Introduction
Secondary organic aerosol (SOA) accounts for a large but undefined fraction
of organic matter in PM2.5, forming through the photooxidation of
biogenic and anthropogenic volatile organic compounds (VOCs) in the gas
phase yielding low-vapour-pressure products that partition into the particle
phase (Kroll and Seinfeld, 2008; Hallquist et al., 2009). The global
fluxes of anthropogenic SOA are poorly constrained and highly uncertain, with
a wide range of estimates from 2 to 25 Tg yr-1 (Volkamer et al., 2006;
Henze et al., 2008). Measurements suggest that anthropogenic precursors form
more SOA than predicted by models (Heald et al., 2005; Volkamer et al.,
2006; Matsui et al., 2009), likely due to incomplete model representation of
SOA formation pathways (Henze et al., 2008),
partitioning (Donahue et al., 2006), ambient conditions
(Ng et al., 2007), and precursors
(Robinson et al., 2007; Fu et al., 2008).
A tracer-based approach has been useful in identifying aerosol sources and
source apportionment (Schauer et al., 1996). SOA can
be linked to its precursor VOC following the SOA tracer approach introduced
by Kleindienst et al. (2007) in which ambient concentration of tracers (or
the sum of thereof) are converted to secondary organic carbon (SOC) or SOA
mass yields using tracer-to-OC or tracer-to-SOA ratios, respectively, that
were determined in chamber studies. For biogenic SOA, relatively well
defined and established tracers are employed, such as methyltetrols for
isoprene and β-caryophyllinic acid for β-caryophyllene
(Kleindienst et al., 2007). In contrast, the
tracer-based approach for aromatic SOA relies on a single molecule
(2,3-dihydroxy-4-oxopentanoic acid (DHOPA)) that is derived from toluene
(Kleindienst et al., 2007). Advancing the
tracer-based approach to anthropogenic SOA apportionment should involve
expanding the number of available tracers, particularly those that form from
aromatic VOCs other than toluene.
Chamber experiments have been conducted to identify SOA products formed
during the photooxidation of aromatic precursors associated with
anthropogenic sources, such as benzene, toluene, ethylbenzene, xylenes
(BTEX), and low-molecular-weight polycyclic aromatic hydrocarbons (PAHs).
Furandiones have been identified as a major product of aromatic VOC
photooxidation in the presence of NOx (Bandow et al., 1985; Forstner
et al., 1997; Hamilton et al., 2003; Koehler et al., 2004).
Nitromonoaromatics (e.g. nitrophenols, methyl-nitrophenols, nitrocatechols,
and nitrosalicylic acids) are likewise products of aromatic VOC
photooxidation in the presence of NOx (Forstner et al., 1997; Jang
and Kamens, 2001; Hamilton et al., 2005; Sato et al., 2012; Irei et al.,
2015), but some of these species have also been detected in the primary emission from
vehicles (Tremp et al., 1993) and biomass burning
(Iinuma et al., 2010). While nitromonoaromatics have been
quantified in ambient aerosol (Dron et al., 2008; Kitanovski et al.,
2012; Kahnt et al., 2013), the extent of their formation from primary and/or
secondary sources has yet to be determined. Phthalic acid is a product of
naphthalene photooxidation (Kautzman et al.,
2010) and has been proposed as a tracer for naphthalene and methylnaphthalenes in
PM2.5 (Kleindienst et al., 2012). However, phthalic
acid has also been observed in the emission of motor exhaust
(Kawamura and Kaplan, 1987), and thus both primary and
secondary sources can contribute to its ambient concentration. These three
classes of compounds can be potentially used a tracers for SOA; however,
further ambient studies are needed to evaluate their detectability, ambient
concentrations, and origins.
There are many desired characteristics for a molecule to be used as a source
tracer. First, it should be unique to the source of origin. For example
DHOPA was previously identified as a unique product of toluene
photooxidation in the presence of NOx (Kleindienst et al., 2004), and
methyltetrols are unique to isoprene (Claeys et al.,
2004a). Second, the tracer should be formed in reasonably high yields so it
has sufficiently high concentrations in the atmosphere to allow for reliable
quantification. Third, the tracer needs to be reasonably stable in the
atmosphere, so that it is conserved between formation and collection at a
receptor location. Fourth, an efficient SOA tracer should have a low vapour
pressure so that it primarily partitioned to the particle phase, which
minimizes possible underestimation from loss to the gas phase. Thus, an
effective SOA tracer will exhibit source specificity, consistent
detectability, atmospheric stability, and partitioning to the aerosol phase.
In this work, we examine and evaluate the efficacy of nitromonoaromatics,
furandiones, and aromatic dicarboxylic acid isomers as potential SOA tracers
in terms of their ambient concentration, gas–particle partitioning, and
source specificity through correlations with established tracers, including
levoglucosan for biomass burning (Simoneit et al.,
1999), hopanes for vehicular emissions (Schauer et
al., 1999), and DHOPA for anthropogenic SOA
(Kleindienst et al., 2004). Sample
preparation procedures were optimized for the simultaneous extraction of
primary and potential secondary source tracers, which were then quantified
by gas chromatography–mass spectrometry (GC-MS). These methods were applied to measure the ambient concentrations
and gas–particle distributions for analytes in fine particulate matter
(PM2.5) collected in Iowa City, IA, in the autumn of 2015. November was
chosen for this study because, in a prior study at this site, biogenic SOA
tracers were detected in this month (Jayarathne et al.,
2016) and aromatic SOA tracers have a less pronounced seasonal variation
than those that are biogenic (Shen et al., 2015; Ding et al., 2012;
Lewandowski et al., 2008). Developing and evaluating these tracers provide
additional tools for better understanding the contribution of aromatic VOCs
to ambient aerosol and will help to expand the current knowledge about the
composition and sources of ambient aerosol, particularly in urban and
peri-urban environments.
Experimental methods
Field sampling
Gas and particle (PM2.5) samples were simultaneously collected daily
for the period 4–15 November 2015 in Iowa City, IA, USA (41.6572∘ N, 91.5035∘ W). The sampler was installed on a wooden platform,
and the inlet was positioned 3.5 m above ground level. The sampling site was
surrounded by an agricultural field and a university parking lot. Sample
collection was preformed using a medium-volume URG air sampler (3000B, URG
Corp.) with a cyclone (URG) operating at a flow rate of 90 L min-1. Air
flow rate was monitored before and after sampling using a rotameter (Gilmont
Instruments). PM2.5 samples were collected on 90 mm quartz fibre filters
(QFF; Pallflex® Tissuquartz™, Pall Life Sciences) that
were pre-cleaned by baking for 18 h at 550 ∘C. Gas samples
were collected on 52 mm polyurethane foam (PUF) plugs placed after the filter
holder (URG-2000-30-52PC). PUF plugs were pre-cleaned using acetone (HPLC
grade, Sigma-Aldrich), hexanes, and acetonitrile (Optima-Fisher
Scientific-Fisher Chemical) by a repeated compression extraction apparatus
adapted from Rogge et al. (2011). This apparatus is
composed of a thick-walled borosilicate glass cylinder equipped with a
polytetrafluoroethylene (PTFE) valve and PTFE plunger that was used to
compress the solvent out of the PUF.
Samples were collected for 23 h, and filter changing was performed at
08:00 (local time). After sampling, filters were transferred to Petri
dishes, lined with pre-baked aluminum foil, and sealed with Teflon tape. PUF
samples were transferred to pre-baked wide-mouth glass jars, capped with a
Teflon-lined cap, and sealed with Teflon tape. Sampled filters and PUF were
transported to the laboratory and stored frozen at -20 ∘C until
analysis. One field blank was collected for every five samples following the
same described procedure, except no air was pulled through the system.
Extraction
All glassware used in this experiment was first baked (500 ∘C for
5 h) to remove organic contaminants and then silanized (using 5 %
solution of dichlorodimethylsilane in toluene) to minimize the sorption of
analytes to the glass surface (Kitanovski et al., 2012).
Filters and PUF were spiked with isotopically labelled internal standards,
representing the different classes of organic compounds reported in this
study. Adding internal standards prior to extraction corrects for loss of
analyte during the extraction process, provided the internal standard
adequately represents the chemical and physical properties of the analyte.
Specifically, internal standards and their corresponding analytes were
3-nitrosalicylic acid-D3 and 5-nitrosalicylic acid; 4-nitrophenol (4NP)-D4 and
other nitromonoaromatics; maleic anhydride-D2 for 2,5-furandione, and succinic anhydride-2,2,3,3-D4 for the three other
furandiones; levoglucosan13C6 for levoglucosan; ketopinic acid (KPA) for DHOPA; phthalic
acid-D4 for aromatic dicarboxylic acids; and acenaphthene-D10, pyrene-D10,
benzo[a]anthracene-D12, coronene-D12 for PAHs. The use of KPA as an internal
standard for DHOPA builds upon prior work by Kleindienst et al. (2007).
Filters were extracted sequentially with three 10 mL portions of
acetonitrile using ultrasonication (Branson 5510, 137 W) for 15 min at
60 sonics per minute. The combined three extracts were reduced to 2 mL by
rotary evaporation at 30 ∘C, 120 rpm, and 200 mbar. The reduced
extracts were filtered with a 0.25 µm PTFE syringe filters and stored
frozen at -20 ∘C until analysis. Immediately prior to analysis,
the extracts were evaporated to 100 µL under a gentle stream of
ultra-pure nitrogen at 30 ∘C. PUF samples were extracted by
three cycles of repeated compression using acetonitrile; extracts were then
combined, evaporated, filtered, and reduced to the final volume using the
same conditions as filter extracts.
Instrumental analysis
OC and elemental carbon (EC) were measured by thermal-optical analysis
(Sunset Laboratory Inc.) on a 1 cm2 filter portion, following Schauer
et al. (2003).
Organic species were analysed using an Agilent 7890A GC, coupled with 5975C
MS (Agilent Technologies). 2,3-Dihydroxy-4-oxopentanoic acid (Toronto
Research Chemicals), phthalic acid isomers, levoglucosan, and biogenic SOA
tracers were trimethylsilylated with
N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA-TMCS; Fluka Analytical 99 %)
prior to analysis. The silylation reaction was performed by evaporating 10 µL of the extract under a gentle stream of nitrogen to dryness,
adding 20 µL of the silylation agent and 10 µL of pyridine
(Burdick & Jackson, Anhydrous), and heating to 70 ∘C for 3 h.
A 2 µL aliquot of the silanized extract was introduced to the
GC-MS equipped with a DB-5 column, electron ionization (EI) source (70 eV),
and a GC inlet temperature of 300 ∘C. Nitromonoaromatics
were also silylated using the same reagent but under different conditions, in
which 20 µL of the extract was evaporated to dryness under a gentle
stream of nitrogen, 10 µL of the silylation agent was added, and then
the mixture was capped and heated for 90 min at 100 ∘C. The
different silylation protocol used for nitromonoaromatics yielded more
symmetrical peak shapes and higher intensities than the
derivatization method used for levoglucosan and phthalic acid isomers that
resulted in asymmetrical nitromonoaromatic peaks with low intensities. The
GC injection volume was 1 µL, and the inlet conditions, column
type, and MS parameters matched those previously described. Furandiones were
analysed using the method developed in our previous work (Al-Naiema
et al., 2017), and PAHs were analysed using a DB-5 column as described
elsewhere (Al-Naiema et al., 2015).
Responses of the analytes were normalized to the corresponding
isotopically labelled internal standards and quantified using a linear
calibration curve with a squared correlation coefficient (R2)≥ 0.995. Analytical uncertainties were propagated from the standard deviation
of the field blank value and 10 % of the measured concentration. For
analytes not detected in the field blank, instrument detection limits were used
in error propagation. All measurements were field-blank-subtracted. Due to
low recoveries of furandiones from PUF, gas-phase concentrations of
furandiones were corrected for the recoveries of the authentic standards.
Particle-phase fraction calculation and model
The fraction of a species in the particle phase (Fp) was
calculated from the ratio of concentration in the particle phase to the
total concentration (sum of gas and particle), following Eq. (1).
Fp=[particle]gas+[particle]
Fp was modelled using the gas–particle partitioning
coefficient (Eq. 2) from absorptive partitioning theory developed by Pankow (1994), and following Yatavelli et al. (2014):
Fp=1+1kom×COA-1,
where COA is the concentration of the organic aerosol (µg m-3)
and kom is the partitioning coefficient (m3 µg-1) described as
kom=RT106PL0EMW,
in which R is an ideal gas constant (8.2 × 10-5 m3 atm mol-1 K-1); T is temperature (averaging 9 ∘C during
this study); 106 is a unit conversion factor (µg g-1);
PL0 is the sub-cooled vapour pressure (atm), obtained from the
Estimation Program Interface suite™ version 4.11 from the Environmental
Protection Agency (EPA); MW is the molar mass (g mol-1); and
E is the activity coefficient (set equal to 0.3, 1, and 3).
Statistical analysis
Inter-species correlations were evaluated using Minitab software (version
16). The Anderson–Darling test for normality indicated that neither ambient
concentrations nor log-transformed concentrations were normally distributed.
Hence, Spearman's rho (rs) was used to assess correlations.
Correlations were interpreted as follows: very high (0.9–1.0), high
(0.7–0.9), moderate (0.5–0.7), low (0.3–0.5), and negligible (0.0–0.3)
(Mukaka, 2012). The statistical significance of correlations was
evaluated at the 95 % confidence interval (p≤0.05).
Results and discussion
Validation of gas–particle partitioning
Fraction of PAHs in the particle phase as a function of the number
of carbon atoms. Circles represent the daily measured Fp
(n=14), the red line represents average Fp, and the dotted line
(black) is the predicted Fp using the absorption model (Table S1).
Select PAHs were used compare ambient measurements and model estimation:
C10 is naphthalene, C12 is acenaphthene, C14 is anthracene,
C16 is pyrene, C18 is benzo(ghi)fluoranthene, C20 is
benzo(b)fluoranthene, and C22 is picene.
PAHs with two to five rings span a range of high to low volatility,
respectively. The accuracy of the measured gas–particle distributions were
evaluated with PAHs that have been extensively discussed in the literature.
The average fractions of PAH in the particle phase (Fp)
measured in Iowa City, IA, USA (Fig. 1), were ∼ 5 % for 10–14 carbon atoms (two–three rings: naphthalene, acenaphthene, and anthracene), 14 %
for 16 carbon atoms (four rings: pyrene), 59 % for 18 carbons (four rings:
benzo(ghi)fluoranthene), and > 98 % for 20 carbon atoms (five
rings: picene). The predicted Fp values estimated using the Pankow
absorption model (1994) following Eqs. (1) and (2) and using
parameters in Table S1 in the Supplement follow the same trend (Fig. 1, dashed line), with a
systematic underestimation for the predicted Fp by ≤ 12 % for most PAHs and 20 % for 18 carbons. Such an underestimation has
been widely documented in comparison of theory to ambient partitioning
studies and is attributed to the omission of PAH sorption on elemental
carbon in the model (Dachs and Eisenreich, 2000; He and Balasubramanian,
2009; Wang et al., 2011). Ambient studies of gas–particle partitioning are
influenced by many factors such as ambient temperatures (Terzi
and Samara, 2004), relative humidity (Pankow et al., 1993), and sampling
technique (reviewed by Kim and Kim, 2015), confounding direct comparisons
between this and other studies. Overall, the general trends observed herein
are consistent with prior studies that report PAHs with two–three aromatic rings
(C10–C14) mainly in the gas phase (Fp≥ 0.93),
those with five or more aromatic rings (≥ C20) mainly in the particle
phase (Fp≥ 0.9), and those with four aromatic rings
(C16 and C18) partitioned between the two phases depending on their
chemical structure and atmospheric conditions (Yamasaki et al., 1982;
Williams et al., 2010; Ma et al., 2011; Kim and Kim, 2015).
Gas-phase sampling using QFF without a denuder upfront is subject to
artefacts caused by vapour adsorption on the filter, resulting in
underestimation of the concentration of the species in the gas
phase, particularly for low-MW PAHs (Delgado-Saborit et al., 2014).
However, comparing the partitioning trend in this study to those sampled
with a denuder during the same season (Possanzini et
al., 2004) shows less than 5 % discrepancies for the low (C ≤ 12) and
high (C ≥ 18) molecular weight PAHs, while for C14 and C16
our Fp measurements were lower by 14 and 6 %,
respectively. These results show a slight underestimation of
Fp rather than an overestimation, which would be expected if
vapour adsorption on the QFF significantly impacted gas–particle
partitioning results. We estimate that the uncertainties associated with our
gas–particle partitioning measurements are ≤ 5 % for species
predominantly in the particle phase (Fp>0.9) or
gas phase (Fp<0.1) and are in the range of 14 %
for semi-volatile species (0.1<Fp<0.9).
The toluene tracer (2,3-dihydroxy-4-oxopentanoic acid)
GC-MS identification
Mass spectrum of the trimethylsilylateted derivatives of
2,3-dihydroxy-4-oxopentanoic acid by electron impact ionization.
2,3-Dihydroxy-4-oxopentanoic acid (DHOPA, also known as T-3) has been
identified as a product of toluene photooxidation by Kleindienst et al. (2004), and their chemical ionization
mass spectrum has been used to identify this tracer in other studies. To
support identification by the more common EI, the
corresponding mass spectrum of its trimethylsilylated (TMS) derivative is
given in Fig. 2. The most abundant ions are m/z 73 and 147, corresponding to
Si(CH3)3+ and (CH3)2 Si = O Si(CH3)3+
fragments, respectively; however, these are common to the
BSTFA-TMCS silylation reagent. Ions at m/z 277, 349, 321, and 364 are unique to DHOPA and
are recommended for quantification. Here, the m/z 277 ion was used for
quantification due to the high relative abundance and low background, and
the other ions were used qualitatively. This mass spectrum obtained from
a pure standard builds upon the previous EI mass spectrum for the DHOPA in
an aerosol sample by Hu et al. (2008) that included some
spectral interferences from adipic acid that co-eluted.
Ambient concentration and gas–particle partitioning
Method performance parameters for nitromonoaromatics,
including GC retention time (tR), instrument detection limit
(IDL), instrument quantitation limit (IQL), and mean extraction recoveries
(±1 standard deviation for n=3).
Nitromonoaromatic
tR
Base peak*
Linear range
Linear regression
IDL
IQL
Extraction recovery
Filter
PUF
(min)
(m/z)
(µg L-1)
R2
(µg L-1)
(µg L-1)
(%)
(%)
4-Nitrophenol-D4 (IS) (4NP-D4)
9.66
200
–
–
–
–
–
–
4-Nitrophenol (4NP)
9.69
196
50–5000
0.999
13.2
43.9
100.5 ± 2.2
97.9 ± 2.2
4-Methyl-2-nitrophenol (4M-2NP)
9.71
210
10–5000
0.999
2.7
8.8
96.7 ± 2.8
100.3 ± 0.6
4-Methyl-3-nitrophenol (4M-3NP)
9.71
208
40–5000
0.999
11.4
38.0
100.2 ± 0.5
97.1 ± 3.2
2-Methyl-4-nitrophenol (2M-4-NP)
11.08
210
50–5000
0.999
14.5
48.2
99.7 ± 0.8
99.0 ± 1.6
4-Nitroguaiacol (4NG)
12.08
211
50–5000
0.999
14.9
49.5
94.2 ± 2.4
97.9 ± 2.2
5-Nitroguaiacol (5NG)
12.40
211
30–5000
0.998
6.7
22.4
96.2 ± 3.3
97.4 ± 1.7
4-Nitrocatechol (4NC)
13.13
284
40–5000
0.999
10.1
33.8
102.5 ± 2.9
49.9 ± 17.0
4-Methyl-5-nitrocatechol (4M-5NC)
13.66
313
40–5000
0.999
9.1
30.3
108.1 ± 1.8
41.5 ± 24.8
4-Hydroxy-3-nitrobenzyl alcohol (4H-3NB)
14.23
298
20–5000
0.998
6.3
21.0
95.6 ± 2.5
103.3 ± 3.5
3-Nitrosalicylic acid-D3 (IS) (3NSA-D3)
14.08
315
–
–
–
–
–
–
5-Nitrosalicylic acid (5NSA)
15.11
312
20–5000
0.996
5.1
16.9
100.2 ± 1.0
95.0 ± 3.1
* m/z are given for trimethylsilylated derivatives of all
analytes.
The average mass concentration of DHOPA ranged from 0.14 to 0.50 ng m-3
and averaged 0.29 ± 0.12 ng m-3 (Fig. 3a). DHOPA was detected
only in the particle phase (Table 2), although the 22.1 ± 13.5 %
extraction recoveries of this species from PUF limited the sensitivity of
gas-phase measurements. Nonetheless, it is reasonable to conclude that this
species does not appreciably partition to the gas phase.
Daily PM2.5 concentrations for 2,3-dihydroxy-4-oxopentanoic
acid (a) and benzene dicarboxylic acids (b), where PhA is
phthalic acid, t-PhA is terephthalic acid, i-PhA is isophthalic acid, and
4M-PhA is 4-methylphthalic acid.
The average concentrations of DHOPA in Iowa City were within the range of
those observed in Bondville, IL, in autumn (Lewandowski et al.,
2008) but were lower by a factor of 45 (on average) than what was detected
in the Pearl River Delta, China, for the same season (Ding et al.,
2012). Although an authentic standard for DHOPA was not previously
available, prior measurements were based upon a surrogate standard response and
are subject to bias.
The contribution of toluene SOA to OC was estimated based on the SOA tracer
method introduced by Kleindienst et al. (2007), where the DHOPA mass
fraction of SOC from toluene was 0.0079 ± 0.0026. Following this estimation method, toluene SOC was estimated to
contribute 36.5 ± 15.0 ngC m-3. The contribution of the
estimated SOC to the total OC in this study ranged 0.3–7 % and averaged
2.2 ± 1.6 %. In other studies, the concentration of toluene SOC was
variable and influenced by seasonal variations and local emission sources
(Kleindienst et al., 2007; Peng et al., 2013). Our estimated SOC levels
were less than half of those observed in the rural Midwestern United States
previously (0.09 µg m-3) during the same season; however the
contribution of the estimated toluene SOC to the total OC was 6 %
(Lewandowski et al., 2008), which is within the upper end of
the range observed in this study. Although toluene SOC concentrations were
much higher in the Pearl River Delta (1.65 µg m-3)
(Ding et al., 2012), its relative contribution to OC (7 %) was
comparable. Because toluene is only one of many aromatic VOC precursors to
SOA, additional tracers are needed to better evaluate the impact of aromatic
VOCs on SOA.
Benzene dicarboxylic acids
Three isomers of benzene dicarboxylic acid and one methyl derivative were
detected in all PM samples. The total (gas plus particle) concentration of
phthalic acid (PhA), the most abundant
isomer, ranged from 4.9 to 21.5 ng m-3 and averaged 13.0 ± 4.3 ng m-3,
while isophthalic acid (i-PhA), terephthalic acid (t-PhA) and
4-methylphthalic acid (4M-PhA) had increasingly lower concentrations in the
range of 0.2 to 6.6 ng m-3 (Fig. 3b). Similar relative abundancies
for these species were observed in other studies, with PhA consistently
being the predominant isomer (Fraser et al., 2003; Mirivel et al., 2011;
Mkoma and Kawamura, 2013). The relatively high ambient mass concentrations
of these dicarboxylic acid isomers at levels that allow for consistent
detection make them promising candidates for tracing aromatic SOA.
Summary table of ambient concentrations of
organic species in gas and particle phases, measured fraction in the
particle phase (Fp), frequency of detection in the particle
phase (FODp), and sources reported in the
literature.
Compound
Mean ambient concentration (±SD)
Fp(%)
FODp(%)
Some of the reported
(ng m-3)
emission sources in the literature
Particle
Gas
Secondary photooxidation
Biomass burning
Vehicle emissions
2,3-Dihydroxy-4-oxopentanoic acid
0.29 (0.12)
ND
100
100
Kleindienst et al. (2004)
Phthalic acid
3.42 (1.92)
9.62 (3.70)
26
100
Kleindienst et al. (2012)
Kawamura and Kaplan (1987)
Terephthalic acid
0.90 (0.58)
ND
100
100
Isophthalic acid
6.21 (4.82)
ND
100
100
Kleindienst et al. (2012)
4-Methylphthalic acid
1.08 (0.51)
0.23 (0.22)
82
100
Kleindienst et al. (2012)
Kawamura and Kaplan (1987)
4-Nitrophenol
0.63 (0.48)
1.47 (1.95)
30
100
Forstner et al. (1997)
Tremp et al. (1993)
4-Methyl-2-nitrophenol
0.26 (0.09)
5.13 (8.57)
5
100
Forstner et al. (1997)
Tremp et al. (1993)
2-Methyl-4-nitrophenol
0.08 (0.05)
0.16 (0.15)
33
93
Forstner et al. (1997)
Tremp et al. (1993)
4-Nitroguaiacol
0.08 (0.02)
0.66 (0.76)
11
86
Iinuma et al. (2007)
4-Nitrocatechol
1.60 (2.88)
0.09 (0.07)
95
93
Lin et al. (2015)
Iinuma et al. (2007)
4-Methyl-5-nitrocatechol
1.61 (1.77)
0.08 (0.06)
95
86
Lin et al. (2015)
Iinuma et al. (2007)
4-Hydroxy-3-nitrobenzyl alcohol
0.06 (0.06)
ND
100
71
Hamilton et al. (2005)
5-Nitrosalicylic acid
0.14 (0.08)
0.04 (0.03)
78
100
Jang and Kamens (2001)
Kitanovski et al. (2012)
2,5-Furandione
0.60 (0.58)
NR
NR
36
Forstner et al. (1997)
Dihydro-2,5-furandione
1.57 (1.34)
5.71 (3.33)
0.16
100
Forstner et al. (1997)
3-Methyl-2,5-furandione
0.44 (0.67)
7.19 (4.55)
0.03
79
Forstner et al. (1997)
Dihydro-3-methyl-2,5-furandione
0.63 (0.97)
5.10 (3.99)
0.02
71
Hamilton et al. (2005)
2-Methylglyceric acid
0.68 (0.80)
0.12 (0.08)
85
100
Claeys et al. (2004b)
2-Methylthreitol
9.90 (12.16)
5.82 (3.89)
63
100
Claeys et al. (2004a)
2-Methylerythritol
12.07 (15.51)
7.20 (4.74)
63
100
Claeys et al. (2004a)
cis-Pinonic acid
2.56 (3.11)
1.57 (1.80)
62
100
Yu et al. (1999)
Levoglucosan
109.68 (68.12)
ND
100
100
Simoneit et al. (1999)
ND – not detected; NR – not reported (see the
text)
The majority of PhA was estimated to be in the gas phase (Fp = 0.26),
in contrast to i-PhA and t-PhA (Fp=1) and
4-M-PhA (Fp=0.82, Table 2). Vapour pressure values and
partitioning theory (Table S1 in the Supplement) cannot explain the observed lower fraction of
PhA in the particle phase compared to i-PhA and t-PhA (Fp= 1).
Instead, the gas-phase measurement of PhA is expected to be positively
biased due to interference by phthalic anhydride, which yields identical
products to PhA when derivatized, hydrolysed, or exposed to high temperatures
(like those encountered in GC analysis). For example, under the conditions
employed in this study, phthalic anhydride and PhA have identical GC
retention time and silylated MS spectra (Fig. S1 in the Supplement). Phthalic anhydride is a
gas-phase product of naphthalene photooxidation (Chan et al., 2009;
Kautzman et al., 2010) and has much higher vapour pressure (7.5×10-6 atm)
than that of PhA (8.9×10-8 atm) (EPA, 2012). As such,
phthalic anhydride will partition to a greater extent to the gas phase,
which is supported by the absorption model estimations
(Fp=4.9×10-5), shown in Table S1.
Thus, PhA concentrations reported here and in prior studies that involve the
use of GC inlet temperatures ≥ 150 ∘C, derivatization, or
hydrolysis (which is common in liquid chromatography) reflect the sum of PhA
and phthalic anhydride. Because phthalic anhydride is primarily in the gas
phase, this causes gas-phase PhA concentrations to be overestimated and
Fp estimates for PhA to be erroneously low. An accurate
determination of Fp for PhA requires collection and analysis
of acid and anhydrides separately by in situ derivatization on
veratrylamine-coated glass fibre filters (OSHA, 1991).
Although PhA and 4M-PhA can be emitted directly from primary sources such
as motor vehicle engines (Kawamura and Kaplan, 1987), there is
a lack of evidence for significant primary source contributions to these
species in ambient air. In contrast, naphthalene, a precursor for secondary
formation of PhA (Kautzman et al., 2010; Kleindienst et al., 2012), was
found to be the most abundant PAH from many combustion sources (Oanh et
al., 1999; Al-Naiema et al., 2015). As shown in Table S3, the concentrations
of PhA and 4M-PhA in the particle phase are highly and significantly
correlated with DHOPA (rs = 0.73, p=0.003; rs = 0.79,
p=0.001, respectively), but they do not correlate with hopane (rs = 0.19, p=0.529),
a fossil fuel combustion biomarker. These correlation data indicate that the
probable origin of these two acids is secondary reactions, rather than
primary emissions. Although i-PhA has a strong correlation with
DHOPA, it also correlates highly and significantly with biomass burning
products (e.g. levoglucosan, 4-nitrocatechol, and 4-methyl-5-nitrocatechol)
and moderately with hopane. The possibility of multiple sources of i-PhA
limits its application as a tracer for anthropogenic SOA. Terephthalic acid
correlates strongly with biomass burning tracers and with hopane, and there
is no evidence supporting secondary formation; hence, t-PhA is not a valid
SOA tracer candidate. The relatively high concentrations of PhA and 4M-PhA
detected in the particle phase and their strong correlations with DHOPA
suggest that these compounds are useful SOA tracers for naphthalene and
methylnaphthalene photooxidation, respectively.
Nitromonoaromatic compounds
Analytical method performance
Selected ion chromatogram for a mixture of nitromonoaromatics
standard (5 ng mL-1), where 4M-2NP is 4-methyl-2-nitrophenol, 4NP is
4-nitrophenol, 4NP-D4 is 4-nitrophenol-D4, 4M-3NP is
4-methyl-3-nitrophenol, 2M-4-NP is 2-methyl-4-nitrophenol, 4NG is
4-nitroguaiacol, 5-NG is 5-nitroguaiacol, 4NC is 4-nitrocatechol, 4M-5NC is
4-methyl-5-nitrocatechol, 3NSA-D3 is 3-nitrosalicylic acid-D3, 4H-3NB is
4-hydroxy-3-nitrobenzyl alcohol, and 5NSA is 5-nitrosalicylic acid.
While many techniques for quantifying nitrophenols in various sample
matrices have been developed and were reviewed by Harrison et al. (2005), our goal was to quantify these compounds in
parallel to other primary and secondary source tracers using GC-MS and
single-filter extraction protocol. Using GC with a DB-5 column, a baseline
separation was achieved for 10 nitromonoaromatic analytes as
trimethylsilylated esters (Fig. 4), with highly symmetrical and narrow peak
shapes. Mass spectrometry was used for identification by comparison of
retention times and mass spectra, and quantification was done based on base
peak area (Table S2). Nitromonoaromatic mass spectra (Table S2) included
mass fragments with m/z [M-60]+ (from the loss of NO2
and CH3), where M is molecular ion for the trimethylsilylated ester.
Save for nitroguiacols and 4-methyl-5-nitrocatechol,
nitromonoaromatics' mass spectra included a mass fragment of [M-15]+
(loss of CH3). The mass spectra for the co-eluting peaks (Fig. 4)
indicate that potential interferences for the 4NP-D4, 4NP, and 4M-2NP
are not appreciably strong (< 1 %), and thus interferences are
expected to be negligible. There is potential for 4M-3NP to interfere with
detection of 4M-2NP, because the former shows a relatively strong signal for
m/z 210 (at 38 % of the base peak signal) that is used to
quantify the latter; however 4M-3NP was not detected in this study, so no
interference is expected in this dataset.
The performance of the GC-MS method was evaluated with respect to linearity,
detection and quantification limits of target analytes, and extraction
efficiency (Table 1). The normalized response for the nitromonoaromatics was
linear (R2≥0.996) from 10 to 5000 µg L-1 with a
constant internal standard concentration of 10 000 µg L-1. This
wide range of linearity indicates the suitability of this method to
determine nitromonoaromatics in different applications. The limit of
detection ranges from 2.7 to 14.9 µg L-1 for the 10
nitromonoaromatic compounds. These method detection limits are higher than
those obtained from liquid chromatography coupled with tandem mass
spectrometry (0.1–0.25 µg L-1) (Kitanovski et
al., 2012) but are sufficient to detect the investigated species in ambient
air. Filter extraction recoveries averaged 99.4 ± 3.8 %,
demonstrating high accuracy and precision of the filter extraction with
acetonitrile and reduction in volume under reduced pressure with rotary
evaporation. For PUF extraction, very high recovery (> 97 %)
was achieved for most compounds (Table 1); however two nitrocatechols, 4NC
and 4M-5NC, had significantly lower (< 50 %) and much more
variable (relative standard deviation (RSD): 34–60 %) recoveries. Similarly low recoveries of 4NC and
4M-5NC have been reported previously (Hawthorne et al.,
1989), which is attributed to the strong interactions of phenols with the
polymeric chains of the PUF. Consequently, gas-phase measurements of 4NC and
4M-5NC are biased low and subject to high uncertainty, such that their
levels and gas–particle partitioning are not reported. Otherwise,
the extraction and analysis method provides high accuracy and reliable
precision for nitromonoaromatics from filters and PUF.
Ambient concentration, gas–particle partitioning, and potential
sources
Time series of ambient PM2.5 concentration of levoglucosan,
4-nitrocatechol (4-NC), 4-methyl-5-nitrocatechol (4-M-5NC), and
5-nitrosalicylic acid (5NSA).
Total concentrations of eight nitromonoaromatics ranged from 0.7 to 17 ng m-3
in the particle phase and from 0.6 to 40 ng m-3 in the gas
phase (Figs. 5 and S3). Average concentrations and Fp are
summarized in Table 2, with daily Fp shown in Fig. S2.
A number of nitromonoaromatics were likely derived from biomass burning, as
evidenced by correlations with a biomass burning marker (levoglucosan) in
this and prior studies. Nitrocatechols were the most abundant particle-phase
species within this compound class, with average concentrations (±standard deviation)
of 1.6 ± 2.9 ng m-3 and 1.6 ± 1.8 ng m-3 for 4NC and 4M-5NC, respectively. These two species have been
previously associated with biomass burning in PM10, via their
correlations with levoglucosan (Iinuma et al.,
2010; Kahnt et al., 2013). The strong correlation of these two species with
levoglucosan extends to PM2.5 in Iowa City (Fig. 5) with very high
correlations with levoglucosan for 4NC (rs= 0.90, p<0.001)
and 4M-5NC (rs=0.85, p<0.001). Although nitrocatechol can be
formed from the toluene photooxidation (Lin et al., 2015), 4NC
correlates weakly with DHOPA (rs≤ 0.2) lacking statistical
significance (Table S3), suggesting that toluene photooxidation is
negligible in relation to biomass burning. Similarly, 5NSA (Fp = 0.73)
was highly correlated with levoglucosan (rs= 0.76,
p=0.002) but moderately correlated with DHOPA (rs=0.49, p=0.078), also
suggesting its primary origin to be biomass burning (Kitanovski et al.,
2012; Zhang et al., 2013) rather than photooxidation (Jang and
Kamens, 2001), in agreement with prior studies. Consequently, these three
species are characteristic of biomass burning, rather than anthropogenic
SOA.
Nitroguaiacol was detected in low concentrations relative to other
nitromonoaromatics. The concentrations of 4NG in PM2.5 ranged from
below the detection limit (BDL) to 0.11 ng m-3, with a frequency of
detection of 86 % (Table 2). Similarly low concentrations were also
reported elsewhere (Kitanovski et al., 2012). In the gas
phase, 4NG was not detected on most of the days, except for 14–16 November,
when gas concentrations reached 0.5–2.1 ng m-3 (Fig. S2). On 14 November,
outdoor festivities, barbecues, and slow-moving traffic occurred
near the sampling site. The possibility of multiple sources and the low
ambient concentrations suggest that this is not a suitable tracer for
anthropogenic SOA.
4H-3NB was detected only in the particle phase (Fp=1), with
a frequency of 71 % and relatively low concentrations ranging from BDL to
0.2 ng m-3. 4H-3NB was identified as a low-abundance product of toluene
photooxidation with hydroxyl radicals (Hamilton et al., 2005; Fang et
al., 2011). Other than toluene photooxidation, there
are no other known emission sources for 4H-3NB. The specificity of 4H-3NB is
supported by the lack of correlation with other biogenic or anthropogenic
tracers (Table S3). Because it is detected only in the particle phase and
is likely specific to toluene photooxidation, it has potential to be a
unique nitromonoaromatic tracer for anthropogenic VOC photooxidation.
However, due to the small number of samples and the frequency of detection
for this tracer, further investigation is recommended to evaluate its
detectability in other environments and source specificity.
In addition, 4NP was consistently detected, with summed gas
and particle concentration of 4NP ranging from 0.3 to 7.3 ng m-3 and
averaging 1.8 ± 2.1 ng m-3. Likewise, two methyl-nitrophenol
isomers (4M-2NP and 2M-4NP) averaged 0.3 ± 1.6 ng m-3
and 5.3 ± 8.5 ng m-3, respectively (Table 2 and Fig. S3). The
higher concentration of 4M-2NP with the higher standard deviation is largely
driven by the aforementioned local source influences on 14 November (32.5 ng m-3), shown in Fig. S2. These three nitromonoaromatics showed
substantial partitioning in the gas phase, with Fp≤ 0.33 (Table 2). The very high correlation of 4NP with 2M-4NP
(rs=0.90, p<0.001) and with 4M-2NP (rs=0.81,
p<0.001) indicates a similar source of origin. These three compounds
have previously been shown to be products from the photooxidation of the
monoaromatic compounds in the presence of NOx (Forstner et al.,
1997; Harrison et al., 2005; Sato et al., 2007) as well as components of
vehicle emissions (Tremp et al., 1993). However, no
significant correlations were observed between these tracers with hopane or
DHOPA. Because of their lack of source specificity and the significant
partitioning in the gas phase, these three nitrophenols are not recommended
for use as tracers of anthropogenic SOA.
Furandiones
Time series of the measured furandiones (2,5-furandione (FD),
dihydro-2,5-furandione (DFD), 3-methyl-2,5-furandione (MFD), and
dihydro-3-methyl-2,5-furandione (DMFD)) detected in gas and particle phases,
with the measured fraction in the particle phase (Fp).
Furandiones were not detected in the gas phase on 4 November 2015. Due to
poor extraction recoveries, the gas-phase concentration of FD was not
reported.
Ambient gas and particle concentrations for the sum of four furandiones and
their Fp are shown in Fig. 6, with individual species data in
Fig. S4. The total furandiones concentration detected in the particle phase
ranged from 0.3 to 4.3 ng m-3 and averaged 1.6 ± 1.1 ng m-3. These concentrations were lower than those detected in our
previous study (9.3 ± 3.0 ng m-3) (Al-Naiema et al., 2017),
which is likely due to the rainy and foggy weather in the autumn of
2015. In the presence of water, anhydrides undergo hydration and ring
opening to form the carboxylic acid derivatives. The relative rate of
hydrolysis for 2,5-furandione (FD) and 3-methyl-2,5-furandione (MFD) are 6
times higher than dihydro-2,5-furandione (DFD) and
dihydro-3-methyl-2,5,-furandione (DMFD) (Trivedi and Culbertson, 1982). The
highest stability against water hydrolysis might explain the higher
concentration of DFD detected in this study compared to other furandiones.
The sum of the gas-phase concentration for the furandiones (DFD, MFD, and
DMFD) averaged 18.0 ± 10.7 ng m-3.
Furandiones were almost entirely in the gas phase (Fig. S4). The measured
Fp values were 0.31 for DFD, 0.08 for MFD, and 0.05 for DMFD, while
this value is not reported for FD, which showed poor extraction recovery
(< 10 %) from the PUF. Low Fp values are expected
for furandiones due to their high vapour pressures (Table S1). The measured
Fp values were substantially higher than those predicted by
Pankow's absorption model by 2 orders of magnitude (Tables 2 and S1).
It is possible that higher-than-predicted Fp values were
driven by furandione adsorption on the front filter or breakthrough from the
PUF (Chuang et al., 1987).
Although no sources other than photooxidation of anthropogenic VOCs are
known to influence the atmospheric concentration of furandiones (Forstner
et al., 1997; Hamilton et al., 2005), only a moderate correlation
(rs=0.50, p=0.064) was observed between the particle
concentrations of furandiones with DHOPA (Table S3). This may be due to the
fact that DHOPA is a tracer specific to toluene, while furandiones can also
form from other aromatic VOCs (Forstner et al., 1997).
Overall, we conclude that furandiones hold a significant importance to serve
as indicators for atmospherically processed aromatic VOCs due to their source
specificity; however the substantial partitioning toward the gas phase and
their water sensitivity limit their application as SOA tracers.
Conclusions
This study evaluates, for the first time, the influence of the source
specificity, ambient concentration, and gas–particle partitioning on the
efficacy of the use of nitromonoaromatics, benzene dicarboxylic acids,
furandiones, and DHOPA as tracers for SOA from anthropogenic VOCs. First and
foremost, DHOPA was detected consistently and only in particle phase
and is specific to toluene photooxidation, making it a good tracer for
toluene SOA despite its relatively low concentrations. Second, PhA is the
most abundant benzene dicarboxylic acid isomer and correlates highly with
DHOPA. Similarly, 4M-PhA correlates highly with DHOPA. Although the measured
Fp values suggest partitioning to the gas phase for these two
species, this is likely due to instrumental interferences from the
corresponding anhydrides. Their particle-phase concentration, nonetheless,
are expected to be useful in tracing naphthalene-derived SOA. Third, 4H-3NB
was detected only in the particle phase and found to be specific to
toluene photooxidation at low levels of NOx. Because of their unique
sources, detectability, and partitioning towards the particle phase, these
species are expected to provide much needed insight to SOA from
anthropogenic origins, which can support a better understanding of the
sources of atmospheric aerosols.
While the above-described species are proposed as tracers of anthropogenic
SOA, structurally similar compounds are largely associated with primary
sources and are not suitable tracers of SOA. For example, t-PhA, 4NC,
4M-5NC, and 5NSA were highly correlated with levoglucosan and known to be
biomass burning products. Other species such as furandiones hold significant
potential to be used as an indicator of processed aromatic VOCs in the
atmosphere due to their source specificity but are not recommended as SOA
tracers because of their substantial partitioning in the gas phase and water
sensitivity. These findings underscore the importance of evaluating and
quantifying potential SOA tracers on an individual species level, as some
species within a compound class may provide source specificity, while others
do not. Given the limited time and geographic distribution for the samples
analysed in this study, further investigation is needed to realize the value
of these compounds as tracers of anthropogenic SOA more broadly.