ARTICLE IN PRESS
Atmospheric Environment ] (]]]]) ]]]–]]]
www.elsevier.com/locate/atmosenv
Formation of secondary organic particle phase compounds
from isoprene gas-phase oxidation products: An aerosol
chamber and field study
Olaf Böge, Yunkun Miao, Antje Plewka, Hartmut Herrmann
Leibniz-Institut für Troposphärenforschung (IfT), Permoserstrasse 15, D-04318 Leipzig, Germany
Received 6 September 2005; received in revised form 6 December 2005; accepted 15 December 2005
Abstract
Acid-catalyzed multiphase reactions of isoprene with hydrogen peroxide have been suggested to produce hydroxylated
oxidation products, namely the diastereoisomeric 2-methyltetrols (2-methylthreitol and 2-methylerythritol), compounds
which were found quite recently in aerosols above forests. In this study aerosol chamber experiments have been performed
reacting isoprene or known isoprene oxidation products with hydrogen peroxide in the presence of acidic particles. Starting
from isoprene, 2-methyl-3-butene-1,2-diol or 2-methyl-2-vinyloxirane the formation of the 2-methyltetrols could be
observed. The reaction was most effective for 2-methyl-2-vinyloxirane. The last process can lead to an annual
2-methyltetrol formation in atmospheric particles of about 1 Tg C. Additionally, results from size segregated 2-methyltetrol
measurements in Melpitz site (Germany) are presented. The concentrations are between 0.5 and 1.7 ng m3. The
2-methyltetrols are enriched in the fine size fraction as expected for secondary organic aerosol compounds.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Secondary organic aerosol; Isoprene; Acidic seed particles; Chamber experiments; 2-Methylthreitol; 2-Methylerythritol
1. Introduction
Organic matter frequently makes up the largest
aerosol fraction over continental regions, especially
in remote areas (Andreae and Crutzen, 1997). The
contribution of secondary organic carbon (SOC) to
the total organic aerosol concentration remains a
controversial issue. The SOC content in tropospheric particles is highly variable and its composition and formation mechanisms are not well
Corresponding author. Tel.: +49 341 235 2446;
fax: +49 341 235 2325.
E-mail address: [email protected] (H. Herrmann).
1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2005.12.025
understood (Pandis et al., 1992; Turpin et al.,
2000). Because of the huge influence of particulate
organic compounds on thermodynamic, microphysical and chemical properties (e.g. Turpin et al.,
2000) a quantitative description of the impact of
organic compounds to aerosol formation and
modification is needed.
Biogenic organic compounds are emitted to the
atmosphere in a substantial amount mainly from
terrestrial vegetation. Total global biogenic organic
emissions are estimated to range from 491 to
1150 Tg year1 exceeding the estimated anthropogenic emissions by as much as an order of
magnitude (Müller, 1992; Guenther et al., 1995).
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After methane and together with the terpenes,
isoprene is the organic compound with the highest
global emission.
It has been well established that terpene oxidation
processes can lead to aerosol formation (Griffin
et al., 1999; Andreae and Crutzen, 1997). However,
on the basis of results from smog chamber experiments (Pandis et al., 1991) it has commonly been
assumed that the photooxidation of isoprene does
not contribute to the production of secondary
organic aerosol (SOA) under ambient conditions
(Seinfeld and Pandis, 1998).
Recently experiments with strongly acidic seed
particles showed that some of the highly volatile
isoprene oxidation products (usually expected to stay
in the gas phase) can lead to formation of SOA (Jang
et al., 2002; Czoschke et al., 2003). In addition the
formation of polymers can occur through heterogeneous reaction of isoprene on an acid impregnated
filter (Limbeck et al., 2003), but it is very difficult to
extrapolate the importance of this filter experiments
to airborne particles in the real atmosphere.
Recently, two diastereoisomeric 2-methyltetrols
(2-methylthreitol and 2-methylerythritol) have been
identified in the natural aerosol from the Amazonian rain forest (Claeys et al., 2004a). The appearance of these compounds was explained by OH
radical-initiated photooxidation of isoprene. The
presence of 2-methyltetrols in natural forest aerosols collected at K-puszta, Hungary (Claeys et al.,
2004b; Ion et al., 2005), has also been recently
reported. The 2-methyltetrols are suggested to form
through acid-catalyzed liquid-phase oxidation of
isoprene by hydrogen peroxide which was demonstrated in a laboratory experiment by mixing water,
sulphuric acid, hydrogen peroxide and isoprene
(Claeys et al., 2004b). 2-Methyltetrols have also been
observed in biomass burning smoke (Schkolnik
et al., 2005) and above a coniferous forest in
Hyytiälä, Finland (Kourtchev et al., 2005). Very
recently, smog chamber experiments examined the
SOA formation from isoprene photooxidation in the
presence and absence of SO2 (Edney et al., 2005).
These experiments indicate that atmospheric photooxidation of isoprene in presence of SO2 could lead
to SOA which consists the isomeric 2-methyltetrols
and methylglyceric acid beside a large amount of not
specified compounds. However, information concerning the mechanisms leading to the 2-methyltetrols was not presented.
Here we show that known isoprene oxidation products, namely, 2-methyl-3-butene-1,2-diol (Ruppert
and Becker, 2000; Jenkin et al., 1998) and 2-methyl2-vinyloxirane (Aschmann and Atkinson, 1994;
Atkinson et al., 1994; Berndt and Böge, 1997), can
contribute to the formation of 2-methyltetrols
through heterogeneous reactions in an aerosol
chamber. Additionally, field measurements of 2methylthreitol and 2-methylerythritol from Germany are presented and compared to measurements
presented in the literature.
2. Experimental
The detailed description of the aerosol chamber
has been reported elsewhere (Iinuma et al., 2004)
and only the essential features are described here.
The chamber has a volume of 9 m3 and a surface/
volume ratio of 3 m1. The chamber was equipped
with a humidifier, a particle generator and probes
for temperature and relative humidity. An injection
port allowed to add gaseous compounds or particles
to the air stream and to carry them along into the
chamber. Polydisperse aerosol was generated by
aspirating an aqueous solution of 0.03 M
(NH4)2SO4 and 0.05 M H2SO4. Outlets are connected with a condensation particle counter (CPC)
for measuring the initial particle number in the
chamber, a particle sampling filter device with a
fixed integrated annular denuder to avoid gaseous
contamination of the deposited particles during
sampling. The inside walls of the denuder are coated
with Apiezon L (Fluka, Germany). The particle size
distributions in the reaction chamber were measured
by a differential mobility particle sizer (DMPS).
Before starting a run the chamber was flushed for
at least 18 h with purified air. Subsequently, the
humidifier was connected to the air stream. At
r.h.445% the particle generator was activated next
to introduce seed particles until the number
concentration reached approximately 15,000 cm3.
The chamber was closed after the introduction of
particles for the experiment. After closing the
chamber first H2O2 was introduced by means of a
syringe within 1 min by an air stream of about
300 l to obtain a chamber concentration of
about 1.5 ppm. After this the organic reactants were
introduced in the same manner. Isoprene and
2-methyl-2-vinyloxirane were introduced neat and
2-methyl-3-butene-1,2-diol and H2O2 as an aqueous
solution. Additional experiments with an isoprene
mixing ratio of 5 ppm have also been performed.
After the reaction was run for 150 min filter samples
were taken on 47 mm quartz filters by an air stream
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of 30 l min1 (optimal efficiency for the integrated
denuder) up to 2 m3. The air stream passed through
the filter was returned to the chamber to avoid large
volume losses. The obtained quartz filter samples
were analysed for the 2-methyltetrols. Blank filter
samples were produced by the above described
procedure without the injection of H2O2 and the
organic 2-methyltetrol precursors.
At the end of sampling the chamber was flushed
again with particle free clean air until the next
experiment. After each set of experiments the
chamber was rinsed out with an aqueous 0.015 M
Na2CO3 solution and afterwards two times with
water.
The field measuring site was the Leibnitz-Institut
für Troposphärenforschung (IfT) research station
near the small village Melpitz (121560 E, 511320 N,
87 m a.s.l.), which is located 45 km in the northeast
of Leipzig (Germany). It is surrounded by mostly
agriculturally used land. During the most dominant
local SW wind direction (about 80% of the time),
the plume of Leipzig can influence the station. A
more detailed description is given by Spindler et al.
(2004).
Particles were collected in summer and autumn
2004 on quartz fibre filters with three high volume
samplers (Digitel DHA-80) with different inlets.
One sampler was equipped with an inlet for particles
o1 mm, the second with an inlet o2.5 mm and the
third had an inlet for particles o10 mm. The
sampling time was 24 h. Samples were collected
over 24 h from midnight to midnight on five days.
The flow rate was 500 l min1. The loaded filters
were wrapped with aluminium foil and stored
frozen until analysis.
The quartz fibre filters were Soxhlet extracted
with CH2Cl2/MeOH (4:1) for 22 h. The Soxhlet
tubes were extracted with CH2Cl2 for 24 h before
use in order to obtain low blank values.
The sample extract was then concentrated to a
volume of about 600 ml, measured with a syringe,
and then separated into three fractions of about
200 ml each. The first fraction was used for the
determination of the nonpolar compounds. The
polar compounds, like dicarboxylic acids and
terpenoic acids were methylated with BF3-MeOH
and were measured in the second fraction. The third
fraction for the determination of 2-methyltetrols
was silylated with BSTFA (N,O-bis(trimethylsilyl)trifluoracetamide) before measuring. The individual
fractions were spiked with 1-chlorhexadecane as
internal standard for quantification. Response
3
factors of this internal standard and the organic
compounds were determined before and included in
the calculation of concentrations. A more detailed
description of this method is given elsewhere
(Plewka et al., 2006).
The reference compounds for the isoprene oxidations products 2-methylthreitol and 2-methylerythritol were synthesized from 2-methyl-2vinyloxirane according to Claeys et al. (2004a).
For the quantification of the 2-methyltetrols the
response factor of erythritol was used, as described
in Plewka et al. (2006).
The recovery of the extraction and derivatization
procedure was determined by spiking 2 mg mesoerythritol on a half 150 mm diameter quartz filter
(Munktell, Sweden). The recoveries were determined twice: 69% and 72%. The coefficient of
correlation for meso-erythritol was 0.9986 for the
calibration curve.
3. Results
Four sets of aerosol chamber experiments were
carried out. The formation of 2-methylthreitol and
2-methylerythritol as result of the reaction of
isoprene, 2-methyl-3-butene-1,2-diol or 2-methyl-2vinyloxirane with H2O2 in the presence of acidic
particles was examined. For each set of experiments
three runs were performed. Table 1 shows the initial
mixing ratios of the organic tetrol precursors and
the H2O2 mixing ratio for the four sets of
experiments. Parameters influencing the formation
of the tetrols, such as r.h., temperature, aerosol
particle number concentration and particle diameter
were held nearly constant in all experimental runs.
The tetrols formed were analysed as trimethylsilylated derivatives by GC-MS. The identification of
Table 1
Initial concentrations in the aerosol chamber and 2-methyltetrol
amounts measured in the particle phase
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4
the tetrols was achieved by their mass spectra and
retention times of the reference mixture cf. Figs. 1
and 2. The assignment of diastereomeric (threo and
erythro) forms was made according to Claeys et al.
(2004a). Fig. 1 in the upper part shows a typical EI
(m/z 219) chromatogram from an experimental run
with 2-methyl-2-vinyloxirane and H2O2. In the
lower part of Fig. 1 the chromatogram of the
reference compounds mixture is shown. Fig. 2
shows the background subtracted mass spectra of
the peaks with retention time 12.27 min (2-methylthreitol) from the chromatograms in Fig. 1. A
detailed discussion of the fragmentation of trimethylsilylated 2-methyltetrols is given by Wang
et al. (2004).
Table 1 summarizes the obtained particulate
tetrol concentrations (sum of the two isomers,
95% confidence interval). In the first run with
1.5 ppm initial isoprene mixing ratio no significant
tetrol formation was observed. With increased
isoprene mixing ratio the formation of small
amounts of 2-methylthreitol and 2-methylerythritol
could be observed. The aerosol phase reaction of
2-methyl-3-butene-1,2-diol or 2-methyl-2-vinyloxirane with H2O2 yielded noticeable higher particle
phase concentrations of the tetrols. It should be
noted that wall effects are not taken into account
for all experiments which might lead to an underestimation of the product yields, especially for the
2-methyl-3-butene-1,2-diol experiments. Wall losses
for this compound are not negligible as measured by
Ruppert and Becker (2000).
Several mechanisms for the oxidation of isoprene
have been suggested to explain the occurrence of 2methylthreitol and 2-methylerythritol in atmospheric particles, cf. Fig. 3. The first mechanism
15000
Abundance
2-methylthreitol
10000
2-methylerythritol
5000
0
12.20
12.60
13.00
Time [min]
13.40
13.80
Abundance
30000
Reference mixture
20000
10000
0
12.20
12.60
13.00
Time [min]
13.40
13.80
Fig. 1. Typical EI (m/z 219) chromatograms from experimental run with 2-methyl-2-vinyloxirane and H2O2 (upper part) and reference
compounds mixture (lower part).
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5
219
15000
73
Abundance
10000
117
147
5000
189
321
243 277
0
50
100
150
200
250
300
350
400
450
350
400
450
m/z
219
73
Abundance
30000
117
20000
147
10000
189
277
321
0
50
100
150
200
250
300
m/z
Fig. 2. Background subtracted mass spectra of the peak with retention time 12.27 min (2-methylthreitol) and from reference compound
(lower part).
proposed the formation of the tetrols in a gas-phase
reaction via reaction (1) followed by reaction (2) cf.
Fig. 2 in Claeys et al., 2004a. The OH-radical
initiated reaction of isoprene leads to the rapid
formation of hydroxyl peroxy radicals. There self
reaction or cross reaction with other peroxy radicals
can lead to 2- and 3-methyl-3-butene-1,2-diol as well
as 2-methyl-2-butene-1,4-diol. The peroxy radicals
can also react with HO2 via reaction (7) producing
almost exclusively organic hydroperoxides and with
NO via reactions (8a,b) forming alkoxy radicals or
nitrates (Lightfoot et al., 1992).
RO2 þ HO2 ! RO2 H þ O2
RO2 þ NO ! RO þ NO2
! RONO2
(7)
(8a)
(8b)
The permutation reactions of peroxy radicals
formed in the OH radical initiated oxidation of
isoprene with peroxy radicals leading to the diols
(2- and 3-methyl-3-butene-1,2-diol, 2-methyl-2-butene-1,4-diol) can only be important in remote
forested regions where ‘low NOx’ conditions prevail, and particularly in the tropics where the
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6
Gas Phase
Particle Phase
phase transfer
HO
OH
OH
R2
OH
1. OH/O2
2. RO2
phase
transfer
+
HO
OH
OH
H2O2/H+
R5
OH
HO
R1 1. OH/O
2
2. RO2
OH
phase
transfer
OH
OH
H2O2/H+
R4
O3 or
R3 NO3
phase
transfer
+
O
O
H2O2/H+
R6
Fig. 3. Proposed reaction pathways for the atmospheric formation of 2-methyltetrols from isoprene.
absolute radical concentrations are also elevated
(Jenkin et al., 1998). In a chamber experiment under
NOx free conditions the yields of 2- and 3-methyl-3butene-1,2-diol were measured to be 4.7% and
2.4%, respectively (Ruppert and Becker, 2000). The
diol yields as sum of all isomers were calculated to
7.2% using a model with an explicit treatment of
peroxy radical reactions (Jenkin et al., 1998). With
increasing NO concentration the diol yields decreases dramatically. For the ‘low NOx’ case with
NO concentrations of 5 109 molecule cm1 and
1.25 109 molecule cm1 a diol yield of just about
1–3% was estimated by Ruppert and Becker (2000).
Taking this value also for the second reaction (2) an
overall yield of just 0.01–0.09% can be estimated for
the tetrol formation in according to reactions (1)
and (2).
Results from field experiments have led to a
reconsideration of the processes by which 2methylthreitol and 2-methylerythritol are formed
from isoprene. The new results showed higher
concentrations of the 2-methyltetrols under high
NOx concentrations during the biomass burning
season compared to those under ‘low NOx’ conditions during the wet season (Claeys et al., 2004b).
The formation of 2-methylthreitol and 2-methylerythritol in atmospheric particles has been explained
by Claeys et al. (2004b) through the acid-catalyzed
reaction of isoprene with hydrogen peroxide
cf. reaction (4) in Fig. 3. While Claeys et al.
(2004b) showed their proposed oxidation mechanism when reacting isoprene with hydrogen peroxide
in an acidic aqueous solution, they did not show
that the reaction proceeds when isoprene and
hydrogen peroxide are reacted in the gas phase in
the presence of seed particles. In the present study
an aerosol chamber experiment was performed with
acidic ((NH4)2SO4/H2SO4) particles and the identification of 2-methylthreitol and 2-methylerythritol
in the aerosol generated in these experiments
indicates that the reaction pathway suggested by
Claeys et al. (2004b) may be relevant to atmospheric
conditions. In the first run with 1.5 ppm initial
isoprene mixing ratio no significant tetrol formation
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was observed. The low amounts of 2-methylthreitol
and 2-methylerythritol in the two sets of experiments starting with isoprene cf. Table 1 are
probably caused by the low isoprene concentration
in the particle phase. The Henry coefficient of
isoprene is only 0.029 M atm1 (Lindinger et al.,
1998). From the aerosol chamber experiments it is
difficult to estimate the importance of the reaction
for the real atmosphere.
A third reaction pathway starting from 2- and
3-methyl-3-butene-1,2-diol cf. reaction (5) is proposed and was tested in the aerosol chamber. The
yields of the 2-methyltetrols with respect to the
reacted educts are higher as in previous experiments, see Table 1. Additionally, the yields are
possibly lowered by distinct wall losses of the
starting material (Ruppert and Becker, 2000).
Taking the formation yield of the diols of 1–3%
for reaction (2) in the ‘low NOx’ case, see above, this
pathway can be an additional possibility for the
occurrence of 2-methyltetrols in atmospheric particles, especially in remote areas.
The last reaction pathway leading to the formation of 2-methylthreitol and 2-methylerythritol
starting from isoprene monoxides was studied in
the aerosol chamber, i.e. reaction (6). The isomeric
isoprene monoxides 2-methyl-2-vinyloxirane and
2(1-methyl-vinyl)oxirane are known reaction products of the ozone isoprene reaction (Aschmann
and Atkinson, 1994; Atkinson et al., 1994) as well as
the nitrate radical initiated oxidation of isoprene
(Berndt and Böge, 1997). Reaction (6) gave the
highest 2-methyltetrol yields in the aerosol chamber
studies presented here.
Additionally to the aerosol chamber experiments
the 2-methyltetrols were measured in atmospheric
particles collected in Melpitz, Germany. The results
of the these measurements are shown in Table 2.
The concentrations of the 2-methyltetrols are
presented as the sum of the two diastereomeric
forms. The concentrations are always below
2 ng m3. The concentrations are significantly below
those obtained for samples collected in Brazil,
Hungary and Finland as cited earlier. The concentrations of the 2-methyltetrols in these measurements are in the range of 50 ng m3 in the Brazil
samples (Claeys et al., 2004a) and about 30 ng m3
in Hungary (Ion et al., 2005) and Finland
(Kourtchev et al., 2005).
Measurements in a Norway spruce forest (Fichtelgebirge, Germany) in summer 2002 showed
concentrations between 1 and 15 ng m3 (Plewka
7
Table 2
Concentration 2-methyltetrols (threo+erythro) in atmospheric
particles from Melpitz site
Sampling date
PM1
(ng m3)
PM2.5
(ng m3)
PM10
(ng m3)
19 Jun 2004
25 Jun 2004
27 Aug 2004
2 Sep 2004
8 Sep 2004
26 Oct 2004
1.270.4
1.170.3
1.270.4
1.470.4
1.170.3
0.570.2
1.670.5
1.270.4
1.770.5
1.370.4
1.470.4
0.570.2
1.170.3
1.070.3
1.770.5
1.570.5
1.270.4
0.870.2
et al., 2006). During the FEBUKO cloud experiment in October/November 2001 at the Schmücke
mountain in the Thüringer Wald (Germany) the 2methyltetrols could be also detected in very low
concentrations (A. Plewka, unpublished results).
4. Atmospheric implications and conclusions
In this study, the formation of 2-methylthreitol
and 2-methylerythritol as particulate products of
the atmospheric oxidation of isoprene was examined in an aerosol chamber. It was shown that the
reaction of known isoprene oxidation products as
well as isoprene itself with hydrogen peroxide on or
in acidic particles can indeed result in the formation
of 2-methyltetrols. The reaction of isoprene itself
with hydrogen peroxide cf. reaction (2) yields only
low amounts of 2-methyltetrols and from the
aerosol chamber experiments presented here it is
difficult to estimate the importance of the reaction
for the real atmosphere. The reaction of 2- and 3methyl-3-butene-1,2-diol cf. reaction (5) was shown
to produce 2-methyltetrols in higher yields, but
these diols are only produced under ‘low NOx’
conditions, i.e. this pathway can only in remote
areas serve as 2-methyltetrols source.
In order to assess the importance of isoprene
monoxides to the formation of 2-methylthreitol and
2-methylerythritol cf. reaction (6) in the real atmosphere the following assumptions have been made.
Taking the rate constants for the reactions of
isoprene with ozone (1.28 1017 cm3 molecule1 s1) and OH with isoprene (1.02 1010 cm3 molecule1 s1) recommended by Calvert et al. (2000), assuming a global average of
50 ppbv for ozone and 2.6 106 molecules cm3 for
OH (Griffin et al., 1999) and neglecting the nitrate
radical reaction it can be calculated that 5.6% of the
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isoprene in the atmosphere are oxidized by ozone on
a global scale. Together with the experimentally
obtained yields (Aschmann and Atkinson, 1994;
Atkinson et al., 1994) of 2-methyl-2-vinyloxirane
and 2(1-methyl-vinyl)oxirane of 2.8% and 1.1%,
respectively, a yield of about 0.22% can be
calculated for the formation of the oxiranes based
on the total atmospheric isoprene conversion. An
annual global emission of isoprene of approximately 500 Tg C (Guenther et al., 1995) indicates an
upper limit for 2-methyltetrols in atmospheric
aerosols formed via reaction (3) followed by
reaction (6) of about 1 Tg C, neglecting all other
sinks for the isoprene monoxides.
In the polluted atmosphere O3 contributes to
about 10% for isoprene conversion, and the
isoprene reaction with NO3 contributes another
10% (Calvert et al., 2000). Therefore, the relative
amount of 2-methylthreitol and 2-methylerythritol
could increase compared to the global average. This
estimation is in line with the observed higher
concentrations of the 2-methyltetrols in forest
aerosols from Amazonia under high NOx conditions during the biomass burning period compared
to those under low NOx conditions during the wet
season (Claeys et al., 2004b). Compared to the total
estimated SOA formation per year from biogenic
sources of 8–40 Tg (Penner et al., 2001), a source
strength of 2-methyltetrols formed by reactions (3)
and (6) should not be neglected.
Additionally to the aerosol chamber experiments
the 2-methyltetrols were measured in atmospheric
particles collected in Melpitz, Germany. As in
previous studies (Claeys et al., 2004a; Schkolnik et
al., 2005), the 2-methyltetrols are enriched in the
fine size aerosol fraction cf. Table 1, as expected for
SOA compounds. The concentrations measured at
Melpitz (Germany) are substantial lower than in
Hungary (Ion et al., 2005), Finland (Kourtchev et
al., 2005) and particularly in Brazil (Claeys et al.,
2004a). However, the isoprene mixing ratio in the
Amazonian rain forest was between 4 and 10 ppb
e.g. (Claeys et al., 2004a,b) whereas those in Melpitz
was usually between 10 and 200 ppt (Gnauk
and Rolle, 1998). Field measurements of 2-methylthreitol and 2-methylerythritol in Melpitz, a region
with low isoprene concentrations, as well as on
different locations in the US (Edney et al., 2005)
in Finland, Hungary and Brazil as cited above
give reason to conclude that this particulate
compounds are ubiquitous products of the isoprene
oxidation.
However, additional mechanistic and kinetic
studies are required to estimate the contribution of
the studied processes to SOA and especially to the
formation of 2-methyltetrols in the real atmosphere
and to clarify if there exist additional sources for 2methyltetrols as assumed by Edney et al. (2005) and
Kourtchev et al. (2005).
Acknowledgements
The authors thank E. Neumann and A. Kappe
for laboratory assistance. This work has been
supported by Deutsche Forschungsgemeinschaft
(DFG) under HE-3086/4-1.
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