Introduction
Organosulfur compounds have been found to contribute a significant mass
fraction of atmospheric organic compounds. A maximum organosulfur
contribution of 30 % to PM10 organic mass was estimated at a forest
site in Hungary by calculating the difference between total sulfur and
inorganic sulfate (Surratt et al., 2008). Using a similar approach, Tolocka and Turpin (2012)
estimated that organosulfur compounds contribute up to 5–10 % of the
total organic mass in southeastern United States, while Shakya and Peltier (2013) reported that organosulfur compounds account for about 1–2 % of
organic carbon in Fairbanks, Alaska. Given their high atmospheric
abundances, it is crucial to understand the composition, formation, and
transformation of organosulfur compounds in the atmosphere.
Organosulfates have been identified as one of the major organosulfur
compounds. The detection of organosulfates in laboratory studies has been
complemented by a number of field observations, which confirm the presence
of organosulfates in atmospheric particles (Iinuma et al., 2007; Surratt et al., 2007, 2008, 2010; Chan et al., 2010; Froyd et al., 2010; Hawkins et al., 2010;
Darer et al., 2011; Frossard et al., 2011; Olson et al., 2011; Stone et al., 2012; Shakya and Peltier, 2013; Budisulistiorini et al., 2015;
Hettiyadura et al., 2015; Huang et al., 2015; Kuang et al., 2016; Rattanavaraha et al., 2016; Riva et al., 2016). Various possible reaction
pathways by which organosulfates form have been suggested. For instance,
Iinuma et al. (2009) and Surratt et al. (2010) showed that organosulfates
can be effectively formed via the reactive uptake of gas-phase epoxides,
which are formed from the photooxidation of various biogenic volatile
organic compounds (e.g., isoprene, α-pinene, and β-pinene),
onto the acidic sulfate seed particles. Rudziński et al. (2009) and
Nozière et al. (2010) suggested that the reactions between sulfate
radical anion and reaction products of isoprene (e.g., methyl vinyl ketone
and methacrolein) can yield a variety of organosulfates.
While the abundance, composition, and formation mechanisms have extensively
been investigated, there is comparably little work understanding how
organosulfates chemically transform in the atmosphere. Organosulfates are
primarily present in the particle phase owing to their low volatility
(Huang et al., 2015; Estillore et al., 2016). They can continuously react with gas-phase oxidants such as hydroxyl (OH)
radicals, ozone (O3), and nitrate (NO3) radicals at or near
the particle surface throughout their atmospheric lifetime. These
heterogeneous oxidative processes have been found to change the size,
composition, and physiochemical properties of both laboratory-generated
organic particles and atmospheric particles (Rudich et al., 2007; George and Abbatt, 2010; Kroll et al., 2015). To gain a better
understanding of how organosulfates chemically transform through
heterogeneous oxidation in the atmosphere, this work investigates the
heterogeneous OH radical-initiated oxidation of sodium methyl sulfate
(CH3SO4Na) particles, the smallest organosulfate detected in
atmospheric particles, using an aerosol flow tube reactor at a high relative
humidity (RH) of 85 %. A soft atmospheric pressure ionization source
(direct analysis in real time, DART) coupled with a high-resolution mass
spectrometer was employed to characterize the molecular composition of the
particles before and after OH oxidation in real time. Sodium methyl sulfate
is detected in atmospheric particles with a concentration of 0.7 and 0.34 ng m-3 during daytime and nighttime, respectively, in
Centreville, Alabama (Hettiyadura et al., 2015). As shown in Table 1, the simple structure of sodium
methyl sulfate allows us to gain a more fundamental understanding of the
heterogeneous oxidation kinetics and chemistry. The sodium salt of methyl
sulfate could be considered as atmospherically relevant since a positive
correlation between sodium ion and organosulfates has been observed over the
coastal areas (Sorooshian et al., 2015; Estillore et al., 2016). The effects of salt (e.g., ammonium and potassium salt) on
the heterogeneous oxidative kinetics and chemistry are also of atmospheric
significance and warrant future study.
Experimental method
An atmospheric pressure aerosol flow tube reactor was used to investigate
the heterogeneous OH oxidation of sodium methyl sulfate droplets. Detail
experimental procedures have been described previously (Davies and Wilson, 2015; Chim et al., 2017). Briefly, aqueous
droplets were generated by a constant output atomizer and mixed with
humidified nitrogen (N2), oxygen (O2), ozone (O3), and hexane
(a gas-phase OH tracer) before introducing into the reactor. The RH inside
the reactor was maintained at 85 % and at a temperature of 20 ∘C. Estillore et al. (2016) measured the hygroscopicity of sodium methyl
sulfate particles and showed that the particles absorb or desorb water
reversibly upon increasing or decreasing RH. These observations suggest that
the particles likely exist as aqueous droplets over a range of RH (10 to 90 %).
In our experiments, since the sodium methyl sulfate droplets are
always exposed to a high RH, they are likely aqueous droplets prior to OH
oxidation.
Sodium methyl sulfate droplets were oxidized inside the reactor by gas-phase
OH radicals that were generated by the photolysis of O3 under
ultraviolet light (254 nm) illumination in the presence of water vapor. The
OH concentration was regulated by changing the O3 concentration and
determined by measuring the decay of hexane using gas chromatography coupled
with a flame ionization detector. The OH exposure, defined as the product of
OH concentration, [OH], and the particle residence time, t, was determined by
measuring the decay of the gas-phase tracer, hexane (Smith et al., 2009).
OH Exposure=-ln[Hex]/Hex0kHex=∫otOHdt,
where [Hex] is the hexane concentration leaving the reactor, [Hex]0 is the
initial hexane concentration, and kHex is the second-order rate constant
of the gas-phase OH–hexane reaction. The aerosol residence time was
determined to be 1.3 min, and the OH exposure was varied from 0 to 1.27 × 1012 molecule cm-3 s.
The particle stream leaving the
reactor was passed through an annular Carulite catalyst denuder and an
activated charcoal denuder to remove O3 and gas-phase species,
respectively.
A portion of the particle stream was sampled by a scanning mobility particle
sizer (SMPS) for particle size distribution measurements. The remaining flow
was delivered into a stainless-steel tube heater, where the particles were
vaporized at 350–400 ∘C. Sodium methyl sulfate particles were
confirmed to be fully vaporized upon heating at 300 ∘C or above
by measuring the size distribution of the particles leaving the heater with
the SMPS in a separate experiment. The resulting gas-phase species were
directed to an ionization region, a narrow open space between the DART
ionization source (IonSense: DART SVP), and the inlet orifice of the
high-resolution mass spectrometer (Thermo Fisher, Q Exactive Orbitrap).
The details of the DART operation have been described elsewhere (Cody et al., 2005). The DART
ionization source was operated in the negative-ion mode. Helium was chosen
as the ionizing gas and entered an ionization chamber, where a high electric
potential of 4 kV was applied. This generates a glow discharge containing
ions, electrons, and metastable helium atoms. A potential of 200 V was
applied to two electrostatic lenses to remove ions and only the metastable
helium atoms exited the chamber. The gas stream was heated to 500 ∘C before leaving the ionization source. The metastable helium
atoms are responsible for ionizing the gas-phase species in the ionization
region (Chan et al., 2014; Cheng et al., 2015, 2016). For ionic compounds like sodium methyl sulfate, negative ions can
be formed via direct ionization in the negative-ion mode (Hajslova et al., 2011); for instance,
pyruvate ions have been detected from ammonium pyruvate using the DART
(Block et al., 2010).
We have run control experiments to investigate the potential volatilization
of the parent compound (i.e., sodium methyl sulfate) and the effect of ozone and
UV light on the composition of the aerosols before oxidation under the same
experimental conditions. To investigate the volatilization of sodium methyl
sulfate, we have measured the mass spectrum by filtering out the aerosols,
and the parent peak is very small, suggesting there is a very small amount of
sodium methyl sulfate present in the gas phase. Volatilization and gas-phase
oxidation of sodium methyl sulfate is expected to be not significant. For
the effect of ozone and UV light, we found that there is no change in
aerosol mass spectra in the presence of ozone without the UV light,
suggesting that the reaction of sodium methyl sulfate with ozone is not
significant. The aerosol mass spectrum is about the same as that obtained in
the absence of ozone with the UV light, suggesting that the photolysis of
sodium methyl sulfate aerosols is not likely to occur.
Results and discussions
Aerosol mass spectra
Figure 1 shows the aerosol mass spectra before and after oxidation. Before
oxidation (Fig. 1a), there is one major peak and some minor background peaks.
The largest peak at m/z 111 has a chemical formula of
CH3SO4-, which is corresponding to the negative ion (i.e.,
anionic form) of sodium methyl sulfate. Figure 1b shows that the intensity of
the parent compound decreases after oxidation. At the maximum OH exposure
(1.27 × 1012 molecule cm-3 s), only one new peak at
m/z 97 evolves, corresponding to the bisulfate ion (HSO4-). As
shown in Fig. 2, its intensity increases significantly after oxidation,
suggesting that the bisulfate ion is likely generated during the oxidation.
Based on the aerosol speciation data measured at different extents of OH
oxidation, oxidation kinetics will be determined in Sect. 3.2 and reaction
mechanisms will be proposed in Sect. 3.3 to explain the formation of major
ions detected in the aerosol mass spectra.
Aerosol mass spectra of sodium methyl sulfate before (a) and after
(b) oxidation.
The kinetic evolution of HSO4- and SO4- as a
function of OH exposure during the heterogeneous OH oxidation of sodium
methyl sulfate. The small uncertainty in ion intensity measurement for
SO4- is not visualized in the figure.
The normalized decay of sodium methyl sulfate as a function of OH
exposure during the heterogeneous OH oxidation.
Oxidation kinetics
The normalized parent decay as a function of OH exposure is shown in Fig. 3 and the OH
radical-initiated decay can be fitted using an exponential
function:
lnII0=-kOH⋅t,
where I is the ion signal at a given OH exposure, I0 is the ion signal
before oxidation, k is the second-order heterogeneous rate constant, and
[OH]⋅t is the OH exposure. The exponential k is determined to
be
(3.79 ± 0.19) × 10-13 cm3 molecule-1 s-1.
Assuming a 24-h average OH concentration of
1.5 × 106 molecule cm-3, the lifetime of sodium methyl
sulfate against heterogeneous OH oxidation is about 20 days. This timescale
is longer than other removal processes such as wet or dry deposition.
Laboratory studies have revealed that primary and secondary organosulfates
are stable against hydrolysis under atmospheric relevant aerosol acidities
and lifetimes, while tertiary organosulfates may undergo hydrolysis
efficiently (Hu et al., 2011). Since sodium methyl sulfate is a primary
organosulfate, it is expected to be stable against hydrolysis. These results
suggest that sodium methyl sulfate is likely chemically stable over
atmospherically relevant timescales. Studying the heterogeneous reactivity of
sodium methyl sulfate towards OH radicals provides a much-needed fundamental
understanding of the oxidation kinetics and pathways, and these data may be
applied in the interpretation of the oxidation of more complex
organosulfates, which may have a range of chemical lifetimes in the
atmosphere. The effective uptake coefficient, γeff, defined
as the fraction of OH collisions that yield a reaction, is computed (Davies
and Wilson, 2015):
γeff=23D0ρmfsNAMwcOH‾k,
where D0 is the mean surface-weighted particle diameter, ρ is the
aerosol density before oxidation, mfs is the mass fraction of solute, NA is
Avogadro's number, Mw is the molecular weight of sodium methyl
sulfate, and cOH‾ is the average speed of gas-phase OH
radicals. The mean surface-weighted particle diameter was 218 nm and
decreased slightly to 211 nm (about 3 % decrease) at the maximum OH
exposure (Fig. 4). Before oxidation, the composition of the droplets (i.e.,
mfs) is derived from the hygroscopicity data reported by Estillore et al. (2016). The particle growth factor, Gf, defined as the ratio of the
diameter at different RH to the dry particle at a reference RH (RH1),
is converted into mfs using the following equation (Ansari and Pandis, 2000; Peng and Chan,
2001):
Gf=mfsRH,1ρRH,1mfsRH,2ρRH,213,
where mfsRH,i and ρRH,i are the mass fraction of
solute and particle density at a given RH, respectively. It is assumed that
sodium methyl sulfate exists as an anhydrous particle at the reference RH
(RH < 10 %) (i.e., mfsRH,1= 1). The particle
density is estimated using the volume additivity rule with the density of
water and sodium methyl sulfate (1.60 g cm-3, Chemistry Dashboard,
2018) with an uncertainty of 20–30 %. The mfs is computed to be 0.34 at
85 %. Using Eq. (3), the γeff is calculated to be
0.17 ± 0.03. Although the γeff is less than 1, as will
be discussed in Sect. 3.3, secondary reactions are likely occurring, leading
to the formation and subsequent reactions of sulfate radical anions
(SO4⋅-).
The surface-weighted particle diameter of sodium methyl sulfate as
a function of OH exposure during heterogeneous OH oxidation.
Reaction mechanisms: OH reaction with sodium methyl sulfate
Sodium methyl sulfate tends to dissociate and exist in its ionic form
because of its high dissociation constant (pKa =-2.4). As shown in
Scheme 1, the oxidation is initiated by hydrogen abstraction from the methyl
group by the OH radical, forming an alkyl radical that quickly reacts with
an oxygen molecule to form a peroxy radical. Based on the well-known
particle-phase reactions (George and Abbatt, 2010), the self-reaction of two peroxy radicals can
form a carbonyl functionalization product (CHSO5-) via the Bennett
and Summers mechanism (Bennett and Summers, 1974) or form both alcohol (CH3SO5-) and
carbonyl functionalization products via the Russell reactions (Russell, 1957).
Alternatively, alkoxy radicals can be produced through the peroxy–peroxy
radical reactions. Once formed, the alkoxy radical can react with an oxygen
molecule to form the carbonyl functionalization product or abstract a
hydrogen atom from the neighboring molecules to form the alcohol
functionalization product. Furthermore, the alkoxy radical can undergo
fragmentation to yield a formaldehyde (CH2O) and a sulfate radical
anion (SO4⋅-). Alternatively, the decomposition of the
alkoxy radical, similar to the OH radical-initiated oxidation of simple
alkyl esters (Sun et al., 2012), could involve the rearrangement of the hydrogen atom from
the alkoxy radical carbon to the oxygen on the methoxy group. The
decomposition of the C-O bond from the methoxy group generates a bisulfate
ion and a formyl radical (CHO). The subsequent reactions of the formyl
radical can yield carbon monoxide (CO) and HO2. Like formaldehyde,
carbon monoxide is volatile and partitions back to the gas phase.
As shown in Fig. 1, neither functionalization nor fragmentation products
are detected. Formaldehyde has a mass which is below the mass range of the
mass spectrometer and, once formed, it is likely partitioned back to the gas
phase due to its high volatility. On the other hand, it is expected that the
two functionalization products can be detected by the DART ionization source if
they were formed in significant amounts. Additional experiments were
performed to verify whether the alcohol and carbonyl functionalization
products can be detected by the DART ionization source. We have measured the
heterogeneous OH radical-initiated oxidation of sodium ethyl sulfate
(C2H5SO4Na) under similar experimental conditions. As shown
in Fig. 5, the negative ions of the alcohol (C2H5SO5-)
and carbonyl (C2H3SO5-) functionalization products are
detected in the aerosol mass spectra. When the sodium ethyl sulfate is
oxidized (Fig. 6), the abundance of these two functionalization products
increases with increasing OH exposure (Fig. 7). Similar to sodium methyl
sulfate, the bisulfate ion has been detected and its intensity increases
after oxidation (Fig. 8). These results suggest that, if functionalization
products are formed during the OH oxidation of sodium methyl sulfate, they
could be detected by the DART ionization source.
Aerosol mass spectra of sodium ethyl sulfate before (a) and after
(b) oxidation.
The normalized parent decay of sodium ethyl sulfate as a function
of OH exposure in the heterogeneous OH oxidation.
The kinetic evolution of carbonyl (C2H3SO5-)
and alcohol (C2H5SO5-) functionalization products as a
function of OH exposure in the heterogeneous OH oxidation of sodium ethyl
sulfate. The small uncertainty in ion intensity measurement for
C2H5SO5- is not visualized in the figure.
The kinetic evolution of HSO4- and SO4- as a
function of OH exposure in the heterogeneous OH oxidation of sodium ethyl
sulfate. The small uncertainty in ion intensity measurement for
SO4- is not visualized in the figure.
Proposed reaction mechanism for heterogeneous OH oxidation of
sodium methyl sulfate (gray arrows denote the minor pathways).
The absence of the functionalization products in the aerosol mass spectra
suggests that the OH reaction with sodium methyl sulfate tends to undergo
fragmentation processes rather than functionalization processes. One
possibility is that, due to the presence of the bulky sulfate group relative to
the methyl group, reaction intermediates resulted from the self-reaction of
two peroxy radicals may not be easily arranged into appropriate
configurations (i.e., cyclic transition states), which are required for the
formation of the functionalization products via the Russell reaction or Bennett
and Summers mechanism. Alternatively, alkoxy radicals are more likely
formed, followed by fragmentation. Although the dissociation energies for
C–O and C–C bonds of sodium methyl sulfate are not known, fragmentation
processes could be enhanced since the decomposition of the alkoxy radical
involves the cleavage of a C–O bond, which is in general thought to be
weaker than a C–C bond (Dean and Lang, 1992). The bond dissociation energy of a C–O bond
might be lowered in the presence of a sulfur atom or sulfur–oxygen bearing
group (Oae and Doi, 1991; Dean and Lange, 1992). One possibility is that a sulfur atom or sulfur–oxygen bearing group
(e.g., sulfate) is more electronegative than a carbon atom, reducing the
electron density and bond strength of the C–O bond by the inductive effect.
Further investigation is required to better understand the effect of sulfate
group on the dissociation energies of the C–O bond for the sodium methyl
sulfate.
Proposed reaction mechanism of heterogeneous OH oxidation of
sodium ethyl sulfate (gray arrows denote the minor pathways).
Formation and reaction of sulfate radical anion in the OH reaction with
sodium methyl sulfate
Scheme 1 shows that sulfate radical anion (SO4⋅-) can be
formed via the fragmentation processes. The sulfate radical anion is a strong
oxidant in aqueous phase. For example, Huie and Clifton (1989) have reported
that hydrogen abstraction by sulfate radical anions on the alkane can result
in the formation of bisulfate ions. They also reported that the hydrogen
abstraction rate is the highest on the tertiary carbon, and the rate is
1 order of magnitude smaller for the secondary carbon, and even smaller for
the primary carbon. The second-order rate constants for SO4⋅-
reactions with alcohols, ethers, alkanes, and aromatic compounds typically
range in value from 106 to 109 M-1 s-1 (Neta et al.,
1977, 1988; Clifton and Huie, 1989; Padmaja et al., 1993). With an
aqueous-phase SO4⋅- concentration of 10-14 M (Herrmann
et al., 2000), the calculated lifetime toward aqueous-phase oxidation with
SO4⋅- ranges from 1.2 days to 3 years. These results suggest
that some organic compounds (e.g., alkanes and alkenes) are stable against
SO4⋅--initiated reactions, but some (e.g., alcohols and
ethers) can react with SO4⋅- efficiently. Future works are
needed to better understand the role of SO4⋅--initiated
oxidation chemistry in the chemical transformation of sodium methyl sulfate
and organic compounds in the atmospheric aerosols. For the OH reaction with
sodium methyl sulfate, it is proposed that sulfate radical anion, once
formed, can abstract a hydrogen atom from the neighboring, unreacted sodium
methyl sulfate, yielding the bisulfate ion, which has a small acid
dissociation constant in equilibrium with sulfate (SO42-) and
hydrogen (H+) ions (Ka = 1.2 × 10-2) (Brown
et al., 2012).
SO4⋅-+CH3SO4-→CH2SO4⋅-+HSO4-,HSO4-⇌H++SO42-.
Moreover, the sulfate radical anion may react with particle-phase water to
form a bisulfate ion and an OH radical (Tang et al., 1988).
SO4⋅-+H2O⇌OH⋅+HSO4-
As shown in Fig. 1b, the bisulfate ion is the second largest peak detected
in the aerosol mass spectrum and its intensity has been found to increase
significantly with increasing OH exposure (Fig. 2). The detection of the
bisulfate ion provides indirect evidence to support the formation and
subsequent reactions of sulfate radical ions. When these reactions occur
(Eqs. 5 and 7), additional sodium methyl sulfate is consumed by the sulfate
radical ions and OH radicals, contributing to the secondary chemistry. It is
also known that the self-reaction of two sulfate radical anions can yield a
peroxydisulfate ion (S2O82-) (Hayon et al., 1972; Tang et al., 1988; Huie et al., 1989, 1991; Das, 2001):
SO4⋅-+SO4⋅-→S2O82-.
Based on its mass-to-charge ratio, the peroxydisulfate ion can be detected
as SO4- at m/z 96 in the aerosol mass spectra (Fig. 1). It is worth
noting that the peak at m/z 96 does not likely originate from the sulfate
radical anions due to its high reactivity. The ion signal intensity of the
SO4- is measured to be smaller than that of the bisulfate ion
(Fig. 2). However, the abundance of these two ions cannot be directly
inferred from their intensities owing to their unknown ionization
efficiencies in the DART ionization source.
Sodium methyl sulfate vs. sodium ethyl sulfate: kinetics and chemistry
We here further examine the results of sodium methyl sulfate and sodium
ethyl sulfate to gain more insights into how the carbon number affects the
kinetics and chemistry for these two small organosulfates (C1 and
C2). Kinetic measurements show that the heterogeneous rate constant and
effective OH uptake coefficient of sodium ethyl sulfate are determined to be
(4.64 ± 0.29) × 10-13 cm3 molecule-1 s-1 and 0.19 ± 0.03, respectively (Table 1 and Fig. 6). These
kinetic parameters are slightly larger than that of sodium methyl sulfate
(3.79 ± 0.19 × 10-13 cm3 molecule-1 s-1
and 0.17 ± 0.03). An additional carbon atom does not significantly
change the heterogeneous OH reactivity. On the other hand, the composition
of the sodium ethyl sulfate (Fig. 5) is different from that of sodium
methyl sulfate after oxidation (Fig. 1). As discussed in Sect. 3.3, the
bisulfate ion (HSO4-) and SO4- have been observed for
both organosulfates. However, the alcohol and carbonyl functionalization
products are only detected in the OH oxidation of sodium ethyl sulfate.
These observations suggest the potential reaction pathways may change with
an increasing carbon number.
As shown in Scheme 2, at the first OH oxidation step of sodium ethyl
sulfate, the hydrogen abstraction can occur either on the primary (Scheme 2,
path A) or the secondary carbon site (Scheme 2, path B). Depending on the
initial OH reaction site, two structural isomers of alcohol
(C2H5SO5-) and carbonyl (C2H3SO5-)
functionalization products can be formed. However, these isomers cannot be
differentiated by exact mass measurements. Although the preferential OH
reaction site is not well understood, we postulate that the formation of the
alcohol and carbonyl functionalization products are likely originated from
the hydrogen abstraction occurred at the primary carbon (Scheme 2, path A).
One likely explanation is that based on the knowledge of the OH reaction
with sodium methyl sulfate (Scheme 1), when the hydrogen abstraction occurs
at a carbon atom adjacent to the sulfate group, an alkoxy radical is likely
formed from the self-reaction of two peroxy radicals and tends to decompose.
It is hypothesized that when the hydrogen atom of the secondary carbon is
abstracted by the OH radical (Scheme 2, path B), similar to the sodium
methyl sulfate, an alkoxy radical is likely generated and fragments into a
sulfate radical anion and an acetaldehyde, which is volatile and likely
partitions back to the gas phase. The sulfate radical anion can subsequently
react with an unreacted sodium ethyl sulfate, leading to the formation of a
bisulfate ion (HSO4-, m/z 97). Alternatively, the self-reactions of
two sulfate radical anions can yield a peroxydisulfate ion
(S2O82-), which can be detected as SO4- at m/z 96 in
the aerosol mass spectra. Future works are needed to verify these
hypotheses.
The surface-weighted particle diameter of sodium ethyl sulfate as
a function of OH exposure during heterogeneous OH oxidation.
Aerosol mass lost via volatilization
While the fragmentation and volatilization processes are likely the dominant
reaction pathways of OH oxidation of sodium methyl sulfate, the diameter of
the particles decreases slightly from 218 to 211 nm at the maximum OH
exposure (Fig. 4). As shown in Scheme 1, when the fragmentation processes
occur, one methyl group is lost via volatilization in the form of
formaldehyde. The methyl group (CH3, Mw= 15 g mol-1)
contributes about 11 % of the total molecular mass (CH3SO4Na,
Mw= 134 g mol-1). At the maximum OH exposure, about 40 %
of sodium methyl sulfate is reacted (Fig. 3). If we assume that only
fragmentation processes occur during OH oxidation, this will lead to a 4.4 %
loss in particle mass via volatilization. The result of this simple
analysis is consistent with the experimental observation that only a small
decrease in particle size (∼ 3 %) is measured after
oxidation. For the sodium ethyl sulfate (Fig. 9), the particle diameter
decreases slightly from 203 to 195.5 nm at the maximum OH exposure
(∼ 4 % decrease in particle diameter). According to Scheme 2,
formaldehyde and acetaldehyde are the volatile fragmentation products,
which are likely partitioned back to the gas phase. Following the above
analysis, if we assume the fragmentation only leads to the formation and
volatilization of the acetaldehyde, this will lead to a maximum 9 % loss
in the particle mass at the highest OH exposure. Similar to sodium methyl
sulfate, the formation and volatilization of fragmentation products do not
cause a significant decrease in particle mass (and diameter) during OH
oxidation.