Introduction
Secondary organic aerosol (SOA) is formed through complex physicochemical
reactions of volatile organic compounds which are emitted into the
atmosphere from biogenic and anthropogenic sources and can constitute a
substantial portion of the continental aerosol mass (Goldstein and Galbally,
2007; Hallquist et al., 2009). Of the volatile organic compounds, isoprene is
the most abundant non-methane hydrocarbon emitted to the atmosphere
(Guenther et al., 1995, 2006). Although the SOA yield of isoprene tends to
be low, its sizable emissions can contribute to a high organic aerosol
loading, making it one of the most studied compounds for aerosol formation
(Guenther et al., 1995; Henze and Seinfeld, 2006; Fu et al., 2008; Carlton
et al., 2009; Hallquist et al., 2009). The primary removal mechanism for
isoprene is by gas-phase reactions with hydroxyl radicals (OH); nitrate
radicals; and, to a lesser extent, ozone. These processes result in the
formation of gas and aerosol products including numerous oxidized SOA
components. Aerosol species previously reported include 2-methyltetrols,
2-methylglyceric acid, C5-alkene triols, and organosulfates (i.e., Edney et al., 2005;
Surratt et al., 2007a, 2010; Riva et al., 2016; Spolnik
et al., 2018). While many of these are formed through multiphase chemistry
(e.g., isoprene epoxydiol (IEPOX) channel), we cannot exclude their gas-phase formation at least
for 2-methyltetrols, probably in part through re-evaporation processes
(Issacman-VanWertz et al., 2016), and for 2-methylglyceric acid, as these
compounds have been linked to gas-phase reaction products from the oxidation
of isoprene (Kleindienst et al., 2009) and in ambient PM2.5 (Xie at
al., 2014). Moreover, these compounds have been identified in ambient PM2.5
in several places around the world, and SOA from isoprene often accounts
for 20%–50% of the overall SOA budget (Claeys et al., 2004a; Wang et al.,
2005; Henze and Seinfeld, 2006; Kroll et al., 2006; Surratt et al., 2006;
Hoyle et al., 2007).
An enhancement of isoprene (ISO)-SOA yields is controlled by various factors
including NOx concentration (Kroll et al., 2006; Chan et al., 2010;
Surratt et al., 2006, 2010) and the acidity of preexisting aerosol (Jang et
al., 2002; Czoschke et al., 2003; Edney et al., 2005; Kleindienst et al.,
2006; Surratt et al., 2007a, 2010; Jaoui et al., 2010; Szmigielski et al.,
2010). The strength of the acidity depends on the aerosol liquid water
content and the relative humidity (RH; Nguyen et al., 2011; Zhang et al., 2011;
Lewandowski et al., 2015; Wong et al., 2015), which are coupled. Smog chamber
experiments have revealed that the yield of isoprene SOA increases under
acidic conditions through an enhanced formation of isoprene-derived
oxygenates by acid-catalyzed reactions (Surratt et al., 2007b, 2008, 2010;
Gomez-Gonzalez et al., 2008; Offenberg et al., 2009). By one mechanism,
isoprene reactions with OH under low- or high-NOx conditions can form
epoxydiols (IEPOX) in high yields followed by their uptake by SOA and
subsequent acid-catalyzed particle reactions (Paulot et al., 2009; Surratt
et al., 2010; Lin et al., 2013; Budisulistiorini et al., 2015; Rattanavaraha
et al., 2016; Gaston et al., 2014a, b; Riedel et al., 2015; Zhang et al.,
2018). However, this type of multiphase chemistry following the uptake of
IEPOX can be highly dependent on the aerosol-phase state and the presence of
aerosol coatings from viscous SOA constituents (Zhang et al., 2018). Such
coatings can cause a substantial diffusion barrier to the availability to an
acidic core.
Atmospheric organosulfates are another class of organic compounds formed
from atmospheric reactions of various precursors, including isoprene, and
have been identified as components of ambient particulate matter (PM; Surratt et al., 2008;
Froyd et al., 2010; Stone et al., 2012; Tolocka and Turpin, 2012). The most
common isoprene organosulfates have been identified both in smog chamber
experiments and in field studies (Surratt et al., 2007a, 2008, 2010;
Gomez-Gonzalez et al., 2008; Shalamzari et al., 2013; Tao et al., 2014;
Hettiyadura et al., 2015; Szmigielski, 2016; Spolnik et al., 2018). For many
of these polar oxygenated compounds, chemical structures, mass spectrometry (MS) fragmentation
patterns, and formation mechanisms have been tentatively proposed (Surratt et
al., 2007a, b; 2008, 2010; Gomez-Gonzalez et al., 2008; Zhang et al., 2011;
Shalamzari et al., 2013; Schindelka et al., 2013; Nguyen et al., 2014; Tao
et al., 2014; Hettiyadura et al., 2015; Riva et al., 2016; Spolnik et al.,
2018). The commonly detected components of isoprene SOA attributed to
processing of isoprene oxidation products (e.g., IEPOX, methacrolein, and
methyl vinyl ketone) have the reported molecular weights (MWs) of 154, 156, 184,
198, 200, 212, 214, 216, 260, and 334 (Surratt et al., 2007b, 2008, 2010;
Gomez-Gonzalez et al., 2008; Kristensen et al., 2011; Zhang et al., 2011;
Shalamzari et al., 2013; Schindelka et al., 2013; Nguyen et al., 2014;
Hettiyadura et al., 2015; Riva et al., 2016). The mechanisms of OS formation
were proposed for the conditions of either acidified or non-acidified
sulfate aerosol seeds (e.g., 2-methyltetrol organosulfates proposed by
Surratt et al., 2007a, and Riva et al., 2016). Whereas Kleindienst et al. (2006)
reported the formation of highly oxygenated products through OH
radical oxidation, Riva et al. (2016) proposed an alternative route through
acid-catalyzed oxidation by organic peroxides. Isoprene organosulfates were
also reported to occur in the aqueous phase through the photooxidation or
dark reactions of isoprene in aqueous solutions containing sulfate and
sulfite moieties (Rudzinski et al., 2004, 2009; Noziere et al., 2010). A
detailed mechanism of this transformation has been tentatively proposed
based on chain reactions propagated by sulfate and sulfite radical anions
(Rudzinski et al., 2009) and confirmed by mass spectrometric studies
(Szmigielski, 2016). The acid-catalyzed formation of 2-methyltetrols has
also been suggested in aqueous-phase oxidation of isoprene with
H2O2 (Claeys et al., 2004b).
To date, few smog chamber studies have examined the effect of relative
humidity on ISO-SOA formation (Dommen et al., 2006; Nguyen et al., 2011;
Zhang et al., 2011; Lewandowski et al., 2015; Wong et al., 2015; Riva et
al., 2016). However, the impact of relative humidity may be an important
parameter, in that it may influence the mechanism of SOA formation and
hence the chemical composition, physical properties, and yield of isoprene
SOA (Vasconcelos et al., 1994; Poulain et al., 2010; Guo et al.,
2014). The chamber studies conducted by Dommen et al. (2006) and Nguyen et al. (2011)
showed a negligible effect of relative humidity on the SOA yield
from the photooxidation of isoprene in the absence of sulfate aerosol. Other
studies suggested that ISO-SOA formation yields under high-NOx
conditions with acidified and non-acidified sulfate aerosol decreased with
an increase in relative humidity while simultaneously the yield of
organosulfates was enhanced (Zhang et al., 2011; Lewandowski et al., 2015).
The latter observation can be explained by transformation of isoprene
propagated by sulfate/sulfite radical anions in the aqueous particle phase
or on the aqueous surface of aerosol particles (Zhang et al., 2011;
Rudzinski et al., 2016; Szmigielski, 2016). The results obtained from the
chamber experiments have been in agreement with recent model approaches
when reactive uptake to aqueous aerosol is used rather than a reversible
partitioning approach (Pye et al., 2013; Marais et al., 2016). A recent
study conducted in our laboratory focused on the effects of relative
humidity on secondary organic carbon (SOC) formation from isoprene
photooxidation in the presence of NOx (Lewandowski et al., 2015). The
study indicated that relative humidity can have a profound effect on the
acid-derived enhancement of isoprene SOC, while an increasing content of
aerosol liquid water suppressed the level of enhancement.
The focus of the present study is to investigate at a molecular level the
role of relative humidity on the chemical composition of isoprene SOA
obtained under acidic and non-acidic conditions. Organosulfate compounds
were analyzed using liquid chromatography–mass spectrometry (LC-MS) measurements (Szmigielski, 2016; Rudzinski et al.,
2009; Darer et al., 2011; Surratt et al., 2007a), while non-sulfate
oxygenated compounds were examined using derivatization followed by gas chromatography–mass spectrometry (GC-MS)
analysis (Jaoui et al., 2004). Here we explored the RH effect of a wide
range of isoprene polar oxygenated products, including 2-methyltetrols,
2-methylglyceric acid, IEPOX, organosulfates), nitroxy-organosulfates (NOSs),
and other selected oxygenates in the presence of acidified and non-acidified
sulfate aerosol. In addition, a chemical analysis of PM2.5
field samples has been conducted to assess the possible
relationship between the laboratory findings and their role in ambient SOA
formation.
Experimental methods
Smog chamber experiments
Chamber experiments were conducted in a 14.5 m3 stainless-steel,
fixed-volume chamber with interior walls fused with a 40 µm PTFE Teflon
coating. Details of chamber operation, sample collection, derivatization
procedure, and the GC-MS analysis method
are described in more detail in Lewandowski et al. (2015) and Jaoui et al. (2004).
A combination of UV-fluorescent bulbs was used in the chamber as
a source of radiation from 300 to 400 nm with a distribution photolytically
comparable to that of solar radiation (Black et al., 1998). The reaction
chamber was operated as a flow reactor with a residence time of 4 h, to
produce a steady-state, constant aerosol distribution which could be
repeatedly sampled at different seed aerosol acidities.
Isoprene and nitric oxide (NO) were taken from high-pressure cylinders each
diluted with N2. Isoprene was obtained from Sigma-Aldrich Chemical Co.
(Milwaukee, WI, USA) at the highest purity available and used without
further purification. Isoprene and NO were added to the chamber through flow
controllers. The temperature in all experiments was ∼ 27 ∘C (Table 1). Dilute aqueous solutions of ammonium sulfate and
sulfuric acid as inorganic seed aerosol were nebulized to the chamber, with
total sulfate concentration of the combined solution held constant to
maintain stable inorganic concentrations in the chamber (Lewandowski et al.,
2015). NO and total oxides of nitrogen (NOx) were measured with a
Thermo Electron NOx analyzer (Model 8840, Thermo Environmental, Inc.,
Franklin, MA, USA). Ozone formed during the irradiation was measured with a
Bendix ozone monitor (Model 8002, Lewisburg, WV, USA). Temperature and relative
humidity were measured with an Omega Digital Thermo-Hygrometer (Model RH411,
Omega Engineering, Inc., Stamford, CT, USA). Isoprene concentrations were
measured by gas chromatography with flame ionization detection
(Hewlett-Packard, Model 5890 GC). Chamber filter samples were collected for
24 h at 16.7 L min-1 using 47 mm glass fiber filters (Pall Gelman
Laboratory, Ann Arbor, MI, USA).
Two sets of experiments were conducted (Table 1) to explore the effect of
humidity and acidity on isoprene SOA products. The non-acidic experiment
(ER667) was conducted at four different humidity levels in the presence of
isoprene, NOx, and ammonium sulfate as seed aerosol
(1 µg m-3). It served as a base case for exploring the changes and nature of
SOA products in the absence of significant aerosol acidity. The second
experiment (ER662, acidic) was similar but run in the presence of acidic seed
aerosol at constant concentration. It included five and four stages differing in
humidity levels for ER667 (9 %, 19 %, 30 %, 39 %, and 49 %) and
ER662 (8 %, 18 %, 28 %, and 44 %), respectively. Aerosol
concentrations are those from Lewandowski et al. (2015).
Ambient aerosol samples
Twenty ambient PM2.5 samples were collected, onto pre-baked quartz
filters using a high-volume aerosol sampler (DHA-80, Digitel), from two sites
(10 samples each) that have strong isoprene emissions: (1) a regional
background monitoring station in Zielonka, in the Kuyavian-Pomeranian
Province in northern Poland (PL; 53∘39′ N, 17∘55′ E),
during the summer 2016 campaign and (2) a regional background monitoring
station in Godów, PL, located in the Silesian Province (49∘55′ N,
18∘28′ E) in the summer 2014 campaign. Sampling times were 12 and 24 h,
respectively. Major tree species at both sites are European oak
(Quercus robur, L.), European hornbeam (Carpinus betulus, L.), Tilia cordata (Tilia cordata Mill), European white birch
(Betula pubescens Ehrh), and European alder (Alnus glutinosa Gaertn). The Zielonka station is in a
forested area, while the Godów station is located near a coal-fired power
station in Dětmarovice (Czech Republic). Godów is also close to the major
industrial cities of the Silesian region in Poland, and thus aerosol samples
collected in Godów were influenced by anthropogenic sources.
Several chemical and physical parameters were measured at the two sites. The
temperature during sampling at both sites ranged from 25 to 28 ∘C. The
relative humidity during sampling was up to 86 % in Zielonka and 94 %
at Godów. Both locations were influenced by NOx
concentration, modestly in Zielonka at 1.3 µg m-3 and at a
level of 30 µg m-3 in Godów, represented by the nearest
monitoring station at Żywiec, PL. The SO2 levels at Zielonka were
approximately 0.6 and 3.0 µg m-3 at Godów. At each site,
organic carbon / elemental carbon ratio (OC/EC) values was determined for each filter using a thermo-optical
method (Birch and Cary, 1996). The organic carbon value at Zielonka was
approximately 1.7 and 5.4 µg m-3 at Godów, although aerosol
masses were not determined.
Instrumentation and analysis methods
Chemicals for extraction and derivatization were obtained from Sigma-Aldrich
Chemical Company. N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) used as
the derivatizing agent included 1 % trimethylchlorosilane as a catalyst.
For the GC-MS analysis, filters were sonicated for 1 h with methanol.
Prior to extraction, 20 µg each of cis-ketopinic acid and
d50-tetracosane was added as internal standards. Following sonication,
the methanol extracts were dried and then derivatized with 200 µL
BSTFA and 100 µL pyridine. Samples were then heated to
70 ∘C to complete the reaction (Jaoui et al., 2004). The derivatized
extracts were analyzed using a ThermoQuest (Austin, TX, USA) GC coupled to an
ion trap mass spectrometer (ITMS). The injector, heated to 270 ∘C,
was operated in splitless mode. Compounds were separated on a 60 m long,
0.25 mm ID RTx-5MS column (Restek, Inc., Bellefonte, PA, USA) with a
0.25 µm film thickness. The GC oven temperature program for the
analysis started isothermally at 84 ∘C for 1 min, followed by a
temperature ramp of 8 ∘C min-1 to 200 ∘C and
a 2 min hold, and was then ramped at 10 ∘C min-1 to 300 ∘C.
The ion source, ion trap, and interface temperatures were 200, 200, and
300 ∘C, respectively. Mass spectra were collected in both the
chemical ionization (CI) and electron ionization (EI) modes (Jaoui et al.,
2004). A semi-continuous OC/EC
analyzer (Sunset Laboratories, Tigard, OR, USA) measured total organic carbon of
the aerosol given the absence of elemental carbon in the reaction system.
Immediately upstream of the analyzer, a carbon-strip denuder was placed in
line to remove gas-phase organic components which could bias the
measurements. The analyses for total OC were made on a 15 min duty cycle.
Silylation of polar compounds results in reduced polarity, enhanced
volatility, and increased thermal stability, and they enable the GC-MS analysis of
many compounds otherwise involatile or too unstable for these techniques.
Therefore, appropriate caution should be taken, for example, with desulfation
reactions associated with primary organosulfates (Takano et al., 1992;
Kolender et al., 2004; Bedini et al., 2016, 2017; Cui et al., 2018), and corrections might
be warranted when analyzing methyltetrols.
For the LC-MS analysis, from each filter, two 1 cm2 punches were taken
and twice extracted for 30 min with 15 mL aliquots of methanol using a
Multi-Orbital Shaker (PSU-20i, BioSan). High-purity methanol (LC-MS
Chromasolv grade; Sigma-Aldrich, PL) was used for the extraction of SOA
filters, reconstitution of aerosol extracts, and preparation of the LC mobile
phase. The two extracts were combined and concentrated to 1 mL using a
rotary evaporator operated at 28 ∘C and 150 mbar
(Rotavapor® R215, Buchi). They were then
filtered with a 0.2 µm PTFE syringe and taken to dryness under a
gentle stream of nitrogen. High-purity water (resistivity
18.2 M Ω × cm-1) from a Milli-Q Advantage water
purification system (Merck, Poland) was used for the reconstitution of
aerosol extracts and preparation of the LC mobile phase. The residues were
reconstituted with 180 µL of 1:1 high-purity methanol/water
mixture (v/v) and then agitated for 1 min. Recoveries were not taken for
compounds analyzed in this study, due to lack of authentic standards;
however,
recovery of 94 %–101 % was measured for appropriate surrogate
compounds.
Extracted-ion chromatograms (KPA: m/z 165, ketopinic acid, IS (internal standards);
IEPOX: m/z 173, two isomers; 2mGA: 321; 2-methylglyceric acid;
2MT: m/z 409, 2-methyltetrols, four isomers; mGAd: m/z 495,
2-methylglyceric acid dimer, three isomers) for non-acidic
isoprene/NOx photooxidation experiments as a function of RH.
Compounds were detected as silylated derivatives. For clarity of the figure, not
all isomers are shown.
Extracts were analyzed by ultra-high performance liquid
chromatography–electrospray ionization (ESI)–quadruple time-of-flight high-resolution mass
spectrometry equipment
consisting of a Waters ACQUITY UPLC I-Class chromatograph coupled to a Waters
SYNAPT G2-S high-resolution mass spectrometer. The chromatographic
separations were performed using an ACQUITY HSS T3 column
(2.1 × 100 mm, 1.8 µm particle size) at room temperature.
The mobile phases consisted of 10 mM ammonium acetate (eluent A) and
methanol (eluent B). To obtain appropriate chromatographic separations and
responses, a gradient elution program 13 min in length was used. The
chromatographic run commenced with 100 % eluent A over the first 3 min.
Eluent B increased from 0 % to 100 % from 3 to 8 min, held constant at
100 % from 8 to 10 min, and then decreased back from 100 % to 0 %
from 10 to 13 min. The initial and final flow was 0.35 mL min-1, while
the flow from 3 to 10 min was 0.25 mL min-1. An injection volume of
0.5 µL was used. The SYNAPT G2-S spectrometer equipped with an ESI
source was operated in the negative-ion mode. Optimal ESI source conditions
were 3 kV capillary voltage with a 20 V sampling cone and full-width-at-half-maximum mass-resolving
power of 20 000. High-resolution mass spectra were recorded from
m/z 50 to 600 in the MS or MS/MS modes. All data were recorded and analyzed
with the Waters MassLynx V4.1 software package. During the analyses, the mass
spectrometer was continuously calibrated by injecting the reference compound,
leucine enkephalin, directly into the ESI source.
Results and discussion
Chemical characterization
Table 1 shows the input and steady-state conditions for all stages of the
chamber experiments, including the values determined for carbon yield,
secondary organic carbon, and organic-mass-to-carbon-mass ratio (OM/OC). The
data indicate that with increasing RH the formation of SOC and carbon yield
is reduced, under both acidic and non-acidic conditions. The results
obtained are consistent with those of Zhang et al. (2011). Secondary organic
aerosol formed under non-acidic conditions was additionally analyzed for
OM/OC and SOA yield. The average OM/OC ratio was 1.92 ± 0.13, and the
average laboratory SOA yield measured in this experiment was 0.0032 ± 0.0004.
For the non-acidic experiment, the carbon yield values range from a
low 0.001 (stage 5, Table 1) at the highest relative humidity to a high of
0.004 at the lowest relative humidity (stage 1, Table 1). For the acidified
experiment, carbon yield declined from above 0.011 at the lowest relative
humidity (8 %) to 0.0013 at the highest relative humidity (44 %).
Although the relative humidity considered for both acidic and non-acidic
experiments do not correspond precisely, an increase of SOC was observed
under acidic conditions at approximately the same relative humidity. The
values of SOA yields agree with previous chamber studies reported in the
literature under the same nominal conditions in the presence of NOx (Edney
et al., 2005; Dommen et al., 2006; Surratt et al., 2007; Zhang et al.,
2011).
Initial and steady-state conditions, yields, and
OM/OC data for chamber experiments on isoprene photooxidation in the
presence of acidic and non-acidic seed aerosol. The initial NOx was entirely
nitric oxide. The non-acidic experiment was conducted with a low-concentration
ammonium sulfate seed (∼ 1 µg m-3). The acidic
experiment was conducted with a higher concentration of inorganic seed
(∼ 30 µg m-3) generated from a nebulized solution
for which half the sulfate mass was derived from sulfuric acid and the other
half from ammonium sulfate (Lewandowski et al., 2015).
Experiment ER662: acidic seed aerosol (1/2 ammonium sulfate,
1/2 sulfuric acid by sulfate mass in precursor solution)
Stage 1
Stage 2
Stage 3
Stage 4
RH (%)
8
28
44
18
Temperature (C)
27.0
27.3
26.9
27.5
Initial isoprene (ppmC)
6.82
6.92
7.01
7.03
Initial NO (ppm)
0.296
0.296
0.296
0.296
Steady-state conditions
O3 (ppm)
0.303
0.292
0.245
0.339
NOx (ppm)
0.220
0.213
0.205
0.234
ΔHC (µg m-3)
3266
3318
3357
3472
Carbon yield
0.0112
0.0027
0.0013
0.0051
SOC (µgC m-3)
32.3
7.9
3.8
15.7
Experiment ER667: Non-acidic seed aerosol (ammonium sulfate)
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
RH (%)
9
19
30
39
49
Temperature (C)
28.2
28.5
27.9
27.8
27.6
Initial isoprene (ppmC)
8.11
8.29
8.25
8.25
8.19
Initial NO (ppm)
0.347
0.347
0.347
0.347
0.347
Steady-state conditions
O3 (ppm)
0.331
0.305
0.329
0.393
0.281
NOx (ppm)
0.260
0.247
0.241
0.229
0.226
ΔHC (µg m-3)
3518
3556
3558
3515
3484
SOA yield
0.007
0.004
0.002
0.002
0.001
Carbon yield
0.0038
0.0022
0.0013
0.0009
0.0010
SOC (µgC m-3)
13.3
7.7
4.6
3.2
3.5
OM/OC
1.96
2.00
2.02
2.03
1.59
The analysis of isoprene SOA from chamber experiments and field samples is
based on the interpretation of mass spectra of the derivatized and
underivatized isoprene SOA products by GC-MS (in EI and CI) and by LC-MS
(negative-ion mode with electrospray ionization), respectively. The
characteristic ions for all BSTFA derivatives are m/z 73, 75, 147, and 149.
In CI mode, adduct ions from the derivatives included m/z M+•+73, M+•+41, M+•+29, and M+•+1, while
fragment ions included m/z M+•-15, M+•-73,
M+•-89, M+•-117, M+•-105, M+•-133, and M+•-207 (Jaoui et al., 2004). The LC-MS analysis used
to identify organosulfates and nitroxy- and nitrosoxy-organosulfates is based
on the deprotonated ions [M–H]- and the corresponding fragmentation
pathways. Organosulfates were recognized by the loss of characteristic ions
of m/z 80 (SO3-), 96 (SO4-), and 97
(HSO4-) (Darer et al., 2011; Szmigielski 2016). The
nitroxy-organosulfates and nitrosoxy-organosulfates were identified from
additional neutral losses of m/z 63 (HNO3) and m/z 47
(HNO3), respectively. Table 2 presents the list of compounds
tentatively identified in the present study along with proposed structures,
MWs, and main fragmentation ions (m/z). Additional
organic acids were tentatively identified in this study, and further work is
being conducted to understand their role in isoprene SOA. At the present
time, the organosulfate (MW 230), 2-methyltartaric acid organosulfate
(MW 244), and 2-methyltartaric acid nitroxy-organosulfate (MW 275) appear not
to have been reported before. An organosulfate with MW 230, but with a
distinct structure, was recently reported in the literature from the
photooxidation of 2-E-pentanal (Shalamzari et al., 2016).
Products detected in SOA samples from chamber experiments
using GC-MS and LC-MS.
Chemical
m/z BSTFA derivative
MW MWBSTFA
Tentative structure*
References
formula
(methane-CI)
(g mol-1)
and chemical name
GC-MS
C5H10O2
247, 231, 157, 147, 73
102 246
3-methyl-3-butene-1,2-diol (C5-diol-1)
Wang et al. (2005) Surratt et al. (2006)
C5H10O2
263, 247, 173, 83, 73,
118 262
2-methyl-2,3-epoxy-but-1,4-diol (IEPOX-1)
Paulot et al. (2009) Surratt et al. (2010) Zhang et al. (2012)
C5H18O3
263, 247, 173, 83, 73
118 262
2-methyl-3,4-epoxy-but-1,2-diol (IEPOX-2)
C4H8O4
337, 321, 293, 219, 203
120 336
2-methylglyceric acid (2-MG)
Claeys et al. (2004a) Surratt et al. (2006) Edney et al. (2005) Szmigielski et al. (2007)
C5H12O4
409, 319, 293, 219, 203
136 424
2-methylthreitol (2MT)
Claeys et al. (2004a) Wang et al. (2004) Edney et al. (2005) Surratt et al. (2006) Nozière et al. (2011)
C5H12O4
409, 319, 293, 219, 203
136 424
2-methylerythritol (2MT)
C8H14O7
495, 321, 219, 203, 73
222 510
2-methylglyceric acid dimer (2-MG dimer)
Surratt et al. (2006) Szmigielski et al. (2007)
Continued.
Chemical
m/z BSTFA derivative
MW MWBSTFA
Tentative structure*
References
formula
(methane-CI)
(g mol-1)
and chemical name
LC-MS
C5H10O6S
197, 167, 97, 81
198
IEPOX-derived organosulfate
Tao et al. (2014)
C4H8O7S
199, 119, 97, 73
200
2-methylglyceric acid organosulfate (2-MG OS)
Surratt et al. (2007a) Gomez-Gonzalez et al. (2008) Shalamzari et al. (2013) Riva et al. (2016)
C5H8O7S
211, 193, 113, 97
212
2(3H)-furanone, dihydro-3,4-dihydroxy-3-methyl organosulfate
Surratt et al. (2008) Hettiyadura et al. (2015) Spolnik et al. (2018)
C5H10O7S
213, 183, 153, 97
214
2,3,4-furantriol, tetrahydro-3-methyl-organosulfate
Hettiyadura et al. (2015) Spolnik et al. (2018)
C5H12O7S
215, 97
216
2-methyltetrol organosulfate (2MT OS)
Surratt et al. (2007a) Gomez-Gonzalez et al. (2008) Surratt et al. (2010)
C5H10O8S
229, 149, 97, 75
230
2-methylthreonic acid organosulfate
This study
C5H9O9S
243, 163, 145, 101
244
2-methyltartaric acid organosulfate
This study
Continued.
Chemical
m/z BSTFA derivative
MW MWBSTFA
Tentative structure*
References
formula
(methane-CI)
(g mol-1)
and chemical name
LC-MS
C5H11NO8S
244, 226, 197, 183, 153, 97
245
2-methyltetrol nitrosoxy-organosulfate
This study
C5H11NO9S
260, 197, 183, 153, 97
261
2-methyltetrol nitroxy-organosulfate
Surratt et al. (2007a) Surratt et al. (2008)
C5H9NO10S
274, 211, 193, 153, 97
275
2-methylthreonic acid nitroxy-organosulfate
This study
* For more stereochemically complex molecules a
representative isomer is shown.
Concentrations of particle-phase products from the non-acidic-seed
experiments (non-acidic) estimated with GC-MS.
Figure 1 presents GC-MS extracted-ion chromatograms (EICs) from the aerosol
obtained during the non-acidic experiment (isoprene non-acidic-seed
irradiation) at a wide range of relative humidities. According to acquired
chromatograms shown in Fig. 1, several isomers associated with the compounds
analyzed can be distinguished, i.e., IEPOX-1 and IEPOX-2, four isomers of
2-methyltetrols, and their relative contributions to SOA masses at various
relative humidity levels.
The formation of isoprene SOA products such as 2-methyltetrols (mT) and
2-methylglyceric acid is well documented in the literature. These
compounds are isoprene SOA markers and have been reported in numerous field
and chamber studies under low- and high-NOx conditions (Claeys et al.,
2004a; Edney et al., 2005; Kroll et al., 2006; Surratt et al., 2006, 2010).
The formation mechanism under low-NOx conditions has been explained by
the reactive uptake of IEPOX onto acidic aerosol seeds
(Paulot et al., 2009; Surratt et al., 2010) and under high-NOx
conditions by the further oxidation of methacryloyl peroxynitrate (MPAN)
(Chan et al., 2010; Surratt et al., 2010; Nguyen et al., 2015).
The LC-MS analyses focused mainly on the formation of the variety of
organosulfates and nitroxy- and nitrosoxy-organosulfates. Mass spectra and
proposed fragmentation pathways of newly identified components are presented
in Sect. 3.4.
Effect of relative humidity and acidity on product formation
Non-acidic aerosol
Table 3 and Figs. 2–3 present the estimated amounts of polar oxygenated
products detected with GC-MS and LC-MS techniques in samples from non-acidic
photooxidation experiments with non-acidic aerosol seeds under various RH
conditions. Six products were quantified (as sums of respective isomers)
based on the response factor of ketopinic acid using GC-MS. Nine other
compounds were detected qualitatively using LC-MS, with chromatographic
responses representing the amounts of respective analytes. Therefore, the
results should be understood as a tendency of product occurrence in the
chamber experiments rather than the real amounts formed. Table 3 does not
contain data on 2-methyltartaric acid organosulfate (MW 244) because it
occurred in the samples merely in trace amounts.
LC-MS chromatographic responses of OS and NOSs from the non-acidic-seed experiments (non-acidic).
The major SOA components detected were 2-methyltetrols, 2-methylglyceric
acid, and its dimer, whose maximal estimated concentrations exceeded 800, 350, and
300 ng m-3, respectively, under low-humidity conditions of RH 9 %
(Fig. 2). At the two lowest humidities, aerosol liquid water is expected to be
very low, and the decrease in these compounds may not be controlled by aerosol
liquid water but possibly by the SOC levels associated with the particles
(Lewandowski et al., 2015), although chamber-related wall effects due to
water vapor might also play some role. Among compounds detected with LC-MS
(Fig. 3) are organosulfates derived from acid-catalyzed multiphase chemistry
of IEPOX (MW 216) and MAE/HMML (methacrylic acid epoxide/hydroxymethyl-methyl-α-lactone) (MW 200) (Surratt et al., 2010; Lin et al.,
2012, 2013; Nguyen et al., 2015). Other components were significantly less
abundant. In most cases, increasing the humidity resulted in decreased yields
of the products detected, although some compounds were observed at higher
concentrations at RH 49 % compared to RH 9 % (i.e., m/z 199:
Fig. 3). As found in Table 1, total SOC decreased with increased humidity.
Generally, the influence of RH on the product yields was modest, consistent
with Dommen et al. (2006) and Nguyen et al. (2011), who saw a negligible
effect of relative humidity on SOA yield in photooxidation of isoprene in the
absence of acidic seed aerosol. By contrast, here the 2-methyltetrols,
2-methylglyceric acid, and 2-methylglyceric acid dimer were found in
significantly larger quantities at RH 9 % compared to RH 49 %. Two
recent studies (Lin et al., 2014; Riva et al., 2016) reported an increase in
aerosol mass with increasing RH. Riva et al. (2016) also reported an increase
in 2-methyltetrols concentrations with increasing RH. However, the initial
conditions for those two studies differed substantially from those in the
present study. Here, isoprene is oxidized in the presence of
NOx and seed aerosol (acidic and non-acidic) under a wide
range of RH. In contrast, in Riva et al. (2016) and Lin et al., 2012, the reactants were hydroxyhydroperoxide
(ISOPOOH) and IEPOX oxidized under NOx-free conditions at
two levels of RH. In addition, organosulfates, 2-methyltetrols, and SOA yields
derived from isoprene photooxidation typically have been enhanced under
acidic conditions (Surratt et al., 2007a, b, 2010; Gomez-Gonzalez et al.,
2008; Jaoui et al., 2010; Zhang et al., 2011). Organosulfates have also been
formed in non-acidic experiments, probably through radical-initiated
reactions in wet aerosol particles containing sulfate moieties (Noziere et
al., 2010; Perri et al., 2010). The NOS and OS compounds detected here could
have been formed via such a mechanism.
Estimated concentrations of reaction products (ng m-3) from
the non-acidic photooxidation experiments (neutral seed
[H+] = 54 nmol m-3 air; Lewandowski et al., 2015).
RH 9 (%)
RH 19 (%)
RH 30 (%)
RH 39 (%)
RH 49 (%)
GC-MS data1
2-methylglyceric acid
379
255
155
171
70
2-Methyltetrols
811
384
371
257
157
2-Methylglyceric acid dimer
308
68
0
0
0
IEPOX-1
5
3
2
0
3
IEPOX-2
37
21
23
12
19
C5-Diol-1
9
6
3
0
0
LC-MS data2
m/z [M–H]-
197
0.28
0.22
0.19
0.37
0.33
199
3.22
2.46
3.60
4.66
4.01
211
0.44
0.20
0.06
0.09
0
213
2.21
1.87
1.52
1.48
0.83
215
17.80
12.30
10.20
9.83
7.24
229
0.70
0.78
1.11
1.29
0.83
244
0.35
0.14
0
0
0.08
260
0.49
0.35
0.32
0.28
0.18
274
0.08
0.10
0.08
0.08
0.07
1 MW as BSTFA derivative. 2 Chromatographic responses
of organosulfates (104).
Acidic seed aerosol
Table 4 and Figs. 4–5 present the estimated amounts of polar oxygenated
products detected using GC-MS and LC-MS techniques in samples from the
acidic photooxidation experiments with acidic aerosol seed under various RH
conditions. We detected the same compounds as in the non-acidic-seed
experiments, with the same analytical limitations of the quantitation. The
presence of 2-methyltetrols and 2-methylglyceric acid and their sulfated
analogues in isoprene SOA at a wide range of RH conditions suggests that
SOA water content does not significantly affect their formation.
Estimated concentrations of reaction products (ng m-3) from the acidic
photooxidation experiments (acidic seed [H+] = 275 nmol m-3 air; Lewandowski et al., 2015).
RH 8 (%)
RH 18 (%)
RH 28 (%)
RH 44 (%)
GC-MS data1
2-Methylglyceric acid
3070
2136
982
473
2-Methyltetrols
5357
4767
1029
341
2-Methylglyceric acid dimer
90
144
102
43
IEPOX-1
1
13
6
0
IEPOX-2
10
3
0
0
C5-Diol-1
53
0
0
0
LC-MS data 2
m/z [M–H]-
197
0.88
0.30
0.21
0.10
199
3.44
1.49
2.62
1.12
211
1.78
0.50
0.76
0.48
213
5.41
1.94
3.40
1.96
215
59.00
18.40
12.30
3.23
229
0.41
0.31
0.39
0.27
244
4.50
1.16
0.72
0.42
260
0.92
0.88
0.45
0.29
274
0.60
0.58
0.36
0.12
1 MW as BSTFA derivative. 2 Chromatographic responses
of selected main organosulfates (104).
Concentrations of particle-phase products from the acidic-seed experiments estimated with GC-MS.
LC-MS chromatographic responses of OS and NOS products from the
acidic-seed experiments.
Early chamber studies on isoprene ozonolysis by Jang et al. (2002) and
Czoschke et al. (2003) showed enhanced SOA yields in the presence of
acidified aerosol seeds. Recent laboratory results showed that the acidity of
aerosol seeds plays a major role in the reactive uptake of isoprene oxidation
products by particle phases (Paulot et al., 2009; Surratt et al., 2010; Lin
et al., 2012; Gaston et al., 2014a, b; Riedel et al., 2015). In our study,
SOC produced in acidic-seed experiments was always higher than in non-acidic-seed
ones under the corresponding RH conditions, while the difference
diminished with increasing RH to a negligible value of
0.3 µg C m-3 at RH 44 %–49 % (Table 1 and Fig. S1
in the Supplement; Surratt et al., 2007a). However, the formation of the
individual organic compounds did not follow the same pattern. As an example,
Fig. 6 shows a comparison of the concentrations of 2-methylglyceric acid
under acidic and non-acidic conditions as a function of relative humidity.
Acidic seed aerosol has a greater effect on 2-methylglyceric acid at lower
relative humidity. Some of the compounds produced in higher quantities in the
acidic-seed experiments included 2-methylglyceric acid, 2-methyltetrols,
furandiol-OS, 2-methyltetrol-NOS, 2-methylthreonic acid NOS, and furanone-OS,
while some others in the non-acidic-seed experiments included IEPOX-2,
2-methylglyceric acid OS, and 2-methylthreonic acid OS. Yields of the remaining
compounds followed an inconclusive pattern (Figs. S1, S2, and S3; Table S1 in
the Supplement). Thus, this study shows that the effect of relative humidity on
the formation of a wide range of isoprene SOA products cannot easily be
predicted, although the majority increases with decreasing relative humidity
under both acidic and non-acidic conditions.
Influence of RH and seed acidity on the estimated
concentration of 2-methylglyceric acid produced in chamber experiments with
non-acidic seeds (red) and with acidic seeds (blue). See Fig. S3 for
additional compounds.
Extracted-ion chromatograms of 2-methylglyceric acid organosulfate
with MW 200 from field studies and chamber experiments.
Extracted-ion chromatograms of furanetriol organosulfate with MW 214
from field studies and chamber experiments.
Extracted-ion chromatograms of 2-methyltartaric acid organosulfate
with MW 244 from field studies and chamber experiments (not detected in
non-acidic sample).
Extracted-ion chromatograms of nitrosoxy-organosulfate with
MW 245 from chamber experiments (not detected in field samples).
Electrospray product ion mass spectrum (-) of 2-methyltetrol
nitrosoxy-organosulfate (MW 245) of the RT = 1.35 min peak (Fig. 10)
acquired for the acidic seed aerosol along with the proposed fragmentation
pathway.
Electrospray product ion mass spectrum (-) of 2-methylthreonic acid
organosulfate (MW 230) at RT = 0.63 min (Fig. S4) acquired for Zielonka
PM2.5 aerosol along with the proposed fragmentation pathway.
Electrospray product ion mass spectrum (-) of 2-methyltetrol
nitroxy-organosulfate (MW 261) eluting at RT = 2.44 min (Fig. S4)
registered for the acidic seed aerosol along with proposed fragmentation
pathway.
Electrospray product ion mass spectrum (-) of 2-methyltartaric acid
organosulfate (MW 244) recorded for the RT = 0.58 min peak (Fig. 9) from
Godów fine aerosol along with the proposed fragmentation pathway.
Chromatographic comparison of chamber experiments and field
samples
We compared the results of chamber experiments to samples of PM2.5
collected at the two rural sites, Zielonka and Godów. To keep the
experimental and ambient conditions as similar as possible, we selected the
experiments carried under the highest relative humidities: ER662 at RH
44 % (acidic seeds) and ER667 at RH 49 % (non-acidic seeds).
Figures 7–10 show the extracted-ion chromatograms of selected components
detected in the respective filter extracts. Several compounds occurred both
in the chamber SOA and in the ambient samples: 2-methylglyceric acid OS
(MW 200), furanetriol OS (MW 214), 2-methyltetrol OS (MW 216),
2-methylthreonic acid OS (MW 230), and 2-methylthreonic acid NOS (MW 275). The
2-methyltartaric acid OS (MW 244) was also found in ambient samples, with only
trace amounts in acidic seed aerosol (Fig. 9). However, 2-methyltetrol
nitrosoxy-organosulfate (MW 245) was detected in the chamber SOA (Fig. 10).
The extracted-ion chromatograms of 2-methyltetrol nitroxy-organosulfate
(MW 261) were insufficient to provide reasonable fragmentation (Fig. S4). The
comparison shows that isoprene SOA in the presence of acidic seed aerosol and
NOx from the chamber studies provide a reasonable
approximation of the ambient processes at both sites even though only Godów
is strongly influenced by anthropogenic pollutants, mainly nitrogen oxides
due to a nearby coal-fired power station. It appears that minor amounts of
NOx in the ambient atmosphere are sufficient to produce
these compounds. These findings will require further confirmation.
Mass spectra and proposed fragmentation pathways of newly
identified organosulfates and nitroxy- and nitrosoxy-organosulfates
Based on the high-resolution mass data and fragmentation spectra recorded
for HPLC-resolved peaks, it is difficult to distinguish between isomers of
the same molecular structure. Moreover, some of the peaks for selected m/z values
in the extracted-ion chromatograms may correspond to more than one
compound. Therefore, identifications for the structures proposed are
tentative. This ambiguity results in the fragmentation spectra having the
fragment ions coming from different precursor ions with the same m/z. Our
proposed structures for the newly identified organosulfates and nitroxy- and
nitrosoxy-organosulfates are based on the accurate mass measurements and the
following assumptions:
All studied compounds have the same carbon backbone of 2-methylbutane.
The presence of the abundant m/z 97 peak corresponding to the HSO4-
ion indicates that the hydrogen atom is present at the carbon atom next to
that bearing HO–SO2–O– moiety (Attygalle et al., 2001). There are,
however, exceptions seen in Figs. 11 and 12.
When the condition given in (b) is not fulfilled, elimination of sulfur
trioxide molecule from the precursor ion can be detected (Szmigielski,
2013).
Elimination of the HONO and HNO3 molecules from the precursor ion is a
diagnostic for the presence of the nitrous (-ONO) and nitric (-ONO2)
esters, respectively. Similar to assumption (a), a β-hydrogen must be
present to enable the β-elimination (Tovstiga et al., 2014).
Electrospray product ion mass spectrum (-) of 2-methylthreonic acid
nitroxy-organosulfate (MW 275) of the RT = 0.83 min peak (Fig. S4)
recorded for Zielonka PM2.5 aerosol along with the proposed
fragmentation pathway.
The 2-methyltetrol nitroxy-organosulfate detected at m/z 260 corresponds to
the major early eluting compounds for the chamber and PM2.5 as seen
in Fig. S4. The minor shifts in retention times of eluting compounds are
generally due to matrix effects (Spolnik et al., 2018). Two partially
resolved peaks with identical MS profiles typically indicate
diastereoisomeric forms. This finding is consistent with earlier studies
(Gomez-Gonzalez et al., 2008; Surratt et al., 2007a). A detailed
interpretation of negative-ion electrospray mass spectra led to a proposed
structure for 2-methyltetrol nitroxy-organosulfates bearing a nitroxy moiety
at the primary hydroxyl group of 2-methyltetrol skeleton and sulfate group at
the secondary hydroxyl group seen in Fig. 13. The main fragmentation pathways
correspond to a neutral loss of 63 u
(HNO3), resulting in m/z 197 as a base peak, and to a bisulfate ion at
m/z 97. Another diagnostic ion at m/z 184 can be attributed to a combined
loss of NO2 and CH2O, suggesting the presence
of a
hydroxymethyl group in the molecule. The presence of m/z 213 and 183 ions
supports the interpretation given above due to a characteristic neutral loss
of a CH2O fragment. A revised structure for the MW 261 SOA
component along with the proposed fragmentation scheme is given in Fig. 13,
where only the mass spectrum of one diastereoisomer is shown.
A second abundant chamber-generated SOA component was detected at m/z 244.
In contrast to 2-methyltetrol nitroxy-organosulfate, the MW 245 unknown was
not detected in PM2.5, which would suggest the compound could play a
relevant role as a reactive reaction intermediate en route to particle
formation through isoprene SOA chains. Two baseline-resolved peaks of
identical electrospray product ion mass spectra could be attributed to
diastereoisomers with an isoprene-retained backbone (Fig. 10). Surratt and
co-workers observed the formation of this compound in the isoprene
photooxidation experiment under high-NOx conditions and
proposed the structure to 2-methylglyceric acid nitroxy-organosulfate
(Surratt et al., 2007a). However, in light of our mass spectral data we
assign the MW 245 unknown to C5 organosulfate, namely 2-methyltetrol
nitrosoxy-organosulfates. The m/z 244 →m/z 226 transition in the
product ion mass spectrum (Fig. 11) points to the intact secondary hydroxyl
moiety of the 2-methyltetrol skeleton. The lack of HNO3 elimination
from the [M–H]- (m/z 244) precursor ion clearly excludes the presence of
the nitroxy group. However, an abundant m/z 197 ion, which forms through the
HNO2 loss, could be associated with the existence of the –O–NO
residue. The structure assigned to the abundant MW 245 component from ER662
(acidic seed aerosol) along with its proposed fragmentation scheme is
presented in Fig. 11.
Additional abundant SOA organosulfates were determined at m/z 229 and 243
for the chamber and PM2.5 as shown in Figs. 12 and 14,
respectively, which do not appear to have previously been detected. The
accurate mass data were recorded for the Godów sample with the following
characteristics: RT = 0.58 min in Fig. 9
(C5H7O9S: 242.9816 Da, error + 0.2 mDa; Fig. 14) and
RT = 0.63 min in Fig. S4 (C5H9NO8S: 229.0011 Da,
error + 0.2 mDa; Fig. 12); this suggested greater oxidation pathways for these
unknown organosulfates compared to those for the formation the of
sulfated-2-methyltetrols. Two partially resolved peaks of identical mass
spectrometric signatures can be noted for these organosulfates, indicating the
presence of two chiral centers in their molecules (Figs. 9 and S4). In either
case, first eluting diastereoisomers give rise to peaks having high
abundances, while the second peak is of a more minor intensity, suggesting the
formation of less hindered compounds both in the chamber experiments and
PM2.5. A detailed interpretation of product ion mass spectra
permitted assignment of structures of the MW 244 and MW 230 unknowns to
2-methyltartaric acid organosulfate and 2-methylthreonic acid organosulfate,
respectively (Figs. 14 and 12 with the mass spectrum of the minor
diastereoisomer not shown). Both spectra display abundant fragment ions
at m/z 163 and 149, which could be explained by the SO3
elimination from their precursor ions. Further fragmentation of m/z 163
ions, i.e., a neutral loss of water followed by decarboxylation, reveals the
simultaneous presence of –O–SO3H and –CO2H residues in the MW 230
diastereoisomeric organosulfates. However, the absence of the bisulfate ion
in the spectrum of the MW 244 organosulfate clearly indicates a lack of a
proton adjacent to the sulfated group and thus suggests the sulfation of a
secondary hydroxyl group. MW 230 organosulfate and the presence of the
bisulfate ion in the MS/MS spectrum does not necessarily reveal unambiguously
the sulfation at a primary hydroxyl group in the molecule. The proposed
fragmentation schemes for the MW 244 and 230 novel organosulfates are
depicted in Figs. 14 and 12. Again, the mass spectra of related
diastereoisomeric organosulfates are not presented.
A final related organosulfate was detected at m/z 274 in substantial
quantities for isoprene SOA from the chamber and rural PM2.5
(Fig. S4). To our knowledge this compound has previously not been reported.
The compound has transitions of m/z 274 →m/z 211 (a loss of
HNO3) and m/z 274 →m/z 97 (a loss of C5H7NO6)
from the product ion mass spectrum from the Zielonka PM2.5 as seen
in Figure 15. The high-resolution data for this organosulfate eluted at RT = 0.83 min (C5H7NO10S:
273.9873 Da, error + 0.4 mDa) clearly points to
nitroxy-organosulfate from isoprene. A detailed explanation of other
diagnostic ions led to a proposed structure of 2-methylthreonic acid
nitroxy-organosulfate (Fig. 15). It could be assumed that due to a high
oxidation state (C/O = 0.5) the MW 275 organosulfate could serves
as an identifying marker of highly processed isoprene aerosol. However,
further study is warranted to rationalize its formation mechanism and
reactivity in the atmosphere.
While these experiments provide an analysis of a wide range of isoprene
reaction products in the aerosol phase as a function of RH and acidity, they
also include a number of shortcomings that need to be addressed in future
work. Perhaps the most significant is the use of authentic standards to
assess the contribution of these products to SOA mass at different RH. In
addition, when the relative humidity is varied, it is important to measure
aerosol liquid water content directly or estimated using thermodynamic
models, such as ISORROPIA (Fountoukis and Nenes, 2007) or AIM (Wexler and
Cregg, 2002), and other gas and particle composition (e.g., inorganic
species). Liquid water inorganic species measurements were not available for
this study.
The use of these marker compounds for ambient air quality models can follow
the approach of Pye et al. (2013). In such an approach, the model is run
using a base case chemical mechanism for isoprene, where there is no
adjustment for acidity and relative humidity. A comparison can then be made
with the same model having such an adjustment incorporated within the
isoprene mechanism. The markers can then serve as constraints to the PM
observations. For the US, the Community Multiscale Air Quality (CMAQ) model
is frequently used for ozone and PM ambient concentrations (Pye et al.,
2013). For Poland, a similar approach can be used with a European model
having the appropriate meteorology and chemical mechanism (Miranda et al.,
2015).