Evidence for Organosulfates in Secondary Organic Aerosol*
* This chapter is reproduced by permission from “Evidence for Organosulfates in Secondary Organic
Aerosol” by J. D. Surratt, J. H. Kroll, T. E. Kleindienst, E. O. Edney, M. Claeys, A. Sorooshian, N. L. Ng,
J. H. Offenberg, M. Lewandowski, M. Jaoui, R. C. Flagan, J. H. Seinfeld, Environmental Science and
Technology, 41: 517-527 2006. Copyright 2006. American Chemical Society.
Evidence for Organosulfates in
Secondary Organic Aerosol
J A S O N D . S U R R A T T ,†J E S S E H . K R O L L ,‡ , X
T A D E U S Z E . K L E I N D I E N S T ,§
E D W A R D O . E D N E Y ,§M A G D A C L A E Y S ,⊥
A R M I N S O R O O S H I A N ,|N G A L . N G ,|
J O H N H . O F F E N B E R G ,§
M I C H A E L L E W A N D O W S K I ,§
M O H A M M E D J A O U I ,#
R I C H A R D C . F L A G A N ,‡A N D
J O H N H . S E I N F E L D *, ‡
Department of Chemistry, California Institute of Technology,
Pasadena, California 91125, Departments of Environmental
Science and Engineering and Chemical Engineering,
California Institute of Technology, Pasadena, California
91125, National Exposure Laboratory, Office of Research and
Development, Environmental Protection Agency, Research
Triangle Park, North Carolina 27711, Department of
Pharmaceutical Sciences, University of Antwerp (Campus Drie
Eiken), Universiteitsplein 1, BE-2610 Antwerp, Belgium,
Department of Chemical Engineering, California Institute of
Technology, Pasadena, California 91125, and Alion Science
and Technology, P.O. Box 12313, Research Triangle Park,
North Carolina 27709
Recent work has shown that particle-phase reactions
contribute to the formation of secondary organic aerosol
(SOA), with enhancements of SOA yields in the presence of
of SOA from the photooxidations of R-pinene and
isoprene, in the presence or absence of sulfate seed
aerosol, is investigated through a series of controlled
chamber experiments in two separate laboratories. By using
electrospray ionization-mass spectrometry, sulfate
esters in SOA produced in laboratory photooxidation
experiments are identified for the first time. Sulfate esters
are found to account for a larger fraction of the SOA
mass when the acidity of seed aerosol is increased, a result
consistent with aerosol acidity increasing SOA formation.
Many of the isoprene and R-pinene sulfate esters
identified in these chamber experiments are also found in
ambient aerosol collected at several locations in the
southeastern U.S. It is likely that this pathway is important
for other biogenic terpenes, and may be important in the
Particle-phase reactions are now understood to play an
(1). Particle-phase oligomerization leads to the formation of
high-molecular-weight (MW) species (2-4); suggested oli-
aldehydes or ketones via peroxyhemiacetal formation (5, 6),
hydration, hemiacetal/acetal formation, and aldol conden-
sation (7, 8). Esterification in isoprene photooxidation (9,
10) has also been reported in SOA formation. The role of
these reactions remains in some doubt as some of the
proposed reactions (e.g., hemiacetal/acetal formation and
ambient conditions (11, 12).
Laboratory chamber studies have demonstrated that the
presence of acidic seed aerosol enhances the SOA yields
observed from the oxidation of various volatile organic
(8, 14, 15), and several model cycloalkenes (2) over those
with a less acidic seed aerosol. Despite recent advances in
understanding particle-phase SOA chemistry, the role of
particle-phase acidity in enhancing SOA formation remains
spectrometric evidence that the reactive uptake of glyoxal
for simplicity, we will use hereafter the term sulfate esters
to also denote sulfate derivatives; i.e. sulfate derivatives
formed from a carbonyl compound) (16-18). In addition,
aerosol collected on filters using Fourier transform infrared
spectrometry (MS) (21, 22). Nevertheless, the importance of
analytical methods, such as gas chromatography/mass
derivatization protocols, such as trimethylsilylation, GC
injection and column temperatures could cause the degra-
hand, ESI-MS has been shown as an effective method for
the detection and quantification of organosulfate species
In the present study ESI-MS is used to detect and
structurally elucidate sulfate esters in SOA formed from the
photooxidations of isoprene and R-pinene under differing
combinations of NOxlevels and seed aerosol acidities. As a
result, the formation of sulfate esters may be a major
contributor to the observed enhancement in SOA yields in
the presence of acidic aerosol.
conditions for all isoprene photooxidation experiments can
be found in Table 1. Isoprene photooxidation experiments
were conducted in Caltech’s dual indoor 28 m3Teflon
chambers (26, 27) and in EPA’s fixed volume 14.5 m3indoor
chamber (15). The temperatures, aerosol size distributions,
and relative humidities, as well as the O3, nitric oxide (NO),
NOx concentrations were continuously measured in both
experiments were conducted in the static mode (i.e., batch
reactor) whereas the EPA experiments were conducted in
Hydroxyl radical (OH) precursors (H2O2 or HONO) were
* Corresponding author phone: (626) 395-4635; fax: (626) 796-
2591; e-mail: firstname.lastname@example.org.
†Department of Chemistry, California Institute of Technology.
‡Departments of Environmental Science and Engineering and
Chemical Engineering, California Institute of Technology.
§Environmental Protection Agency.
⊥University of Antwerp.
|Department of Chemical Engineering, California Institute of
#Alion Science and Technology.
XCurrent address: Aerodyne Research, Inc., 45 Manning Road,
Billerica, MA 01281.
Environ. Sci. Technol. 2007, 41, 517-527
10.1021/es062081q CCC: $37.00
Published on Web 12/07/2006
2007 American Chemical SocietyVOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9517
used H2O2and an initial amount of NO (∼ 800 ppb), or with
three initial inorganic seed aerosol conditions were used:
by nucleation; (2) ammonium sulfate (AS) aerosol; and (3)
acidified ammonium sulfate (AAS) aerosol. Concentrations
of the aqueous solutions that were introduced into the
chambers by atomization are shown in Table 1. The initial
1.0-µm pore size, Teflon filters) were collected for offline
at which the aerosol volume reached its maximum value, as
determined by the differential mobility analyzer (DMA).
All experiments were carried out at relative humidities
high-pressure cylinders to the reaction chamber through a
mixing manifold. The steady-state nature of chamber opera-
tion allows for filter sampling for extended periods for
determining the composition of the resultant SOA. Once
steady-state conditions were attained (∼24 h), samples for
determining the composition of the SOA were collected on
glass fiber filters preceded by a carbon strip denuder. Two
sets of EPA experiments were conducted. In the first set, the
acid, with each of the aqueous solutions atomized into the
chamber by atomization. The initial aerosol concentrations
were 0.1, 30.0, and 30.0 µg/m3, for EPA-299 stage 1, EPA-299
of EPA experiments, EPA-199 stage 1 and EPA-199 stage 2,
acidic aerosol was generated by adding 60 and 200 ppb of
both EPA experiments (i.e., SOA yields, gas-phase products,
trends, etc.) will be discussed in more detail in forthcoming
publications; evidence for organosulfates is the focus here.
r-Pinene Chamber Experiments. All R-pinene experi-
ments were conducted in the EPA dynamic chamber (15).
Conditions for each experiment are listed in Table 2. The
of hydrocarbons containing R-pinene were irradiated in the
presence of NOx. For some of these experiments, SO2was
same collection protocol was used here as that employed in
the EPA isoprene experiments.
Ambient Aerosol Collection. Ambient aerosol was col-
lected from the Southeastern Aerosol Research and Char-
suburban) site pairs at locations across the southeast U.S.
and was initiated in mid-1998 to carry out systematic
TABLE 1. Summary of Experimental Conditions and Sulfate Ester Formation from Isoprene Photooxidation
[M-H]-detected sulfate ester ions (m/z)
EPA-299 stage 1
EPA-299 stage 2
153, 155, 169, 215, 333, 451
199, 215, 244
139, 153, 155, 197, 199, 215, 244, 260, 301, 346
199, 215, 260, 333
153, 155, 167, 169, 181, 197, 199, 215, 244,
153, 155, 157, 167, 169, 181, 197, 199, 215, 244,
197, 199, 215, 244, 260, 301, 317, 333
155, 169, 197, 199, 215, 244, 260, 301, 317,
EPA-299 stage 3H2SO4only
EPA-199 stage 1
EPA-199 stage 2
60 ppb SO2
200 ppb SO2
aAS ) 15 mM (NH4)2SO4; AAS ) 15 mM (NH4)2SO4+ 15 mM H2SO4for Caltech experiments and 0.31 mM (NH4)2SO4+ 0.612 mM H2SO4for
EPA-299 experiments; H2SO4only ) 0.92 mM H2SO4; EPA-199 had no seed nebulized but instead used the photooxidation of SO2to generate
TABLE 2. Summary of Experimental Conditions and Sulfate Ester Formation from r-Pinene Photooxidation
ester ions (m/z)a
EPA-211 stage 1
EPA-211 stage 2
EPA-211 stage 3
EPA-211 stage 4
EPA-211 stage 5
EPA-211 stage 6
249, 265, 294, 310, 412, 426
265, 279, 294, 310, 326, 412, 426
249, 265, 279, 294, 310, 326
265, 294, 310, 412, 426
aIsoprene sulfate ester products like those in Table 2 were also detected only when SO2and isoprene were copresent. No discernible toluene
sulfate ester products were detected.bThis compound was not present during the experiment.
5189ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 2, 2007
measurements of temporal and spatial variability of PM, in
particular PM2.5, gases relevant to secondary O3formation,
and surface meteorology (22). Twenty-four h composite
quartz filters were taken on 4 days at four different sites
during June 2004: Birmingham, AL (BHM, urban site),
Centerville, AL (CTR, rural site outside of BHM), Jefferson
Street (JST, near downtown Atlanta, GA), and Pensacola, FL
(PS, marine influenced urban site). Details of these sites
(terrain, vegetation, transportation, and industrial sources),
sample collection and handling procedures, and specific
aerosol and gas-phase measurements obtained are given
elsewhere (22, 30).
Filter Extraction and Chemical Analyses. Detailed ex-
elsewhere (9, 22). Glass-fiber filters were extracted in the
same manner as Teflon filters (9), except resultant extracts
were filtered through a PALL Life Sciences Acrodisc CR 25
mm syringe filter (PTFE membrane, 0.2 µm pore size) to
remove filter fibers. All sample extracts were analyzed by a
a ThermoElectron LCQ ion trap mass spectrometer (ITMS),
both equipped with an ESI source operated in the negative
(-) ionization mode. Details of the operating conditions for
these instruments are described elsewhere (9). Briefly, all
samples were analyzed on the LC/MS instrument in the full
mode of analysis. Comparison of the resulting mass spectra
produced from these two modes of analyses on the LC/MS
instrument allows for some structural information to be
obtained on the detected SOA components. Samples were
to confirm these results, and in some cases, provide further
structural elucidation. Sulfate standards of sodium propyl
sulfate (City Chemical, 98% purity), sodium lauryl sulfate
(City Chemical, 98% purity), and 1-butyl-3-methylimidazo-
lium 2-(2-methoxyethoxy)ethyl sulfate (Sigma-Aldrich, 95%
common product ions associated with sulfate esters. Ad-
sampler (PILS) with subsequent offline analysis by ion
chromatography (IC) (31). The PILS/IC technique allows for
so for these experiments only inorganic ions are measured.
sulfate esters produce abundant [M-H]-ions, and upon
80 (SO3-‚) product ions (21, 24, 25). In conjunction with the
of sodium propyl sulfate (anionic mass ) 139 Da), sodium
lauryl sulfate (anionic mass ) 265 Da), and 1-butyl-3-
methylimidazolium 2-(2-methoxyethoxy)ethyl sulfate (an-
ionic mass ) 199 Da) were analyzed by the (-)LC/ESI-MS
technique in the full scan mode of analysis followed by the
upfront CID mode of analysis to generate MS and MS/MS
data, respectively. As shown for the sodium propyl sulfate
m/z 97 and 80 product ions.
Sulfate Esters from Isoprene Oxidation. Previously
characterized (9, 10) isoprene SOA products were observed
focus here will be on the identification of sulfate esters.
employing no sulfate aerosol to those with sulfate aerosol
sulfate aerosol was present. To understand the nature of
FIGURE 1. (-)LC/ESI-MS upfront CID mass spectra for selected isoprene sulfate ester SOA products shown in Table 2. (A) Product ion
mass spectrum for sodium propyl sulfate standard (anionic mass of intact propyl sulfate ester ) 139 Da). (B) Product ion mass spectrum
for a 2-methyltetrol sulfate ester detected in a Caltech high-NOx H2O2 AS seed photooxidation experiment. (C) Product ion mass
spectrum for a hemiacetal dimer sulfate ester detected in a Caltech low-NOxAAS seed photooxidation experiment. (D) Product ion mass
spectrum for a C5trihydroxy nitrate sulfate ester detected in EPA-299 stage 2.
VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9519
TABLE 3. Proposed Isoprene Sulfate Ester SOA Products
in prior studies by Surratt et al. (9) and/or Szmigielski et al. (10) and/or Edney et al. (15).cDetected in ambient aerosol collected from SEARCH
are neutral losses observed upon (-)ESI-MS/MS.fInferred precursor due to the MS/MS fragmentation of its respective organosulfate product;
this parent isoprene product goes undetected by (-)ESI-MS and GC/MS methods.gSome evidence for its existence in first-order mass spectra.
hDetected in ambient aerosol by Matsunaga et al. (32).
5209ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 2, 2007
these compounds, tandem MS techniques were employed.
Figure 1B-D shows the LC/ESI-MS upfront CID mass
spectra collected for large chromatographic peaks common
to many of the sulfate aerosol experiments listed in Table 1.
The [M-H]-ions associated with these chromatographic
of these [M-H]-ions yielded m/z 97 and 80 product ions.
In addition, these ions also had an isotopic distribution
common to sulfur, and as a result, these compounds were
on the chemical structures of the identified sulfate esters.
Proposed sulfate ester structures for these ions and all other
esters shown in Table 3 were formed from previously
identified isoprene SOA products, including aldehydes,
dicarbonyls, hydroxycarbonyls, alcohols, and acids contain-
ing an alcohol moiety (9, 10, 32). Sulfate esters formed from
small volatile oxidation products, such as glyoxal, hydroxy-
acetone, and glycolaldehyde, were only detected in experi-
listed in Table 3 eluted from the reverse-phase LC column
within 3 min, indicating their high water solubility. For
example, the m/z 215 sulfate ester had a retention time of
∼1.4 min, close to that of the inorganic sulfate (first peak to
to be slightly less polar with isomers eluting at 2.4, 2.7, and
2.9 min. The presence of a nitrate group was confirmed by
its even-mass [M-H]-ion and the observation of a 63 Da
given in Table 3 containing nitrate groups were detected
only in experiments containing NOx.
In several previous studies, the presence of acidic sulfate
of SOA formed by the photooxidation of isoprene (9, 14, 15).
In the present study, it appears that sulfate ester formation
may be similarly enhanced by the introduction of an acidic
sulfate, no sulfate esters were detected by LC/ESI-MS in
any of the isoprene systems considered here. When experi-
ments were carried out in the presence of AS aerosols, a few
sulfate esters were detected, including [M-H]-ions at m/z
199, 215, 244, 260, or 333. Experiments carried out under
acidic conditions with AAS aerosol produced a considerably
wider array of detectable sulfate ester compounds. In
addition, the peak areas of several ions observed in both the
AS and AAS experiments were found to be larger in the AAS
experiments. For example, in the low-NOxexperiments, the
LC/MS peak area for the m/z 215 sulfate ester was found to
aerosol. Although quantitative data could not be obtained
for either the sulfate ester concentrations or the effective
ester formation is enhanced by the presence of an acidic
may be contributing to the increased SOA mass detected
previously under acidic conditions. Further work is needed
in order to accurately quantify these sulfate esters. It was
found that (-)LC/ESI-MS calibration curves generated by
surrogate standards lacking sulfate groups (such as meso-
erythritol) were not suitable for quantifying the identified
sulfate esters, resulting from these standards having lower
the experimental section were also not suitable for quan-
tification because these compounds had retention times
much greater than (1 min of the retention times for the
isoprene sulfate esters. Also, these standards lack many
structural features common to the identified sulfate esters;
therefore, synthesis of more representative standards is
Sulfate ester aerosol was atomized from a standard
solution of sodium propyl sulfate and analyzed directly by
the PILS/IC technique; no significant levels of inorganic
sulfate were detected, suggesting that organosulfates are
thermally stable at the operating conditions of this instru-
ment. In addition, no chromatographic peak in the IC data
inorganic sulfate if sulfate ester formation occurs.
The time evolution of the SO42-and NH4+aerosol mass
concentrations obtained using the PILS/IC technique for a
Caltech low-NOxisoprene AAS seed aerosol experiment is
compared to that of a Caltech high-NOx AS seed aerosol
experiment in Figure 2. As shown in Figure 2A (although not
by ∼20 and 14%, respectively, over 9 h), a typical profile for
most Caltech isoprene experiments in Table 1, ammonium
and sulfate typically decreased slowly with time due to wall-
loss processes. However, in the experiment shown in Figure
2B, in which sulfate ester concentrations were exceedingly
faster (i.e., SO42-decayed by ∼60% over 6 h) than wall loss,
reaction. It should be noted that the initial NH4:SO4molar
experiment with AS seed aerosol. (B) Caltech low-NOxisoprene experiment with AAS seed aerosol. A control experiment was conducted
in which seed aerosol is atomized from a solution of 0.015 M AS into the Caltech experimental chamber, and no other reactants such
as VOCs or NOxwere present. This control experiment produced a similar result to that of Figure 2A (although not evident from the time
scale presented, SO42-and NH4+decay by ∼20 and 14%, respectively, over 9 h), indicating that the only loss mechanism for sulfate in
this case was wall loss. Of the Caltech isoprene experiments, only the low-NOx AAS seed aerosol experiment showed a significant
decrease in the SO42-aerosol mass concentration, indicating that it was likely lost to reaction.
VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9521
of ammonium volatilization previously characterized (31).
The significant decrease in the SO42-mass concentration
observed for the Caltech low-NOxAAS seed aerosol experi-
yields (9), strongly suggesting that particle-phase sulfate
∼5-7 h after the experiment was initiated; sulfate esters are
formed by this point in the experiments as shown in
Sulfate Esters from r-Pinene Oxidation. As in the
produce abundant [M-H]-ions, corresponding34S isotopic
ions, m/z 97 and 80 product ions, and were not observed in
experiments without SO2(Table 2). Isoprene sulfate esters
photooxidation; for simplicity, these esters are not listed in
ion spectra for representative R-pinene sulfate esters; these
and 326, respectively. Analogous to some of the isoprene
sulfate esters, the m/z 294, 310, and 326 R-pinene sulfate
and/or 47 Da (HONO). The MS/MS spectra of the [M-H]-
ions for the four R-pinene sulfate esters yielded m/z 97
only for the m/z 265 ion due to mass range limits on the
mass spectrometer. It should be noted that the MS3spectra
of high-mass product ions shown in Figure 3 (e.g., m/z 250
in Figure 1A) did yield the m/z 97 and m/z 80 product ions,
thus supporting that the [M-H]-ions at m/z 294, 310, and
326 contain a sulfate group. These results were confirmed
reverse-phase LC column at much later RTs (∼10-26 min)
than those formed in isoprene oxidation, indicating differ-
ences in water solubility. Identified R-pinene sulfate esters
listed in Table 4 were formed from the reactive uptake of
previously identified gas-phase oxidation products (33, 34),
consistent with previous work (17, 18). Except for pinonal-
dehyde, no sulfate esters have been identified to form from
previously identified R-pinene SOA products; however,
further investigation is warranted. For quality control pur-
poses, solid-phase extraction (SPE) was used on duplicate
filters collected from selected experiments (EPA-211) to
remove excess inorganic sulfate; it was found that the
at higher [M-H]-ion abundances, indicating that these
sulfate esters are not a result of inorganic sulfate clusters in
the mass spectrometer.
the LC/MS extracted ion chromatograms (EICs) of m/z 215
obtained from a Caltech low-NOxisoprene AAS experiment
to that of two SEARCH field samples (JST and BHM,
respectively). Both the RTs and mass spectra of the chro-
matographic peaks shown in the EICs of m/z 215 are the
same in all samples, strongly suggesting that this isoprene
sulfate ester is present in ambient aerosol. In addition,
ambient aerosol recently collected at K-puszta, Hungary
indicates that the m/z 199 and 260 isoprene sulfate esters
FIGURE 3. (-)ESI-ITMS product ion mass spectra for sulfate esters of r-pinene oxidation products. (A) Product ion mass spectrum for
m/z 294 detected in EPA-211 stage 5. (B) Product ion mass spectrum for m/z 265 detected in EPA-211 stage 4. (C) Product ion mass spectrum
for m/z 310 detected in EPA-211 stage 5. (D) Product ion mass spectrum for m/z 326 detected in EPA-211 stage 5. These sulfate esters were
always present when r-pinene was photooxidized in the presence of SO2.
5229ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 2, 2007
from the SEARCH network.
Figure 5A-C compares the LC/MS EICs of m/z 294
326, respectively) and with a SEARCH field sample collected
at the BHM field site in June 2004. This figure indicates that
aerosol, consistent with previous work (22). Other R-pinene
sulfate esters identified in this study have been observed in
ambient aerosol in the southeastern U.S (22). It is possible
that the m/z 294 sulfate ester in the ambient aerosol could
also result from the oxidation of other monoterpenes owing
to the lack of detailed connectivity of specific functional
groups (e.g., sulfate esters and hydroxyls) provided by ESI-
our laboratory experiments are relevant to the conditions in
the southeastern U.S., even though the laboratory aerosol
was generated from much higher VOC mixing ratios, lower
RHs, and likely higher aerosol acidities observed in the
esters elucidated in this study were formed only during SOA
formation and not on the filter or during the ESI process,
several quality control tests were conducted. First, a filter
TABLE 4. Proposed r-Pinene Sulfate Ester SOA Products
aPositional isomers containing nitrate or sulfate groups at other hydroxylated positions are possible.bCompounds listed in parentheses are
neutral losses observed upon ESI-MS/MS.cPreviously detected R-pinene oxidation product by Aschmann et al. (33, 34).dProposed by Liggio et
al. (17) to form from pinonaldehyde reactive uptake onto acidic seed particles; however, structure was not confirmed. In the current study, we
confirm its structure with (-)ESI-MS.eDetected in ambient aerosol collected from SEARCH network June 2004 for first time.fESI-MS cannot
differentiate between which product is being detected; for completeness both structures are shown here.gObserved in an ambient study by Gao
et al. (22).hNo structural information was provided in the previous study by Gao et al. (22).
VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9523
extract from a Caltech low-NOxisoprene nucleation (i.e., no
two parts. One part was spiked with a high concentration of
(NH4)2SO4and the other part was spiked with pure H2SO4.
test samples, demonstrating that the sulfate esters detected
The use of reverse phase chromatography allowed for
inorganic sulfate not to be confused with organosulfates,
where inorganic sulfate was the very first peak to elute from
the column. In the two test samples discussed above, the
was very similar to the seeded experiments listed in Table
1; however, this tailing seems to have no effect on the
formation of sulfate esters.
As a second test, a meso-erythritol (a surrogate for the
SO4and the other with pure H2SO4. As for the first quality
control test above, these two samples produced no sulfate
esters in (-)LC/ESI-MS.
Last, a filter extract from an EPA R-pinene experiment
spiked with pure H2SO4. Again, no sulfate esters were
detected. These results strongly suggest the organosulfates
an artifact of sampling or measurement.
of gas-phase alcohols (e.g., methanol and ethanol) and
lower stratosphere have been suggested to occur in the
was found to increase their uptake. Some of these studies
proposed that the observed uptake of the alcohols and
no product studies were conducted. It has also been shown
that reactive uptake of butanol and ethanol onto sulfate
aerosols occurs at room temperature (38, 39). Esterification
was recently shown to occur from the photooxidation of
of organic acids with alcohols (9, 10). The large amounts of
organic acids formed during the photooxidation were
proposed to drive these reactions.
Figure 6 shows the general reactions proposed for the
as model compound) and sulfate derivatives from carbonyl
compounds (pinonaldehyde used as model compound). In
the leaving group. The resulting carbocation becomes a
nucleophilic site for the unshared pair of electrons on one
humidities of these experiments, this likely shifts the equi-
librium in favor of sulfate ester formation. In the case of
likely involves the electron pair of the carbonyl oxygen
ion, and making it more susceptible to nucleophilic attack
from an unshared pair of electrons from one of the oxygen
atoms on sulfate. It should be stressed that other reaction
FIGURE 4. (-)LC/ESI-MS extracted ion chromatograms for m/z 215. The retention times of the m/z 215 EICs are the same as well as the
mass spectra associated with each chromatographic peak; therefore, the comparison of these EICs suggests that the photooxidation of
isoprene in the presence of acid seed produces these sulfate esters observed in the ambient aerosol. In all chamber experiments involving
isoprene in the presence of AS seed aerosol, AAS seed aerosol, or SO2, the m/z 215 ion was detected.
5249ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 2, 2007
mechanisms are also possible, including:
addition of H2SO4to an aliphatic double bond, addition of
formation and sulfate ester oligomerization could also take
place in the case of polyols; however, (-)ESI-MS is not
sensitive to such neutral species, no such products have yet
been identified. In prior work (9) we reported oligomeric
signatures (14, 16, 18 Da differences) and compounds with
masses up to ∼620 Da in matrix-assisted laser desorption
FIGURE 5. (-)LC/ESI-MS extracted ion chromatograms for m/z 294. The retention times of the m/z 294 compounds were the same as well
as the mass spectra associated with each chromatographic peak; therefore, the comparison of these EICs suggests that the photooxidation
of r-pinene in the presence of NOxand acid seed produces these sulfate esters in ambient aerosol. No m/z 294 compounds were detected
in experiments involving only isoprene and acid seed (or SO2).
FIGURE 6. Proposed reactions for the formation of sulfate esters from 2-methyltetrol and pinonaldehyde, a representative alcohol and
aldehyde generated by the photooxidation of isoprene and r-pinene, respectively. Solid boxes indicate (-)ESI-MS detected species.
Dashed boxes indicate other proposed products possibly formed.
VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9525
ionization (MALDI)-MS data collected for Caltech low-NOx
seeded experiments; AAS seed cases produced the most
diester or sulfate ester oligomerization reactions.
Isoprene Sulfate Esters and the role of NOx. Surratt et
al. (9) observed a significant increase in the SOA yield in the
low-NOxAAS seed aerosol experiments, whereas very little
(if any) increase in the SOA yield was observed in the high-
NOx AAS seed aerosol experiments. This difference likely
occurs because of the large abundance of organic acids
formed under high-NOxconditions competing with sulfuric
acid (or sulfate) for esterification with alcohols. The low-
NOxSOA was found to comprise largely of neutral polyols
(e.g., 2-methyltetrols and hemiacetal dimers in Table 3) and
hydroperoxides, which may react readily with sulfuric acid
to produce sulfate esters. In previous work (9) we reported
detecting no SOA components with (-)LC/ESI-MS for the
low-NOxseeded cases; reanalysis of that data indicates that
sulfate esters (m/z 215, 333, and 415) were in fact detected,
but because they eluted very closely to inorganic sulfate
(within 1-1.5 min), they were believed to be an artifact.
Atmospheric Implications. Sulfate esters identified pre-
viously in ambient aerosol (21, 22) appear to be secondary
significance is the detection of sulfate esters from isoprene
for the acid induced reaction pathway. There is also the
other than R-pinene as well as sesquiterpenes, could lead to
the formation of aerosol-bound sulfate esters. These esters
could contribute significantly to the HULIS fraction of
ambient aerosol due to their high water-solubility, acidity,
thermally stability, and high molecular weights, all of which
are common properties of HULIS (41). Strong chemical
evidence is presented here for the substantial occurrence of
sulfate esterification in both laboratory-generated and ambi-
for the observed increase in SOA yields in response to
increasing aerosol acidity. Additional studies are needed to
determine the mass fraction of sulfate esters in ambient
aerosols and the factors that influence their formation, such
as relative humidity, temperature, and initial sulfate aerosol
Research at Caltech was funded by the U.S. Environmental
grant no. RD-83107501-0, managed by EPA’s Office of
Research and Development (ORD), National Center for
CR-831194001, and by the U.S. Department of Energy,
05ER63983. This article has been jointly developed and
by EPA personnel under EPA scientific and technical peer
on its scientific merit, technical accuracy, or contribution to
However, the Agency’s decision to publish the article jointly
with Caltech is intended to further the public purpose
supported by Cooperative Agreement no. CR83194001 and
not to establish an official EPA rule, regulation, guidance, or
policy through the publication of this article. Further, EPA
does not endorse any products or commerical services
mentioned in this publication. J.D.S. was supported in part
under the Science to Achieve Results (STAR) Graduate
Fellowship Program. Research at the University of Antwerp
was supported by the Belgian Federal Science Policy Office
and the Research Foundation-Flanders (FWO). The Electric
Power Research Institute provided support for the SEARCH
network. We thank Rafal Szmigielski for his discussions on
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Received for review August 30, 2006. Revised manuscript
received October 18, 2006. Accepted November 3, 2006.
VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9527