Introduction
Recent years have seen a growing interest in the atmospheric mechanisms
leading to the formation of secondary organic aerosols, SOAs (Chan et al.,
2010). In this sense, ozonolysis of terpenes has been reported as a
potential source of new particles under natural conditions (Saathoff et al.,
2009; Sipilä et al., 2014; Newland et al., 2015a). The oxidation of
terpenes leads to the formation of extremely low volatility organic
compounds that can enhance or even dominate the formation and growth of
organic aerosol particles in forested areas. Dimers of these species are
considered large enough to act as nano-condensation nuclei (Ehn et al.,
2014). Likewise, the fast reactions of other alkenes with ozone can
contribute to the production of non-volatile species that could condense and
contribute to the total mass of pre-existing particles or even induce
nucleation events in both rural and urban atmospheres.
As is known (Johnson and Marston, 2008), ozone adds to double bonds,
producing an energy-rich primary ozonide which promptly decomposes, giving off a
carbonyl molecule and an excited carbonyl oxide reactive intermediate
(Criegee intermediate, CI). For cyclic alkenes, the ozonolysis just opens
the cycle producing larger CIs than the equivalent linear alkenes.
Furthermore, the carbonyl functional group is included in the CI molecule.
Thus, products with higher molecular weight, lower volatility and higher
potential capacity to contribute to SOA formation are expected for chemicals
with endocyclic double bonds. The excited CI can be stabilised
by collisions with gas molecules or undergo fragmentation or unimolecular
rearrangement (Anglada et al., 2011). Then, the stabilised CI (sCI) can
react with other molecules in the gas phase such as H2O, HOx,
aldehydes, organic acids, RO2 or NOx (Vereecken et al., 2012).
The reaction with water molecules is expected to be one of the main fates of
sCI, producing α-hydroxi-hydroperoxides (Ryzhkov and Ariya, 2004;
Anglada et al., 2011).
On the other hand, experimental results show that SO2–sCI reactions are
a potential source of SOA. Very recently, it has been found that a
significant fraction of ground level sulfuric acid originates from the
oxidation of sulfur dioxide by sCI to SO3 (Mauldin et al., 2012), and,
thus, several studies have been carried out on the reactions of Criegee
intermediates with SO2 (Boy et al., 2013; Berndt et al., 2014a; Stone
et al., 2014; Newland et al., 2015b; Liu et al., 2016). The relative
contribution of SO2 and water vapour to the CI removal in the
atmosphere depends on the CI structure (Berndt et al., 2014a; Stone et al.,
2014) and may have an important effect on the SOA formation yields.
In this work we report the study of the ozonolysis of 2,5-dihydrofuran
(2,5-DHF) and 2,3-dihydrofuran (2,3-DHF) under variable concentrations of
water vapour and SO2.
2,5-DHF+O3→…SOA2,3-DHF+O3→…SOA
Very recently, it has been found that different dihydrofurans may be
involved in the SOA formation from alkane photooxidation (Loza et al., 2014;
Zhang et al., 2014). Alkoxy radicals are initially generated from the
reaction with OH. They can subsequently isomerise into δ-hydrocarbonyl intermediates which undergo cyclisation and dehydration,
producing dihydrofurans (Martín et al., 2002).
2,3-Dihydrofuran is also found in the atmosphere due to the emissions from
biomass burning (Lemieux et al., 2004) and is included in atmospheric
chemistry models aimed at the emissions of aerosols (Freitas et al., 2011).
Furthermore, during the last few years, different oxygenate chemicals have
been tested as fuel additives or components of biofuels. Thus, furans and
derivatives are potential second-generation biofuels since they could be
produced from fructose and glucose (Román-Leshkov et al., 2007). In
this sense, the ignition characteristics of 2,5-DHF have been reported and
compared to those of other heterocyclic compounds (Fan et al., 2016).
Schematic set-up of the experimental system.
A previous study has reported the tropospheric oxidation study of 2,5-DHF and
2,3-DHF (Alwe et al., 2014). The measured rate constants with ozone are
1.65 and 443.2 × 10-17 cm3 molecule-1 s-1 for
2,5-DHF and 2,3-DHF, respectively. Alwe et al. (2014) show that
the dominant pathway of tropospheric degradation of 2,5-DHF is the reaction
with OH, whereas for 2,3-DHF it is the reaction with O3. So, ozonolysis of
2,3-DHF is a potential source of CI in the troposphere. Although the
reaction of 2,5 DHF with ozone is slower, the result for this unsaturated
cyclic ether may serve as a reference for other substituted dihydrofurans.
Up to now, no studies have been carried out to assess the potential
capacity of the title compounds to generate condensable matter and SOA.
Thus, in this work we study the ozonolysis reactions of 2,5-DHF and 2,3-DHF
following the conditions that lead to the formation and growth of new
particles. The effect of water vapour and SO2 concentrations during the
process are also studied and discussed. The experimental work is supported
by theoretical calculations to explore the key steps involved within the
reaction mechanism leading to SOA.
Methods and instrumentation
Experimental set-up and methods
A schematic diagram of the experimental system is shown in Fig. 1. A 200 L
capacity FEP collapsible chamber was used to carry out most of the
experiments under atmospheric pressure at 297 ± 1K. The reactants were
sequentially sampled in a volume-calibrated glass bulb (1059 L) using
capacitance pressure gauges (MKS 626AX, 100 and 1000 torr full scale) and
then flushed to the reactor using synthetic air through a mass flow
controller (MFC 1179BX, MKS). The concentrations of each substance were
calculated from their partial pressure in the bulb and the dilution factor
considering the glass bulb volume and the final volume of the Teflon
reactor.
Ozone was produced by an ozone generator (BMT Messtechnik 802N) fed with
pure oxygen. The ozone–oxygen mixture was introduced simultaneously to the
sampling bag and to a 19.8 cm long quartz cell to measure the absorption of
radiation at 255 nm using a UV–vis Hamamatsu spectrometer (C10082CAH) with a
1 nm resolution. The ozone concentration in the bulb was then calculated
from the known value of the ozone absorption cross section. Further dilution
of ozone enabled the required range of ozone concentrations in the reactor.
Water vapour was generated using a glass bubbler placed just before the
reactor inlet. The amount of water required for a given relative humidity (RH)
was calculated and injected into the bubbler. All the sampled water was then
evaporated and flushed inside the reactor through the flow of synthetic air.
The certified water impurity content in the synthetic air used was below
2 ppm. Although it is generally negligible, it could be significant if water
molecules were involved in fast reactions. To properly asses the effect of
water in the studied reactions, for the experiments at RH = 0 %, the
flow from the air cylinder was passed through a trap containing molecular
sieve 5A (Supelco) cooled down to 157 K with an ethanol–liquid nitrogen
slush bath. Some experiments were also carried out using a liquid nitrogen
trap and checking that no condensed oxygen was visible in the trap. For such
experiments water concentration in the reactor was estimated as low as
20 ± 10 ppb by introducing the dried air and known concentrations of
SO3 and monitoring the time for new particle formation (see Supplement,
Fig. S1 and related discussion).
The sequence of reactants entering the Teflon bag was as follows. First,
2,5-DHF (or 2,3-DHF) was introduced, then the OH scavenger (cyclohexane),
then SO2 and finally ozone. In the experiments carried out with water
vapour, water was introduced in the bubbler at the beginning, being
evaporated by the air carrying the DHF, the scavenger and SO2.
The injection of ozone constitutes the ignition of the 2,5-DHF (or
2,3-DHF)–ozone reaction. To completely wash away the ozone from the bulb
and transport it to the reactor, a flow of synthetic air was required for
45 s. During this time period the Teflon reactor was shaken to accelerate
the mixing of reactants.
The formation of particles was followed continuously by a TSI condensation particle
counter (CPC) 3775, and, at given times, samples were derived to a fast
mobility particle sizer (FMPS 3091, TSI) to obtain the particle size
distribution and total mass of particles with diameters within the range
5.6–560 nm (Aranda et al., 2015). The total aerosol mass concentration was
calculated from the measured particle size distribution assuming unit
density and spherical particles.
Prior to each experiment the Teflon bag was repetitively filled and emptied
with clean synthetic air to remove particles remaining from previous
experiments. The process was continued until the level of particles was below
1 cm-3.
For some experiments, a fluorescence SO2 analyser (Teledyne Instruments
101-E) was coupled to the reactor to measure the SO2 concentration
profiles during the experiments. Previous runs with only SO2/air
samples in the reactor showed that SO2 wall losses were negligible.
Additionally, no interference in the SO2 fluorescence signal from the
rest of co-reactants was observed in the range of concentrations used in
this study.
A series of experiments was also carried out monitoring the O3
concentration using an ozone analyser (Environnement O342M) to follow the
decay of O3 and validate the simulated decays of reactants due to the
ozonolysis reactions.
Computational methods
All the calculations were performed with the Gaussian09 program package
(Frisch et al., 2009). The BMK formulations (Boese and Martin, 2004) of the
density functional theory (DFT) combined with the Pople triple split-valence
basis set 6-311++G(3df,3pd) were employed. In all cases, the structural
parameters were fully optimised via analytic gradient methods. The
synchronous transit-guided quasi-Newton (STQN) method was employed to
locate transition structures. For a better estimation of energies,
single-point calculations at ab initio level CCSD(T) were performed using the
6-311G(d,p) basis set (Bartlett, 1989; Cizek, 2007).
Reagents
All the substances used were of the highest commercially available purity: 2,5-DHF
97 %, Aldrich; 2,3-DHF 99 %, Aldrich; SO2 99.9 %, Fluka; cyclohexane 99.5 %, Sigma-Aldrich;
synthetic air 3X, Praxair; O2 4X, Praxair; SO3, Sigma-Aldrich > 99 %.
Liquid reagents were purified by successive trap-to-trap distillation.
Results and discussion
2-5-Dihydrofuran
Conditions for SOA formation
Cyclohexane in excess was used as an OH radical scavenger in concentration
ratios so that > 95 % of OH formed in the reactions was removed
(Ma et al., 2009). Different series of experiments were carried out to
characterise the formation and growth of particles that originated from the
reaction of ozone with 2,5-DHF.
In Fig. 2 we show a typical experiment carried out with 0.5 ppm of 2,5-DHF,
1ppm of O3 and 0.5 ppm of SO2 with zero RH initial concentrations.
Since new particles are readily formed due to this reaction, low-volatility
substances must be produced and exceed their saturation vapour pressure
several times over, initiating homogeneous nucleation (Kelving effect;
Pruppacher et al., 1998). The experimental data obtained for the particle
number concentration are presented together with the simulated profile of
2,5-DHF (from the known initial concentrations and the gas-phase rate
constant; Alwe et al., 2014), the experimental and simulated ozone profiles,
and the total concentration of consumed 2,5-DHF (Fig. 2). Small particles
with diameters above 4 nm were quickly detected by the CPC. A maximum in the
particle number concentration (PNC) was observed around 8 min, and then PNC
progressively decreased with time mainly due to coagulation of particles and
wall losses. For reaction times longer than 2 h and PNC below
1 × 104 cm-3 its decrease may be attributed solely to
wall losses. The plot of Ln(PNC) against time for different experiments
provided linear fittings and an average Kw of
(5 ± 2) × 10-5 s-1.
The total mass concentration (or particle mass concentration, PMC) of
measured aerosol is also shown in Fig. 2 multiplied by a factor scale since
the units are different to the rest of the magnitudes. The PMC profile arises
after the nucleation event and continues growing until approximately 30 min. Then a low decrease of the mass concentration is observed with
time. The profile of total concentration of consumed 2,5-DHF is very similar
to that of the mass concentration. This fact suggests that Reaction (R1) is
the limiting step in the production of condensable matter. Beyond 30 min
the mass concentration decreases, mainly due to wall losses, although
2,5-DHF is still reacting. Better fits of these two profiles are obtained
when the wall rate constant is considered.
A series of experiments with lower concentrations of reactants was conducted
to characterise the nucleation conditions. Thus, for example, in Fig. 2 we
can also see the profile of the particle number concentration for initial
concentrations of 2,5-DHF, ozone and SO2 of 0.1, 0.2 and 0.1 ppm,
respectively. In such experiment no particles were detected before 3 min
total time. For such reaction time and conditions, the reacted concentration
of 2,5-DHF amounts to 3.5 × 1010 molecule cm-3. From the average of the
different experiments, we estimate that an upper-limit concentration of
3.5 ± 1 × 1010 molecule cm-3 is required for the direct
gas-phase product from Reaction (R1) to initiate the nucleation event.
Effect of SO2 and water
When the experiments were carried out in the absence of SO2, no
particles were observed. On the other hand, when SO2 was added, an
increase in PNC and in the total condensed mass was observed for increasing
concentrations of SO2 (Fig. S2). Even for
relatively low SO2 concentrations high concentrations of particles
(above 105 cm-3) were measured. This series of experiments was
conducted with dried synthetic air with water concentration around 20 ppb.
A series of experiments was conducted also following the temporal profile of
SO2 for initial concentrations in the range of 0.5 ppm down to 10 ppb and
for higher 2,5-DHF and ozone concentrations in the range of 0.5 to 1 ppm. In
all cases the SO2 concentration remained neatly constant during these
experiments (Fig. S3), showing that, although sulfur
dioxide participates in the mechanism leading to particles, it is released
again as free gas-phase SO2.
From the experimental SO2 profiles a first-order loss rate constant may
be inferred – for example from the experiments with 10 ppb of SO2 k= 8.5 × 10-6 s-1.
To check the possibility of SO3 production, we
can assume a simple mechanism where any lost SO2 molecule would be
converted exclusively into SO3:
SO2→SO3k=8.5×10-6s-1.
SO3 would then exclusively react with water to produce
H2SO4 (Jayne et al., 1997):
H2O+SO3→H2SO4k=3.90×10-41exp(6830.6/T)[H2O]2.
Temporal profiles for particles and gases. Initial concentrations:
0.5 ppm of 2,5-DHF, 1 ppm of O3, 0.5 ppm of SO2 and RH = 0.
When simulating the SO2, SO3 and H2O profiles for a 20 ppb water
concentration and for 10 ppb initial SO2 concentration, it would
require more than 1 h to generate 5 × 106 molecule cm-3 of
H2SO4, which is the approximate concentration able to nucleate
(Metzger et al., 2010). For 20 ppb initial SO2 concentration it would
require 28 min. Nevertheless, for these experiment nucleation was almost
instantaneous if we take away the mixing time of reactants. So, for the
experiments with the lower concentrations of SO2, the results suggest
that the reaction of SO3 with water cannot be responsible for the
formation of particles and that an alternative “dry channel” is able to
produce organic non-volatile species able to condense. For higher SO2
concentrations (in the range of 0.5 ppm) small changes at the level of the
uncertainty of the SO2 measurements cannot be completely excluded as a
possible source of SO3.
Under atmospheric conditions, reaction with water vapour is expected to be
one of the main fates of CI intermediates (Ryzhkov and Ariya, 2004). For
2,5-DHF, Fig. S4 shows that increasing the RH within the range 0 to 40 %
had no significant effect on the measured values of both PNC and PMC. Thus,
concerning the potential competing reactions of the sCI with water vapour
and SO2, the results suggest that water reaction contribution is
negligible.
Concerning the effect of ozone and 2,5-DHF initial concentrations, different
series of experiments were carried out. As discussed above, the ozone
reaction with 2,5-DHF was the rate-limiting step in the production of SOA.
Thus, the increase of [O3] or [2,5-DHF] led to the acceleration of the
process and the increase of the measured PNC and PMC. See Figs. S5–S7 for further details.
2,3-Dihydrofuran
Effect of SO2
In Figs. 3 and S8 we show the results for typical experiments carried out
with 1 ppm of ozone and 0.5 ppm of 2,3-DHF with dried synthetic air. When
the experiments were carried out in the absence of SO2, no particles
were observed. On the other hand, when SO2 was added as a co-reactant
(see for example the profile of the experiment with 0.5 ppm initial
concentrations of SO2 and 0 % RH), particles suddenly originated
approximately 1 min after the introduction of ozone in the reactor. The
number of particles still grew until approximately 5 min and then
decreased during the rest of the experiment.
PNC and PMC profiles
for different SO2 initial concentrations (0.05, 0.2 and 0.5 ppm). This
series of experiments was carried out under dry conditions and with 0.5 ppm
of 2,3-DHF and 1 ppm of ozone initial concentrations. Simulated temporal
profile of 2,3-DHF, red solid line.
In Fig. 3 we can also see the experimental O3 profile and the simulated
profile for [2,3-DHF]. The ozone–2,3-DHF reaction is very fast, and 2,3-DHF
is consumed within the first minute of the experiment while the mass of
condensed matter starts growing after nucleation and continues beyond 20 min. These results show that the initial ozone–2,3-DHF reaction itself
is not the rate-limiting step in the mechanism leading to new particles, in
contrast to the results for 2,5-DHF.
Initial concentrations of reactants and SOA data for the ozonolysis
reaction of 2,3-DHF. PNC, PMC and diameter reported values are the maximum
data registered for each magnitude during the experiment. All the
experiments reported in this table were carried out under RH = 0.
2,3-DHF
O3
SO2
PNC
PMC
Diameter
(ppm)
(ppm)
(ppm)
(cm-3)
(µg m-3)
(nm)
0.2
0.5
0.5
5.7 × 106
2
25
0.5
0.5
0.5
5.8 × 105
1.9
30
1
0.5
0.5
3.0 × 104
0
–
2
0.5
0.5
1.0 × 103
0
–
3
0.2
0.5
27
0
–
3
0.5
0.5
47
0
–
3
1
0.5
5.3 × 103
0.12
80
0.5
1
0.5
1.70 × 106
10.1
40
0.5
2
0.5
2.10 × 106
18.9
50
0.5
3
0.5
3.30 × 106
33.5
50
0.5
4
0.5
3.40 × 106
38
60
For experiments with increasing concentrations of SO2, nucleation was
found after less time, and higher PNC values were obtained. The total mass
and diameter of particles also increased with SO2 (Figs. 3 and S9).
For the smaller SO2 concentration (0.05 ppm), a clear delay of the burst
of nucleation is observed (Fig. 3). These results suggest that SO2 is
involved within the first steps of the mechanism driving the nucleation
event. For this reason the potential secondary ozonide formed from the sCI
reaction with SO2 has been characterised; see below.
A series of experiments was conducted following the temporal profile of
SO2 for initial concentrations in the range 0.01 to 0.5 ppm and for higher
2,3-DHF and ozone concentrations (0.5 to 1 ppm, respectively). As in the
case of Reaction (R1), for Reaction (R2) the SO2 concentration remained
constant during these experiments. Likewise, SO3–water reaction is not
expected to contribute to nucleation, at least in the experiments with
SO2 below 0.02 ppm. The facts that new particles are generated only in
the presence of SO2 and that SO2 does not decrease during the
experiments suggest that sulfur dioxide can behave as a catalyst in the
production of condensable products leading to particles.
Effect of 2,3-DHF and ozone initial concentrations
Two series of experiments were carried out changing the ratio of 2,3-DHF
over ozone for a fixed concentration of SO2 and in the absence of water
vapour. The results are summarised in Table 1. When 2,3-DHF was in excess
over ozone, it led to a fast consumption of ozone (during the first minute).
As shown in the table, for a fixed low initial ozone concentration, the
higher the concentration of 2,3-DHF, the lower the number concentration.
Nucleation in these experiments (hardly observed) was also almost
instantaneous, but the PNC significantly decreased (at least 2 orders of
magnitude) compared to the reference experiments (stoichiometric conditions),
and the total mass fell to virtually zero. For those experiments, a
significant difference was observed in the number concentration data from
the FMPS and the CPC. The FMPS hardly detected particles (they must have
diameters above 5.6 nm to be detectable), while the CPC can detect smaller
particles (down to 4 nm). The results showed that the particles did not grow
when ozone was the stoichiometry-limiting reactant. Thus, further ozone may
be involved after the initial O3–2,3-DHF reaction to produce SOA. These
results are also consistent with the series of experiments with ozone in
excess over 2,3-DHF. With ozone in excess, the particle number concentration
and the total mass of particles increased with the ozone concentration. As
found in Table 1 or Fig. S10, the size of the particles also grew as the
initial ozone concentration was increased.
Effect of water vapour on PNC, on PMC and on the particle diameters'
profiles: dry conditions versus 50 % RH. Both experiments were carried out
with 0.5, 1.0 and 0.5 ppm initial concentrations of 2,3-DHF, ozone and
SO2, respectively.
Relative energies to reactants sCI and SO2 (ΔE +
ZPE) in kcal mol-1 for stationary points of the all reaction.
Energies were obtained at CCSD(T)/6-311G(d,p) level of theory and zero point
energies were obtained at the BMK/6-311++G(3df,3pd) level of theory.
A
B
C
D
D'
E
E'
sCI1
72.21
2.57
-30.39
-12.58
-8.42
-59.58
-117.58
sCI2
93.45
2.01
-28.01
-11.32
-9.40
-55.87
-116.39
sCI3
93.45
2.01
-26.61
-10.32
-6.07
-69.28
-115.45
Effect of water vapour
Two series of experiments have been carried out in this work to study the
effect of water. First, for the experiments conducted in the presence of
water (RH from 0 to 50 %) and in the absence of SO2, no particle
formation was observed. So, if water reacts with the 2,3-DHF CI, this
reaction does not produce particles in the absence of SO2 (Table S1
in the Supplement).
In the second series of experiments SO2 was also present, with all the
experiments being carried out under the same initial concentrations of 2,3-DHF, ozone
and SO2 with RH ranging from 0 to 50 %. As shown in Figs. 4 and S11, the increase in RH reduced the particle number concentration and the
mass of formed SOA. Figure 4 shows the results for two experiments, under
dried air conditions and under 50 % RH. In the presence of 50 % RH, the
particle number concentration was significantly lower, and the measured mass
concentration fell from 13 to approximately 3 µg cm-3. Concerning
the growth of particles, their final diameters were also smaller in the
presence of water.
These effects may be due to the reaction of water with the Criegee
intermediate which would compete with the sCI reaction towards SO2.
From previous studies (e.g. Anglada et al., 2011), this reaction is expected to
proceed through the addition of the water oxygen atom to the carbon atom in
the carbonyl oxide and the simultaneous transfer of one hydrogen atom from
the water molecule to the terminal O atom in the CI. The expected products
are α-hydroxi-hydroperoxides that can subsequently decompose or
react with other atmospheric species.
(a) General schematic potential energy surface for the
reactions sCI + SO2→ products. (b) Geometries of
stationary points involved in the sCI + SO2 reaction at the
BMK/6-311++G(3df,3pd) level of theory.
If we take the dry experiments (0 % RH) as a reference, then we can assume
that for the experiments carried out in the presence of water the decrease
in mass concentration, ΔPMC, is due exclusively to the reaction
of water molecules with the sCI. Water would competitively consume sCI, with
a kinetic rate constant kH2O, against the reaction with SO2, with
a kinetic rate constant kSO2. Thus, the ratio
(kH2O.[H2O]) / (kSO2.[SO2]) can be obtained from the ratio
ΔPMC / PMC, that is, from the drop in the mass concentration
values measured at a given relative humidity (ΔPMC drop is
attributed solely to the reaction with H2O) and the actual mass
concentration obtained in such conditions (PMC, attributed to the reaction
with SO2). Thus, the ratio for the rate constants of the 2,3-DHF
Criegee intermediate with water vapour and SO2 was obtained plotting
ΔPMC/PMC versus [H2O] for constant and known SO2
concentration (Fig. S12), kH2O / kSO2= (9.8 ± 3.7) × 10-5.
This ratio is similar to other reported values for different CIs
(Berndt et al., 2014a). This result may be used to assess different
atmospheric conditions. For example, for relatively dry conditions (20 %
RH) in a polluted atmosphere with 75 ppb of SO2 (the current 1 h
NAAQS standard) at 25 ∘C, approximately 11 % of the sCI
produced in the ozonolysis of 2,3-DHF would react with SO2, with
potential to yield new particles and nucleation events. In the lower
troposphere this source is not unique in the sense that other pollutants
would be simultaneously producing condensable products. Nucleation under
real atmospheric conditions is due to the cumulative sources of non-volatile
species, and so these events are expected to generate under lower
concentrations than those found in the laboratory for the individual
sources.
Theoretical insights
Recent studies have revealed that sCI may react with SO2 and other
trace gases several orders of magnitude faster than assumed so far (Welz et
al., 2012). Therefore sCI reactions have emerged as a potential source of
tropospheric sulfate to be assessed in the predictions of tropospheric
aerosol formation. Even though the atmospheric fate of the smaller carbonyl
oxides is mainly dominated by the reaction with water molecules or water
dimers (Berndt et al., 2014a; Chao et al., 2015), the reactivity of sCIs is
strongly dependent on the structure.
Previous theoretical studies have shown that the first step of the sCI with
SO2 is the formation of a cyclic secondary ozonide (SOZ) that can
undergo decomposition to the corresponding carbonyl and SO3 or
isomerisation involving a 1,2-H-shift leading to an organic acid and
SO2 (Jiang et al., 2010; Kurten et al., 2011). The release of SO3
would produce H2SO4 in the presence of water molecules. Vereeken
et al. (2012) also described the formation of a stable
ester sulfinic acid from the opening of the SOZ.
R1R2COO+SO2→R1R2CO+SO3→R1COOH+SO2(only ifR2=H)→R1C(O)OS(O)OH(only ifR2=H)
In this sense we have studied the transition states for both 2,5-DHF and
2,3-DHF Criegee intermediate reactions with SO2. Figure 5a shows a
schematic representation of the general potential energy surface for the
studied reactions. Figure 5b displays the optimised geometries of the
stationary points for all reactants, products and stationary states from the
sCI obtained at the BMK/6-311++G(3df,3pd) level of theory. The sCI
+SO2 system has been taken as a zero-energy reference. As shown in
Table 2, the first step of the ozonolysis is highly exothermic, so
unimolecular decomposition and stabilisation through collision would compete
in the formation of both sCIs. Concerning the reaction mechanism, in the case
of 2,5-DHF reaction with O3, only one Criegee intermediate (sCI1) may
be formed. On the other hand, due to the asymmetry of 2,3-DHF, two possible
Criegee intermediates can be obtained (sCI2 and sCI3) from its reaction with
O3. For all these intermediates, there are two possible configurations:
syn and anti. The latter, anti, was used in all cases to perform the
potential energy surface calculations since they were more stable than the
syn isomers at 1.08, 3.67 and 3.65 kcal mol-1.
The reaction of sCI1 with SO2 first evolves to the formation of an
intermediate adduct M1, which then may react through two channels. On the
one hand, through the transition state TS1.1, SO3 and an aldehyde are
formed. If the adduct M1 evolves via the transition state TS1.2, SO2 is
regenerated and an organic acid is produced. Due to the fact that both
transition states lie below the energy of reactants, the most favourable
reaction results in the regeneration of SO2 and the organic acid with
reaction energy of -117.58 kcal mol-1 (Table 2).
For the case of 2,3-DHF, the same mechanism is observed in the reactions of
sCI2 and sCI3 with SO2. First the intermediate adducts, M2 and M3, are
generated, and then they evolve through two possible reaction pathways:
through the transition state TS2.1 (or TS3.1) or transition state TS2.2 (or
TS3.2), resulting in production of SO3 and an aldehyde or SO2 and
an organic acid, respectively. In both cases, similarly to what has been
described previously for sCI1, the regeneration of SO2 and an
organic acid is the more favourable pathway since the intermediate states lie
below the energy of reactants. The reaction energies are -116.39 and
-115.45 kcal mol-1 for the organic acids 2 and 3, respectively (Table 2).
These theoretical results are consistent with the experimental findings
described previously: no evidence of SO3 release was observed, and
SO2 concentrations remained unchanged during the laboratory
experiments. Thus, both computational and experimental results suggest that
SOA formation may generate from Reaction (R4) for both 2,5-DHF and 2,3-DHF
ozonolysis reactions. Concerning the organic acids from Reaction (4) – organic acids “1”, “2” and “3”: HC(O)CH2OCH2C(O)OH
(formylmethoxy-acetic acid), HC(O)OCH2CH2COOH (3-formyloxy-propanoic
acid) and HC(O)CH2CH2OCOOH (2-formyl-etoxy-formic acid), respectively
(Fig. 5) – there are no vapour pressure data available in the literature. Similar
chemicals like HC(O)OCH(CH3)COOH (2-formyloxy-propionic acid) and
CH3C(O)OCH2COOH (acetoxyacetic acid) are solids at room
temperature and have high melting and boiling points (Zanesco, 1966;
Buckingham and Donaghy, 1982). Furthermore, previous studies (e.g. Donahue et al.,
2011) provide correlations to estimate vapour pressures based on group
contribution factors and the oxygenation ratio O : C. For the involved acids,
with four carbon atoms and four oxygen atoms (1:1 O : C ratio), saturation
concentrations as low as 5 × 1011molecule cm-3 are predicted. Thus,
the organic acids expected from the SO2 catalytic pathway could be the
species responsible for the nucleation events observed under laboratory
conditions.
Conclusions
2,5-DHF and 2,3-DHF show a different behaviour during the ozonolysis, showing
that the reactivity of the CI is clearly dependent on the structure. For
2,5-DHF the mass increase of particulate matter correlates in time with the
total concentration of reacted alkene. On the other hand, for 2,3-DHF, the
particle mass concentration rises well after the total consumption of this
cyclic alkene. Furthermore, no effect of RH on the production of SOA was
found for the ozonolysis of 2,5-DHF, while the increase of RH inhibits the
SOA production from to the ozonolysis of 2,3-DHF.
As found in this work, the experimental results for the reaction of ozone
with 2,5-DHF show that the presence of small amounts of SO2 is required
to generate SOA. Nevertheless, the main atmospheric fate of 2,5-DHF is the
reaction with OH radicals, and so a small contribution to new particle
formation in the atmosphere is expected from Reaction (R1). Nevertheless,
the information reported in this work contributes to the sparse current
knowledge of the reactivity of CI intermediates with SO2 and H2O,
and so it may be helpful in the understanding of the behaviour of other
cyclic alkenes.
On the other hand, Reaction (R2) is dominant over the OH reaction in the
troposphere, and, as has been found in this work, the ozonolysis of
2,3-DHF leads to the formation of SOA in the presence of SO2. The
results show that under increasing amounts of water vapour the yields of SOA
decrease, suggesting the competing reactions of the sCI with SO2 and
water molecules.
The experimental findings reported in this work show that SO2
participates as a catalyst in the production of new particles from the
ozonolysis of both 2,5-DHF and 2,3-DHF. Under the experimental conditions in
this study, at least for the experiments with low SO2 concentrations,
the oxidation of SO2 to SO3 through reaction with the corresponding
sCI seems negligible, contrary to the results found for smaller carbonyl
oxides (Berndt et al., 2014b). Likewise, the pathway producing ester sulfinic
acids (Reaction 5) can be ruled out for 2,5-DHF and 2,3-DHF. Thus, the
organic acids produced through Reaction (4) are expected to be the key
species in the formation of SOA.