Introduction
Atmospheric chemical processes exert a major influence on atmospheric
composition. Identified gas-phase oxidants include the OH radical, ozone,
NO3 and under certain circumstances other species such as halogen
atoms. Reactions with these oxidants can lead to (for example) chemical
removal of primary air pollutants, formation of secondary pollutants (e.g.
ozone, harmful to human and environmental health, and a greenhouse gas), and
the transformation of gas-phase species to the condensed phase (e.g.
SO2 oxidation leading to the formation of sulfate aerosol, and the
formation of functionalised organic compounds leading to secondary aerosol
formation, which can influence radiation transfer and climate).
Stabilised Criegee intermediates (SCIs), or carbonyl oxides, are formed in
the atmosphere predominantly from the reaction of ozone with unsaturated
hydrocarbons, though other processes may be important in certain conditions,
e.g. alkyl iodide photolysis (Gravestock et al., 2010), dissociation of the
dimethyl sulfoxide (DMSO) peroxy radical (Asatryan and Bozzelli, 2008), and reactions of peroxy
radicals with OH (Fittschen et al., 2014). SCIs have been shown in laboratory
experiments and by theoretical calculations to oxidise SO2 and NO2
(e.g. Cox and Penkett, 1971; Welz et al., 2012; Taatjes et al., 2013; Ouyang et
al., 2013; Stone et al., 2014) as well as a number of other trace gases
found in the atmosphere. Recent field measurements in a boreal forest (Mauldin III et al., 2012) and at a coastal site (Berresheim et al.,
2014) have both identified an apparently missing process oxidising SO2
to H2SO4 (in addition to reaction with OH) and have implied SCIs as
a possible oxidant, acting alongside OH. Assessment of the importance of
SCIs for tropospheric processing requires a quantitative understanding of
their formation yields and atmospheric fate – in particular, the relative
importance of bimolecular reactions (e.g. with SO2), unimolecular
decomposition, and reaction with water vapour. Here we describe an
experimental investigation into the formation and reactions of the SCIs
derived from isoprene (the most abundant biogenic volatile organic compound (VOC)), formed through the
ozonolysis process, which dominates atmospheric SCI production, and studied
under boundary layer conditions to assess their potential contribution to
tropospheric oxidation.
Stabilised Criegee intermediate kinetics
Ozonolysis-derived CIs are formed with a broad internal energy distribution,
yielding both chemically activated and stabilised CIs. SCIs can have
sufficiently long lifetimes to undergo bimolecular reactions with H2O
and SO2, amongst other species. Chemically activated CIs may undergo
collisional stabilisation to an SCI (Scheme 1), or unimolecular
decomposition or isomerisation.
Simplified generic mechanism for the reaction of Criegee
intermediates (CIs) formed from alkene ozonolysis.
To date the majority of studies have focused on the smallest SCI,
CH2OO, because of the importance of understanding simple SCI systems
(this species is formed in the ozonolysis of all terminal alkenes) and the
ability to synthesise CH2OO from alkyl iodide photolysis, with
sufficient yield to probe its kinetics. However, the unique structure of
CH2OO (which prohibits isomerisation to a hydroperoxide intermediate)
likely gives it a different reactivity and degradation mechanism to other
SCIs (Johnson and Marston, 2008).
Recent experimental work (Berndt et al., 2014; Newland et al., 2015; Chao et
al., 2015; Lewis et al., 2015) has determined the predominant atmospheric
fate for CH2OO to be reaction with water vapour. Some of these
experiments (Berndt et al., 2014; Chao et al., 2015; Lewis et al., 2015)
have demonstrated a quadratic dependence of CH2OO loss on [H2O],
suggesting a dominant role for the water dimer, (H2O)2, in
CH2OO loss at typical atmospheric boundary layer H2O
concentrations. For larger SCIs, both experimental (Taatjes et al., 2013;
Sheps et al., 2014; Newland et al., 2015) and theoretical (Kuwata et al.,
2010; Anglada et al., 2011) studies have shown that their kinetics, in
particular reaction with water, are highly structure dependent. syn-SCIs (i.e. those
where an alkyl-substituent group is on the same side as the terminal oxygen
of the carbonyl oxide moiety) react very slowly with H2O, whereas
anti-SCIs (i.e. with the terminal oxygen of the carbonyl oxide moiety on the same
side as a hydrogen group) react relatively fast with H2O. This
difference has been predicted theoretically (Kuwata et al., 2010; Anglada et
al., 2011) and was subsequently confirmed in recent experiments (Taatjes et al., 2013; Sheps et al., 2014) for the two CH3CHOO
conformers. Additionally, it has been predicted theoretically (Vereecken et al., 2012) that the relative reaction rate constants for the
water dimer vs. water monomer,
k(SCI + (H2O)2)/k(SCI + H2O) of larger SCIs (except
syn-CH3CHOO) will be over 70 times smaller than that for CH2OO,
suggesting that reaction with the water dimer is unlikely to be the dominant
fate for these SCIs under atmospheric conditions.
An additional, and potentially important, fate of SCIs under atmospheric
conditions is unimolecular decomposition (denoted kd in Reaction R4). This is
likely to be a significant atmospheric sink for syn-SCIs because of their slow
reaction with water vapour. Previous studies have identified the
hydroperoxide rearrangement as dominant for SCIs with a syn configuration,
determining their overall unimolecular decomposition rate (Niki et al.,
1987; Rickard et al., 1999; Martinez and Herron, 1987; Johnson and Marston,
2008). This route has been shown to be a substantial non-photolytic source
of atmospheric oxidants (Niki et al., 1987; Alam et al., 2013). CIs formed
in the anti configuration are thought to primarily undergo rearrangement and
possibly decomposition via a dioxirane intermediate (“the acid/ester
channel”), producing a range of daughter products and contributing to the
observed overall HOx radical yield (Johnson and Marston, 2008; Alam et
al., 2013).
For CH2OO, rearrangement via a “hot” acid species represents the lowest
accessible decomposition channel, with the theoretically predicted rate
constant being rather low, 0.3 s-1 (Olzmann et al., 1997). Recent
experimental work supports this slow decomposition rate for CH2OO
(Newland et al., 2015; Chhantyal-Pun et al., 2015). However, Newland et al. (2015) suggest the decomposition of larger syn-SCIs to be considerably
faster, albeit with substantial uncertainty, with reported rate constants
for syn-CH3CHOO of 288 (±275) s-1 and for (CH3)2COO
of 151 (±35) s-1. Novelli et al. (2014) estimated decomposition of
syn-CH3CHOO to be 20 (3–30) s-1 from direct observation of OH
formation, while Fenske et al. (2000) estimated decomposition of CH3CHOO
produced from ozonolysis of trans-but-2-ene to be 76 s-1 (accurate to within
a factor of 3).
Alkene+O3⟶k1ϕSCI+(1-ϕ)CI+RCHOSCI+SO2⟶k2SO3+RCHOSCI+H2O⟶k3ProductsSCI⟶kdProductsSCI+(H2O)2⟶k5Products
Isoprene ozonolysis
Global emissions of biogenic VOCs have been estimated to be an order of
magnitude greater, by mass, than anthropogenic VOC emissions (Guenther et
al., 1995). The most abundant non-methane biogenic hydrocarbon in the
natural atmosphere is isoprene (2-methyl-1,3-butadiene, C5H8),
with global emissions estimated to be 594 (±34) Tg yr-1
(Sindelarova et al., 2014). While the vast majority of these emissions are
from terrestrial sources, there are also biogenic emissions in coastal and
remote marine environments, associated with seaweed and phytoplankton blooms
(Moore et al., 1994). Isoprene mixing ratios (as well as those of some
monoterpenes) have been reported to reach hundreds of pptv (parts per
trillion by volume) over active phytoplankton blooms in the marine boundary
layer (Sinha et al., 2007; Yassaa et al., 2008), with the potential to
impact local air quality (Williams et al., 2010).
Removal of isoprene from the troposphere is dominated by reaction with the
OH radical during the day and reaction with the nitrate radical during the
night (Calvert et al., 2000). The ozonolysis of isoprene is also a
non-photolytic source of HOx radicals (Atkinson et al., 1992; Paulson
et al., 1997; Malkin et al., 2010), with measured yields of OH between 0.25
(Paulson et al., 1997) and 0.27 (Atkinson et al., 1992) (with a current
recommended yield of 0.25; Atkinson et al., 2006). Isoprene ozonolysis also
leads to the formation of a range of multi-functional oxygenated compounds,
some of which can form secondary organic aerosol (Noziere et al., 2015).
Mechanism of formation of the nine possible Criegee intermediates
(CIs) from isoprene ozonolysis.
Isoprene ozonolysis yields five different initial carbonyl oxides (Scheme 2). The three basic species formed are formaldehyde oxide (CH2OO),
methyl vinyl carbonyl oxide (MVKOO) and methacrolein oxide (MACROO) (Calvert
et al., 2000; Atkinson et al., 2006). MVKOO and MACROO both have syn and anti
conformers, and each of these can have either cis or trans configuration (Zhang et
al., 2002; Kuwata et al., 2005) with easy inter-conversion between the cis and
trans conformers (Aplincourt and Anglada, 2003). The kinetics and products of
isoprene ozonolysis have been investigated theoretically by Zhang et al. (2002).
They predicted the following SCI yields: CH2OO, 0.31; syn-MVKOO, 0.14;
anti-MVKOO, 0.07; syn-MACROO, 0.01; and anti-MACROO 0.04. This gives a total SCI yield of
0.57. They predicted that 95 % of the chemically activated CH2OO
formed will be stabilised, considerably higher than the experimentally
determined stabilisation of excited CH2OO formed during ethene
ozonolysis (35–54 %) (Newland et al., 2015). This is because the
majority of the energy formed during isoprene ozonolysis is thought to
partition into the larger, co-generated, primary carbonyl species (Kuwata et
al., 2005) (i.e. methyl vinyl ketone (MVK) or methacrolein (MACR)). The
predicted stabilisation of the other SCIs ranges from 20 to 54 % at
atmospheric pressure. It is relevant to note that the total SCI yield from
isoprene ozonolysis used in the Master Chemical Mechanism, MCMv3.2 (Jenkin
et al., 1997; Saunders et al., 2003), is considerably lower at 0.22, as a
consequence of the MCM protocol, which applies a weighted mean of total SCI
yields measured for propene, 1-octene and 2-methyl propene (Jenkin et al.,
1997). However, the relative yield of CH2OO (0.50) compared to the
total SCI yield in the MCM is very similar to that calculated by Zhang et al. (2002) (0.54).
Dimethyl sulfide (DMS)
The largest natural source of sulfur to the atmosphere is the biogenically
produced compound dimethyl sulfide, DMS (CH3SCH3), which has
estimated global emissions of 19.4 (±4.4) Tg yr-1 (Faloona,
2009). DMS is a breakdown product of the plankton waste product
dimethylsulfoniopropionate (DMSP). Jardine et al. (2015) have also recently
shown that vegetation and soils can be important terrestrial sources of DMS
to the atmosphere in the Amazon Basin, during both the day and at night, and
throughout the wet and dry seasons, with measurements of up to 160 pptv
within the canopy and near the surface. The oxidation of DMS is a large
natural source of SO2, and subsequently sulfate aerosol, to the
atmosphere and is therefore an important source of new particle formation.
This process has been implicated in an important feedback leading to a
regulation of the climate in the pre-industrial atmosphere (Charlson et al.,
1987). The two most important oxidants of DMS in the atmosphere are thought
to be the OH and NO3 radicals (Barnes et al., 2006) (Reactions R6 and
R7). Because of its photochemical source, OH is thought to be the more
important oxidant during the day in tropical regions, while NO3 becomes
more important at night, at high latitudes, and in more polluted air masses
(Stark et al., 2007). Certain halogenated compounds, e.g. Cl (Wingenter et al.,
2005) and BrO (Wingenter et al., 2005; Read et al., 2008), have also been
suggested as possible oxidants for DMS in the marine environment.
DMS+OH→CH3SCH2+H2O→CH3S(OH)CH3
DMS+NO3→CH3SCH2+HNO3
Experimental
Experimental approach
The EUPHORE facility is a 200 m3 simulation chamber used primarily for
studying reaction mechanisms under atmospheric boundary layer conditions.
Further details of the chamber setup and instrumentation are available
elsewhere (Becker, 1996; Alam et al., 2011), and a detailed account of the
experimental procedure, summarised below, is given in Newland et al. (2015).
Experiments comprised time-resolved measurement of the removal of SO2
in the presence of the isoprene–ozone system, as a function of humidity or
DMS concentration. SO2 and O3 abundance were measured using
conventional fluorescence and UV absorption monitors, respectively; alkene
abundance was determined via FTIR spectroscopy. Experiments were performed
in the dark (i.e. with the chamber housing closed; j(NO2)≤10-6 s-1),
at atmospheric pressure (ca. 1000 mbar) and temperatures between 287
and 302 K. The chamber is fitted with large horizontal and vertical fans to
ensure rapid mixing (3 min). Chamber dilution was monitored via the
first-order decay of an aliquot of SF6, added prior to each experiment.
Cyclohexane (ca. 75 ppmv) was added at the beginning of each experiment to act
as an OH scavenger, such that SO2 reaction with OH was calculated to be
≤ 1 % of the total chemical SO2 removal in all experiments.
Experimental procedure, starting with the chamber filled with clean air,
comprised addition of SF6 and cyclohexane, followed by water vapour (or
DMS), O3 (ca. 500 ppbv) and SO2 (ca. 50 ppbv). A gap of 5 min
was left prior to addition of isoprene to allow complete mixing. The
reaction was then initiated by addition of the isoprene (ca. 400 ppbv), and
reagent concentrations followed for 30–60 min; typically ca. 25 % of
the isoprene was consumed after this time. Nine isoprene + O3
experiments, as a function of [H2O], were performed over separate days.
Each individual run was performed at a constant humidity, with humidity
varied to cover the range of [H2O] = 0.4–21 × 1016 molecules cm-3, corresponding to a relative humidity (RH) range of 0.5–27 % (at 298 K).
Five isoprene + O3 experiments as a function of DMS were also
performed. Measured increases in [SO2] agreed with measured volumetric
additions across the SO2, humidity and DMS ranges used in the
experiments.
Analysis
As in our previous study (Newland et al., 2015), from the chemistry
presented in Reactions (R1)–(R5) SCIs will be produced in the chamber from the
reaction of the alkene with ozone at a given yield, φ. A range of
different SCIs are produced from the ozonolysis of isoprene (see Scheme 2:
nine first-generation SCIs present), each with their own distinct chemical
behaviour (i.e. yields, reaction rates). It is not feasible (from these
experiments) to obtain data for each SCI independently; consequently, for
analytical purposes we adopt two alternative analyses to treat the SCI
population in a simplified (lumped) manner:
In the first of these, we make the approximation that all SCIs may be
considered as a single species (defined from now on as ISOP-SCI).
Alternatively, the SCI population is grouped into two species, the first of
which is CH2OO (for which the kinetics are known) and the second
(hereafter termed CRB-SCI) represents all isomers of the other SCI species
produced, i.e. Σ(MVKOO + MACROO). The implications of these
assumptions are discussed further below, but a key consequence is that the
relative rate constants obtained from the analysis presented here are not
representative of the elementary reactions of any single specific SCI isomer
formed but rather represent a quantitative ensemble description of the
integrated system, under atmospheric boundary layer conditions, which may be
appropriate for atmospheric modelling.
Following formation in the ozonolysis reaction, the SCIs can react with
SO2, with H2O, with DMS (if present), or with other species, or
undergo unimolecular decomposition, under the experimental conditions
applied. The fraction of the SCIs produced that reacts with SO2 (f) is
determined by the SO2 loss rate (k2[SO2]) compared to the sum
of the total loss processes of the SCIs (Eq. 1) :
f=k2[SO2]k2[SO2]+k3[H2O]+kd+L
Here, L accounts for the sum of any other chemical loss processes for SCIs in
the chamber, after correction for dilution, and neglecting other
(non-alkene) chemical sinks for O3, such as reaction with HO2
(also produced directly during alkene ozonolysis; Alam et al., 2013; Malkin
et al., 2010), which was indicated through model calculations to account
for < 0.5 % of ozone loss under all the experimental conditions.
SCI yield calculation
ΔSO2 vs. ΔO3 during the excess
SO2 experiments ([H2O] < 5 × 1015 cm-3). The gradient determines the minimum SCI yield (φmin) from isoprene ozonolysis.
The value for the total SCI yield of ISOP-SCI, φISOP-SCI, was
determined from an experiment performed under dry conditions (RH < 1 %) in the presence of excess SO2 (ca. 1000 ppbv), such that SO2
scavenged the majority of the SCIs (> 95 %). From Eq. (2),
regressing dSO2 against dO3 (corrected for chamber dilution),
assuming f to be unity (i.e. all the SCIs produced reacts with SO2),
determines the value of φmin, a lower limit to the SCI yield.
Figure 1 shows the experimental data, from which φmin was
derived.
d[SO2]d[O3]=ϕ⋅f
The lower-limit criterion applies because in reality f will be less than 1, at
experimentally accessible SO2 levels, as a small fraction of the SCIs
will still react with trace H2O present or undergo decomposition. The
actual yield, φISOP-SCI, was determined by combining the
result from the excess-SO2 experiment with those results from the series of
experiments performed at lower SO2, as a function of [H2O], to
determine k3/k2 and kd/k2 (see Sect. 2.2.2) through an
iterative process to determine the single unique value of φISOP-SCI which fits both data sets.
k(SCI+H2O)/k(SCI+SO2) and kd/k(SCI+SO2)
By rearranging Eq. (1), the following equation (Eq. 3) can be derived.
Therefore, in order to determine the relative rate constants
k3/k2 and (kd+L)/k2, a series of experiments were performed
in which the SO2 loss was monitored as a function of [H2O] (see
Sect. 2.1).
[SO2]1f-1=k3k2[H2O]+kd+Lk2
Cumulative consumption of SO2 and O3, ΔSO2
versus ΔO3, for the ozonolysis of isoprene in the presence of
SO2 at a range of water vapour concentrations, from 4 × 1015 to 2.1 × 1017 cm-3. Symbols are
experimental data corrected for chamber dilution. Lines are smoothed fits to
the experimental data.
From Eq. (2), regression of the loss of ozone (dO3) against the loss
of SO2 (dSO2) for an experiment at a given RH determines the
product f⋅φ at a given point in time. This quantity will vary through
the experiment as SO2 is consumed, and other potential SCI co-reactants
are produced, as predicted by Eq. (1). A smoothed fit was applied to the
experimental data for the cumulative consumption of SO2 and O3,
ΔSO2 and ΔO3 (as shown in Fig. 2), to determine
dSO2/dO3 (and hence f⋅φ) at the start of each experiment, for
use in Eq. (3). This fit was derived using a box model run in FACSIMILE
(Curtis and Sweetenham, 1987) with a chemical scheme taken from MCMv3.2
(http://mcm.leeds.ac.uk/MCM), with additional updated SCI chemistry
constrained by the experimental measurements. The start of each experiment
(i.e. when [SO2] ∼ 50 ppbv) was used as this corresponds to
the greatest rate of production of the SCIs, and hence largest experimental
signals (i.e. O3 and SO2 rate of change; greatest precision) and is the
point at which the SCI + SO2 reaction has the greatest magnitude
compared with any other potential loss processes for either reactant species
(see discussion below). The value [SO2]((1/f)-1) can then be regressed
against [H2O] for each experiment to give a plot with a gradient of
k3/k2 and an intercept of (kd+L)/k2 (Eq. 3). Our
data cannot determine absolute rate constants (i.e. values of k2, k3,
kd) in isolation but are limited to assessing their relative values,
which may be placed on an absolute basis through use of an (external)
reference value (k2(CH2OO + SO2) in this case).
k(SCI+DMS)/k(SCI+SO2)
A similar methodology was applied to that detailed in Sect. 2.2.2 to
determine the relative reaction rate of ISOP-SCI with DMS
k(SCI+DMS)/k(SCI+SO2), k8/k2. Here, the SO2 loss was
determined as a function of [DMS] rather than [H2O]. [H2O] was
< 1 × 1016 molecules cm-3 for all
experiments.
SCI+DMS⟶k8Products
Equation (3) is modified to give Eq. (4) by the addition of the DMS term.
The gradient of a plot regressing [SO2]((1/f)-1) against [DMS] is then
k8/k2 and the intercept is k3/k2[H2O] + (kd+
L)/k2. Using this intercept, these experiments can also be used to
validate the k3/k2 and (kd+L)/k2 values derived from the
experiments described in Sect. 2.2.
[SO2]1f-1=k8k2[DMS]+k3k2[H2O]+kd+Lk2
Isoprene + ozone as a function of [H2O]
SCI yield
Figure 1 shows the derived φmin for isoprene, 0.55, determined
from fitting Eq. (2) to the experimental data. φmin was
then corrected (< 3 %) as described in Sect. 2.2.1 using the
k3/k2 and kd/k2 values determined from the measurements
shown in Fig. 3 using Eq. (5). The corrected yield, φISOP-SCI, is 0.56 (±0.03). Uncertainties are ±2σ and represent the combined systematic (estimated measurement uncertainty)
and precision components.
Application of Eq. (5) to derive relative rate constants for
reaction of the isoprene-derived SCIs with H2O (k3/k2) and
decomposition ((kd+L)/k2). Y= [SO2]((1/f)-1)-k9[acid] /k2.
Literature yields for SCI production from isoprene ozonolysis are given in
Table 1. The value derived for the yield in this work agrees very well with
the value of 0.58 (±0.26) from a recent experimental study
(Sipilä et al., 2014) which used a similar single-SCI analysis approach.
Total isoprene SCI yields derived in this work and reported in the
literature.
φISOP-SCI
Reference
Methodology
0.56 (±0.03)
This work
SO2 loss
0.58 (±0.26)
Sipilä et al. (2014)
Formation of H2SO4
0.30 (φCH2OO)a
Neeb et al. (1997)
HMHPb yield
0.26
Hasson et al. (2001)
Sum of difference between HMHP and H2O2 yields at high/low [H2O]
0.28
Rickard et al. (1999)
Assumes stabilisation of 40 % of CH2OO produced + difference between MVK and MACR production at high/low [SO2]
0.53
Rickard et al. (1999)
Assuming 95 % of CH2OO is stabilised (after Zhang et al., 2002) + difference between MVK and MACR production at high/low [SO2]
0.57
Zhang et al. (2002)
Theoretical
0.22
MCMv3.2c
Based on a weighted average of the yields for propene, 1-octene and 2-methyl propene.
Uncertainty ranges (±2σ, parentheses) indicate combined
precision and systematic measurement error components for this work, and are
given as stated for literature studies. All referenced experimental studies
produced SCIs from C5H8+ O3 and were conducted between 700
and 760 Torr. a Yield of stabilised CH2OO only. b
Hydroxymethyl hydroperoxide (a first-order product of CH2OO +
H2O). c http://mcm.leeds.ac.uk/MCM/ (Jenkin et al., 1997).
Earlier experimental studies have reported lower values (by up to a factor
of 2) for the total isoprene SCI yield. Rickard et al. (1999) derive a total yield
of 0.28 from the increase in primary carbonyl yield (MVK and MACR) in the
presence of a suitable SCI scavenger (excess SO2). However, owing to
the fact that they could not measure a formaldehyde yield, in their analysis
it was assumed that 40 % of the chemically activated CH2OO formed
was stabilised (derived from the measured CH2OO SCI yield for ethene
ozonolysis), corresponding to their determination of a CH2OO SCI yield
of 0.18 for isoprene ozonolysis. If it is assumed that 95 % of the
CH2OO formed was actually stabilised, as calculated by Zhang et al. (2002), then this yield increases to 0.43, giving a total yield, φISOP-SCI, of 0.53, in excellent agreement with the current work.
Hasson et al. (2001) calculated a total SCI yield of 0.26 by measuring the
sum of the difference between (i) the H2O2 production under dry
and high-RH conditions (to give the non-CH2OO SCI yield) and (ii) the
difference between hydroxymethyl hydroperoxide (HMHP) production under dry
and high-RH conditions (to give φCH2OO). One potential reason
for the significantly lower total SCI yield calculated by Hasson et al. compared
to this work is the low value of φCH2OO determined,
potentially due to HMHP losses. Neeb et al. (1997) determined a value for
φCH2OO approximately twice that determined by Hasson et al.,
using a similar methodology. This discrepancy may be owing to the fact that
Hasson et al. do not account for the formation of formic acid, which is a
degradation product of HMHP. From theoretical calculations, Zhang et al. (2002) predicted a yield of 0.31 for CH2OO, the most basic SCI, 0.14
for syn-MVKOO, 0.07 for anti-MVKOO, 0.04 for anti-MACROO and 0.01 for syn-MACROO. This gives a
sum of SCI yields of 0.57, again in very good agreement with the overall
value derived here. The MCM (Jenkin et al., 1997; Saunders et al., 2003)
applies a φISOP-SCI of 0.22, based on the limited experimental
data available at the time of its original release (Jenkin et al., 1997).
Although this total value is slightly lower than the experimental
measurements reported prior to the release of MCMv3.2 (i.e. Rickard et al., 1999; Hasson et al., 2001), the protocol uses a similar relative yield
for stabilised CH2OO (0.50) compared to the total SCI yield as reported
by Zhang et al. (2002). A probable reason for the low SCI yields in the MCM
is the assumption of low stabilisation of the chemically activated CI
formed.
The CH2OO yield (φCH2OO) from isoprene ozonolysis derived
in this work can be calculated by multiplying the total SCI yield (0.56) by
the fraction of the total SCI yield predicted to be CH2OO by Zhang et al. (2002) (0.54). This gives a yield of stabilised CH2OO from this work
of 0.30. This is in very good agreement with Neeb et al. (1997), who
derived a yield of stabilised CH2OO from isoprene ozonolysis of 0.30 by
measuring HMHP (the product of
CH2OO + H2O) formation.
Analysis 1: single-SCI (ISOP-SCI) treatment
Figure 2 shows the cumulative consumption of SO2 relative to that of
O3, ΔSO2 versus ΔO3 (after correction for
dilution), for each isoprene ozonolysis experiment as a function of
[H2O]. A fit to each experiment, which has the sole purpose of
extrapolating the experimental data to evaluate dSO2/dO3 at t=0 (start of each experimental run) for use in Eqs. (1)–(3), is also
shown. This fit is derived using a box model run in FACSIMILE (Curtis and
Sweetenham, 1987) as described in Sect. 2.2.2. The overall change in
SO2, ΔSO2, is seen to decrease substantially with
increasing humidity over a relatively narrow range of [H2O] (0.4–21 × 1016 cm-3). This trend is similar to that
seen for smaller, structurally less complex alkene ozonolysis systems
(Newland et al., 2015), and is as would be expected from the understood
chemistry (Reactions R1–R5), as there is competition between SO2, H2O,
and decomposition for reaction with the SCIs formed.
Other potential fates for SCIs under the experimental conditions presented
here include reaction with other reactants/co-products: ozone (Kjaergaard
et al., 2013; Vereecken et al., 2014; Wei et al., 2014), other SCIs (Su et
al., 2014; Vereecken et al., 2014), carbonyl products (Taatjes et al.,
2012), acids (Welz et al., 2014), or the parent alkene itself (Vereecken et
al., 2014). Sensitivity analyses were performed using a box model run in
FACSIMILE (Curtis and Sweetenham, 1987) with a chemical scheme taken from
the MCM, with additional updated SCI chemistry. Based on reported reaction
rates of ozonolysis products with SCIs, these analyses indicate that the
only reaction partners likely to compete significantly with SO2,
H2O or unimolecular decomposition under the experimental conditions
applied here are organic acids (i.e. HCOOH and CH3COOH); these formed
during the experiments, at concentrations reaching up to 2.5 × 1012 cm-3. All other potential co-reactants listed above were
calculated to account for < 10 % (for the worst-case run) of the
total SCI loss under the experimental conditions applied.
Model runs were performed in which a rate constant of 1.1 × 10-10 cm3 s-1 was used for reaction between SCIs and formic
and acetic acids (HCOOH, CH3COOH), as given by Welz et al. (2014) for
CH2OO + HCOOH, together with an acid yield of 0.5 from the reactions
of isoprene-derived SCI species with water, which gave a good agreement with
the experimentally determined acid yields measured by FTIR. The reduction in
SO2 loss between the model runs with the SCI + acid reaction
included, and those without the reaction, varied between 7 and 17 %.
Equation (3) can be extended to explicitly account for the presence of acids
by inclusion of a further term (Eq. 5). This requires a value for
k9/k2, the ratio of the rate constants for SCI reactions with acids
and with SO2. Here, we employ a value of 3.0, derived from the mean of
the recently reported rates of reaction of CH2OO with HCOOH and
CH3COOH (Welz et al., 2014), and the rate constant for CH2OO +
SO2 reported by Welz et al. (2012) – although in reality this
term represents potential reaction of all SCIs present with multiple acid
species. The acid concentrations are taken from FTIR measurements during the
experiments.
SCI+acid⟶k9Products
[SO2]1f-1-k9k2[Acid]=k3k2[H2O]+kd+Lk2
Figure 3 shows a fit of Eq. (5) to the data shown in Fig. 2, giving a
gradient of k3/k2 and an intercept of the (relative) rate of
SCI decomposition (kd+L)/k2. The results are well described by
the linear relationship (Eq. 5) across the full range of experimental
conditions. This suggests that the analytical approach described – of
treating the SCIs produced from isoprene ozonolysis as a single system –
provides a good quantitative description of the
ISOP-SCI / O3/ H2O / SO2 system under atmospheric boundary layer
conditions, and hence provides a good approximation for use in atmospheric
modelling studies. Reaction with the water dimer is not considered in this
analysis (see discussion below). From Fig. 3 it is apparent that the
observations can be described well by a linear dependence on [H2O]
across the full range of experimental conditions applied. However, the
humidity levels accessible in these experiments were limited (constrained by
the operational range of the FTIR retrievals), and [H2O] can range up
to ∼ 1 × 1018 cm-3 in the atmosphere; the
derived relationship may work less well at these high RHs as the role of the
water dimer becomes more important; this is considered further in Sect. 3.3 (below), in which the SCI mix formed during isoprene ozonolysis is
separated into CH2OO and the other SCIs formed.
Isoprene-derived SCI relative and absolute rate constants derived
in this work a.
SCI
105k3/k2
1015k3 (cm3 s-1)
10-11kd/k2 (cm-3)
kd (s-1)
k8/k2
1010k8 (cm3 s-1)
CH2OOb
3.3 (±1.1)
1.3 (±0.4)
-2.3c (±3.5)
-8.8c (±13)
ISOP-SCI
3.1 (±0.5)
1.2 (±0.2)
3.0 (±3.2)
12 (±12)
3.5 (±2.2)
1.4 (±0.7)
CRB-SCI
2.9 (±0.7)
1.1 (±2.7)
6.6 (±7.0)
26 (±27)
Uncertainty ranges (±2σ, parentheses) indicate combined
precision and systematic measurement error components. a Scaled to an
absolute value using k2(CH2OO) = 3.9 × 10-11 cm3 s-1 (Welz et al., 2012). b From Newland et al. (2015). c Values are indistinguishable from zero within the
measurement uncertainties.
From Fig. 3, the derived relative rate constant for reaction of ISOP-SCI
with water vs. SO2, k3/k2, is 3.1 (±0.5) × 10-5 (Table 2). Newland et al. (2015) recently reported a
k3/k2 relative rate constant for CH2OO of 3.3 (±1.1) × 10-5 using the same experimental approach as used in this
study. The value derived for ISOP-SCI here is the same, within uncertainty,
as that derived for CH2OO, suggesting that the other SCIs formed during
isoprene ozonolysis have a mean k3/k2 similar to that of CH2OO.
No absolute values of k2 (SCI+SO2) have been measured for
ISOP-SCI. However, Welz et al. (2012) obtained an absolute value of k2
(298 K) for CH2OO (3.9 × 10-11 cm3 s-1),
using direct methods at reduced pressure (a few Torr). If this value is used
as an approximation for the k2 value of ISOP-SCI (at atmospheric
pressure and ambient temperature), then a k3 (ISOP-SCI + H2O)
value of 1.2 (±0.2) × 10-15 cm3 s-1 is
determined (assuming the reaction between ISOP-SCI and water vapour is
dominated by reaction with the water monomer, rather than the dimer, as
discussed above).
From Eq. (5), the intercept in Fig. 3 gives the term (kd+L)/k2.
(kd+L) will be dominated by kd under the experimental conditions
applied and analysis extrapolation to the start of each experimental run;
however, the possibility of other chemical loss processes (see below)
dictates that the derived value for kd is technically an upper limit.
From Fig. 3, kd/k2 is determined to be 3.0 (±3.2) × 1011 cm-3 (Table 2). Using the k2 value
determined by Welz et al. (2012) to put kd/k2 on an absolute scale
(as above for k3) yields a kd of ≤12 (±12) s-1.
Newland et al. (2015) recently determined kd for CH2OO to be ≤ 4.7 s-1. This suggests
that kd for the non-CH2OO
SCIs within the ISOP-SCI family is relatively low, i.e. a few tens per second, and/or
that CH2OO dominates the ISOP-SCI population. The limited precision
obtained for these kd values reflects the uncertainty in the intercept
of the regression analysis shown in Fig. 3.
Sipilä et al. (2014) recently reported a value of
kloss/k2 for isoprene ozonolysis-derived SCIs, treated using a
single-SCI approach, which is analogous to the value (k3[H2O] +
kd)/k2 reported in this section. They derive a value of 2.5 (±0.1) × 1012 cm-3 at [H2O]
= 5.8 × 1016 cm-3. From the k3 and kd values derived
in the single-SCI analysis in this work (Table 2), we calculate a value of
2.1 (±0.6) × 1012 cm-3 at the same [H2O], in
good agreement.
The results presented here suggest that while SCI- and conformer-specific
identification is important to determine the product yields, it does not
appear to be important when solely considering the combined effects of
isoprene ozonolysis products on the oxidation of SO2 under the
experimental conditions applied.
Analysis 2: two-SCI species (CH2OO + CRB-SCI) treatment
In the preceding section, the combined effects of the five SCIs initially
produced during isoprene ozonolysis were treated as a single pseudo-SCI,
ISOP-SCI. In this section an alternative approach is presented, in which the
SCI family is split into two components. These are CH2OO, for which
the reaction rates with water and the water dimer have been quantified in
recent experimental studies, and the sum of the MVKOO and MACROO SCI,
denoted CRB-SCI.
To date, the effects of the water dimer, (H2O)2, have only been
determined experimentally for CH2OO (Berndt et al., 2014; Chao et al.,
2015; Lewis et al., 2015; Newland et al., 2015). Theoretical calculations
(Vereecken et al., 2012) predicted the significant effect of the water dimer
compared to the monomer for CH2OO, but also that the ratio of the SCI
+ (H2O)2: SCI + H2O rate constants, k5/k3, of
the larger, more substituted SCIs, anti-CH3CHOO and (CH3)2COO, are
2–3 orders of magnitude smaller than for CH2OO (Vereecken et al.,
2012). This would make the dimer reaction negligible at atmospherically
accessible [H2O] (i.e. < 1 × 1018 cm-3) for
SCIs larger than CH2OO. The results presented in Sect. 3.2 show that,
under the single-SCI treatment of the isoprene ozonolysis SCI chemistry, a
water-monomer-only approach is able to describe the experimental data. Hence
the effect of the water dimer reaction on CRB-SCI is not considered in this
analysis (the water dimer reaction is included for CH2OO).
[SO2]1f-1-k9k2[Acid]=γAk3A[H2O]+k5A[(H2O)2]+(kdA+LA)k2A+γCk3C[H2O]+(kdC+LC)k2A
where A denotes CH2OO and C denotes CRB-SCI.
Application of Eq. (5) to derive relative rate constants for
reaction of the sum of the MVKOO and MACROO SCI (CRB-SCI) with the water
monomer, and the decomposition rate. Red line: water-monomer-only reactions;
blue dashed line: water monomer reaction and CH2OO water dimer reaction
rate from Newland et al. (2015); green dotted line: CH2OO water dimer
reaction rate from Chao et al. (2015). Shaded areas indicate reported
uncertainties on dimer reaction rates. Y = [SO2]((1/f)-1)-k9[acid] /k2.
Figure 4 shows three fits, obtained using Eq. (6) and corresponding to
different treatments for the reaction of CH2OO with H2O and with
(H2O)2, to the measured data presented in Fig. 3. For all three
scenarios, the relative contribution of the two SCI components to the total
SCI yield (γ) was assumed to be γA=0.54 and
γC=0.46, after Zhang et al. (2002).
k3A/k2A is assumed to be 3.3 × 10-5 after Newland et al. (2015).
The solid red line in Fig. 4 is a linear fit to the data to determine
k3C and kdC. The CH2OO + (H2O)2 rate
constant, k5A, was assumed to be zero to reduce the number of free
parameters. This assumption is reasonable considering the apparent linear
dependence of the presented measurements on [H2O] across the full range
of conditions applied. The linear fit determines a value of
k3C/k2A=2.9 (±0.7) × 10-5 and a value of
(kdC+LC)/k2A(CRB-SCI) = 6.6 (±7.0) × 1011 cm-3 (Table 2). Again, as for the single species
analysis, the decomposition term is poorly constrained.
The dashed blue line fits Eq. (6) using the parameters derived above for
CRB-SCI and the water dimer relative reaction rate for CH2OO determined
in Newland et al. (2015), k5/k2=0.014 (±0.018). This still gives
a good fit to the data in Fig. 4. The dotted green line is a similar fit
but uses the recently directly determined CH2OO + (H2O)2
rate, k5A, of 6.5 (±0.8) × 10-12 cm3 s-1 by
Chao et al. (2015). It is seen that this fit considerably overestimates
the observations at higher [H2O]. However, owing to the quadratic
relationship of [(H2O)2] to [H2O], a small difference in the
rate constant can have a large effect, especially at higher [H2O].
Possible explanations for this discrepancy are (i) that the kinetics
observed for CH2OO as formed from CH2I2 photolysis are not
representative of the behaviour of the CH2OO moiety as formed
through alkene ozonolysis (although the conditions are such that a
thermalised population would be expected in both cases); (ii) that the
fraction of the total isoprene SCI yield that is CH2OO is lower than
that predicted by Zhang et al. (2002), and hence the effect of the (H2O)2
reaction overall is reduced – however, the predicted yield is in good
agreement with those determined experimentally, albeit using indirect
methods, so it seems unlikely that the actual CH2OO yield is
considerably lower; and (iii) that multiple effects are affecting the curvature of
the results shown in Fig. 4. Analogous plots for CH3CHOO shown in
Newland et al. (2015) displayed a shallowing gradient with increasing [H2O]
(i.e. the opposite curvature to that caused by the (H2O)2 reaction).
The probable explanation for the curvature observed for CH3CHOO is the
presence of a mix of syn and anti conformers (Scheme 2) in the system and the
competing effects of the different kinetics of these two distinct forms of
CH3CHOO. A similar effect may arise for the isoprene-derived CRB-SCI
which include multiple syn and anti conformers (see Scheme 2). The competition of
this effect with that expected from the water dimer reaction may effectively
lead to one masking the other under the experimental conditions applied.
Rate data for the reactions of isoprene-derived SCIs obtained using both
analytical approaches described are given in Table 2.
Atmospheric implications
Treatment of the SCIs produced from isoprene ozonolysis as a single-SCI
system appears to describe the observations well over the full range of
experimental conditions accessible in this work (Sect. 3.2). The derived
values for k3(ISOP-SCI) reported here, obtained by fitting Eq. (5)
to the measurements, placed on an absolute basis using the measured
k2(CH2OO + SO2) of 3.9 × 10-11 cm3 s-1; Welz et al., 2012), corresponds to a loss rate for ISOP-SCI from
reaction with H2O in the atmosphere of 340 s-1 (assuming
[H2O] = 2.8 × 1017 molecules cm-3, equivalent to
an RH of 65 % at 288 K). Comparing this to the derived kd value, 12
(±12) s-1, it is seen that reaction with H2O is predicted
to be the main sink for isoprene-derived SCI in the atmosphere, with other
sinks, such as decomposition and other bimolecular reactions, being
negligible. Hence kd is neglected in the following analysis.
An estimate of a mean steady-state ISOP-SCI concentration in the background
atmospheric boundary layer can be calculated using Eq. (7).
[ISOP-SCI]ss=[Isoprene][O3]k1ϕk3[H2O]
Using the data given below, a steady-state SCI concentration of 4.1 × 102 molecules cm-3 is calculated for an isoprene
ozonolysis source. This assumes an ozone mixing ratio of 40 ppbv, an
isoprene mixing ratio of 1 ppbv, an SCI yield φ of 0.56, and a
reaction rate constant k1 (isoprene – ozone) of 1.0 × 10-17 cm3 s-1 (288 K) (Atkinson et al., 2006), k2
(ISOP-SCI + SO2) of 3.9 × 10-11 cm3 s-1, and
k3 (ISOP-SCI + H2O) of 1.2 × 10-15 cm3 s-1, and with [H2O] of 2.8 × 1017 cm-3 (RH
∼ 65 % at 288 K). A typical diurnal loss rate of SO2
to OH (kOH[OH]) is 9 × 10-7 s-1 (Welz et al., 2012),
while the SO2 loss rate arising from reaction with ISOP-SCI, using the
values above, would be 1.6 × 10-8 s-1. This suggests, for
the conditions given above, the diurnally averaged loss of SO2 to SCIs to be a very small fraction (1–2 %) of that due to OH.
This analysis neglects additional chemical sinks for SCIs, which would reduce
SCI abundance, and the possibility of other alkene ozonolysis products
leading to SO2 oxidation, which may increase the impact of alkene
ozonolysis upon gas-phase SO2 processing (Mauldin III et al., 2012; Curci
et al., 1995; Prousek, 2009). However, the analysis also neglects additional
sources of SCIs, e.g. photolysis of alkyl iodides (Gravestock et al., 2010; Stone
et al., 2013), dissociation of the DMSO peroxy radical
(Asatryan and Bozzelli, 2008; Taatjes et al., 2008), and reactions of peroxy
radicals with OH (Fittschen et al., 2014), which are currently poorly
constrained and may even dominate SCI production over an ozonolysis source
in some environments.
SCI concentrations are expected to vary greatly depending on the local
environment, e.g. alkene abundance may be considerably higher (and with a
different reactive mix of alkenes giving a range of structurally diverse
SCIs) in a forested environment, compared to a rural background. Furthermore,
isoprene emissions exhibit a diurnal cycle in forested environments owing to
a strong temperature dependence; hence they are predicted to change significantly
in the future as a response to a changing climate and other environmental
conditions (Peñuelas and Staudt, 2010).
Cumulative consumption of SO2 and O3, ΔSO2
versus ΔO3, for the ozonolysis of isoprene in the presence of
SO2 at a range of DMS concentrations, from 6 to 55 ppbv.
[H2O] in all experiments was < 9 × 1015 cm-3. Markers are experimental data, corrected for chamber dilution.
Solid lines are smoothed fits to the experimental data.
Isoprene + ozone as a function of DMS
Results
A series of experiments analogous to those reported in Sect. 3 were
performed as a function of dimethyl sulfide concentration, [DMS], rather
than [H2O]. Figure 5 shows that SO2 loss in the presence of
isoprene and ozone is increasingly inhibited by the presence of greater
amounts of DMS. Under the experimental conditions applied, it is assumed
that the SCIs produced in isoprene ozonolysis are reacting with DMS in
competition with SO2 (Reaction R8).
Equation (4) is analogous to Eq. (3) but for varying [DMS] rather than
[H2O]. However, as for the isoprene + O3 as a function of water
experiments described in Sect. 3, there is potential for the acid products
of the isoprene ozonolysis reaction to provide an additional sink for SCIs in
the chamber. Using the same methodology as described in Sect. 3.2, an
explicit acid term was included in Eq. (4) to give Eq. (8).
[SO2]1f-1-k9k2[Acid]=k8k2[DMS]+k3k2[H2O]+kd+Lk2
Application of Eq. (8) to derive rate constants for reaction
of ISOP-SCI with DMS (k8) relative to that for reaction with SO2. Y= [SO2]((1/f)-1)-k9[acid] /k2.
Figure 6 shows a fit of Eq. (8) to the experimental data. This yields a
gradient of k8/k2 and an intercept of (k3[H2O] +
kd+L)/k2. The derived relative rate constant of
k(SCI+DMS)/k(SCI+SO2), k8/k2, using this method is 3.5
(±1.8). Using the absolute value of k2(CH2OO + SO2)
derived by Welz et al. (2012) (as described previously) determines a value of
k8=1.4 (±0.7) × 10-10 cm3 s-1 (Table 2).
The intercept of the linear fit in Fig. 6 is 1.0 (±1.7) × 1012 cm-3. This represents (k3[H2O] +
kd+L)/k2 and hence can also be compared with the kinetic
parameters derived in Sect. 3 from the isoprene + O3 as a function
of H2O experiments. From Fig. 3, (kd+L)/k2=3.0 (±3.2) × 1011 cm-3 and k3[H2O] /k2=2.5
(±0.4) × 1011 cm-3 (with [H2O] = 8 × 1015 cm-3, the mean of the values for the five DMS
experiments (6.7–8.8 × 1015 cm-3)), giving a combined
value of 5.5 (±3.2) × 1011 cm-3. These two values
therefore agree within the precision of the data.
Experimental uncertainties
As noted above, this analysis assumes that the multiple SCI species in
reality present in the ozonolysis system may be analysed as a single species
(or exhibit the same reactivity). While the data indicate that this
approximation satisfactorily describes the observed behaviour under the
conditions applied, other work (e.g. Taatjes et al., 2013) has shown that
reactivity of different SCIs, and different conformers of the same SCIs, can
differ, affecting the retrieval of kinetics in multi-SCI ozonolysis systems;
Newland et al. (2015) illustrate this effect in the case of syn- and
anti-CH3CHOO. Similarly, the response of the SCI population to reaction
with organic acids is approximated by a single reaction with those species
observed (i.e. HCOOH, CH3COOH). A further assumption made is that the
mean isoprene–SCI + SO2 reaction rate may be represented by that
directly measured for CH2OO with SO2 (Welz et al., 2012). These
approximations introduce systematic uncertainty into the derived rate
constants, but given the lack of fundamental data for individual SCI
isomers, it is not possible to evaluate this. The data obtained are well
within the capability of the experimental approaches: DMS levels were
inferred from the known volumetric addition to the chamber and are thought
unlikely to be significantly in error. O3 and isoprene were monitored
using well-established techniques at levels well above their detection
limits. The observed changes in SO2 removal upon addition of DMS (as
shown in Fig. 5) were substantial, well in excess of the sensitivity limit
and uncertainty of the SO2 monitor. However, it is important to note
that no constraints regarding the products of the proposed DMS + SCI
reaction were obtained; OH reaction with DMS is complex, proceeding through
both abstraction and addition/complex formation channels, the latter
rendered partially irreversible under atmospheric conditions through
subsequent reaction with O2 (Sander et al., 2011). The observed
behaviour (Fig. 5) is not consistent with reversible complex formation
dominating the SCI-DMS system under the conditions used; however it is
possible that decomposition of such a complex to reform DMS, or its further
reaction (e.g. with SO2, analogous to the secondary ozonide mechanism
proposed by Hatakeyama et al., 1986), would be consistent with the observed
data, and also imply that the reaction may not lead to net DMS removal.
Time-resolved laboratory measurements and product studies are needed to
provide a test of this mechanistic possibility.
Discussion and atmospheric implications
To the authors' knowledge, this is the first work to show the relatively
fast (in relation to other recently determined SCI bimolecular reactions,
e.g. SCI + SO2 and NO2, and the well-established OH + DMS
reaction) rate of reaction of SCIs with DMS, although the products have yet
to be identified. While this work presents only SCIs derived from isoprene
ozonolysis, it seems likely that the fast reaction rate will apply to all
SCIs (though the precise rate will be structure-dependent).
DMS is mainly produced as a by-product of phytoplankton respiration, and so
the highest concentrations are found in marine coastal environments or above
active phytoplankton blooms. Furthermore, Jardine et al. (2015)
recently showed that DMS mixing ratios within and above a primary Amazonian
rainforest ecosystem can reach levels of up to 160 pptv, in canopy and above
the surface, for periods of up to 8 h during the evening and into the
night, with levels peaking at 80 pptv above canopy.
SCIs can also be expected to be present in the marine environment. As already
discussed, mixing ratios of isoprene (Sinha et al., 2007; Yassaa et al.,
2008) and monoterpenes (Yassaa et al., 2008) have been reported to reach in
the region of hundreds of pptv over active phytoplankton blooms in the
marine boundary layer. Additionally, the emission of small alkenes from
coastal waters has been observed (Lewis et al., 1999). Furthermore, the
photolysis of alkyl iodides (prevalent in the coastal environment; Jones et
al., 2010) may be a significant source of SCIs (Stone et al., 2013).
Berresheim et al. (2014) suggested that small SCIs derived from
alkyl iodide photolysis may be responsible for observed H2SO4
production, in excess of that expected from measured SO2 and OH
concentrations, at the coastal atmospheric observatory Mace Head, Ireland.
Jones et al. (2014) proposed SCIs produced from alkyl iodide
photolysis as a possible source of surprisingly high formic acid
concentrations observed in the marine environment in the European Arctic.
Other non-ozonolysis sources of SCIs include dissociation of the DMSO peroxy radical (Asatryan and Bozzelli, 2008; Taatjes et
al., 2008) (which could be an important source in the marine environment,
where DMSO is an oxidation product of OH + DMS), and potentially from
reactions of peroxy radicals with OH in remote atmospheres (Fittschen et
al., 2014).
From the analysis in Sect. 3.4 a concentration of ISOP-SCI of 4.1 × 102 molecules cm-3 was calculated, assuming an isoprene
concentration of 1 ppbv. In a remote marine environment isoprene
concentrations are probably an order of magnitude lower than this, and
consequently [ISOP-SCI] would be calculated to be on the order of 4 × 101 molecules cm-3. However, some regions will be
impacted by both high isoprene and DMS concentrations, for example tropical
islands, such as Borneo, which can have high isoprene concentrations and are
strongly influenced by marine air masses (MacKenzie et al., 2011), as well
as significant terrestrial sources from vegetation and soils in the Amazon,
especially into the evening and at night (Jardine et al., 2015), when
ozonolysis chemistry is at its most effective relative to photochemical OH
chemistry. High sulfate composition of organic aerosols collected from the
Borneo rainforests likely arises from the chemical processing of oceanic
emissions of DMS and SO2 (Hamilton et al., 2013). The sulfate
content of aerosols was observed to increase further over oil palm
plantations in Borneo, where isoprene concentrations may reach levels on the
order of tens of ppbv (MacKenzie et al., 2011), indicating scope for alkene
ozonolysis–DMS chemical interactions to become significant. If a
diurnally averaged [OH] is taken as 5 × 105 molecules cm-3, then the loss rate of DMS to OH
is ∼ 3.5 × 10-6 s-1, while the loss to ISOP-SCI, at a concentration of 1 × 102 cm-3, is ∼ 2 × 10-8 s-1,
i.e. about 0.4 % of the loss to OH. However in an environment with
particularly high isoprene mixing ratios, such as over the oil palm
plantations in Borneo, this could rise to a few percent.
SCIs derived from isoprene ozonolysis are unlikely to compete with OH during
the daytime or NO3 during the night, as an oxidant of DMS. However,
alternative SCI sources have been suggested which may lead to significantly
higher SCI concentrations in marine environments those predicted from
ozonolysis alone. Further investigation is required to clarify the reasons
for the observed discrepancies in SO2 and DMS oxidation and the
possibility that these may be, at least in part, explained by the presence
of SCIs, dependent on the products of SCI–DMS interactions. SCIs are most
likely of a similar importance to other minor reaction channels for DMS
processing such as reaction with atomic chlorine or BrO, reported to have a
reaction rate constant of ∼ 3.4 × 10-10 cm3 molecule-1 s-1 at 298 K (Atkinson et al., 2004) and marine boundary
layer concentrations on the order of 103–104 molecules cm-3
(von Glasow and Crutzen, 2007). SCIs may be most important for
DMS oxidation during the evening period and early morning periods, when OH
and NO3 production are both relatively low.