Introduction
Epidemiological studies have linked human exposure to fine particulate
matter (PM2.5, aerodynamic diameter < 2.5 µm) to
increased morbidity and mortality from respiratory and cardiovascular
diseases (e.g., asthma, myocardial infarction, stroke; Brook et al.,
2010; Chen et al., 2013; Davidson et al., 2005; Jansen et al., 2005;
Katsouyanni et al., 1997; van Eeden et al., 2005). Primary combustion
particulates (e.g., wood smoke particles, vehicle emissions) are known to be
causative agents of these diseases (Danielsen et al., 2011; Nel, 2005);
however, increasing attention is also being paid to secondary organic
aerosols (SOAs; Fujitani et al., 2012; Jang et al., 2006; Kramer et al.,
2016; Lin et al., 2016; McDonald et al., 2010; McWhinney et al., 2013; Tuet
et al., 2017a, 2017b), which are produced from the atmospheric
transformation of hydrocarbons (HCs) in the presence of atmospheric oxidants
(e.g., NOx, OH radicals, O3; Hallquist et al., 2009).
Although SOA comprises a large fraction of PM2.5 (20–90 %; Gelencsér et al., 2007; Kanakidou et al., 2005), its mechanistic
role in causing adverse health effects remains unclear.
The toxicity of organic aerosols has been ascribed to the generation of
reactive oxygen species (ROS) and the modification of biomolecules (e.g.,
DNA and cellular enzymes; Danielsen et al., 2011; Nel, 2005). ROS can
induce oxidative stress in pulmonary systems, followed by a cascade of
inflammation responses and ultimately the apoptosis of lung cells
(Danielsen et al., 2011; Li et al., 2003, 2008). Particulate
organic compounds such as quinones and polyaromatic hydrocarbons can react
with cellular reducing agents (e.g., NADPH) and form ROS (i.e.,
H2O2 and O2-; Kumagai et al.,
2012). To efficiently determine the oxidative potential (the ability to
generate ROS) of different types of particulate matter at a laboratory
benchtop scale, a low-cost acellular technique, dithiothreitol (DTT)
assay, has been widely used (Antiñolo et al., 2015; Cho et al., 2005;
Hedayat et al., 2014; Janssen et al., 2014; Kramer et al., 2016; Verma et
al., 2015). DTT acts as a surrogate for biological reducing agents owing to
its two sulfhydryl groups. A recent study (Tuet et al., 2017a) has reported a positive
nonlinear correlation between DTT activities and the production of ROS in murine
alveolar macrophages. Some quinones (e.g., 1,4-naphthoquinone, NQN, and
9,10-phenanthrenequinone, PQN) can efficiently consume DTT via a catalytic
redox cycle, during which quinones are reduced to semiquinones or
hydroquinones (Chung et al., 2006; Li et al., 2003). Hence, quinone
compounds, commonly found in primary combustion particulates (Danielsen
et al., 2011; Jakober et al., 2007), are known to be important contributors
to the DTT response of combustion particles.
(a) Simplified mechanisms for the formation of alkyl and acyl
hydroperoxides, PANs, electron-deficient alkenes, and
quinones (Eddingsaas et al., 2012b; Jang and Kamens, 2001; Saunders et
al., 2003, 1997; Wyche et al., 2009; Xu et al., 2014). Photooxidation
products are not limited to the compounds shown. (b) Possible reaction
mechanisms between sulfhydryl groups in DTT (represented by
R-SH) and SOA products (Grek et al., 2013; Kumagai et al., 2002; Mudd,
1966; Mudd and McManus, 1969; Nair et al., 2014). EWG represents the
electron-withdrawing group attached to an alkene.
Unlike combustion PM, biogenic SOA and most aromatic SOAs (except naphthalene
SOA) contain little or no quinones (Forstner et al., 1997; Hamilton et
al., 2005; McWhinney et al., 2013; Pindado Jiménez et al., 2013);
however, recent work has shown that the DTT activity of SOA (toluene,
1,3,5-trimethylbenzene (TMB), isoprene, and α-pinene) was high and
even comparable to that originating from combustion particulates (e.g., wood
smoke particles; Jiang et al., 2016), suggesting that
there must be unidentified mechanisms underlying DTT consumption other than
the catalytic act of quinones.
In this study, three groups of SOA products were introduced to explain the
mechanistic role of SOA products in DTT consumption (Fig. 1a and b). First,
non-catalytic particulate oxidizers in SOA, such as organic hydroperoxides
(alkyl hydroperoxides and acyl hydroperoxides) and peroxy acyl nitrates
(RC(O)OONO2; PANs), can oxidize sulfhydryl groups in DTT to form
disulfides, sulfenic acids (RSOH), sulfinic acids (RSO2H), or sulfonic
acids (RSO3H; Grek et al., 2013; Mudd,
1966). These non-catalytic particulate oxidizers are abundant in SOA sourced
from various hydrocarbons (Docherty et al., 2005; Sato et al., 2012).
Second, catalytic particulate oxidizers, such as quinoid substances, can
oxidize sulfhydryl groups through a redox cycle (Cho et al.,
2005; Kumagai et al., 2002). A trace amount of quinones can be found in
aromatic SOA products (Forstner et al., 1997). Third,
electron-deficient alkenes in SOA can react with the sulfhydryl groups of
DTT via a Michael addition (Nair et al., 2014). Alkenes
substituted with an electron-withdrawing group (e.g., conjugated carbonyls)
are commonly found in ring-opening products from the photooxidation of
aromatic HCs (e.g., toluene; Jang and Kamens, 2001; Saunders et al.,
2003, 1997; Wyche et al., 2009). The contributions of all three groups of
SOA products to DTT activity can be influenced by the type of precursor HC
(aromatics vs. biogenics) and by NOx (NO + NO2) levels
(HC / NOx ratios; Eddingsaas et al., 2012b; Jang and Kamens, 2001;
Wyche et al., 2009; Xu et al., 2014).
Advanced analytical instruments (e.g., aerosol mass spectrometers and liquid
chromatograph mass spectrometers integrated with soft ionization) have
innovated the characterization of SOA compositions; however, their data are
limited to elemental analysis (Xu et al., 2014) or the
identification of some chemical species (e.g., carboxylic acids and
carbonyls) by a unique fragmentation (Sato et al., 2012; Shiraiwa et al.,
2013). Particulate oxidizers (e.g., PANs and organic hydroperoxides) are
thermally unstable and can decompose during chemical injection at high
temperature, making it difficult to characterize SOA compositions using mass
spectrometers (Zheng et al., 2011). This
difficulty is also compounded by a lack of authentic standards suitable for
the analysis of diverse and complex particulate oxidizers.
Outdoor chamber experiment conditions.
HC and date
Initial
Initial NOx
Initial
[SOA]maxb
ΔHCc
Y
Mid-
RHe
Tempe
Chemical
HC
(HONO)a
HC / NOx
collection
K
assayf
ppb
ppb
ppbC ppb-1
µg m-3
ppb
%
timed
%
Toluene
13 Feb 2016
641
525 (193)
9
229
403
15.1
13:40
22–63
281–303
DTT
01 May 2016
935
766 (133)
9
348
631
14.6
14:20
18–46
294–316
DTT, PAN
01 May 2016
938
301 (73)
22
292
542
14.3
12:10
21–48
294–315
DTT, PAN
23 May 2016
691
906 (250)
5
148
546
7.1
13:20
18–60
288–315
DTT, Enhance
23 May 2016
735
313 (86)
16
147
421
9.3
15:40
15–60
288–316
DTT, Enhance
18 Aug 2016
640
783 (179)
6
178
517
9.1
12:30
24–61
297–319
DTT, OHP
06 Aug 2016
610
240 (55)
18
75
216
9.2
12:30
43–59
297–305
DTT
18 Aug 2016
342
107 (24)
22
44
227
5.2
14:20
20–38
303–321
OHP
17 Nov 2016
622
179 (43)
24
139
452
8.1
13:20
12–56
282–309
DTTg
TMB
04 Oct 2015
613
920
6
201
613
6.7
14:40
20–43
290–310
DTT
04 Oct 2015
657
310
19
207
542
7.8
13:20
24–46
290–306
DTT
20 Feb 2016
589
1024
5
150
548
5.6
13:00
14–60
282–311
DTT
20 Feb 2016
583
156
34
128
455
5.7
14:40
16–61
282–311
DTT
11 Jan 2016
595
256
21
114
414
5.6
15:50
23–81
274–298
Enhance
Isoprene
23 Apr 2016
2693
2680
5
352
2693
4.7
12:00
18–48
290–314
DTT
23 Apr 2016
2755
430
32
93
2755
1.2
13:30
23–51
290–312
DTT
14 May 2016
2928
2800
5
406
2928
5.0
14:20
17–47
292–315
DTT, Enhance
14 May 2016
2858
423
34
107
2858
1.3
12:00
25–55
293–312
DTT
22 Jul 2016
2525
2423
5
246
2525
3.5
13:20
20–55
297–320
PAN (gas)h
22 Jul 2016
2718
473
29
70
2718
0.9
12:50
23–58
297–320
PAN (gas)h
20 Aug 2016
3060
3300
5
279
3060
3.3
12:30
20–58
296–321
DTT, OHP, PAN
20 Aug 2016
3173
583
27
125
3173
1.4
11:50
25–61
297–318
DTT, OHP, PAN
α-Pinene
25 Feb 2016
319
639
5
257
319
14.5
15:00
21–63
278–299
DTT
25 Feb 2016
323
91
36
650
323
36.1
13:30
25–67
278–298
DTT
18 Jan 2016
257
144
18
223
257
15.6
15:50
25–78
275–297
Enhance
a For toluene experiments, NOx was contributed by NO,
NO2, and HONO. The concentration of HONO was estimated using the difference in
the NO2 signal with and without the base denuder (1 %
Na2CO3+1 % glucose). b [SOA]max is the maximum SOA concentration during the aerosol
collection. c ΔHC is the consumption of HC when the SOA concentration
reached a maximum during the aerosol collection.
d This column is the mid-collection time (based on Eastern
Standard Time; EST) of SOA sampling.
e The RH and temperature conditions shown in Table 1 were recorded
from the beginning of photooxidation (sunrise) until the ending of PILS
sampling. f The SOA samples were applied to a series of chemical assays, namely
DTT assay (DTT), DTT enhancement (Enhance), organic hydroperoxides analysis
(OHP), and PAN analysis (PAN).
g For DTT measurement of toluene SOA sample collected on 17 November 2016,
the concentration of the potassium phosphate buffer (0.8 mM) in the first step
of the DTT assay was 2 times higher than the typical buffer concentration (0.4 mM). The DTTm of the toluene SOA
sample (17 November 2016) is shown in Fig. 3. h The concentration of gaseous PAN products (collected by an impinger)
was measured by the Griess assay.
The purpose of this study is to characterize the effect of SOA products on
DTT consumption. SOAs were generated from the photooxidation of different HCs
under varied environmental conditions (NOx levels) using a large
outdoor photochemical smog chamber. The two most abundant anthropogenic HCs
(i.e., toluene and TMB) in the ambient atmosphere and the two ubiquitous
biogenic HCs (i.e., isoprene and α-pinene) were chosen as SOA
precursors. Aerosols were collected using an online technique with a
particle-into-liquid sampler (PILS). Selected toluene and isoprene SOA
samples were immediately applied to the DTT assay and the quantification of
particulate oxidizers. The amount of PAN was measured using the Griess assay
and that of organic hydroperoxides was measured using the
4-nitrophenylboronic acid (NPBA) assay. The contribution of quinones to the
oxidative potential of SOA was estimated by the enhancement of DTT
consumption in the presence of 2,4-dimethylimidazole, a co-catalyst for the
redox cycling of quinones (Dou et al., 2015). In addition to
particulate oxidizers, the contribution of electron-deficient alkenes to DTT
activity was investigated for aromatic SOA (toluene SOA). Although the
chemical assays (e.g., NPBA assay and Griess assay) used in this study have
limitations (e.g., providing structural details of organic compounds), they
are user-friendly and can accurately quantify the total amount of organic
hydroperoxides and PANs, both of which are important for understanding the
role of SOA in cellular oxidative stress at the molecular level. The quality
control (QC) of the chemical assays used in this study will be discussed.
Results and discussion
DTT activity of SOA
The SOA yield (Y) represents a ratio of organic mass formed to HC consumed
(Odum et al., 1996). As shown in Table 1, the Y values of
toluene, TMB, isoprene, and α-pinene SOA ranged from 5 to 15, 6 to 8, 1 to 5, and 14 to 36 %,
respectively. Except isoprene SOA, the SOA yields in this study were
consistent with those reported in previous studies (Eddingsaas et al.,
2012a; Healy et al., 2008; Odum et al., 1996; Sato et al., 2007). Our SOA
yields for isoprene SOA were lower than those reported in other studies
(Carlton et al., 2009; Xu et al., 2014) because the
temperatures in our outdoor experiments were higher than those sourced from
indoor chambers. Within the NOx conditions (HC / NOx= 5–36 ppbC ppb-1) in this study, SOA yields of high-NOx isoprene were much
higher than those of low-NOx isoprene, and SOA yields of the other three
types of SOA under high-NOx conditions were generally lower than those
under low-NOx conditions. Aromatic hydrocarbons (toluene and
1,3,5-trimethylbenzene) are mainly oxidized by OH radicals, while biogenic
hydrocarbons (isoprene or α-pinene) are oxidized by both OH radicals and ozone. Based on
the integrated reaction rate (IRR) analysis, the oxidation of isoprene by OH
radicals is at least 3 times higher than that by ozone under the low-NOx
condition (HC / NOx= 17 ppbC ppb-1). The oxidation of biogenic
hydrocarbons was dominated by OH radicals, particularly in the morning.
DTTt of chamber-generated SOA under varied NOx
conditions (HNOX: high NOx; LNOX: low NOx) and positive controls
(i.e., PQN and NQN). The number above each column represents the initial
HC / NOx ratio. The x axis represents the mid-collection time (Table 1).
The DTTt of PQN and NQN is divided by 400 and 100, respectively. Each
error bar was calculated by t0.95×σ/n, where t0.95
is the t score (4.303 for n= 3 replicates) with a two-tail 95 %
confidence level.
The DTT consumption rate, DTTt (pmol min-1 µg-1),
was defined as DTT consumption (ΔDTT, pmol) per minute of reaction
time (t, min) per microgram of SOA mass (mSOA, µg):
DTTt=ΔDTTmSOAt.
Figure 2 illustrates the DTTt (t= 30 min) of SOA produced from four
different HCs under varied NOx conditions. Overall, the influence of
NOx on DTTt varied, depending on the type of HC. The DTTt of
toluene SOA was insensitive to NOx for samples collected within a
similar sampling period, but it decreased with increasing aging time. The
DTTt of toluene SOA reached approximately 70 pmol min-1 µg-1 by
13:00 under both high-NOx and low-NOx conditions but
decreased by about 40–50 % in the late afternoon. For aged toluene SOA,
the decline in DTTt might reflect the decay of photooxidation products
that could potentially react with DTT (e.g., electron-deficient alkenes that
can react with OH radicals; Finlayson-Pitts and Pitts,
2000). The lifetime of two semi-volatile electron-deficient alkenes
(4-oxo-2-butenoic acid and 2-hydroxy-3-penten-1,5-dial) that were reported
in a previous study (Jang and Kamens, 2001) was estimated using a
structure–reactivity relationship for the reaction with OH radicals
(typically 2 × 10-4 ppb under chamber conditions; Finlayson-Pitts and Pitts, 2000; Jang and Kamens, 2001; Kwok and
Atkinson, 1995). If these two electron-deficient carbonyls are oxidized by
OH radicals in the gas phase, the estimated lifetime of 4-oxo-2-butenoic
acid and 2-hydroxy-3-penten-1,5-dial is estimated to be 134 min and 43 min,
respectively (Sect. S5). The actual lifetime of these compounds will be
shorter than our estimation since they can also be oxidized in the particle
phase. Furthermore, some particulate oxidizers might also photochemically
decompose with increasing oxidation time (Sect. 3.3). For isoprene SOA,
DTTt was significantly affected by NOx. There was a 38 to
69 % decrease in isoprene DTTt when the HC / NOx (ppbC ppb-1)
ratio was reduced from 30 to 5. Under high-NOx conditions, the
DTTt of less-aged isoprene SOA was about 50 % lower than that of
less-aged toluene SOA. However, under low-NOx conditions, the DTTt of isoprene SOA was comparable
to that of toluene SOA. The DTTt of
TMB and α-pinene SOA was much lower than that of toluene and
isoprene SOA, and they were not affected significantly by NOx
conditions. The DTTt values of this study were also compared with those
reported in previous studies. The DTTt values of α-pinene SOA
in this study were close to those reported by Tuet et al. (2017b). The DTTt values of
isoprene SOA were, however, higher than those observed in Tuet et al. (2017b) and Kramer et al. (2016). This difference might be caused
by the degree of aerosol aging under different NOx conditions, initial
OH radical sources, humidity, and temperature.
Traditional DTTt has been used to measure the oxidative potential
originating from the catalytic redox cycling of particulate constituents
(e.g., quinones and metals; Charrier and Anastasio, 2012; Cho et al.,
2005; Kumagai et al., 2002). When governed by such catalytic reactions, DTT
consumption increases linearly with reaction time (Fig. S3). To demonstrate
the time dependency of DTT consumption, the reaction time of DTT assay was
extended to 2 h for isoprene SOA and toluene SOA, which both had high
DTTt. The mass-normalized DTT consumption (DTTm, nmol µg-1) was defined as the ratio of ΔDTT (nmol) to
mSOA (µg):
DTTm=ΔDTTmSOA.
In Fig. 3, the NOx effect on DTTm was consistent with that on
DTTt (Fig. 2): no NOx effect was observed on the DTTm of
toluene SOA, and the DTTm of low-NOx isoprene SOA was much higher
than that of high-NOx isoprene SOA.
The time profile of DTTm for toluene and isoprene SOA under
different NOx conditions. To achieve the completion of the reaction
between DTT and SOA, the DTTm of toluene sample (initial
HC / NOx= 24 ppbC ppb-1 collected on 17 November 2016) was measured with a 0.8 mM
potassium phosphate buffer in the first step of DTT assay (2 times higher
than the typical buffer concentration; 0.4 mM). Each error bar was
calculated by t0.95×σ/n using three replicates, where
t0.95 is the t score (4.303 for n= 3 replicates) with a
two-tail 95 % confidence level.
Figure 3 shows that the increase in DTTm with time for both isoprene
and toluene SOA was nonlinear, suggesting that DTT consumption by SOA
products was governed by non-catalytic processes. For example, DTT
consumption by isoprene SOA was nearly completed within 2 h. For toluene SOA
(initial HC / NOx= 6 or 18 ppbC ppb-1), the increase in DTTm also
appeared to slow down over a 2 h reaction, although the DTTm did not
reach a plateau under the same DTT assay conditions (i.e., the same buffer
concentration). Medina-Ramos et al. (2013) reported that the electron transfer rate between glutathione (GSH) and
an electro-generated mediator ([IrCl6]2-) exhibited a slight
acceleration when the phosphate buffer concentration was increased from 0 to
50 mM at pH = 7.0. To achieve the completion of the reaction between
particle oxidizers in SOA and DTT, the DTTm of toluene SOA
(HC / NOx= 24 ppbC ppb-1) was measured with a 0.8 mM potassium phosphate
buffer in the first step of DTT assay (2 times higher than the typical
buffer concentration; 0.4 mM). As shown in Fig. 3, the DTTm of the
toluene SOA (HC / NOx=24 ppbC ppb-1) reached a plateau within 2 h,
proving that DTT consumption by toluene SOA was controlled by non-catalytic
mechanisms. Under high-NOx conditions, the DTTm (t= 2 h) of toluene
SOA was 4–5 times higher than that of isoprene SOA. This difference was
about 2 times greater than that for DTTt (Fig. 2); therefore, we
concluded that DTTm is more suitable than DTTt for estimating the
oxidative potential of SOA, given that DTTm can determine the maximum
capacity of non-catalytic modulators in SOA to consume DTT.
DTT modulator: quinones
To illustrate the role of quinones in modulating the DTT responses of SOA,
the enhanced DTT consumption rate (t= 30 min) in the presence of
2,4-dimethylimidazole was measured. The enhancement factor (pmol min-1 µg-SOA-1 µmol-imidazole-1)
was estimated by Eq. (4):
enhancement factor=ΔDTTmix-ΔDTTSOA-ΔDTTimidazolemSOAnimidazolet,
where nimidazole (µmol) is the moles of 2,4-dimethylimidazole
added to the DTT reaction mixture, ΔDTTmix (pmol) is the DTT
consumption by the mixture of SOA and 2,4-dimethylimidazole, ΔDTTSOA (pmol) is the DTT consumption by SOA only, and ΔDTTimidazole (pmol) is the DTT consumption by 2,4-dimethylimidazole
only. As shown in Fig. 4, the enhancement factors of the four SOA were 2–3
orders of magnitude lower than those of the reference quinone compounds
(i.e., NQN and PQN), suggesting that the redox cycling of quinones was not
the major mechanism underlying the DTT consumption by the SOA. Hamilton et
al. (2005) reported that the total
amount of identified quinones (i.e., 5-methyl-o-benzoquinone,
2-methyl-p-benzoquinone, 2-hydroxy-5-methyl-p-benzoquinone) from the
photooxidation of toluene was less than 0.07 % of the total aerosol mass.
In a model compound study, Kumagai et al. (2002)
also reported that the oxidation of DTT by most benzoquinones (e.g.,
1,4-benzoquinone, 2-methyl-p-benzoquinone) was negligible under simulated
physiological conditions (pH = 7.5, 37 ∘C).
Enhancement factors (pmol min-1 µg-SOA-1 µmol-imidazole-1)
of SOA in the presence of 2,4-dimethylimidazole. The
label above each column represents the initial HC / NOx ratio. The
enhancement factor is expressed as the mean (±σ) of three
replicates. The enhancement factors of PQN and NQN are divided by 60.
DTT modulator: non-catalytic particulate oxidizers
In-depth investigations on the roles of non-catalytic particulate oxidizers
in DTT consumption were performed for isoprene SOA and toluene SOA, which
led to high DTTt. Organic hydroperoxides and PANs can oxidize
sulfhydryl groups (oxidation state of S[-2]) to disulfides (S[-1]) or to
even higher oxidation states (S[0], S[+2], S[+4]; Fig. 1b; Grek et
al., 2013; Mudd, 1966; Mudd and McManus, 1969). Under low-NOx
conditions, alkyl peroxy radicals (RO2) dominantly react with HO2
radicals, producing alcohols, alkyl hydroperoxides, and carbonyls
(Finlayson-Pitts and Pitts, 2000; Kroll et al., 2006; Ng et al.,
2007b). Under high-NOx conditions, RO2 radicals mainly react with
NO generating aldehydes (Finlayson-Pitts and Pitts, 2000).
The reaction of aldehydes with OH radicals followed by the reaction with
molecular oxygen yields peroxy acyl radicals (RC(O)OO; Finlayson-Pitts and Pitts, 2000). RC(O)OO can react with
NO2 to form PANs and react with HO2 radicals to form RC(O)OOH
(Finlayson-Pitts and Pitts, 2000; Nguyen et al., 2012; Xu et al.,
2014). In this study, the nanomoles of organic hydroperoxides per microgram
of SOA were quantified using the NPBA assay, represented by [OHP]m (nmol µg-SOA-1). The nanomoles of PANs per microgram of SOA
were measured by the Griess assay, represented by [PAN]m (nmol µg-SOA-1).
(a) Concentration of organic hydroperoxides in SOA, [OHP]m (nmol µg-1), measured by NPBA assay. (b) Concentration of PANs in SOA, [PAN]m (nmol µg-1), measured by Griess assay. The number
above each column represents
the initial HC / NOx ratio. The x axis represents the mid-collection time
(Table 1). (c) Comparison of DTTm (t= 2 h) with the sum of
[OHP]m and [PAN]m. The [OHP]m, [PAN]m, and DTTm are expressed as the mean (±σ)
of three replicates. HNOX
represents high-NOx conditions, and LNOX represents low-NOx
conditions.
As shown in Fig. 5a, by increasing the HC / NOx (ppbC ppb-1) from 5 to 27,
[OHP]m in isoprene SOA increased 2 times owing to the organic
hydroperoxides formed from the RO2+HO2 reaction pathway under
low-NOx conditions. Under the experimental conditions of this study,
the influence of NOx on [OHP]m in toluene SOA was insignificant.
Presumably, the aging process reduced the significance of the NOx
effect on [OHP]m. Low-NOx toluene SOA was collected about 2 h
later (i.e., a greater degree of aging) than high-NOx toluene SOA. The
organic hydroperoxides in the low-NOx toluene experiment degraded more
through photolysis or photooxidation (Lee et al., 2000)
than those in the high-NOx toluene experiment. The effect of the aging
process on toluene [OHP]m was consistent with that on toluene DTTt
(Fig. 2).
As shown in Fig. 5b, [PAN]m was found to be 1 order of magnitude
lower than [OHP]m. With the decrease in HC / NOx (ppbC ppb-1) from
about 22 to 9, [PAN]m in the toluene SOA increased 3 times as a
result of PANs production from the RO2+NO reaction pathway under high-NOx conditions (Fig. 1a; Xu et al., 2014). For
isoprene, the moles of both aerosol phase PANs and gas phase PANs per cubic
meter of air volume were significantly greater at higher NOx levels (Fig. S10). Most PAN products from the photooxidation of isoprene existed in the
gas phase and the amount of PAN in the particle phase was trivial (Fig. S10);
for example, aerosol phase PAN products were only 0.5 % of gas phase PAN
products.
To underline the contribution of organic hydroperoxides and PANs to the
DTTm of SOA, the DTTm values of toluene and isoprene SOA were also
compared with the sum of [OHP]m and [PAN]m. Figure 5c shows that
organic hydroperoxides were the major products that induced the oxidative
potential of isoprene SOA. For toluene SOA, only 45–65 % of DTTm
could be ascribed to organic hydroperoxides, and the remaining fraction was
attributed to other organic compounds in SOA. We propose that
electron-deficient alkenes, abundant in toluene SOA (Jang and
Kamens, 2001), can substantially modify sulfhydryl groups in DTT via a
Michael addition (Fig. 1b; Nair et al., 2014). In the next
section, the reactivity of electron-deficient alkenes with DTT will be
demonstrated using selected model compounds.
The DTTt (t= 30 min) of four different electron-deficient
alkenes. Each error bar was calculated by t0.95×σ/n using
three replicates, where t0.95 is the t score (4.303 for n= 3
replicates) with a two-tail 95 % confidence level. EWG in the
mechanism represents an electron-withdrawing group (Nair et
al., 2014).
DTT modulator: electron-deficient alkenes
Figure 6 illustrates the DTTt (t= 30 min) of four electron-deficient
alkenes (i.e., acrolein, methacrolein, 2,4-hexadienal, and mesityl oxide).
Acrolein showed much higher DTTt than the other compounds. The
susceptibility of an alkene to a Michael addition reaction depends on the
nature of the electron-withdrawing group coupled to the C = C bond
(Nair et al., 2014). The methyl group of methacrolein and
mesityl oxide is an electron-donating group that increases the electron
density on the C = C bond, thus decreasing the reactivity of the C = C bond
with DTT. The extended conjugation (C = C-C = C-C(O)H) in 2,4-hexadienal
stabilizes the C = C bond, leading to an extremely low DTTt.
The alkenes from the photooxidation of toluene were usually coupled with
electron-withdrawing groups such as carbonyls, nitrates, and carboxylic
acids (Jang and Kamens, 2001). These electron-withdrawing groups
enable the alkenes to be reactive with DTT. Compared with toluene SOA, TMB
SOA will have more alkyl-substituted alkenes owing to the three methyl groups on the aromatic ring of TMB, and it will therefore be less reactive with DTT. This
tendency partially explains why the DTTt of TMB SOA was significantly
lower than that of toluene SOA (Fig. 2). Based on aerosol composition
predictions using predictive SOA models such as the Unified Partitioning
Aerosol Phase Reaction (UNIPAR) model, the mass fraction of
electron-deficient alkenes in high-NOx toluene SOA should be more than
50 % (Im et al., 2014); therefore, the gap between toluene
DTTm and concentrations of non-catalytic particulate oxidizers (Fig. 5c) might be filled by abundant electron-deficient alkenes.
Atmospheric implications and conclusions
The influence of NOx on the oxidative potential of SOA was investigated
using DTTt (Fig. 2). Among four HCs, only isoprene SOA was
significantly sensitive to NOx levels, showing much higher DTTt at
lower NOx conditions. The DTTt of toluene SOA was found to be
lower with a longer aging time, regardless of NOx conditions.
For SOA consisting of non-catalytic redox compounds, DTTm is more
appropriate than DTTt for assessing oxidative potential because of the
nonlinear relationship between DTT consumption and reaction time (Fig. 3).
A decrease in isoprene DTTm was observed with increasing NOx
levels, but no significant NOx effect on DTTm was observed for
toluene SOA within a 2 h reaction. To apply the DTTm results of this
study to ambient atmosphere, DTTm should be coupled with SOA mass
concentrations. Under high-NOx conditions, the DTTm of toluene SOA
was almost 5 times higher than that of isoprene SOA, underlining the
importance of toluene in urban areas, despite its lower SOA yield (Table 1) in the urban environment (i.e., higher NOx conditions). In spite of
relatively low DTTm for high-NOx isoprene SOA, isoprene could
still play a substantial role in the oxidative potential of ambient urban
aerosols because of its abundance (Guenther et al.,
2006) and high SOA yields (Table 1) under high-NOx conditions. The
NOx effect on the DTTm of isoprene SOA is limited to the NOx conditions applied in this study and should be extended to a variety of
HC / NOx ratios in further studies.
As discussed in Sect. 3.1 and 3.2, the DTT consumption by SOA was not
sourced from quinones, which can catalytically yield ROS. Hence, the
contribution of non-catalytic particulate oxidizers, especially organic
hydroperoxides, to the oxidative potential of SOA was highlighted in this
study. Non-catalytic particulate oxidizers account for almost 100 % of
isoprene DTTm and 45–65 % of toluene DTTm (Fig. 5c). In
addition to non-catalytic particulate oxidizers, electron-deficient alkenes
in toluene SOA can potentially react with DTT via a Michael addition
(Nair et al., 2014).
The results of this study also show that some of the oxidizers (e.g., PANs)
formed from the photooxidation of hydrocarbons predominantly exist in the
gas phase (Fig. S10). Future studies should further consider how, through
absorption into the bio-system, gas phase oxidizers may be effectual for
inducing oxidative stress.