ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-8137-2018Different roles of water in secondary organic aerosol formation from toluene
and isopreneDifferent roles of water in SOA formationJiaLongXuYongFuxyf@mail.iap.ac.cnState Key Laboratory of Atmospheric Boundary Layer Physics and
Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of
Sciences, Beijing 100029, ChinaDepartment of Atmospheric Chemistry and Environmental Sciences,
College of Earth Sciences, University of Chinese Academy of Sciences,
Beijing 100049, ChinaYongFu Xu (xyf@mail.iap.ac.cn)8June201818118137815417November201713December20178May201820May2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/8137/2018/acp-18-8137-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/8137/2018/acp-18-8137-2018.pdf
Roles of water in the formation of secondary organic aerosol (SOA) from the
irradiations of toluene-NO2 and isoprene-NO2 were
investigated in a smog chamber. Experimental results show that the yield of
SOA from toluene almost doubled as relative humidity increased from 5 to
85 %, whereas the yield of SOA from isoprene under humid conditions
decreased by 2.6 times as compared to that under dry conditions. The distinct
difference of RH effects on SOA formation from toluene and isoprene is well
explained with our experiments and model simulations. The increased SOA from
humid toluene-NO2 irradiations is mainly contributed by
O–H-containing products such as polyalcohols formed from
aqueous reactions. The major chemical components of SOA in
isoprene-NO2 irradiations are oligomers formed from the gas phase.
SOA formation from isoprene-NO2 irradiations is controlled by stable
Criegee intermediates (SCIs) that are greatly influenced by water. As a
result, high RH can obstruct the oligomerization reaction of SCIs to form
SOA.
Introduction
Water is an important environmental factor that can influence
the formation of secondary organic aerosol (SOA) through the physical or
chemical processes, and is often represented with relative humidity (RH) or
liquid water content (LWC). Toluene and isoprene are two important precursors
of SOA, which are representatives of volatile organic compounds (VOCs) from
anthropogenic and biogenic sources. Both toluene and isoprene can produce
glyoxal during their oxidation processes in the atmosphere. As widely
reported, glyoxal is a typical precursor of SOA formed in the aqueous phase
(Volkamer et al., 2009; Lim et al., 2010, 2013; Ervens et al., 2011; Shen et
al., 016). The difference is that toluene contains an aromatic ring, which is
mainly oxidized by OH radicals, while isoprene contains two
C=C bonds, which can also be oxidized by O3 in
addition to OH. Thus, toluene and isoprene can provide insight into the roles
of water in SOA formation from different kinds of VOCs.
Sadezky et al. (2006, 2008) reported that stable Criegee intermediates (SCIs)
(CH2OO, C2H4OO, C3H6OO, and C4H8OO) play a
central role in SOA formation from the ozonolysis of ethyl butenyl ether,
trans-3-hexene, 2, 3-dimethyl-2-butene, and trans-4-octene. They further
suggested that SCI-derived oligomers are formed by the reactions of
RO2 with SCIs. Sakamoto et al. (2013) showed that the reactions of
SCIs with hydroperoxides from ethylene can form SOA. Inomata et al. (2014)
and Riva et al. (2017) showed that the reaction of an SCI with carboxylic
acids or hydroperoxides can contribute to SOA formation from the ozonolysis
of isoprene. Zhao et al. (2015, 2016) also showed that the SOA generated from
the ozonolysis of trans-3-hexene and α-cedrene is primarily composed
of oligomers formed from the addition of SCIs to RO2 radicals.
Although these studies have demonstrated the importance of SCI-derived
oligomers in SOA formation from the ozonolysis of alkenes, the role of SCIs
in SOA formation from isoprene-NO2 irradiations has not been
reported.
RH has a positive correlation with the mass yield of SOA from aromatics, such
as p-xylene (Healy et al., 2009), toluene (Kamens et
al., 2011; White et al., 2014), o-, p-xylene (Zhou et al., 2011), benzene and
ethylebenzene (Jia and Xu, 2014). This has been mainly attributed to
aqueous-phase reactions, such as active uptake of glyoxal in particle water.
An exception is from the study of Cocker et al. (2001), who found that the
yield of SOA from m-xylene and 1,3,5-trimethylbenzene in the presence of
propene was unaffected by RH (5 and 50 %). This is probably due to the
presence of propene in their reaction systems, which can reduce the OH
radicals, leading to the decrease in the yield of SOA (Song et al., 2007).
SOA from isoprene has been widely studied, as summarized by Carlton et
al. (2009). An earlier study from Dommen et al. (2006) showed that RH had
little effect on the SOA yield from isoprene-NOx (NO, NO2)
irradiations in the absence of seed particles at 2 and 84 % RH. A study
from Zhang et al. (2011) showed that RH had a negative effect on SOA
formation from isoprene-NOx irradiations with seed particles of
(NH4)2SO4 under two RH conditions (15–40 and 40–90 %) and
ascribed the rise of SOA yield under lower RH to the enhancement of
2-methylglyceric acid (2-MG) and its corresponding oligomers. Nguyen et
al. (2011b) found that RH did not affect the yields of SOA from isoprene in
their isoprene-NOx-H2O2 irradiations without seed
particles under dry (2 %) and humid (90 %) conditions, but they
observed enhancement of 2-MG-derived oligomers under low RH, which is
consistent with Zhang et al. (2011). Zhang et al. (2012) studied SOA
formation from methacrolein (MACR, one of major products from isoprene) under
different ratios of MACR / NO. Their results showed that the effect of RH
on formation of SOA depended on the yields of SOA precursors (e.g.,
methacryloyl peroxynitrate, MPAN). In addition, isoprene-derived
organosulfates (Zhang et al., 2011, 2012) and isoprene epoxydiols
(IEPOX)-derived products (Nguyen et al., 2014) are enhanced under higher RH.
A recent study from Lewandowski et al. (2015) showed that the aerosol yield
from isoprene-NO irradiations decreased with increasing RH (9 to 49 %).
The role of water in SOA formation is so complex that more research is still
required to understand mechanisms of SOA formation.
MPAN is one of key precursors of SOA from isoprene under high NOx
conditions (Surratt et al., 2010), which can react with OH to produce
epoxides (methacrylic acid epoxide, MAE, hydroxymethyl-methyl-a-lactone,
HMML). Lin et al. (2013) reported that MAE was an important precursor to
2-MG, a tracer of isoprene-derived SOA. Nguyen et al. (2015) showed that HMML
could form SOA. Since SCIs, IEPOX, MPAN, HMML, and MAE co-exist in
isoprene-NO2 irradiations, there are cross-reactions in the system.
Thus, the study is still needed to demonstrate the role of these precursors
in oligomer formation from isoprene-NO2 irradiations.
Both toluene and isoprene can produce glyoxal during their oxidation
processes. Why was the positive effect of RH on the SOA yield of isoprene
not observed? We consider that different chemical processes are likely
responsible for the different effects of RH on the SOA yields from toluene
and isoprene. One of the most important differences between isoprene and
toluene reaction systems is oxidation pathways. To clarify the different
mechanisms of SOA formed under different humid conditions, this paper
presents the experimental results of mass yields and chemical components of
SOA from toluene and isoprene under controlled RH conditions, as well as the
explanation of the mechanism of SOA formation.
Experimental section
All the experiments were carried out in a 1.3 m3 FEP reactor (DuPont
500A, USA). The equipment and experimental procedures were similar to our
previous works (Jia and Xu, 2014, 2016; Ge et al., 2017). Thus, only a brief
introduction is given here. Two ionizing air blowers were around the reactor
to remove the electric charge on the surface of the reactor. A light source
was provided by black lamps (F40BL, GE, USA), with a center wavelength of
365 nm. The photolysis rate of NO2 was determined to be
0.35 min-1 inside the reactor. The humidity was controlled by bubbling
the high pure water (18.2 MΩ cm at 25 ∘C,
Millipore/Direct-Q3). NaCl seed particles were prepared by a constant output
atomizer (Model 3076, TSI, USA).
Background air was prepared by a Zero Air Supply (model 111 and model 1150,
Thermo Scientific, USA) with three additional VOC traps (BHT-4, Agilent).
NOx, O3, and SO2 were measured by corresponding
analyzers of Thermo model 42C, model 49C, and model 45i (trace level). The
concentrations of NOx, O3, and SO2 in background air
were determined to be less than 1 ppb. The particles in background air could
not be detected with SMPS in the absence of irradiations, but the particle
number concentration of 104 cm-3 was obtained at an irradiation
time of about 5 h. From some experiments and model simulations the particles
were considered to be H2SO4 (less than 1 µgm-3),
which was formed from oxidation of SO2 by OH radicals.
Gas-phase organics were measured with a gas chromatograph-mass spectrometer
(GC-MS: Agilent model 7890A GC and Agilent model 5975C mass selective
detector, USA), which was equipped with a thermal desorber (Master TD, Dani,
Italy). Particle number and mass concentrations were determined by SMPS (TSI
model 3936, composed of DMS TSI 3080 and CPC TSI 3776). LWC was determined
following the method of the reduced Dry-Ambient Aerosol Size Spectrometer
(DAASS) (Engelhart et al., 2011). During the dry mode, the SMPS was modified
by adding a large diameter Nafion dryer (Permapure MD-700-48F-3; the RH of
the sample air can be reduced from 85 to 3.5 %) to the sampling inlet and
a multi-tube Nafion dryer (Permapure PD-200T-24E-M; the RH of sheath can be
reduced from 85 to 7 %) to sheath flow. During the humid mode, the humid
air in SMPS was quickly replaced by humid air in the chamber by venting the
sheath air at 10 L min-1, and then the humid aerosol was measured by
SMPS. As a result, the LWC was determined by the difference of the particle
mass concentrations between dry and humid modes.
To analyze the chemical components of SOA, the particles that ranged from
100 to 650 nm were collected on a 25 mm disk using a Dekati low-pressure
impactor (DLPI, Dekati Ltd., Finland) at 10 L min-1. Organic
functional groups of SOA were measured by a Fourier transform infrared (FTIR)
spectrometer (Nicolet iS10, Thermo Scientific, USA). The mass spectra of SOA
were measured by an electrospray ionization high-resolution mass spectrometer
(ESI-HRMS, Exactive-Orbitrap mass spectrometer, Thermo Scientific, USA). The
average molecular size information of the humic-like substances (HULIS)
present in SOA was determined by UV-Vis spectroscopy (Lambda 25,
Perkin–Elmer, USA) based on the ratio of E2 / E3, in which E2 and E3
denote the absorbance at 250 and 365 nm, respectively (Peuravuori and
Pihlaja, 1997; Duarte et al., 2005).
The liquid reactants of toluene (99.8 % purity, Xilong Chemical Co.,
Ltd.), isoprene (99.9 % purity, Alfa Aesar), or H2O2 (30 weight
% in H2O) were injected into the airline and were evaporated with
background air. NO2 (520 ppm in N2, Beijing Huayuan Gas
Company) was injected into the reactor directly. For the experiments of the
oxidation of isoprene by OH, OH radicals were generated from the photolysis
of H2O2 by UV lights (UVA-340, Q-Lab Corporation, USA). For the
experiments of isoprene-O3 dark reaction, O3 was produced by
an ozone generator with pure O2 (99.995 %). N-hexane
(> 97 % purity, Beijing Tongguang Fine Chemicals Company) was used as
an OH scavenger in the ozonolysis of isoprene. To evaluate the possible
contributions of SOA from n-hexane in the ozonolysis of isoprene with
n-hexane, two experiments of the irradiations of hexane-H2O2 were
performed for 6 h, in which no SOA was observed by SMPS under both dry and
humid conditions.
The initial conditions and purposes for the experiments are listed in
Table 1, most of which are the irradiations of toluene-NO2 and
isoprene-NO2. The initial concentrations of isoprene and toluene were
about 0.90 and 0.85 ppm, respectively, and initial NO2
concentrations were about 320 ppb. At the end of each experiment, isoprene
was almost completely consumed after 6 h reactions, and about 400 ppb of
toluene was reacted at the end of 7 h reactions. The RH was controlled to be
dry (6∼10 % RH) or humid (78∼88 % RH) conditions
for different experiments. Two sets of experiments with artificially added
NaCl seeds (about 10 µgm-3) were performed to quantify the
role of particle water in SOA formation in humid toluene and isoprene
reactions. To find out how RH affects the oxidation pathways of isoprene by
OH and O3 in isoprene-NO2 irradiations, additional
experiments of isoprene-H2O2 irradiations and isoprene-O3
reactions were carried out. The initial H2O2 and O3
concentrations were around 5 and 1.5 ppm, respectively.
To evaluate the potential contribution of SOA precursors (e.g., glyoxal,
IEPOX, MPAN, HMML, MAE, and SCIs) from toluene and isoprene reaction systems,
a model of the Master Chemical Mechanism (MCM v3.3.1, website:
http://mcm.leeds.ac.uk/MCM, last access: 1 May 2016, Jenkin et
al., 2015) was used, which includes the chamber-dependent reactions. To
examine the influence of RH on oligomer formation from SCIs, the reactions of
SCIs with carbonyls were added to MCM, which were expressed with X+SCI=X(SCI)1, X(SCI)1+SCI=X(SCI)2…X(SCI)n-1+SCI=X(SCI)n, where n= 1–10 and X represents carbonyls. The
rate constant for these reactions was set to be 5×10-12 cm3 molecule-1 s-1 (Vereecken et al., 2012).
Since most of RO2 was consumed by NOx,
SCI +RO2 reactions were not included in our model. The carbonyls
were chosen based on the results of mass spectra data from
isoprene-NO2 irradiations shown in Sect. 3.4. A set of ordinary
differential equations was built and solved using Matlab.
Experimental conditions of toluene and isoprene irradiations.
VOCsNo.T / KRH / %VOC / ppmNO / ppbNO2 / ppbAimToluene130460.9154.3307.72304850.8045.3303.63304840.9331.5293.9SOA size and yield430461.0370.2323.3530260.87912.0328.06303100.9171.9326.7FTIR7303810.8467.8301.0830470.9309.0325.0FTIR with NaCl seeds9303810.90610.0334.01030490.91410.6386.5UV/Vis11304800.9107.7364.112305790.92712.1288.1LWC by FTIR13305790.91810.6294.7LWC by SMPSIsoprene1430270.8967.0353.0size and yield15302850.8046.0364.01630170.8500.0311.717302800.8440.3308.51830370.9010.0299.5FTIR19303810.7990.3270.220302800.8280.2273.02130180.7900.0283.12230390.8733.0301.0FTIR with NaCl seeds23303790.8274.0325.02430380.8230.1332.5UV/Vis25303810.8770.3363.026305810.8311.5288.8LWC by FTIR27304780.8230.5313.5LWC by SMPS2830370.8100.2295.1ESI-HRMS29303850.8040.5290.2Results and discussionRH effects on SOA yieldsDetermination of LWC
The LWC in particles makes up a great percentage under humid conditions (as
shown in Fig. 1). To calculate the yield of SOA, the LWC has to be excluded.
On the other hand, since LWC was only measured at the end of the reaction, to
obtain the time evolution of SOA concentrations we needed to deduct LWC
during the whole reaction period. The value of LWC depends on chemical
components of particles and environmental conditions (temperature and
humidity). The volume growth factor (VGF) was used to estimate the
contributions of LWC in particles, which was defined by Engelhart et
al. (2011) as the ratio of the particle volume at humid air to the particle
volume at dry air. Assuming that all the particles are spheres and have the
same growth factor, the VGF is equal to the growth factor (GF) cubed as
VGF=VhydratedVdried=DhydratedDdried3=GF3.Vhydrated and Vdried indicate the total measured
volumes of hydrated or dried particles, respectively. Dhydrated
and Ddried are the diameter of hydrated or dried particles,
respectively, calculated from their volumes.
VGF is determined to be 1.28 (GF = 1.09, RH = 78 %) for the
particles from toluene-NO2 irradiations, 1.18 (GF = 1.06,
RH = 79 %) from isoprene-NO2 irradiations, 1.40
(GF = 1.12, RH = 77 %) from isoprene-H2O2 irradiations,
and 1.30 (GF = 1.09, RH = 88 %) from isoprene-O3 reaction
systems. There have been many studies about the growth factor of SOA. Aklilu
and Mozurkewich (2004) gave a GF range of 1.05–1.12 for atmospheric organic
particles (79 % RH). Stroud et al. (2004) predicted a GF of 1.1 for the
organic aerosols from toluene-NO-isopropyl nitrite irradiations at 79 %
RH. Prenni et al. (2007) reported the GF of 1.065±0.02 at 85 % RH
for SOA formed from toluene. Jimenez et al. (2009) obtained
GF =1.057±0.02 at 95 % RH for SOA from isoprene. In general,
our results of GF are in good agreement with previous estimates, indicating
that the LWC measured by our modified SMPS is reliable.
Mass concentration distributions of both dried and hydrated
particles from both toluene and isoprene systems at 3 h after the initiation
of reaction.
Mass concentration time profiles of SOA from different toluene and
isoprene reaction systems under dry and humid conditions. An SOA density of
1.4 g cm-3 was used and applied to the SMPS mass correction (Dommen et
al., 2006; Sato et al., 2007). The blue square markers are the number
concentration of SOA from isoprene-H2O2 irradiations at 6 % RH.
The wall loss rate constant of particles was less dependent on RH conditions,
so an average value of 4.8×10-3 min-1 was used to correct
the SOA formation.
SOA yields
We assumed that the VGF did not change during the reaction course. Thus, the
LWC from toluene and isoprene under humid conditions can be determined by
VGF. Figure 2 shows that in touene-NO2 irradiations, the mass
concentrations of SOA at 81 % RH are much larger than those at 10 %
RH, with a ratio of maximum mass concentration of SOA at 81 % RH to that
at 10 % being 2.2. However, in isoprene-NO2 irradiations, the
mass concentrations of SOA at 80 % RH are much lower than those at
7 % RH, with the ratio of maximum mass concentration of SOA being 0.57,
which is almost the same as that from isoprene-O3 reactions (the
ratio is 0.45). For isoprene-H2O2 irradiations, the mass
concentrations of SOA from humid conditions are generally larger than those
under dry conditions. Nevertheless, the maximum mass concentration of SOA
from humid conditions is 177.9 µgm-3 (the ratio is 1.01),
which is close to that from dry conditions.
The mass yield of SOA generally increases with time. The maximum yields
during the experimental course were used for the following discussion. The
mean maximum yields of SOA from toluene were obtained to be
5.58±0.76 % (dry) and 8.97±0.84 % (humid),
respectively (Fig. 3). Our results are within the range obtained by other
investigators (Kamens et al., 2011; Odum et al., 1997; Ng et al., 2007).
Previous studies (Kamens et al., 2011; Zhou et al., 2011; Jia and Xu 2014;
Wang et al., 2016) mainly ascribed the positive effect of RH on SOA yields
from aromatics to LWC, which can enhance the formation of SOA by aqueous
reactions, such as reactive uptake of glyoxal in aerosol water. Our yields of
SOA from toluene are smaller than those from Ng et al. (2007) (around
11 % at 4 % RH) and Hildebrandt et al. (2009) (11–17 % at
21 % RH), which is probably due to the additional and excessive OH
radical sources (HONO or H2O2) used in their experiments. In
addition, the temperature in this study is higher than the previous studies,
which may be another reason accounting for the lower SOA yields in this work.
Maximum yields of SOA from toluene and isoprene under dry (red
color) and humid (black color) conditions (∘:
toluene-NO2-hν; △: isoprene-NO2-hν;
⋆: isoprene-O3; ∗: isoprene-H2O2-hν).
A negative effect of RH on SOA yields was observed in the systems of
isoprene-NO2-hν and isoprene-O3. The mean maximum yields
from isoprene-NO2 irradiations are reduced from
3.14±0.35 % (dry) to 1.19±0.38 % (humid) (Fig. 3).
This negative RH effect is in good agreement with the corresponding results
from Zhang et al. (2011) and Lewandowski et al. (2015). The yields of SOA
from our isoprene-O3 reactions are 3.00 % (dry) and 1.40 %
(humid), which are quite close to the results from the isoprene-NO2
system, while RH has a very weak effect on the SOA yields from
isoprene-H2O2 irradiations in our study. The maximum yields of SOA
were determined to be 7.7 % (dry) and 7.8 % (humid) from
photooxidation of isoprene-H2O2, which are in good agreement with
the results (around 8 %) of isoprene-H2O2 irradiations under dry
conditions from Clark et al. (2016). A similar yield (7 %) of SOA from
photooxidation of isoprene-NOx-H2O2 was also obtained in
the results from Nguyen et al. (2011b). Based on the experimental conditions
in Nguyen et al. (2011b), we estimated that for their reaction system over
99 % of isoprene was oxidized by OH and the remaining 1 % by
O3 by using simulations based on the MCM. Thus, the reaction system
by Nguyen et al. (2011b) can be considered to be closer to the
isoprene-H2O2 system. In addition, some other previous studies
(Gaston et al., 2014; Riedel et al., 2015; Zhang et al., 2018) showed that RH
had a negative effect on the formation of SOA from isoprene-OH systems due to
an acid dilution effect. In these studies, acidic sulfate seed particles were
used and the acid-catalyzed effect was very obvious. Thus, higher RH can
reduce the acidity of the seed particles by particle water. In our study
acidic seed particles were from a little amount of H2SO4 formed from
the gas-phase reaction of SO2 and OH. It was estimated that the mass
concentration of H2SO4 particles was less than
1 µgm-3. When liquid water content increased from
1 µgm-3 to the maximum 54 µgm-3 under humid
conditions, the pH value was estimated to be in the range of 2 to 3.7,
indicating that the pH variation was small in our experimental conditions.
Therefore, compared with previous studies, the acid dilution effect was not
remarkable in our work. These results show that high RH can reduce the
maximum yields of SOA from the reaction channel of isoprene with O3
(O3 channel) and that RH has little effect on the maximum yields from
the reaction channel of isoprene with OH (OH channel) without sufficiently
high mass concentrations of acid particles. Thus, it shows that the
ozonolysis of isoprene is probably a key pathway influencing SOA formation in
isoprene-NO2 irradiations in our experimental conditions, which will
be further discussed in a later section.
In our isoprene-NO2 irradiations, based on the MCM simulation (Exp.
25), the amount of isoprene oxidized by OH, O3, and NO3 is
59, 25, and 16 % at the end of reactions, respectively. There are
cross-reactions when NO2 and O3 are both present. Thus, we
cannot deduce SOA contribution simply by initial ratios of isoprene oxidized
by OH and O3. Since SOA is mainly formed by the secondary or later
generation products, we can evaluate the contribution of reaction pathways to
the formation of SOA in terms of SOA precursors from different channels. As
described previously, SCIs can be taken as the SOA precursors from the
O3 channel, while IEPOX, MPAN, HMML, and MAE can be used as SOA
precursors from the OH channel. The MCM simulations show that the total yield
of SCIs was dominant as compared to OH channel precursors such as IEPOX,
MPAN, HMML, and MAE. The former accounts for 70 % of total concentrations
(ppb) of SOA precursors, and the latter (IEPOX + MPAN + HMML + MAE)
30 % at the end of reaction in isoprene-NO2 irradiations.
Therefore, even though 59 % of isoprene was consumed by OH and only
25 % by O3, the formation of SOA in isoprene-NO2 was
mainly from the O3 channel. For these three oxidation channels, RH
has little effect on SOA yields from OH channel oxidization. Previous studies
have shown that humidity has little effect on SOA formation from NO3
oxidation of alkenes (Bonn and Moorgat, 2002; Fry et al., 2009; Boyd et
al., 2015). Thus, only the O3 channel is greatly influenced by RH.
The maximum possibility is that the O3 channel can produce SCIs that
can be consumed by water. Thus, although most of the isoprene was oxidized by
OH and the SOA yield from the OH channel was over 2 (5) times greater than
that from the O3 channel under dry (humid) conditions, the O3
channel was still a major pathway influenced by water vapor in the
isoprene-NO2 system, which will be discussed in the following
section.
UV-Vis spectra of SOA from toluene (a) and
isoprene (b) photooxidations under dry and humid conditions.
UV-Vis spectra of SOA
The molecular sizes of SOA can reveal the degree of oligomerization
reactions. Larger molecules displayed higher absorbance in longer wavelength
regions as surmised by Mostafa et al. (2014). Thus, we used the UV-Vis
spectra to determine the molecular size information of SOA, which can provide
information about the oligomerization degree of molecular size through the ratio of
E2 / E3 (absorbance at 250 nm divided by absorbance at 365 nm)
indirectly. Lower E2 / E3 ratios are associated with higher molecular
weight (Peuravuori and Pihlaja, 1997; Duarte et al., 2005). All the spectra
are characterized by a continuous absorption that increases with decreasing
wavelength from 200 nm up to about 1100 nm (Fig. 4), which indicates the
presence of conjugated double bond molecules (such as oligomers). The ratios
of E2 / E3 are 1.27 (1.53), 1.29 (2.25), 1.69 (1.97), and
1.55 (1.73) under dry (humid) conditions from our systems of
toluene-NO2, isoprene-NO2, isoprene-O3, and
isoprene-H2O2, respectively. The E2 / E3 ratios show that high
RH can indeed reduce molecular sizes of SOA and suppress the oligomerization
reactions. Zhang et al. (2011) and Nguyen et al. (2011b) both reported that
oligomers were greatly reduced under humid conditions, and considered that
high RH suppressed the oligomerization reactions with water as a product.
Nevertheless, if the suppression of the oligomerization reactions under humid
conditions is the main reason for the decrease in SOA yield from isoprene,
why is the maximum yield from isoprene-H2O2 irradiations unchanged
under humid conditions? Besides the weak acid dilution effect in our
experimental conditions, there must be an intrinsic mechanism regarding the
influences of RH on the SOA yield from isoprene. In addition, oligomers have
also been identified as important products of SOA from aromatics, and water
is a byproduct during the oligomerization process (Kalberer et al., 2004; Lim
et al., 2010; Gaston et al., 2014). However, a negative effect of RH on SOA
yield from aromatics has never been observed. This is because there are
likely competing processes that are responsible for SOA formation from
aromatics under humid conditions. Oligomers are generally inhibited by higher
RH, while the organics formed by aqueous reactions are enhanced.
Infrared spectra of SOA from toluene irradiations under
humid (a) and dry (b) conditions. The infrared spectrum
of (c) was obtained after evaporation of SOA from (a) for
15 min at 110 ∘C; the difference spectrum between (a) and
(c) is in the blue area (d); the infrared spectrum of
(e) is with extra LWC by NaCl seeds. The main bands are the hydrogen
bonded O–H stretch in alcohols or acids
(2400–3700 cm-1), the carbonyl (C=O) band at
1727 cm-1, the organonitrate (ONO2) bands at 1636, 1278, and
855 cm-1, the C–OH band at 1423 cm-1, and the
C–OH stretch at 1080 cm-1.
IR spectra of SOAToluene
Figure 5a and b show the typical infrared spectra of SOA from the
irradiations of toluene under both dry and humid conditions. The prominent
features on SOA spectra are the board hydrogen bonded O–H
stretching, the carbonyl C=O stretching, the organic
nitrate (ONO2) bands, the C–OH bands of alcohols or
polyalcohol. The bands all greatly increase by over 2 times as RH increases
from 10 to 81 %. These changes in band strength with RH are quite similar
to the changes in the SOA mass yield with RH.
To further reveal the chemical properties of the increased products formed
from humid conditions, the SOA sample from humid conditions was evaporated.
First, we found that the IR spectrum of SOA almost did not change after being
heated at 100 ∘C for 15 min. Then the sample was further evaporated
at 110 ∘C for 15 min. After the evaporation at 110 ∘C, the
major absorption bands were considerably changed. As a result, the spectrum
is almost the same as the spectrum of SOA collected under dry conditions
(Fig. 5c). The major reduced absorptions are from the O–H and
C–OH bands (Fig. 5d). These bands are assigned to hydrates of
glyoxal and other water-soluble compounds in SOA (Volkamer et al., 2009; Lim
et al., 2010; Kamens et al., 2011; Jia and Xu, 2014; Wang et al., 2016).
Therefore, alcohols (such as hydrates) are considered major contributors to
toluene SOA under humid conditions.
LWC is an important factor that can greatly influence the contribution of SOA
from aqueous reactions. The maximum LWC was measured to be about
44 µgm-3 from humid toleuene-NO2 irradiations. To
determine the role of LWC in SOA formation, extra LWC was introduced into the
reaction system by adding 10 µgm-3 of NaCl particles. The
initial LWC was determined to be 30 µgm-3, and maximum LWC
was 74 µgm-3 during 6 h of reaction. The SOA mass
concentration was obtained by subtracting the mass concentrations of NaCl,
NaNO3, and LWC from total mass concentration of particles. Compared
with the experiment without NaCl, the SOA mass concentrations increased by
16 % in the experiment with NaCl, and all the bands assigned to
O–H, C=O, and C–OH were
enhanced by 50, 29, and 35 %, respectively (Fig. 5e). This demonstrates
that the increase in LWC can greatly enhance the formation of SOA from
hydration of glyoxal. Therefore, it is concluded from our study that the
formation of SOA from toluene is controlled by LWC under humid conditions,
and that most SOA is formed by aqueous reactions in touene-NO2
irradiations.
Infrared spectra of SOA from isoprene irradiations under dry and
humid conditions. The bands at 1636, 1282, and 855 cm-1 are from
ONO2. The bands at 1170 and 1121 cm-1 are assigned to
C–O–C in oligomers or C–O in carboxylic
acids, and the band at 1055 cm-1 is from alcohols. The absorption
shoulder from 927 to 1080 cm-1 is assigned to C–O and
O–O in the peroxide group (C–O–O)
(Pretsch et al., 2009).
IsopreneIsoprene-NO2 system
The spectra of SOA from the irradiations of isoprene-NO2 are
characterized by the high abundance of C=O and
ONO2 groups (Fig. 6). There are three bands assigned to different
kinds of C–O or C–O–O groups in the
region of 927–1243 cm-1 under dry conditions (Pretsch et al., 2009).
These bands are indicators of alcohols and polymeric structures (Czoschke et
al., 2003). Thus, oligomers and organic nitrates are dominant species in SOA.
Under humid conditions, the absorption intensities of the bands
(O–H, C=O, ONO2,
C–O, or C–O-O) are all reduced by 2
times. The tert-nitrate can hydrolyze in particle water by the replacement of
ONO2
with the
-O–H group (Liu et al., 2012). Because such replacement hardly
changes the vapor pressure of corresponding species (Pankow and Asher, 2008),
newly formed alcohols should remain in the aerosol phase. We also did extra
experiments to test the hydrolysis of organic nitrates. After the SOA sample
from dry isoprene-NO2 irradiation was exposed to humid air (90 %
RH) for 1 h, we did not find any apparent change in the ONO2 group.
Meanwhile, the peak height ratios of ONO2/O–H
from SOA are almost the same under dry and humid conditions. Thus, the
hydrolysis of nitrates is not the major reason for the decrease in
particle-phase organic nitrates. It also indicates that aerosol phase
oligomers can hardly be influenced by RH. Then, high RH likely inhibited the
formation of particle-phase organics by reducing the oligomerizations in the
gas phase (e.g., SCI-derived oligomers).
RH generally enhances SOA formation by the aqueous reactions. Similarly, the
aqueous reactions also exist in isoprene-NO2 irradiations. However,
the maximum LWC from humid isoprene-NO2 irradiations was measured to
be 8 µgm-3 at the end of reaction, which is much smaller as
compared to 44 µgm-3 in toluene irradiations. Taking glyoxal
as an example, although the maximum concentrations of glyoxal were simulated
to be 39 ppb in isoprene-NO2 irradiations, which is only 60 % of
its maximum concentration of 65 ppb from toluene irradiations, due to the
limitation of LWC, the SOA from the aqueous reactions was significantly
reduced in isoprene-NO2 irradiations. To further confirm the role of
LWC, we did an additional experiment with NaCl seeds (initial LWC of
30 µgm-3) in isoprene-NO2 irradiations. The results
show that the absorptions of the bands from O–H and
C–OH increase by 20 to 30 % as compared to those without
additional LWC (Fig. 6). It is true that increasing LWC can indeed enhance
SOA formation in isoprene-NO2 irradiations; however, the absorptions
of C=O, ONO2, and C–O from dry
conditions are still 2 times larger than those from the experiment with extra
LWC. This demonstrates that the increase in SOA through aqueous reactions is
far less than the decrease due to H2O-related reactions under humid
conditions. Thus, high water vapor can probably inhibit some key processes
responsible for SOA formation from isoprene-NO2 irradiations, which
will be discussed in the following contents.
Isoprene-H2O2 and isoprene-O3 systems
To determine which process responds to the decrease in SOA under humid conditions from
isoprene-NO2 irradiations, IR spectra of SOA from the OH and
O3 channels were studied, respectively. Since isoprene is the chain
unit of terpenes, the abundance of functional groups in oxidation products
from isoprene and terpenes is expected to be close.
O–H-containing products from terpene are 10
times more
enriched from the OH channel oxidation than from the O3 one
(Calogirou et al., 1999). Our extra experiments show the similar
characteristics of IR spectra of SOA from both isoprene and α-pinene
(figure not shown). The peak height ratio of
O–H/C=O is 0.24 in the SOA from the
α-pinene-O3 system, while it is as high as 2.19 in the SOA
from the α-pinene-OH system. Here the absorption ratio of
O–H to C=O was used to examine the
difference between the O3 and OH oxidation channels.
The IR spectra of SOA from the isoprene-H2O2 system are
characterized by strong absorptions of both hydrogen bonded
O–H and C–OH and by weak absorption of
C=O under both dry and humid conditions (Fig. 7, top),
with the peak height ratios of
O–H/C=O being 1.63 (dry) and 1.45
(humid), which strongly supports the claim that alcohols or polyalcohols are
major components of SOA from isoprene-H2O2 irradiations. Under humid
conditions, the peak at 1090 cm-1 assigned to the
C–O–C group from esters is slightly decreased, while
the band at around 3200 cm-1 from O–H absorption is
broadened as compared to the dry condition. It indicates that esters (e.g.,
oligomers) decrease while the compounds containing O–H
increase under humid conditions. Nevertheless, the relative abundances of
O–H, C=O, and C–OH groups
are almost the same between dry and humid conditions, which shows a weak
effect of RH on SOA from isoprene-H2O2 irradiations as compared to
isoprene-NO2 irradiations. In the OH channel, isoprene can be
oxidized to form RO2 (ISOPO2). If there is no NO,
ISOPO2 will be further oxidized to isoprene epoxydiols (IEPOX) by OH
and HO2 radicals. IEPOX are key intermediates of SOA in isoprene-OH
reactions (Surratt et al., 2010). Under dry conditions, IEPOX can be adsorbed
on H2SO4 seeds to form polyalcohols (e.g., 2-methyltetrols) through
acid-catalyzed heterogeneous reactions, which can further form oligomers by
esterification (Lin et al., 2012). Under humid conditions, IEPOX can be
absorbed into particle water to produce polyalcohols (Nguyen et al., 2014).
In addition, the decrease in the C–O–C group indicates
that the formation of oligomers is inhibited by the abundance of particle
water as discussed in Sect. 3.2, which is in agreement with the result of Lin
et al. (2014). Because polyalcohols (dominant) and IEPOX-derived oligomers
are all in the aerosol phase, the total mass concentration of SOA does not
change much under humid conditions in isoprene-H2O2 irradiations. In
other words, RH does not change the partition of IEPOX in our experimental
conditions. This is consistent with the result of Riva et al. (2016) that
water has a weaker impact on IEPOX-derived SOA yield.
Infrared spectra of SOA from isoprene with different oxidants under
dry and humid conditions. The bands at 1051 and 960 cm-1 are assigned
to C–O and O–O groups in peroxide
O–O–O (Pretsch et al., 2009).
In the isoprene-O3 systems, if the bands from ONO2 are
excluded, both the shape and band intensities of IR spectra of SOA are quite
similar to those of SOA from isoprene-NO2 irradiations. All the bands
assigned to O–H, C=O, and
C–O are reduced by over 2 times under humid conditions (bottom
panel of Fig. 7). The ratios of
O–H/C=O are 0.36 (0.44) under dry
(humid) conditions. The results are consistent with our expectation that
lower ratios of O–H/C=O should be in
SOA from the ozonolysis of isoprene. Since OH radicals were well removed in
our experiments, SCIs became the key intermediates of SOA. The
C–O–O group is an indicator of the participation of
SCIs in SOA from the ozonolysis of isoprene. The C–O–O
group is very apparent under dry conditions, which decreases by 60 %
under humid condition. Oligomer products in SOA have been found to be formed
by the reactions of n (n= 1–10) SCIs with RO2 in the
ozonolysis of small enol ethers and trans-3-hexene (Sadezky et al., 2008;
Zhao et al., 2015). Thus, SCI-derived oligomers are also deduced to be the
key components in SOA from the O3 channel of isoprene. The
model-simulated results show that when RH increases from 10 to 88 %, the
consumption of SCIs by water increases from 13 to 58 %, while the
SCI-derived oligomers decrease from 87 to 42 %. The reaction products of
SCIs with H2O have relatively high vapor pressures as compared to
oligomers, so they are mainly in the gas phase. Therefore, humid conditions
can reduce the SOA formed by SCI-related reactions in the isoprene-O3
systems.
In isoprene-NO2 irradiations, the ratios of
O–H/C=O are 0.35 (0.36) under dry
(humid) conditions, which are almost the same as the corresponding values in
isoprene-O3 but totally different from the values in
isoprene-H2O2. The yields IR spectra (ratios of
O–H/C=O) and the influence of RH on
SOA production from the isoprene-O3 system is almost the same as
those from isoprene-NO2 irradiations. In isoprene-NO2
irradiations, even though 60 % of isoprene was oxidized by OH, because of
the presence of NO, most of the ISOPO2 from oxidation of OH could be
quickly consumed by NO to form MPAN (around 15 ppb under both dry and humid
conditions) and other products, leading to the decrease in IEPOX from
224.0 ppb (in isoprene-H2O2) to 41.2 ppb (in
isoprene-NO2). The yield of MACR is generally greater in
isoprene-NO2 irradiations and isoprene-O3 systems than that
in isoprene-H2O2 irradiations. MACR can react to form MPAN in the
presence of NO2, which can be oxidized by OH to form SOA precursors
of epoxides (e.g., HMML, MAE), such as in the Nguyen et al. (2011b) work.
Epoxides can further be oxidized to produce 2-MG and related oligomers
(Surratt et al., 2010; Lin et al., 2013; Nguyen et al., 2015). 2-MG-derived
oligomers can be enhanced under lower RH (Zhang et al., 2011). Both the
results from Nguyen et al. (2014) and MCM simulations further show that if
there are enough OH radicals, most MPAN can be further oxidized by OH to
produce epoxides. However, since there were no extra OH sources in our
systems, MCM simulations show that only 12 % (24 %) of MPAN under dry
(humid) conditions was oxidized by OH to produce HMML and MAE. The maximum
concentrations of HMML and MAE were only 6.8 and 2.7 ppb under dry
conditions (Fig. 13), which is too small to explain the yields of SOA in
isoprene-NO2 irradiations. If we simply assume that the
concentrations of SOA were proportional to the IEPOX concentration as in
isoprene-H2O2 irradiations, over 70 % of SOA should come from
IEPOX in dry or humid isoprene-NO2 irradiations. However, the IR
spectra of SOA from dry or humid isoprene-NO2 are totally different
from those in isoprene-H2O2 irradiations. In contrast, they are
similar to those from the isoprene-O3 system. Thus, IEPOX is not the
major contributor to SOA in isoprene-NO2 systems. On the other hand,
similar to the isoprene-O3 system, SCI-related reactions in the
isoprene-NO2 system were probably key pathways.
Positive ion mode ESI-Orbitrap mass spectra of SOA from
isoprene-NO2 irradiations under dry (a) and
humid (b) conditions.
Mass spectra of isoprene SOA
To further determine whether SCI-derived oligomers are the major components
of SOA from isoprene-NO2 irradiations, the high-resolution mass
spectra of SOA under dry and humid conditions were obtained with ESI-HRMS
(Fig. 8). The mean molecular size of SOA was reduced from 352 under dry
conditions to 295 under humid conditions, which is in good agreement with the
results by UV/Vis spectra. The peaks on the spectrum show highly regular mass
differences, especially in the range of 300∼800m/z, which is a
typical structure for polymers or oligomers (Kalberer et al., 2004). The
total intensity of peaks in the range of 300 to 800 m/z under humid
conditions is reduced by 75 % as compared to that under dry conditions.
This demonstrates that oligomers are probably a major component of SOA from
isoprene-NO2 irradiations, which are greatly reduced under humid
conditions. The mass spectrum of SOA from the ozonolysis of isoprene is
similar to the one from the isoprene-NO2 system. The spectrum of SOA
from isoprene-H2O2 (Fig. 9) shows a very different feature from that
of the SOA from isoprene-NO2. It does not reveal obviously regular
structures of the peaks for oligomers.
To further characterize whether SCIs are the major building blocks of the
oligomers in SOA from isoprene-NO2 irradiations, a Kendrick mass
defect (KMD) analysis was used. The KMD analysis is a standard method to
visualize the complex organic mass spectra (Kendrick, 1963). The Kendrick
mass (KM) is converted from the IUPAC mass M by multiplying a factor of
NMbase/Mbase (i.e., the factor is
14.00000 / 14.01565 for the base unit of CH2) using Eq. (2).
NMbase is the exact mass Mbase rounded to the nearest
integer. KMD is calculated as the difference between the nominal KM (NKM) and
KM using Eq. (3). The basic principle of the KMD method is that a homologous
series of compounds differing only by a number of base units have identical
KMD values. Thus, the KMD analysis allows for the rapid identification
oligomers by a plot of KMD vs. KM, in which homologous compounds can line up
in the horizontal direction. Since the KMD analysis has a great advantage to
clearly determine the molecular composition of hundreds of individual
compounds in SOA samples, it has been applied extensively for complex SOA
sample analyses using HR-MS (Reinhardt et al., 2007; Walser et al., 2008;
Nguyen et al., 2010, 2011a; Nizkorodov et al., 2011). In addition, since
different series of homologous oligomers may have similar KMD values, the KMD
data need to be pre-sorted by the z* value which is calculated by
Eq. (4) (Hsu et al., 1992).
KM=M×NMbaseMbaseKMD=NKM-KMz*=moduloNMNMbase-NMbase
Positive ion mode ESI-Orbitrap mass spectra of SOA from
isoprene-H2O2 irradiations under dry conditions.
The correlation of yields of the top five SCIs (CH2OO,
MACROO and MVKOO, MGLOO, and GLYOO) and the ratios of CH2O2,
C4H6O2, C3H4O3, and C2H2O3 based oligomers to
total mass under dry and humid conditions from isoprene-NO2
irradiations. Only the oligomers that belong to families (M-[SCI]n, n=0,1,2,3…) with n≥2 were taken into consideration.
Base units of oligomers: SCIs
There are 16 kinds of SCIs produced in isoprene-NO2 irradiations
based on MCM v3.3.1 simulation, in which CH2OO (CH2O2, with
a yield of 50.1 %), MACROO (C4H6O2, 18.3 %), MVKOO
(C4H6O2, 12.2 %), MGLOO (C3H4O3, 11.3 %), and
GLYOO (C2H2O3, 2.6 %) account for 95 % of total SCIs. To
explain that these SCIs exist in SOA as base units of CH2O2,
C4H6O2, C3H4O3, and C2H2O3, a wide set of other
base units (OH, CO, NO2, ONO2, CH2, CH2O, and
COO) are also included for KMD analysis. The ratio of oligomers with a
given base unit to total mass is defined to characterize the contribution of
different base units to SOA. It should be pointed out that large
uncertainties exist in the estimate of relative contributions of different
units because of the poor quantification performance using ESI-MS techniques.
Due to the cross containing of units in oligomer molecules, the sum of the
ratios is larger than 100 %. The oligomers with the same base unit
(M-[base unit]n, n=0,1,2,3…) that contains at least three
compounds are considered one class of oligomers. The results show that only
the ratios of oligomers with the base units of CH2O2,
C4H6O2, C3H4O3, and C2H2O3 are proportional to
the yields of corresponding SCIs from the isoprene-NO2 system.
Figure 10 displays the correlation diagram between the ratios of oligomers
(with CH2O2, C4H6O2, C3H4O3, and
C2H2O3 as repeating units) to total mass and the top five SCI
yields (CH2OO, MACROO, MVKOO, MGLOO, and GLYOO). It shows that the
ratios linearly increase with increasing yields under both dry and humid
conditions. Thus, this demonstrates that the oligomers with CH2O2,
C4H6O2, C3H4O3, and C2H2O3 repeat units are
from contributions of these SCIs of CH2OO, MACROO (and MVKOO), MGLOO,
and GLYOO in the isoprene-NO2 system. Therefore, these five SCIs are
chosen as the base units for KMD analysis, which shows that the ratios of
compounds containing SCI units are reduced by 45 % on average as RH
increases from 7 to 85 %. This is also in good agreement with MCM
simulation of decrease in SCI-derived oligomers by 44 % and with the
decrease in intensity of peroxide C–O–O absorption in
FTIR (Fig. 6). In addition, the KMD analysis is also used to determine the
components of oligomers in SOA from isoprene-H2O2. All the above
base units are tested, and the results show that CH2O-containing
oligomers are the major products in SOA, and the chain lengths of oligomers
are much shorter than those from isoprene-NO2. The maximum repeat
unit number of n is less than 3 in most families of oligomers in SOA from
isoprene-H2O2. By contrast, the maximum value of n is larger than 5
in oligomer families of SOA from isoprene-NO2. This indicates that
SCIs incline to produce long chain oligomers.
Base unit of CH2OO
It has been considered that the CH2OO radical can serve as an
oligomer unit in SOA from the ozonolysis of ethylene (Sakamoto et al., 2013).
CH2OO has the highest yield (50.1 %) of all the SCIs from
isoprene. The KMD analysis shows that the masses of CH2OO-containing
oligomers account for 46.2 % (29.4 %) of the total mass on the MS
under dry (humid) conditions. Figure 11 displays the selected mass spectra of
oligomers with CH2OO as chain units and their corresponding KMD plots
under dry and humid conditions, which shows that both the length of oligomer
chains and the number of oligomers are greatly reduced under humid
conditions. The number of oligomers under humid conditions is reduced by
64 % as compared to dry conditions. Another feature of
CH2OO-based oligomers is that the sizes of their end groups are larger than 300 (C14–C17), which probably come from other
oligomers formed during reactions. This indicates that most
CH2OO-based oligomers are formed in the particle phase.
Positive mode mass spectra of oligomers with CH2OO as chain
units in SOA from isoprene-NO2 irritations under dry and humid
conditions (a) and corresponding plots of KMD (CH2OO) vs.
nominal KM (CH2OO) (b). The horizontal lines connect the
family of compounds with an equal elemental composition differing only by
[CH2OO]n (n=0,1,2,3…) groups.
Mass spectra of oligomers with C4H6O2 (MACROO and MVKOO)
as the repeating unit and their Kendrick plots using C4H6O2 as the
Kendrick base. Species separated by C4H6O2 groups fall on the
horizontal lines.
Base units of MACROO and MVKOO
Figure 12 shows the mass spectra and KMD plot of oligomers with
C4H6O2 (MACROO and MVKOO) as base units. The KMD analysis results
show that the ratio of C4H6O2--based oligomers to total compounds is 39.7 % (17.2 %)
under dry (humid) conditions. In addition to CH2OO, MACROO and MVKOO
based oligomers have the second highest contribution to SOA among all the
SCIs. Similar to CH2OO, the maximum number of chain units is 6 in
oligomers from C4H6O2. However, the size of
end groups is much smaller than that
from CH2OO. The most frequent end group is C3H6O2 as shown in Fig. 12. Based on the Chemspider
database and MCM simulation, we deduced that C3H6O2 is from
hydroxyacetone (ACETOL), which is the most abundant carbonyl-containing
products in the isoprene-NO2 reaction system. The maximum
concentration of ACETOL is over 90 ppb based on our experimental conditions.
C2H2O3 is deduced to be glyoxylic acid that is one of products from
isoprene irradiation. SCIs can react with carbonyl and alcohol products
(e.g., ACETOL), RO2, and H2O (Calvert et al., 2000; Chao et
al., 2015; Tobias and Ziemann, 2001). However, different from the
isoprene-O3 system, MCM simulations show that most RO2 is
consumed by NO in isoprene-NO2 irradiations. Thus carbonyl and
alcohol products become the major initiators of SCI oligomerizations.
List of major base units and their corresponding ratios of oligomers
to total mass from SOA in isoprene-NO2 irradiations.
Base unitUnit nameSCI yields by MCMRatio of oligomers to total mass Dry/%Humid/%Delta/%CH2O2CH2OO50.146.229.436.3C4H6O2MACROO and MVKOO30.639.717.256.7C3H4O3MGLOO11.319.13.780.4C2H2O3GLYOO2.63.20.680.1C4H6O3dehydrated 2-MG (or CH3OC3H3OO)(1.4)25.39.064.6CH2––76.462.717.9CH2O––78.567.613.8
An addition of a C–O–O group can change the vapor
pressure of oligomers (containing n SCI units) by a factor of 2.5×10-3 (Pankow and Asher, 2008). The vapor pressures of SCI-derived
oligomers (e.g., C3H3O2–[C4H6OO]n) are estimated to be
less than 10-7 atm (n≥2) and 10-12 atm (n≥4). The
compounds can self-nucleate as their vapor pressures are less than
10-9 atm (Kamens et al., 1999). It indicates that the initial particles
in the O3 oxidation channel of isoprene are formed by the
self-nucleation of oligomers (n≥4). The oligomers with n≥2
probably further condensed on these particles. Thus, MACROO- or MVKOO-based
oligomers can be formed in the gas phase (e.g., Reactions R1 and R2). With
the increase in chain units, these oligomers can either self-nucleate or
further oligomerize in the aerosol phase.
Other base units of oligomers
It is worth noticing that the yield of CH3OC3H3OO
(C4H6O3) is only 1.4 % based on the MCM simulation. However,
the contribution of C4H6O3 to oligomers is as high as 25.3 %
(9.0 %) under dry (humid) conditions (Table 2). Thus, C4H6O3 is
not totally from CH3OC3H3OO. 2-MG usually serves as molecular
tracers for isoprene SOA (Kleindienst et al., 2007). As reported by Zhang et
al. (2011) and Nguyen et al. (2011b), C4H6O3 was the repeated unit
of 2-MG's corresponding oligomers in SOA from isoprene-NOx
irradiations. Lin et al. (2013) reported that C4H6O3 was from MAE
in MACR-NOx irradiations. Thus, C4H6O3 is probably
formed from dehydration of 2-MG and MAE in oligomers. Considering the low
yield of MAE in our system, we considered that most C4H6O3-based
oligomers are probably contributed by 2-MG in our work. The ratios of
CH2OO, MACROO, and MVKOO based oligomers are almost 2 times larger
than that from 2-MG under both dry and humid conditions. Thus, even though
the ratio of C4H6O3-based oligomers was decreased by 65 % as RH
increased from 7 to 85 %, 2-MG derived oligomers would not be the major
reason for the decrease in SOA yield from isoprene-NO2 irradiations.
In addition to C4H6O3, CH2 and CH2O based
oligomeric compounds also have high ratios in isoprene-NO2 systems,
which have been also reported as the most prominent units in SOA products
from the ozonolysis of isoprene in the Nguyen et al. (2010) study under dry
conditions. However, different from SCI based oligomers, the ratios of
CH2 and CH2O based oligomers decreased by 18 and 14 % as
RH increased from 7 to 85 %, respectively. Thus, the reduction of SCI
based oligomers is the major reason for the decrease in SOA yields from
isoprene-NO2 photooxidations.
Mechanisms for the different roles of water in isoprene-NO2
systemsVapor wall loss vs. SCI–H2O reaction
It is noted that the wall loss of semi-volatile organic compounds (SVOCs) can
lead to the underestimation of the yield of SOA (Matsunaga and Ziemann, 2010;
Loza et al., 2010; Zhang et al., 2014; Yeh and Ziemann, 2015; Ye et
al., 2016; Palm et al., 2016; Krechmer et al., 2016; La et al., 2016; Nah et
al., 2017). Since SCI-derived oligomers are the major products of SOA from
isoprene-NO2 irradiations, a question arises about which process is
dominant for the reduction of SOA production under humid condition, wall loss
of SCI related oligomers (in gas phase), or the reaction between SCI and
H2O. The MCM simulation shows that SCIs are so reactive that most of
them are consumed by reactions before they are lost to the wall (Fig. 14).
The percentage of the SCIs consumed by H2O was increased from 6 to
46 % as RH increased from 5 to 85 % due to the extremely high
concentration of gas H2O. The removal of SCIs by H2O
(85 % RH) can lead to a decrease in SCI-derived oligomers by 43 % as
compared to 5 % RH. The result is comparable with the decrease in SOA
yields of 62 % from isoprene-NO2 irradiations. Meanwhile, as
discussed in the previous section, the vapor pressures of SCI-derived
oligomers were so low that they were ready to condense on particles. The
upper limit of the wall loss rate constant of
4.8×10-4 s-1 for SVOC was calculated from the equation
given by McMurry and Grosjean (1985), while the condensation rate constant of
SVOC to the particles was calculated to be over 0.65 s-1 in our study
based on the equation from La et al. (2016). This indicates that the
condensation rate of gas-phase oligomers to particles is much faster than
that to the wall. Therefore, the reactions between SCIs and H2O
rather than the wall loss of SVOC are the major cause of the decrease in SOA
formation from isoprene-NO2 irradiations in this work.
MCM-simulated time profiles of SOA precursors in
isoprene-NO2 irradiations. SCI-derived oligomers were from the
reactions of SCIs with glyoxylic acid and ACETOL (solid lines for dry
conditions, dashed lines for humid conditions).
Effects of water on SOA formation: O3 vs. OH
To quantify the RH effect of SOA and relatively possible contribution of
SCI-derived oligomers from isoprene-NO2 irradiations, the reactions
of SCIs with formic acid, glyoxylic acid, and ACETOL were added into MCM, in
which the reaction of SCIs with formic acid does not form oligomers.
Simulations show that the total mass concentration of oligomers from these
reactions was 558.4 (271.2) µgm-3 at 7 % (80 %) RH,
and the mass concentrations from other SOA precursors
IEPOX, MPAN, HMML, and MAE were 182.8 (167.0), 27.4 (28.9), 28.1 (27.4), and
11.2 (10.9) µgm-3 at 7 % (80 %) RH (Fig. 13). It is
obvious that the mass concentrations of SCI-derived oligomers reduced by
51 % as RH increased from 7 to 80 %, while the concentrations of
other precursors had little change under different RH conditions. Thus,
SCI-derived oligomers should have a great potential for formation of SOA,
compared to other precursors.
Wall losses vs. gas-phase reactions between H2O and SCIs.
Mechanisms for SOA formation from the O3 and OH oxidation
channels of isoprene.
Our results clearly show that the different effects of RH on SOA yields
originate from the oxidation channels (Fig. 15). Both the OH and O3
channels can well explain the differences in results of isoprene-NO2
irradiations from the Zhang et al. (2011) and Nguyen et al. (2011b) studies.
In the Zhang et al. (2011) study, there were no additional OH radical sources
in their systems. Thus, the SOA was mainly from the O3 channel.
Similar to our isoprene-O3 systems, a negative effect of humidity on
SOA yield was observed in their work. In the Nguyen et al. (2011b) work, due
to sufficient OH radical source, over 99 % of isoprene was oxidized by
OH, and SCI concentrations were very low. Even though high NOx
was used, most MPAN could be further oxidized by OH to produce epoxides.
Therefore, SOA was mainly from the OH channel in Nguyen et al. (2011b)'s
work. This is why the yield of SOA in their work was not influenced by RH.
Our results obviously show that SOA is formed by reactive uptake of SOA
precursors (e.g., IEPOX) in the OH channel, and by the condensation of
SCI-derived oligomers in the O3 channel. In the presence of
NO2, the formation of SOA is also controlled by the SCI-related
reactions without extra OH sources. However, the SCI-related reactions
(SCI-derived oligomers) can be inhibited by high water vapor. In previous
studies, SOA was usually modeled based on the vapor pressures of SVOC, which
only considers the effect of temperature. Our study strongly suggests that RH
is also a key factor in SOA formation.
Conclusion
Opposite effects of RH on SOA formation from the
irradiations of toluene and isoprene have been elucidated in our work.
Different influences of RH on both SOA yields and mean molecule size
demonstrate the different mechanisms related to SOA formation from the
irradiations of toluene and isoprene. High RH can greatly enhance the SOA
formation in the toluene-NO2 system, so that the maximum yields of
SOA from toluene increased from 5.58 % (dry) to 8.97 % (humid). FTIR
spectra show that the increased part of SOA under humid conditions was mainly
contributed by aqueous reactions of water-soluble products (e.g., glyoxal).
Different from toluene-NO2 irradiations, water has a complex role in
isoprene systems. In isoprene-H2O2 irradiation systems, RH has no
remarkable effects on SOA yields. FTIR spectra show that water can inhibit
the oligomerization reactions from polyalcohols; however, polyalcohols were
still the major products in both dry and humid conditions from
isoprene-H2O2 irradiation, which was mainly from the reactive uptake
of IEPOX in the presence of H2SO4 particles from background gas. In
isoprene-O3 and isoprene-NO2 irradiation systems, high RH has
a negative effect on SOA yields, which decreased from 3.14 % (dry) to
1.19 % (humid). According to the FTIR, ESI-HRMS, KMD analysis and MCM
simulations, the oligomers with SCIs as base units were considered to be the
major products of SOA in isoprene-O3 systems and isoprene-NO2
irradiation systems. Under humid conditions, the SCIs can be consumed by
water in the gas phase, leading to the decrease in the formation of oligomers
from SCIs.
Data are available by contacting the corresponding author.
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(no. 41375129) and the National Key R&D Program of China (2017YFC0210005).
Edited by: Jason Surratt
Reviewed by: two anonymous referees
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