution at di†erent stages of the reaction. A dilution
system using pressureÈvolume measurements was employed to
introduce known amounts of the various substances into the
reactor. Synthetic air or nitrogen was used as bath gas. A
detailed description of the set-up can be found elsewhere.21
The concentration of reactants and products in the experi-
ments were determined by scaled subtraction using spectra of
reference compounds at known concentrations. The scale
factors were linear to concentration, i.e. BeerÏs law obeyed,
over the concentration intervals employed. However, for 2-
nitrooxy acetaldehyde (2-NAA) there was no possibility to
achieve a good calibration owing to the instability of this
compound. Its concentration in the reference spectrum was
therefore estimated as described below for the unidentiÐed
3
the authors reported a 68.7 and 13% yield of acetone and
organic nitrates, respectively. Work on products and mecha-
nisms by Alvarado et al.13 for MBO232 reacting with OH
radicals has also shown the formation of acetone, aldehydes
and organic nitrates in the presence of NO. Formation of car-
bonyls and organic nitrates plays an important role in the
atmosphere since they may be photolysed and thereby be a
source of free radicals. Organic nitrates may act as a reservoir
for NO and could be subject to long range transport.15,16 A
x
strong source of acetone is considered as being of importance
for global atmospheric chemistry.17h20
In this work, Ðve structurally similar unsaturated alcohols,
including some of the MBOs, were examined to clarify their
degradation pathways via oxidation initiated by NO radicals.
3
2-propene-1-ol (allyl alcohol), 3-butene-2-ol, 2-methyl-3-
butene-2-ol (MBO232), 2-butene-1-ol and 3-methyl-2-buten-1-
ol (MBO321) were investigated. The last four compounds may
be regarded as methyl substituted varieties of the basic com-
pound 2-propene-1-ol. Fig. 1 illustrates the common structure
within the dotted-line box and the Ðve examined compounds.
IdentiÐcation and yield of some products, leading to mass
balance is presented. Tentative reaction schemes are proposed
from the yield of products and the inÑuence of the methyl
group substitution is discussed.
organic nitrates. The NO radicals were generated by thermal
3
decomposition of N O . The N O was synthesised in a
2
5
2 5
separate set-up by the reaction of excess O with NO accord-
3
2
ing to reaction (1) followed by reaction (2). The N O was
then trapped and stored at 195 K.
2
5
NO ] O ] NO ] O
(1)
(2)
2
3
3
2
NO ] NO % N O
2
3
2 5
The experiments were conducted by Ðrst introducing the
unsaturated alcohol to the reactor, conÐrming its concentra-
tion by FTIR spectroscopy and then adding N O by a
Experimental set-up and procedures
2
5
stream of bath gas. As an exception, for a few experiments
The experiments were carried out in a 0.153 m3 borosilicate
glass reactor at 297(^2) K and 1020(^5) mbar pressure.
Reactants and products were measured by long optical path
Fourier transform infrared (FTIR) spectroscopy using a
Nicolet Magna 560-spectrometer equipped with an MCT
detector. Spectra were collected at 1.0 cm~1 resolution using a
with MBO232 and MBO321, N O was prepared in situ by
2
5
mixing O with excess NO . A relatively high concentration
3
2
of NO (2.0 ] 1014 molecule cm~3) was present when
2
MBO232 or MBO321 was added, to ensure that no O was
left. Further, some other experiments with even higher initial
3
NO concentration (4.9 ] 1014 molecule cm~3) were con-
2
Fig. 1 Structure of the investigated compounds. The common structure possesses allyl alcohol-like carbon skeleton with a hydroxy group in the
a position and double bond between the b and c carbon atoms.
2556
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ducted to assess the role of NO in the reaction mechanisms.
method, the total organic nitrates measured contain all the
2
Initial concentrations of the unsaturated alcohols and N O
products that have absorption between 800 and 900 cm~1.
Table 2 shows the list of organic nitrates and their cross sec-
tions from previous studies.21,24h26 The values indicate that
the cross sections do not change drastically despite the di†er-
ence in molecular structure of the organic nitrates. Therefore,
it is concluded that using the average cross section value is an
acceptable way of estimating the total yield of organic
nitrates.
2
5
are listed in Table 1. In all experiments, the reactions were
monitored by FTIR spectroscopy, until concentration changes
ceased. Yields were obtained from graphs of product formed
vs. alcohol lost. For curved plots, e.g. for organic nitrates
(RÈONO ), the Ðrst few points after the concentration of the
2
transient peroxy nitrate had decreased to less than 3% of
reacted alcohol, were used to obtain an estimate of lower limit
yields. The end RONO measured at the termination of an
experiment represents a situation where less volatile products
The stability and wall adsorption characteristics of the indi-
vidual alcohols were investigated by introducing them into the
reactor and monitoring the concentration over time. Typi-
cally, less than 1% h~1 was lost and with 3% for MBO321
being the extreme case. This loss rate was insigniÐcant com-
pared to the rate of reaction, thus it was not taken into
account for the yield calculation.
2
have equilibrated between the gas phase and reactor walls
and/or aerosol particles. The error for the average values is
given as one standard deviation. Further discussion of the
inÑuence of peroxynitrate formation on product yield will
follow later in this paper.
In order to estimate the yield of total organic nitrates in the
Chemicals used were: 2-propene-1-ol ([95%, Merck), 3-
butene-2-ol ([97%, Merck), MBO232 ([98%, Aldrich), 2-
butene-1-ol ([97%, mixture of cis and trans, Aldrich),
MBO321 ([99%, Aldrich), and glycolaldehyde dimer ([98%,
Fluka). For all the investigated compounds, freezeÈpumpÈ
thaw cycles were repeated several times to eliminate dissolved
experiments, a C
(IBA) absorption cross section for the ONO nitrooxy group
band located between 800 and 900 cm~1,22 was used in
analogy to previous studies to estimate the concentration of
nitratesÏ average integrated band area
1h10
2
one or more organic nitrates formed.21,23h26 With this
Table 1 Experimental conditions. All experiments were made at 297(^2) K and 1020(^10) mbar
Initial concentration/molecule cm~3
Investigated
compound
[N O ]
[Unsaturated alcohol]
[NO ]
2
5
2
Allyl alcohol
Allyl alcohol
3-Butene-2-ol
3-Butene-2-ol
3-Butene-2-ol
3-Butene-2-ol
3-Butene-2-ol
MBO232
MBO232
MBO232
MBO232
MBO232
MBO232
MBO232
MBO232
2-Butene-1-ol
2-Butene-1-ol
MBO321
MBO321
MBO321
1.9 ] 1014
2.0 ] 1014
1.9 ] 1014
2.0 ] 1014
2.0 ] 1014
2.0 ] 1014
1.9 ] 1014
1.5 ] 1014
1.5 ] 1014
2.2 ] 1014
2.2 ] 1014
1.8 ] 1014
1.7 ] 1014
1.8 ] 1014
1.9 ] 1014
2.4 ] 1014
1.4 ] 1014
1.2 ] 1014
9.1 ] 1013
1.5 ] 1014
1.5 ] 1014
1.6 ] 1014
1.8 ] 1014
3.2 ] 1013
2.7 ] 1014
8.7 ] 1014
8.7 ] 1014
8.9 ] 1014
8.4 ] 1014
8.5 ] 1014
1.0 ] 1015
1.7 ] 1015
1.3 ] 1015
2.0 ] 1014
2.4 ] 1014
2.4 ] 1014
1.5 ] 1015
1.5 ] 1015
1.6 ] 1015
1.8 ] 1015
9.3 ] 1014
9.2 ] 1014
2.8 ] 1014
2.0 ] 1014
2.5 ] 1014
2.4 ] 1014
2.7 ] 1014
3.0 ] 1014
3.7 ] 1013
3.8 ] 1014
È
È
È
È
4.9 ] 1014
4.9 ] 1014
4.9 ] 1014
D2.0 ] 1014
D2.0 ] 1014
È
È
È
4.9 ] 1014
4.9 ] 1014
4.9 ] 1014
È
È
D2.0 ] 1014
D2.0 ] 1014
È
È
È
È
È
È
MBO321
MBO321
MBO321
MBO321
MBO321
Table 2 Absorption cross sections used to obtain an average value for this study21,24h26a
IBA absorption cross section
Compound
/cm molecule~1 (base 10)
Ref.
Methyl nitrate
Ethyl nitrate
Isopropyl nitrate
3-Nitroxy-2-butanone
2,3-Butyl di-nitrate
3-Nitroxy-2-butanol (RR; SS)
3-Nitroxy-2-butanol (RS; SR)
2-Nitro-3-butyl nitrate
C10-alkyl nitrates
0.854(^0.02)
1.09(^0.02)
1.09(^0.05)
1.31(^0.04)
1.37(^0.05)b
1.23(^0.02)
1.18(^0.02)
1.46(^0.07)
1.46(^0.09)
25
25
25
21
21
21
21
21
24, 26
Average
1.25(^0.20)
a The average nitrate absorption cross section (1.25 ] 10~17 cm molecule~1) was used to calculate the yield of total organic nitrate. The stated
errors are at the 95% conÐdence level. b The value indicated is one half of the original cross section.
Phys. Chem. Chem. Phys., 2000, 2, 2555È2564
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gases before use in the dilution system. Synthetic air (20% O
often been observed that the less stable radical is also formed
2
in N 99.996%, AGA Gas AB) was employed as bath gas for
in a signiÐcant yield and it is therefore necessary to consider
2
the experiments. Reference spectra used for the authentication
both possibilities.28
and quantiÐcation of the products were taken in nitrogen
(99.996%, AGA Gas AB).
One of the products, 2-nitrooxy acetaldehyde (2-NAA),
from NO oxidation of allyl alcohol, 3-butene-2-ol and
3
MBO232 was synthesised for qualitative analysis. This com-
pound was prepared by nitration of glycolaldehyde with
(3a)
(3b)
N O in much the same way as described by Kames et al. for
2
5
preparation of various organic nitrates from alcohols.27 First,
0.5 g (0.0042 mol) of glycolaldehyde dimer (C H O) was sus-
2
4
2
pended and partly dissolved in CCl (8 g) at room tem-
4
perature. The suspension was then solidiÐed by cooling to 197
K, a vacuum applied, and 0.7 g (0.0065 mol) of N O was
2
5
sublimated onto the frozen solid mixture. The temperature of
the mixture was then raised to 273 K, again taking the liquid
suspension form, stirred for 20 min and further gradually
raised to 293 K with continuous stirring for another 20 min. A
greenish coloured liquid, containing impure 2-NAA, was
Under atmospheric conditions and in the present experi-
ments, i.e. in the presence of oxygen, the initially formed alkyl
radical in reaction (3a) or (3b), will add O to form a nitrooxy-
2
substituted alkylperoxy radical in reaction (4a) or (4b).
separated on top of the CCl phase. Attempts to remove
4
HNO that was also formed during the reaction were only
3
partially successful. Repeated extraction with cold, NaCl-
saturated water Ðnally gave an aqueous phase with pH 5È6
indicating that the HNO was essentially removed and that
3
the ester was not hydrolysed rapidly. However, on standing at
room temperature for 20 min, the product again became
(4a)
(4b)
acidic and gave o† NO . Thus, the compound is thermally
2
unstable and difficult to prepare in a pure state for quantitat-
ive work. Organic nitrates are potentially dangerous because
of explosion hazard; therefore, anyone contemplating their
synthesis and use should employ accepted safety procedures.
The product was analysed with GC/MS and FTIR. The
mass spectrum showed the molecular ion peak at m/z 105 and
The peroxy radicals thus formed may react with NO, NO ,
the NO typical for organic nitrates at m/z 46. The gas phase
2
2
NO , HO or other peroxy radicals in the atmosphere.2,29
FTIR spectrum showed strong characteristic organic nitrate
3
2
However, in the present experiments NO is excluded as reac-
absorbance at 1678, 1285, and 842 cm~1.
tant. In the presence of NO , an alkyl peroxynitrate may be
2
formed in equilibrium with the alkylperoxy radical and NO
2
Results and discussion
according to reaction (5).
Table 3 shows the observed products and the fractional molar
yield of identiÐed compounds. In addition, estimated yields of
unidentiÐed compounds, that are useful for assessing reaction
paths, are given. The degradation of unsaturated organic com-
pounds by NO radicals is initiated by addition to one of the
3
carbon atoms in the double bond.2 The tendency of NO
(5)
3
addition to the less substituted carbon atom and to produce
the more stable radical product is in accordance with the
““anti-MarkovnikovÏsÏÏ rule in reaction (3a). However, it has
For all the experiments, formation of an alkyl peroxynitrate
was shown by the presence of IR bands at D1730 and D790
Table 3 Summary of the molar based product yieldsa
Nitrooxy
acetaldehyde
(%)
Initial RÈONO
(estimate)
(%)
End RÈONO
(estimate)
(%)
2
2
Acetone
(%)
Acetaldehyde
(%)
HCHO
(%)
Glycolaldehyde
(%)
Compound
Allyl alcohol
n \ 2
ND
ND
37(^1)
2(^2)
2(^1)
4(^1)
1(^0)
10(^3)
11(^3)
\2
41(^7)
32(^6)
33(^4)
67(^8)
63(^5)
ND
53(^10)
41(^6)
30(^1)
32(^5)
45(^5)
63(^6)
64(^14)
27(^1)
29(^6)
3-Butene-2-ol
n \ 2
ND
27(^7)
28(^6)
ND
ND
3-Butene-2-olb
n \ 3
ND
ND
54(^10)
71(^12)
71(^10)
38(^6)
MBO232
n \ 5
63(^6)
59(^1)
ND
ND
MBO232b
n \ 3
ND
ND
2-Butene-1-ol
n \ 2
12(^2)
ND
7(^2)
29(^10)
MBO321
n \ 8
21(^6)
ND
60(^39)
a n \ number of experiments carried out. ND \ not detected. b High initial NO (4.9 ] 1014 molecule cm~3) concentration.
2
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ROO ] HO ] ROOH ] O
(6)
2
2
This channel is very important in certain cases, e.g. the reac-
tion between the methylperoxy radical and HO produces
2
methyl hydrogenperoxide in a yield close to 100%.30
The further reaction of the oxy radicals is the crucial step to
determine the product distribution.31,32 The possible reaction
channels for the oxy radicals produced from the investigated
compounds are shown in Figs. 3È7.
As can be seen in Table 3, a closed mass balance has not
been achieved from the detected products for any of the inves-
tigated compounds. These poor mass balances need to be con-
sidered, since no signiÐcant remaining unidentiÐed absorption
feature could be observed in the residual infrared spectra from
any experiment.
One possible cause is production of compounds with func-
tional groups that do not absorb infrared radiation strongly,
and therefore, can not be detected. As an example, the epoxide
functional group does not absorb strongly in the IR. Epoxides
Fig. 2 An example of peroxy nitrate and total organic nitrates for-
mation in a MBO232 experiment. Spectra are scaled, o†set and sub-
tracted with MBO232, HCHO, acetone, HNO , N O , NO , H O
3
2
5
2
2
and HO NO . The absorbance is given in arbitrary units.
2
2
have been observed as major products from NO initiated
3
cm~1. Fig. 2 illustrates the formation and disappearance of
the peroxynitrate as time advances. The peroxynitrate serves
as a temporary storage for the peroxy radical. To obtain more
accurate values, when estimating the total alkyl nitrate con-
centration, IR absorption due to the presence of the nitrooxy
peroxynitrate was subtracted.
degradation of unsaturated compounds under conditions of
low pressure and low oxygen concentration.33 Thus, taking
the conditions of the present investigation into account, sig-
niÐcant epoxide formation appears unlikely. However, no ref-
erence IR spectra were available for identiÐcation or to set
any upper limits for epoxide yield. Another type of compound
for which no reference spectra were available is hydro-
peroxide. Hydroperoxide may be formed from nitrated peroxy
Products from the reaction of a peroxy radical with another
peroxy radical may be the corresponding oxy radicals.2,29
Another possible outcome from the reaction between two
peroxy radical entities is the formation of an alcohol and a
ketone by elimination of molecular oxygen. For the investi-
gated compounds, this may produce tri-functional species
(hydroxycarbonyl nitrate, dihydroxynitrate). This process is
not accessible for the tertiary alkylperoxy radicals.2 Also, the
HO and NO reactions with peroxy radicals could give
radicals reacting with HO according to reaction (6).
Another cause may be loss of substances from the gas
2
phase. The loss may take place in two ways. A substance may
either be adsorbed to the walls of the reactor or it may form
an aerosol. The Ðrst event was indicated by the formation and
decay of the total organic nitrates (alkyl nitrates) with time. In
all experiments, the decay continues after all N O has disap-
peared. No other conceivable reactant is present at a concen-
tration high enough to cause such loss. Unimolecular
decomposition is a possibility, but the decomposition should
2
3
2 5
alkoxy radicals in high yield. The reaction with HO can
2
produce an organic hydroperoxide in a second channel as is
shown in reaction (6).
Fig. 3 Possible degradation paths of alkoxy radicals originating from the NO initiated oxidation of allyl alcohol. Cases a and b refer to attack
3
at the least and most substituted carbon atom respectively. Yields of the quantiÐed products are given as molar percentages. The \60% value is
an estimated upper limit yield.
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Fig. 4 Possible degradation paths of alkoxy radicals originating from the NO initiated oxidation of 3-butene-2-ol. Cases a and b refer to attack
3
at the least and most substituted carbon atom respectively. Yields of the quantiÐed products are given as molar percentages. The \68% value is
an estimated upper limit yield.
give detectable products and no such products were seen. The
total organic nitrate eventually approaches an equilibrium
between gas phase and wall surface (Table 3). Until the equi-
librium is reached, the composition of the gas phase organic
nitrates changes with time. This is due to the fact that di†erent
nitrates have di†erent affinities for the wall surface. This is
illustrated by the peak shape of the nitrate band between 1600
and 1700 cm~1 as is shown in Fig. 2 for an MBO232 experi-
ment. As time progressed, the shape of the peak and its inte-
grated band area changed because of selective adsorption of
nitrates to the reactor wall.
The second type of event, aerosol formation, is sometimes
but not always manifested by a wavelength-dependent dis-
placement of the baseline in FTIR spectra. This is caused by a
Fig. 5 Possible degradation paths of alkoxy radicals originating from the NO initiated oxidation of MBO232. Cases a and b refer to attack at
3
the least and most substituted carbon atom respectively. Yields of the quantiÐed products are given as molar percentages. The \32% value is an
estimated upper limit yield.
2560
Phys. Chem. Chem. Phys., 2000, 2, 2555È2564

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Fig. 6 Possible degradation paths of alkoxy radicals originating from the NO initiated oxidation of 2-butene-1-ol. Cases a and b refer to
3
addition at the b and c carbon atom respectively. Yields of the quantiÐed products are given as molar percentages. The \80% value is an
estimated upper limit yield.
particle size-dependent scattering of IR radiation. In order to
have this scattering e†ect, the particle concentration must be
high enough and the particles should have physical dimen-
sions similar to the wavelength of the radiation. Energy will
then be lost by scattering radiation from the optical path. The
spectrometer will interpret this as a broad absorption, as is
illustrated in Fig. 8 (a) and (b). Such a baseline shift is a strong
indication of the presence of aerosols. On the contrary, the
absence of a baseline shift can not rule out the existence of
particles, since a low concentration of large particles could
pass unnoticed while the same condensed mass distributed in
smaller particles may cause noticeable scattering.
As indicated above, both wall adsorption and aerosol for-
mation occurred in the present investigation and provide an
explanation to the notably poor mass balances. The main pro-
perty causing molecules to adsorb or condense is low vapour
pressure, which is expected from high molecular weight com-
pounds and from polar substances. High molecular weight
Fig. 7 Possible degradation paths of alkoxy radicals originating from the NO initiated oxidation of MBO321. Cases a and b refer to attack at
3
the least and most substituted carbon atom respectively. Yields of the quantiÐed products are given as molar percentages. The \64% and \25%
values are estimated upper limit yields.
Phys. Chem. Chem. Phys., 2000, 2, 2555È2564
2561

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in agreement with the 33(^4)% yield of 2-NAA. It is con-
cluded that path 1a accounts for about 30% of the reacted
alcohol, with no inÑuence of the initial NO concentration.
2
Cleavage of the original double bond is an insigniÐcant route
as shown by the 2% yield of formaldehyde.
A high NO concentration will reduce the initial concentra-
2
tion of NO by shifting the N O equilibrium, however, no
3
2 5
e†ect of this shift was observed. Also, a high NO concentra-
tion would lower the concentrations of peroxy radicals by
2
promoting the formation of RÈOONO and HO NO . This
2
2
2
trapping phenomenon for peroxynitrate was observed as a
prolonged time of high concentration of RÈOONO and
2
HO NO in the experiments. Formation of organic nitrates
2
2
through reaction between alkoxy radicals and NO according
Fig. 8 IR spectra of reacted MBO321 (a) and MBO232 (b) with
2
to reaction (7) is also possible.
NO . Spectra were taken 1 min after mixing the alcohols with N O .
3
2 5
An example of MBO321 FTIR spectrum (a) with base line shift due to
the presence of aerosol.
RÈO~ ] NO ] M ] RONO ] M
(7)
2
2
The total organic nitrate yield at the end of experiment for
products are not anticipated in the present investigation, since
the starting material contains Ðve or fewer carbon atoms. The
other crucial factor, the polarity of a substance, is often depen-
dent on the number and type of the substituent groups. No
volatility information is, to the best of the authorsÏ knowledge,
available for the triply substituted products, which are illus-
trated in the reaction schemes or for the hydroperoxides that
are products from reaction (6). To have some idea about the
physical properties of these compounds, one can consider
similar substances such as glyceraldehyde (2,3-dihydroxypro-
panal) and dihydroxyacetone (1,3-dihydroxypropanone). The
melting point of dihydroxyacetone is 363 K and glycer-
aldehyde has a vapour pressure of 1 mbar at 418 K.34,35 It is
therefore reasonable to assume that the volatility of triply sub-
stituted products such as those indicated in Figs. 3È7 is low.
Thus, the most likely explanation for the poor mass balances
is that such compounds disappear from the gas phase.
the low NO case indicates 2-NAA to be the only nitrate left
2
in the gas phase. However, high NO experiments showed a
2
small but signiÐcant increase of about 10%, in organic nitrate
end concentration. This could be due to formation of di-
nitrates that have about twice the nitrate band absorption
cross section of mono-nitrates (cf. Table 2).
MBO232
Acetone was produced in a yield of 63(^6)% with no signiÐ-
cant di†erence between ““lowÏÏ and ““highÏÏ NO experiments.
2
The only way of obtaining acetone is via path 1a in Fig. 5.
2-Nitrooxy-acetaldehyde is also a product from path 1a and
was observed in a yield of 67(^8)%. Our acetone yield is in
accordance with the value presented by Fantechi et al.11 of
68.7(^7.1)%; however, their nitrate yield of 13% is signiÐ-
cantly lower than ours. Acetone and 2-NAA are only produc-
ed in path 1a (Fig. 5), and this conÐrms bond rupture next to
the double bond as the dominant process. 3(^2)% of formal-
dehyde was observed, showing that paths 2a and 2b are not
very active. Analogous to allyl alcohol and 3-butene-2-ol,
2-NAA is the most abundant organic nitrate of the measured
total alkyl nitrates at the end of the reaction. It is thus con-
cluded that paths 3a and/or 3b account for a maximum of
D30% of the alcohol lost. Again, products from paths 3a and
3b are suspected to have properties such that would make
them disappear from the gas phase with time. Mechanistic
The suggested triply substituted compounds formed in path
3 (Figs. 3È7) originate from H atom abstraction by O giving
2
an HO radical. HO is also formed from degradation via
2
2
path 1. In the presence of NO , HO may form peroxynitric
acid (HO NO ). In all experiments, HO NO was observed
although the concentration varied signiÐcantly both between
di†erent reacting compounds and between di†erent experi-
ments. The reason for the variability in HO NO concentra-
2
2
2
2
2
2
2
2
2
tion is that it strongly depends on the rate of HO production,
total conversion of alcohol at the particular NO concentra-
studies of MBO232 reacting with OH, Cl, or NO indicated
2
tion, and temperature of the experiments. The HO eventually
2
3
the possibility of multifunctional compounds being produc-
disappears via its self-reaction or slow loss on the reactor
ed.11,14 However, there is so far, no direct evidence for the
walls, in addition to the initial possible loss by a reaction (6)
producing organic hydroperoxides.
formation of triply substituted compounds. The high NO
concentration experiments did not show a signiÐcant di†er-
2
ence in product distribution.
Allyl alcohol
2-Butene-1-ol
Allyl alcohol yielded 37(^1)% of HCHO, 41(^7)% of
2-NAA, and 53(^10)% of total initial nitrates including the
2-NAA and unidentiÐed organic nitrates, most of which disap-
peared with time. Path 1a is the only route to HCHO since no
glycolaldehyde was seen and all the 37% of HCHO has to be
formed this way. This is supported by the estimated yield of
41% of 2-NAA. The amount of alcohol reacting according to
path 1a is thus about 40%. It is suggested that the remaining
60% either produced hydroperoxide e.g. via reaction (6) or
triply substituted compounds via paths 3a or 3b in Fig. 3.
The main carbonyl products were 10(^3)% of formaldehyde,
12(^2)% of acetaldehyde, and 7(^2)% of 2-hydroxy-1-acetal-
dehyde (glycolaldehyde). The reaction scheme presented in
Fig. 6 thus demonstrates that paths 2a and/or 2b are active.
However, it is only possible to identify D20% of the reacted
alcohol as gas phase products. Logically, it follows that paths
3a and/or 3b could account for the remaining 80%. The total
organic nitrate observed in these experiments initially reached
a maximum of D40% and then it decayed to D30% at the
end of the experiments. The total organic nitrate at the end
stage is suspected to be 2-nitrooxy propanelaldehyde (via path
1a). This compound is equivalent to the 2-NAA formed in
experiments with allylalcohol, 3-butene-2-ol and MBO232 via
path 1a. This possible product, 2-nitrooxy propanelaldehyde,
absorbs at slightly lower wavenumbers compared to 2-NAA
(Fig. 9).
3-Butene-2-ol
Two initial NO concentrations were used for this compound
2
(cf. Table 1). For ““lowÏÏ NO concentration experiments, the
2
NO present originated only from the dissociation of N O .
The average acetaldehyde yield from both the ““highÏÏ and
2
2 5
““lowÏÏ NO experiment was 28(^5)% with no distinctive dif-
2
ference between the two initial concentrations of NO . The
The use of a mixture of cis and trans isomers as starting
material will not inÑuence the product distribution since the
2
only plausible way to form acetaldehyde is path 1a in Fig. 4,
2562
Phys. Chem. Chem. Phys., 2000, 2, 2555È2564

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Table 4 Number of adjacent carbon atoms attached to a, b and c
carbon atoms for the investigated compounds. The double bond is
located between the b and c carbon atoms
Number of neighbouring carbon at position
Compound
a
b
c
Allyl alcohol
3-Butene-2-ol
MBO232
Allyl alcohol
2-Butene-1-ol
MBO321
1
2
3
1
1
1
2
2
2
2
2
2
1
1
1
1
2
3
Fig. 9 Comparison of the reference spectrum of 2-NAA (a) with
organic nitrate absorption resulting from oxidation of the Ðve unsatu-
rated alcohols. All spectra are scaled, o†set and subtracted with
unsaturated alcohol, HCHO, acetone, HNO , N O , NO , H O and
Comparing the distribution between paths 1È3 within case a
(initial addition to c carbon atom) for the Ðrst series, there are
at least two observations that can be made. Firstly, there is
only a few percent of products via path 2. This is interpreted
as breakage of the aÈb bond in the alkoxy radical being more
favoured than breakage of the bÈc bond. The second obser-
vation is that the relative importance of path 1 increases with
the number of methyl groups in the a position. The change is
in accordance with suggestions from the literature stating that
increasing decomposition yield is related to an increasing sta-
bility of the produced fragments.32 The stability of the radical
3
2
5
2
2
HO NO . (b) Allyl alcohol, (c) 3-butene-2-ol, (d) MBO232, (e) 2-
2
2
butene-1-ol, (f) MBO321.
initial attack by the NO radical will break up the double
3
bond and subsequently give the same radical [cf. reaction (3)].
MBO321
The measurements with MBO321 were technically more diffi-
cult to make because this compound has a lower vapour pres-
leaving group C(CH ) H
increases as n increases from 0
3 n (2~n)
sure. For MBO321,
a yield of 21(^6)% acetone and
to 2 while the remaining fragment is identical in the series.
A similar type of analysis can be made for the second series,
allyl alcohol, 2-butene-1-ol, and MBO321. In this series, the
number of methyl group substituents increases at the c posi-
tion instead of the a position as was the case for the Ðrst
series. As the number of methyl group substituents increases,
29(^10)% glycolaldehyde indicate that paths 2a and/or 2b in
Fig. 7 are the products forming pathways. 11(^3)% of formal-
dehyde shows that path 1a is also active. About 64% of the
alcohol reacted is thus unaccounted for. One remaining route
is path 3a that would form 3-nitrooxy-3-methyl-2-butanon-1-
ol. Since the tri-substituted compound would most likely be
lost from the gas phase, most of the total alkyl nitrate at the
end of the experiments should be 2-nitroxy-2-methyl-
propenelaldehyde, which is produced from path 1a in Fig. 7.
Oxidation of MBO321 gave the clearest case of aerosol for-
mation that was observed in this investigation (Fig. 8). It is
not immediately obvious why the triply substituted products
from MBO321 should have properties favouring aerosol for-
mation when similar products from some of the other alcohols
do not form any visible aerosol. The answer is likely to be
it is more probable for the NO attack to take place at the b
3
carbon atom instead of the c carbon atom (Table 4). This
causes path 2b to be more active which increases the yield of
products in path 2. However, the greater number of methyl
group substituents at the c carbon atom increases the stability
of the radical leaving groups produced via path 2a. Analogous
to the radical produced via path 1a it can be represented as
C(CH ) H
and its stability increases as the value of n
3 n (2~n)
increases. The absence of a clear trend in the second series
may be explained by a combination of the e†ects discussed
above.
found in the fact that MBO321 reacts very fast with NO .
3
Condensing products formed at a high rate may not have suf-
Ðcient time to reach the walls, but will cause high supersatu-
ration in the gas phase followed by homogeneous nucleation.
The apparent yield of total organic nitrates has a strong
dependence on time. From the experiments, one can derive the
total organic nitrates as two di†erent yields, one at short reac-
tion time and one at long reaction time, the latter only corre-
sponding to nitrates with high enough vapour pressure. The
values obtained for the initial and Ðnal total organic nitrates
yield were 60(^39)% and 29(^6)%.
Conclusions
This study showed that closed mass balances for the investi-
gated unsaturated alcohols were impossible to achieve by just
considering the products remaining in the gas phase. Strong
evidence exists that products are lost from the gas phase. This
is shown by the decrease of IR nitrate absorption with time to
approach a value determined by the equilibrium between gas
phase and the reactor wall. The change in nitrate peak size
and shape with time is an indication of a selective loss of
organic nitrates of low volatility. The formation of aerosol
General trends
According to previous studies28 the most likely primary target
was positively observed in the reaction between NO and
MBO321. It is clear that the products lost must have a low
3
for the NO attack on a double bond is the carbon atom that
3
is less alkyl substituted. Table 4 indicates the number of
vapour pressure. A plausible channel producing nitrated com-
pounds with a molecular structure that is expected to give a
low vapour pressure is that proceeding via hydrogen
abstraction by molecular oxygen, i.e. path 3 above. Based on
the arguments given above, it is suggested that the hydrogen
abstraction from the oxy radical is an important pathway for
all compounds except MBO232. In the case of MBO232, the
bond cleavage between the a and b carbon atoms is the most
important process as was shown by the [60% yield of
acetone and 2-NAA. As expected, this bond breakage was
more pronounced when the number of methyl substituents
was increased from zero to two at the a carbon atom because
the stability of the leaving group also increases.32
carbon atoms adjacent to the a, b and c carbon atom for the
investigated compounds. The atom with the OH group is
assigned to be the a carbon. For the Ðrst series, allyl alcohol,
3-butene-2-ol, and MBO232, the c carbon atoms only have 1
adjacent carbon atom compared to 2 for the b carbon atom.
NO is here expected to preferentially add to the c carbon
3
atoms. The number of carbon atoms adjacent to the a-carbon
atom plays a minor role for the initial attack of the NO
radical. From the experimental data, no evidence for addition
3
to the b carbon atom was seen, i.e. case b in Figs. 3È5 and all
identiÐed products are explained by degradation following
addition to the c carbon atom, case a in Figs. 3È5.
Phys. Chem. Chem. Phys., 2000, 2, 2555È2564
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Pierce, B. Scholes, R. Steinbrecher, R. Tallamraju, J. Taylor and
In the series with increasing number of methyl substituents
at the c carbon atom, the tendency for bond breakage
between the b and c carbon atom was increased. It is pro-
P. Zimmerman, J. Geophys. Res., 1995, 100, 8873.
R. Atkinson, J. Phys. Chem. Ref. Data, 1997, 26, 215.
P. D. Goldan, C. Kuster and F. C. Fehsenfeld, Geophys. Res.
L ett., 1993, 20, 1039.
P. Harley, V. Fridd-Stroud, J. Greenberg, A. Guenther and P.
Vasconcellos, J. Geophys. Res., 1998, 103, 25479.
A. Guenther, P. Zimmerman, L. Klinger, J. Greenberg, C. Ennis,
K. Davis, W. Pollock, H. Westberg, G. Allwine and C. Geron, J.
Geophys. Res., 1996, 101, 1345.
2
3
posed that this is an e†ect of NO attack occurring more fre-
3
quently at the b carbon atom with increasing substitution at
4
5
the c carbon atom. This accordingly leads to an increased
Ñow through path 2b with an increased number of c carbon
atom methyl substituents. The e†ect of increased stability of
the leaving radical with additional methyl groups may also
contribute to the increased bond breakage through path 2a.
6
7
G. Konig, M. Brunda, H. Puxbaum, C. N. Hewitt, S. C.
Duckham and J. Rudolph, Atmos. Environ., 1995, 29, 861.
C. E. Canosa-Mas, P. S. Monks and R. P. Wayne, J. Chem. Soc.,
Faraday T rans., 1992, 88, 11.
Atmospheric implications
8
9
R. Atkinson, J. Phys. Chem. Ref. Data, 1991, 20, 459.
R. Atkinson and J. Arey, Acc. Chem. Res., 1998, 31, 574.
This study is concerned with the oxidation mechanisms of
unsaturated alcohols, which are emitted to the atmosphere
from biogenic sources. The oxidation produces aldehydes,
ketones, and organic nitrates as major products. One of the
strongly emitted compounds, MBO232, showed a large yield
of 2-NAA and acetone. Since acetone has a signiÐcant role in
the upper troposphere as a precursor for peroxyacetylnitrate
(PAN), such a high yield should be regarded as an important
fact to consider for upper troposphere and lower stratosphere
10 G. Fantechi, N. R. Jensen, J. Hjorth and J. Peeters, Int. J. Chem.
Kinet., 1998, 30, 589.
11 G. Fantechi, N. R. Jensen, J. Hjorth and J. Peeters, Atmos.
Environ., 1998, 32, 3547.
12 G. Fantechi, N. R. Jensen, O. Saastad, J. Hjorth and J. Peeters, J.
Atmos. Chem., 1998, 31, 247.
13 A. Alvarado, E. C. Tuazon, S. M. Aschmann, J. Arey and R.
Atkinson, Atmos. Environ., 1999, 33, 2893.
14 C. Ferronato, J. J. Orlando and G. S. Tyndall, J. Geophys. Res.,
1996, 103, 25579.
chemistry. Production of nitrates converts NO into a form
15 R. K. Talukdar, S. C. Herndon, J. B. Burkholder, J. M. Roberts
and A. R. Ravishankara, J. Chem. Soc., Faraday T rans., 1997, 93,
2787.
x
that could be subjected to long range transport before being
released via photolysis. NO can be formed either directly via
2
16 K. R. Talukdar, J. B. Burkholder, M. Hunter, M. K. Gilles, J. M.
Roberts and A. R. Ravishankara, J. Chem. Soc., Faraday T rans.,
1997, 93, 2797.
17 H. B. Singh, D. OÏHara, D. Herlth, W. Sachse, D. R. Blake, J. D.
Bradshaw, M. Kanakidou and P. J. Crutzen, J. Geophys. Res.,
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18 H. B. Singh, M. Kanakidou, P. J. Crutzen and D. J. Jacob,
Nature, 1995, 378, 50.
19 F. Arnold, J. Schneider, K. Gollinger, H. Schlager, P. Schulte,
D. E. Hagen, P. D. WhiteÐeld and P. van Velthoven, Geophys.
Res. L ett., 1997, 24, 57.
20 F. Arnold, V. Burger, B. Droste-Franke, F. Grimm, A. Krieger, J.
Schneider and T. Stilp, Geophys. Res. L ett., 1997, 24, 3017.
21 M. Hallquist, PhD Thesis, Gothenburg University, Gothenburg,
1998.
path 2 or by oxidation of NO as result of the production of
HO from path 1. For all the investigated compounds except
2
MBO232, path 1 was found dominant over path 2. Notable is
also that the two degradation paths for the hydroxynitrooxy
substituted alkoxy radical, e.g. from oxidation of allyl alcohol,
give back di†erent odd electron species as illustrated by reac-
tion (8a) and (8b).
CH OHÈCHO~ÈCH ONO ]
2
2
2
HOÈCH ONO ] HCHO ] HO (8a)
2
2
2
CH OHÈCHO~ÈCH ONO ]
2
2
2
CH OHÈCHO ] HCHO ] NO (8b)
22 H. Bandow, M. Okuda and H. Akimoto, J. Phys. Chem., 1980,
2
2
84, 3604.
The implications for the atmosphere will then be di†erent.
In the atmosphere, HO could convert NO to NO giving
OH, which maintains the oxidative degradation cycle of
23 M. Hallquist, I. Wangberg, E. Ljungstrom, I. Barnes and K. H.
Becker, Environ. Sci. T echnol., 1999, 33, 553.
2
2
24 I. Wangberg, I. Barnes and K. H. Becker, Environ. Sci. T echnol.,
1997, 31, 2130.
organic compounds. The NO can be formed either directly in
2
25 S. Langer and E. Ljungstrom, J. Chem. Soc., Faraday T rans.,
1995, 91, 405.
path 2 or via oxidation of NO as a result of the production of
HO in path 1. For most of the investigated compounds path
2
26 M. Hallquist, I. Wangberg and E. Ljungstrom, Environ. Sci.
T echnol., 1997, 31, 3166.
1 is dominant over path 2.
27 J. Kames, U. Schurath, F. Flocke and A. Volz-Thomas, J. Atmos.
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Acknowledgements
28 R. P. Wayne, I. Barnes, P. Biggs, J. P. Burrows, C. E. Canosa-
Mas, J. Hjorth, G. Le Bras, G. K. Moortgat, D. Perner, G.
Poulet, G. Restelli and H. Sidebottom, in T he Nitrate Radical:
Physics, Chemistry and the Atmosphere, Air Pollution Research
Report 31, CEC Brussels, 1990.
Dr N. R. Jensen (Environmental institute JRC, Ispra, Italy)
and Dr K. Wirtz (Centro de Estudios Ambientales del Medi-
terraneo, Valencia, Spain) are greatly acknowledged for pro-
viding us with the reference spectra of glycolaldehyde.
We would like to thank Dr Lars Cedheim, SP Swedish
National Testing and Research Institute, Chemistry and
Material Technology, Boras, Sweden, for valuable discussions.
The Foundation for Strategic Environmental ResearchÈ
MISTRA, the Swedish Natural Science Research Council and
the EC-RADICAL project No. ENV4-CT97-O419 are grate-
fully acknowledged for Ðnancial support of this work.
29 P. D. Lightfoot, R. A. Cox, J. N. Crowley, M. Destriau, G. D.
Hayman, M. E. Jenkin, G. K. Moortgat and F. Zabel, Atmos.
Environ., Part A, 1992, 26, 1805.
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32 R. Atkinson, Int. J. Chem. Kinet., 1997, 29, 99.
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34 T. Berndt and O. Boge, J. Atmos. Chem., 1995, 21, 275.
35 Handbook of Chemistry and Physics, ed. D. R. Lide, CRC Press,
Inc., Boca Raton, 73rd edn., 1991.
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Phys. Chem. Chem. Phys., 2000, 2, 2555È2564