The
effect of relative humidity (RH) on secondary organic aerosol (SOA) formation
from the photooxidation of m-xylene initiated by OH radicals in the absence
of seed particles was investigated in a Teflon reactor. The SOA yields were
determined based on the particle mass concentrations measured with a scanning
mobility particle sizer (SMPS) and reacted m-xylene concentrations
measured with a gas chromatograph–mass spectrometer (GC-MS). The SOA
components were analyzed using a Fourier transform infrared (FTIR)
spectrometer and an ultrahigh-performance liquid chromatograph–electrospray
ionization–high-resolution mass spectrometer (UPLC-ESI-HRMS). A significant
decrease was observed in SOA mass concentration and yield variation with the
increasing RH conditions. The SOA yields are 14.0 %–16.5 % and
0.8 %–3.2 % at low RH (14 %) and high RH (74 %–79 %),
respectively, with the difference being nearly 1 order of magnitude. Some of
the reduction in the apparent yield may be due to the faster wall loss of
semi-volatile products of oxidation at higher RH. The chemical mechanism for
explaining the RH effects on SOA formation from m-xylene–OH system is
proposed based on the analysis of both FTIR and HRMS measurements, and the
Master Chemical Mechanism (MCM) prediction is used as the assistant. The FTIR
analysis shows that the proportion of oligomers with C-O-C groups from
carbonyl compounds in SOA at high RH is higher than that at low RH, but
further information cannot be provided by the FTIR results to well explain
the negative RH effect on SOA formation. In the HRMS spectra, it is found
that C2H2O is one of the most frequent mass differences at low
and high RHs, that the compounds with a lower carbon number in the formula at
low RH account for a larger proportion than those at high RH and that the
compounds at high RH have higher O : C ratios than those at low RH. The
HRMS results suggest that the RH may suppress oligomerization where water is
involved as a by-product and may influence the further particle-phase
reaction of highly oxygenated organic molecules (HOMs) formed in the gas
phase. In addition, the negative RH effect on SOA formation is enlarged based
on the gas-to-particle partitioning rule.
Introduction
Secondary organic aerosol (SOA) is a significant component of atmospheric
fine particulate matter in the troposphere (Hallquist et al., 2009; Spracklen
et al., 2011; Huang et al., 2014), causing serious concern as it has a
significant influence on air quality, the oxidative capacity of the
troposphere, global climate change and human health (Jacobson et al., 2000;
Hansen and Sato, 2001; Kanakidou et al., 2005; Zhang et al., 2014). In a
previous study from a global model simulation, it has been found that SOA
represents a large fraction, approximately 80 % of total organic
aerosol sources (Spracklen et al., 2011).
The formation of SOA in the atmosphere is principally via the oxidation of
volatile organic compounds (VOCs) by common atmospheric oxidants such as
O3, OH and NO3 radicals (Seinfeld and Pandis, 2016). Aromatic
compounds mainly from anthropogenic sources, including solvent usage,
oil-fired vehicles and industrial emissions, contribute 20 %–30 % to
the total VOCs in urban atmosphere, which play a significant role in the
formation of ozone and SOA (Forstner et al., 1997; Odum et al., 1997; Calvert
et al., 2002; Bloss et al., 2005; Offenberg et al., 2007; Ding et al., 2012;
Zhao et al., 2017). Amongst aromatics, m-xylene is significant, and the
mean concentration of which together with p-xylene in the daytime was
determined to be up to 140.8 µg m-3 in the atmosphere of
urban areas in developing countries (Khoder, 2007).
The oxidation of aromatics in the troposphere is mainly initiated through OH
radicals, and is affected by many chemical and physical factors. The
concentrations of oxidant species, VOCs and NOx
concentrations, as well as the ratio of VOCs to NOx (Ge et
al., 2017b) determine the main chemical mechanism. Light intensity (Warren et
al., 2008), temperature (Qi et al., 2010) and relative humidity (RH) are the
most significant physical parameters that affect the chemical process. RH
governs the water concentration in the gas phase and the liquid water content
(LWC) in the particle phase. Water plays a significant role as a reactant,
product and solvent to directly participate in chemistry (Finlayson-Pitts and
Pitts Jr., 2000) and indirectly affect the reaction environment such as the
acidity of particles (Jang et al., 2002). The acid catalysis of heterogeneous
reactions of atmospheric organic carbonyl species in particle phase can lead
to a large increase in SOA mass, while this process can be suppressed by
lower acidity at high RH (Czoschke et al., 2003). In addition, RH can
change the viscosity of SOA and further affect the chemical processes of SOA
formation (Kidd et al., 2014; Liu et al., 2017).
Investigations of the RH effects on aromatic SOA have been conducted in many
previous works. In the presence of NOx, it was observed that
RH significantly enhanced the yield of SOA from benzene, toluene,
ethylbenzene and xylene photooxidation, which was explained by a higher
formation of HONO, particle water, aqueous radical reactions and hydration
from glyoxal (Healy et al., 2009; Kamens et al., 2011; Zhou et al., 2011; Jia
and Xu, 2014, 2018; Wang et al., 2016). Meanwhile, under
low-NOx conditions, wherein no NOx was
introduced artificially and photolysis of H2O2 was as an OH
radical source, it has been observed that deliquesced seed contributed to the
enhancement of SOA yield from toluene (Faust et al., 2017; Liu et al., 2018).
However, under low-NOx levels, it has been found that, in
the study on toluene SOA formation, a moderate RH level (48 %) leads to a
lower SOA yield than low-RH level (17 %–18 %; Cao and Jang, 2010).
In a most recent study on SOA formation of toluene (Hinks et al., 2018), high
RH led to a much lower SOA yield than low RH under low-NOx
levels, which is attributed to condensation reactions that remove water,
leading to less oligomerization at high RH. In a study on the chemical
oxidative potential of SOA (Tuet et al., 2017) under low-NOx
conditions, it was observed that the mass concentration of SOA from
m-xylene irradiation under dry conditions was much larger than that under
humid conditions, whereas the study did not focus on the mechanism of the RH
effect on m-xylene SOA formation. These demonstrate that the RH effects on
aromatic SOA yields, especially m-xylene, have not been fully understood,
and RH effects are controversial under various NOx levels
and seed particle conditions.
The chemical components of SOA are important, and are on which climate- and
health-relevant properties of particles are dependent. Chemical composition
of SOA from aromatics–NOx photooxidation has been
investigated by gas chromatograph–mass spectrometer (GC-MS) analysis
(Forstner et al., 1997). Nevertheless, this study was only performed at a
limited RH range of 15 %–25 %. GC-MS in this study may not be the
optimal technique for analysis of SOA components as high temperature at GC
injection ports can easily decompose some low-volatility substances in SOA.
FTIR was also used to study the chemical composition of SOA from
aromatics–NOx photooxidation under different RH conditions,
in which the information of functional groups in SOA was provided (Jia and
Xu, 2014, 2018). In these studies, O-H, C=O, C-O and C-OH were found to be
the main functional groups, intensities of which largely increased with
increasing RH. Compounds in SOA with the O-H group mainly contributed to the
increase of SOA, such as polyalcohols formed from aqueous reactions. A recent
study on SOA components from toluene–OH system under both dry and humid
conditions were analyzed via high-resolution mass spectrometry (HRMS) (Hinks
et al., 2018). Although some chemical composition in SOA has been identified,
the analysis and the mechanism of RH effects still need to be further
studied. The RH effects on SOA formation from m-xylene under
low-NOx conditions have not been studied well. In the
present study, we present the results from the experiments about the SOA
formation from the OH-initiated oxidation of m-xylene in the absence of
seed particles in a Teflon bag. The SOA yields at different RHs and the
chemical components under both low- and high-RH conditions will be reported.
The underlying mechanism of SOA formation for these different conditions will
be also discussed.
Experimental materials and methodsEquipment and reagents
Experiments on m-xylene photooxidation were performed in a 1 m3
air-tight Teflon FEP film reactor (DuPont 500A, USA), which is similar to our
previous works (Jia and Xu, 2014, 2016, 2018; Ge et al., 2016, 2017c, b, a).
A light source was provided by 96 lamps (F40BLB, GE; UVA-340, Q-Lab, USA)
surrounding the Teflon bag to simulate the UV band of the solar spectrum in
the troposphere. The NO2 photolysis rate was determined to be
0.23 min-1, which was used to reflect the light intensity in the
reactor. To remove the electric charge on the surface of the FEP reactor, two
ionizing air blowers were equipped outside the Teflon bag and were used
throughout each experiment (McMurry and Rader, 2007).
The background gas was zero air, which was generated from Zero Air Supply and
CO Reactor (Model 111 and 1150, Thermo Scientific, USA) and further purified
by hydrocarbon traps (BHT-4, Agilent, USA). The humid zero air was obtained
by bubbling dry zero air through ultrapure water (Milli Q, 18MU, Millipore
Ltd., USA). To obtain the different desired RH in the reactor, a different
ratio of dry and humid zero air was mixed. The RH and temperature in the
reactor were measured by a hygrometer (Model 645, Testo AG, Germany).
Throughout each experiment, the background NOx concentration
in the reactor was lower than 1 ppb and OH radicals were provided from
H2O2 photolysis. Hydrogen peroxide was introduced into the
reactor along with the zero air flow over a period of 30 min via an
injection of H2O2 solution (30 wt %) into a three-way tube
using a syringe to the desired concentration of 20 ppm. Though the
H2O2 level was not measured, it was estimated through the
measured volume of H2O2 solution evaporated. m-Xylene
(99 %, Alfa Aesar) was introduced to the reactor subsequently using the
same approach. No seed particles were introduced artificially. All reactants
were introduced initially and then the lights were turned on and the reaction
started. The experiments were conducted for 4 h. Thus, the “end” of the
experiment in this study refers to the experiment at 4 h of reaction time.
Monitoring and analysis
The concentration of m-xylene in the reactor was measured with a gas
chromatograph–mass spectrometer (GC-MS, Model 7890A GC and Model 5975C mass
selective detector, Agilent, USA), which was equipped with a thermal desorber
(Master TD, Dani, Italy). The size distribution and concentrations of
particles were monitored with a scanning mobility particle sizer (SMPS, Model
3936, TSI, USA). In our enclosed Teflon bag, our sampling instruments
consumed 10 % of reactor volume during the photooxidation and we did not
use make up air to dilute the bag. The particle wall loss constant has been
determined to be 3.0×10-5 s-1 and 6.0×10-5 s-1 at low-RH and moist conditions, respectively. Though
the particle wall loss constant is size dependent, it is not a strong
function of particle size for the relatively narrow size distributions in
smog chamber experiments (Park et al., 2001). Here we approximate the wall
loss as size independent. In experiments under moist conditions, particles
measured by SMPS consisted of liquid water content (LWC) and SOA. In low-RH
experiments, as SOA hardly absorbs aerosol water, LWC can be negligible.
Thus, the SOA mass can be directly measured by SMPS in low-RH experiments. To
obtain the SOA mass in high-RH experiments, LWC should be excluded from total
particle mass. The method for the measurement of LWC was already described in
the previous study (Jia and Xu, 2018), so here only a brief introduction is
provided. During each moist experiment, the SMPS measured the humid
particles. After 4 h from the start of oxidation reaction in each moist
experiment, the SMPS was modified to the dry mode. In the dry mode, a Nafion
dryer (Perma Pure MD-700-12F-3) was added to the sampling flow and a Nafion
dryer (Perma Pure PD-200T-24MPS) was added to the sheath flow. After the
modification of SMPS, the humid air in SMPS was quickly replaced by dry air
through venting the sheath air at 5 L min-1, so that the RH in the
sheath air could decrease to 7 %. Then, SMPS at this dry mode measured
dry particle concentrations as the RH in the sample air decreased to 10 %
at this time. The LWC was determined by the difference of the particle mass
concentrations before and after the SMPS modification to the dry mode.
The chemical composition of SOA originated from m-xylene–OH irradiation
was investigated using a Fourier transform infrared (FTIR) spectrometer, which
can provide information on the functional groups. The particles were
collected on a ZnSe disk using a Dekati low-pressure impactor (DLPI, Dekati
Ltd., Finland) at the end of each experiment (Ge et al., 2016; Jia and Xu,
2016). The duration of DLPI for FTIR was 15 min, and this sampling was taken
just after 4 h of each experiment. Then, the ZnSe disk was directly put in a
FTIR (Nicolet iS10, Thermos Fisher, USA) for the measurement of functional
groups of the chemical composition in SOA samples.
To obtain the detailed information of chemical composition, SOA particles
were sampled using the Particle into Liquid Sampler (PILS, model 4001, BMI,
USA). The PILS samples water-soluble species in particles. As
low-NOxm-xylene SOA is composed of almost all
water-soluble species, it is reasonable and reliable to use PILS to sample
SOA for the analysis of the chemical composition. The flow rate of the sample
gas was around 11 L min-1, and the output flow rate of liquid sample
was 0.05 mL min-1. Two denuders were used to remove the VOCs and acids
in the sample gas. SOA liquid samples collected by PILS were finally
transferred into vials for subsequent analysis of mass spectrometry. The
duration of PILS was 5 min, and this sampling was taken just after 4 h of
each experiment. Operatively, the blank measurements were obtained by
replacing the sample gas with zero air collected in vials. It is well known
that the PILS samples water-soluble species in the SOA with high efficiency.
In addition, it is reported that the PILS can also sample slightly
water-soluble organic compounds with average O : C ratios higher than
0.26 instead of the total SOA composition and the collection efficiency can
exceed 0.6 (Zhang et al., 2016). Thus, the PILS can sample the overwhelming
majority of the SOA system in our study, though PILS cannot sample
water-insoluble species in the SOA.
The accurate mass of organic compounds in SOA was measured by a
ultrahigh-performance liquid chromatograph (UPLC, Ultimate 3000, Thermo
Scientific, USA) with a heated electrospray ionization high-resolution
Orbitrap-based mass spectrometer (HESI-HRMS, Q Exactive, Thermo Scientific,
USA). Methanol (Optima™ LC/MS Grade, Fisher
Chemical, USA) was used as the eluent in UPLC system. The elution flow rate
was 0.2 mL min-1, and the overall run time was 5 min. The injection
volume was 20 µL. In this study, the UPLC was only used as the
injection system of HRMS. The acquired mass spectrum of SOA was in the range
of 80–1000 Da. The HESI source was conducted in positive and negative ion
modes using the optimum method for characterization of organic compounds. We
used the Thermo Scientific Xcalibur software (Thermo Fisher Scientific Inc.,
USA) to analyze the data from HRMS. To calculate the elemental composition of
compounds, the accurate mass measurements were used. The reaction pathways
and products of m-xylene–OH photooxidation in the Master Chemical
Mechanism (MCM v3.3.1, the website at http://mcm.leeds.ac.uk/MCM; last
access: 16 October 2017) was used for analysis of the products measured by
HRMS (Jenkin et al., 2003; Jia and Xu, 2014).
Experimental conditions and results at 4 h of experiments in
m-xylene–H2O2 photooxidation system.
a Calculated using the density and mass concentration
of the injected H2O2 solution, and the volume of the reactor.
b The mass concentration at 4 h of reaction time with particle
wall loss corrected. An SOA density of 1.4 g cm-3 was used to obtain
the SOA mass concentrations (Song et al., 2007). c In
Experiments. 3–4 and 10–11, the initial concentrations of m-xylene were
calculated using the density and the volume of the injected m-xylene, and
the volume of the reactor; the reacted concentrations of m-xylene were
estimated using 40 % of the initial concentrations of m-xylene.
Results and discussionRH effects on SOA yields
Eleven experiments were conducted. Experimental conditions and results at
4 h of experiments in the m-xylene–H2O2 photooxidation
system are summarized in Table 1. Experiments 1–4 were conducted in dry zero
air, and are defined as the low-RH experiments. Experiments 8–11 were
conducted in humid zero air, and are defined as the high-RH experiments.
Experiments 5–7 were conducted in the mixed air of dry and humid zero air,
and are defined as the intermediate-RH experiments. The initial
concentrations of m-xylene and the consumed m-xylene proportion
(typically ∼40 %) were approximately the same under different RH conditions.
Under intermediate- and high-RH conditions, LWC accounts for a certain
proportion of particles (Jia and Xu, 2018). To obtain the time evolution of
SOA concentrations, the LWC has to be subtracted during the whole
photooxidation period. Since LWC was only measured at the end of the
reaction, the volume growth factor (VGF) was used to estimate the
contribution of LWC in particles, which was defined as the ratio of the humid
particle volume to the dry particle volume (Engelhart et al., 2011). It was
assumed that the VGF did not change during the whole photooxidation period.
The removal of aerosol water during the LWC measurement may cause dissolved
species that are probably volatile/semi-volatile compounds to evaporate back
into the gas phase (El-Sayed et al., 2015, 2016). Glyoxal is a typical
semi-volatile compound with a high Henry's law constant, and is involved in
SOA formation in m-xylene–OH system of our study. The Henry's law constant
of glyoxal in pure water is as high as 4.19×105 M atm-1 at
298 K (Ip et al., 2009). Only 1 in 10 000 glyoxal
molecules can dissolve in the LWC, whose
concentration was obtained in our study. Thus, SOA concentrations for
intermediate- and high-RH conditions were slightly underestimated, but the
underestimation is extremely minimal and can be neglected. To obtain the mass
concentrations of SOA, an SOA density of 1.4 g cm-3 was used (Song et
al., 2007).
Mass concentration time profiles of SOA from
m-xylene–H2O2 photooxidation at 14 % (Exps. 3–4) and
76 %–77 % (Exps. 10–11) RH (corrected by particle wall loss and for
the amount of LWC in particles).
In Fig. 1, the wall-loss-corrected SOA mass concentrations are plotted as a
function of photooxidation reaction time for m-xylene–OH systems at low
(Exps. 3–4) and high (Exps. 10–11) RHs. It can be clearly observed that
there is a large difference in the maximum mass concentration between
14 % and 76 % RHs. In Table 1, the maximum mass concentrations are
95.5–150.3 µg m-3 at low RHs, whereas they are
7.5–27.9 µg m-3 at high RHs, with the largest difference
being over a factor of 10. The RH effect was reproducible when the initial
m-xylene concentration was slightly changed under similar conditions.
We used the definition of the ratio of the SOA mass to the consumed
m-xylene mass to calculate the SOA yield at the end of each experiment. In
Table 1, the SOA yields at low RH are 14.0 %–16.5 %, while those at
high RH are only around 0.8 %–3.2 %. SOA yields at low RH are nearly
1 order of magnitude larger than those at high RH. Though temperatures at
high RH are slightly higher than those at low RH as shown in Table 1, which
can lead to a higher SOA yield, the difference of temperatures between low-
and high-RH conditions is lower than 2 ∘C, which cannot lead to a
significantly different SOA yield to affect the result (Qi et al., 2010).
Seed aerosols were not artificially introduced throughout all the
experiments, which could lead to the underestimation of SOA, as SOA-forming
vapors partly condense to the reactor walls instead of particles (Matsunaga
and Ziemann, 2010; Zhang et al., 2014). The extent to which vapor wall
deposition affects SOA mass yields depends on the specific parent hydrocarbon
system (Zhang et al., 2014, 2015; Nah et al., 2016, 2017). Zhang et
al. (2014) have estimated two m-xylene systems under
low-NOx conditions and concluded that SOA mass yields were
underestimated by factors of 1.8 (Ng et al., 2007) and 1.6 (Loza et al.,
2012) under low-RH conditions. In addition, the excess OH radicals in our
experimental system lead to less underestimation of SOA formation as the
losses of SOA-forming vapors can be mitigated via the use of excess oxidant
concentrations (Nah et al., 2016). Thus, the underestimation of SOA formation
can be limited. In fact, the wall loss of m-xylene was not taken into
consideration for the calculation of mass yields, which generally
overestimates.
The wall loss of organic compounds that is sensitive to humidity can affect
the RH effect on SOA yields, as the reduction of SOA yields at high humidity
can be due to the loss to the wet reactor wall. There are thousands of SOA
precursors from m-xylene–H2O2 photooxidation. However, as far
as we know, there is no previous study that investigates the RH effect on the
loss of these organic compounds to the wet reactor wall. Thus, we select
three organic compounds that have relevant experimental data to estimate how
much the wall loss of chemical species affects SOA formation at different
RHs.
The first compound is glyoxal, a typical SOA precursor. Glyoxal can easily
dissolve in the aqueous phase due to the large Henry's law constant of 4.19×105 M atm-1 at 298 K (Ip et al., 2009), and is very
sensitive to humidity. Loza et al. (2010) found that the wall loss of glyoxal
was minimal at 5 % RH, with kW=9.6×10-7 s-1, whereas kW was 4.7×10-5 s-1
at 61 % RH. Obviously, there is a large difference in wall loss between
low and high RHs. We assume that kW linearly increases with RH,
and the kW value is estimated to be 6.1×10-5 s-1 at 80 % and 7.4×10-6 at 13 % RH, with
the difference being a factor of 8.2. According to the wall loss of glyoxal,
glyoxal only decreased by 10 % at the end of our experiment at low RH,
while glyoxal decreased by 59 % at high RH. This means that the SOA yield
was underestimated by 59 % at high RH and by 10 % at low RH when
the glyoxal lost to the wall was completely transformed into SOA. When this
wall effect of SOA precursors was taken into consideration, the SOA yields at
74 % (Exp. 8) and 14 % (Exp. 2) RHs were 6.1 % and
15.5 %, respectively, still with a difference of a factor of nearly 3.
Thus, the loss of organic vapor to the wet wall with a Henry's law constant
as high as glyoxal's cannot completely explain the large difference of SOA
formation at low and high RH in our study.
In a recent study, the signal decay of two compounds (C5H8O2
and C5H9O4N) generated from isoprene oxidation at RH = 5 %, 50 % and >90 % has been presented (Huang et al., 2018);
these were selected as the second and third compounds. C5H8O2
decreased by 10 % after 8.3 h at 5 % and 50 % RH, while
C5H9O4N decreased by 20 % and 40 % at 5 % and
50 % RH after 8.3 h, respectively. In our study, the SOA yield at the
end of our experiment decrease by 71 % at 51 % RH (Exp. 6) relative
to that at 14 % RH (Exp. 2). According to the wall loss of
C5H8O2, the SOA yield would not decrease at intermediate RH,
while taking C5H9O4N, as it only decreased by 10 % at low
RH and decreased by 22 % at intermediate RH, the SOA yield at 51 % RH
(Exp. 6) would decrease by 13 % relative to that at 14 % RH (Exp. 2).
Obviously, the decay characteristic of the two compounds cannot explain the
more than factor of 3 difference of SOA formation at 14 % and 51 % RH
in our experiments. If we take an extreme case of >90 % RH to estimate
the impact of the semi-volatile organic compound (SVOC) wall losses on SOA
formation in our experiments, the results are indeed different.
C5H8O2 and C5H9O4N decreased by 90 % and
70 % after 2 h (kW=3.2×10-4 and 1.7×10-4 s-1), respectively and subsequently remained steady,
indicating the saturation of the wet wall to absorb the organic vapor under
humid conditions of >90 % RH. Taking the decay characteristics of
C5H8O2, it is estimated that the SOA yield at the end of 2 h
would decrease by 90 % and would not further decrease after 2 h.
However, our experimental results show that the yield at the end of 2 h is
1.4 %, a decrease of 87 % relative to that at low RH, and then it
further decreased by 82 % at the end of 4 h. It seems that for VOCs that
generate intermediate SVOCs with a high-RH effects of wall losses, the RH
effect of their SOA yields can be explained by SVOC wall losses during the
first period. However, these SVOCs generally have saturation characteristics,
which cannot explain our observed RH effect of SOA formation. In fact, there
were many different SOA precursors from the m-xylene oxidation system that
probably have a much smaller Henry's law constant relative to that of
glyoxal, 4.19×105 M atm-1. Thus, we considered that other
mechanisms might exist to explain the negative RH effect on SOA formation
from m-xylene photooxidation.
SOA yields as a function of RH for different aromatic (toluene and
m-xylene) oxidation under low-NOx conditions with
photolysis of H2O2 as the OH source. The hollow circles
represent where no seed particles were introduced and the circles with a
cross where seed particles were introduced. The size of markers indicates the
magnitude of the amount of reacted VOC.
For comparison and discussion of the results of SOA formation with other
previous studies, Fig. 2 was plotted to show the SOA yields as a function of
RH for the two aromatic compounds (toluene and m-xylene) oxidation under
low-NOx conditions with the photolysis of H2O2
as the OH source. In Fig. 2, the hollow circles represent where no seed
particles were introduced and the circles with a cross represent where seed
particles were introduced, and the size of markers indicates the magnitude of
the amount of reacted VOC. In the most recent study on toluene SOA formation
conducted without seed particles (Hinks et al., 2018), the SOA yield at a
low-NOx level was 15 % under dry conditions (<2 %
RH) and 1.9 % under humid conditions (89 % RH), with the ratio of two
yields of dry to humid conditions being over 7.5. The toluene SOA produced
under high-RH conditions was significantly suppressed, in which the tendency
of RH effects on SOA yield was very similar to our study, though the
difference of SOA yield in the range of low- and high-RH conditions in Hinks
et al. (2018) was slightly smaller than that in this study. The small
difference of RH effects between Hinks et al. and our study is likely
associated with the difference in experimental conditions, including RHs,
initial and reacted VOCs and H2O2 concentrations, in addition to
different species. This comparison demonstrates that different species of
toluene and m-xylene of aromatics pose very similar RH effects under
low-NOx conditions. Hinks et al. attributed the suppression
of SOA yields by elevated RH to the lower level of oligomers generated by
condensation reactions and the reduced mass loading at high RH.
In a study on an SOA model for toluene oxidation, the negative RH effect on
SOA formation was also found in the presence of seed particles (Cao and Jang,
2010). In their study, the SOA yield at a low-NOx level was
28 %–30 % under low-RH conditions (17 %–18 % RH) and
20 %–25 % under moderate-RH conditions (48 % RH; Cao and Jang,
2010), but they did not focus on the RH effect to give an explanation.
Furthermore, their RH only changed from 17 % to 48 %, the reacted
parent VOC was smaller and the seed particles were present, so the RH effect
on SOA yields was not as significant as that in Hinks et al. and our study.
Ng et al. have investigated the yields of SOA formed from m-xylene–OH
system at low RH (4 %–6 %) under low-NOx conditions
(Ng et al., 2007). They obtained that the SOA yields were in the range of
35.2 %–40.4 % in the presence of seed particles. The SOA yields were
larger than those of our study, as they conducted the experiments under a
different irradiation time and with inorganic seed particles. These seed
particles can provide not only a surface for chemical reactions, but also
acidic and aqueous environments that can promote SOA formation (Jang et al.,
2002; Liu et al., 2018; Faust et al., 2017). The reacted concentrations of
the parent VOC was close between Cao and Jang and Ng et al. though the
species were different. The results from these two studies can be considered
together, since their experiments all had seed particles. As shown in Fig. 2,
the negative RH effect on SOA yields can be found. In addition to these three
previous studies shown in Fig. 2, a study on the chemical oxidative potential
of SOA (Tuet et al., 2017) found that the concentration of SOA from
m-xylene irradiation at a low-NOx level under dry
conditions was much larger than that under humid conditions
(89.3 µg m-3 at <5 % RH and 13.9 µg m-3
at 45 % RH), but they did not calculate the m-xylene SOA yields or give
an explanation for the RH effect.
FTIR spectra of particles from photooxidation of m-xylene–OH
experiments under 14 % (Exp. 2) and 74 % RH (Exp. 8) conditions.
RH effects on functional groups of SOA
Figure 3 shows the FTIR spectra of particles from the photooxidation of
m-xylene–OH experiments under both 14 % (Exp. 2) and 74 % (Exp. 8)
RH conditions. The DLPI sample flow rate was 10 L min-1, and the
sampling duration was 15 min. We used the same sampling flow rate and
duration for both RH conditions. DLPI has 13 stages, and it can collect
particles in the size range of 30 nm–10 µm. When we sampled using DLPI, the four plates for stages 4–7 were
removed, so that particles in the range of 108–650 nm were collected on the
third plate. As shown in Fig. S1 in the Supplement, the particles in the
range of 108–650 nm can represent the total SOA from m-xylene oxidation
in this study. The mean collection efficiency of the DLPI was 83 % for
stages 4–7 (Durand et al., 2014). Thus, the SOA mass collected on the ZnSe
window was 10.3 and 3.0 µg at 14 % RH (Exp. 2) and 74 % RH
(Exp. 8), based on the SMPS measurement and the DLPI collection efficiency.
As shown in Fig. 2, the SOA from m-xylene–OH experiments can be clearly
observed under both RH conditions. The intensities of all functional groups
from the 14 % RH experiment are much higher than those from the 74 %
RH experiment, which is consistent with the reduced SOA yields under elevated
RH conditions.
Absorbance positions of functional groups and the abundance at
14 % (Exp. 2) and 74 % (Exp. 8) RHs.
AbsorptionFunctionalityIntensity (×10-3) Ratio ∗frequencieslow RHhigh RH3235O-H5.91.90.323000C-H4.51.40.311720C=O5.11.50.291605C-C of aromatic rings4.42.80.64and conjugated C=O1415CO-H4.82.40.501180C-O-C, C-O and OH2.91.40.48of COOH1080C-C-OH5.31.80.34
∗ Ratio of the intensity at 74 % RH to that at 14 % RH.
The assignment and the intensity of the FTIR absorption frequencies at
14 % (Exp. 2) and 74 % (Exp. 8) RHs is summarized in Table 2. The
broad absorption at 2400–3600 cm-1 is
O-H stretching vibration in phenol, hydroxyl and carboxyl groups (Stevenson
and Goh, 1971; Santos and Duarte, 1998; Duarte et al., 2005). The band at
3000 cm-1 is C-H stretching vibration (Stevenson and Goh, 1971; Santos
and Duarte, 1998; Duarte et al., 2005). The sharp absorption at
1720 cm-1 is the C=O stretching vibration in carboxylic acids, formate
esters, aldehydes and ketones (Stevenson and Goh, 1971; Santos and Duarte,
1998; Duarte et al., 2005). The absorptions at 1605 cm-1 match C-C
stretching of aromatic rings and the C=O stretching of conjugated carbonyl
groups. The absorptions at 1415 cm-1 match the deformation of CO-H,
phenolic O-H and C-O (Coury and Dillner, 2008; Ofner et al., 2011). The
absorptions at 1180 cm-1 match the C-O-C stretching of polymers, C-O
and OH of COOH groups (Jang and Kamens, 2001; Jang et al., 2002; Duarte et
al., 2005). The absorptions at 1080 cm-1 match the C-C-OH stretching of
alcohols (Jang and Kamens, 2001; Jang et al., 2002).
Background-subtraction HESI-Q Exactive-Orbitrap MS results of SOA in
positive (a) and negative (b) ion modes from the
photooxidation of m-xylene–OH under both 14 % (Exp. 2) and 74 %
(Exp. 8) RH conditions (note that the y axis scales for low and high RH are
largely different: 106 at low RH and 105 at high RH).
The absorption intensity at ∼3200 cm-1 that is identified as the
hydroxyl group is used as a representative of the reflection of the SOA
formation. In addition, Table 2 gives the ratio of intensities at 74 % RH
(Exp. 8) to those at 14 % RH (Exp. 2) to compare the differences of the
relative intensities of functional groups. The intensities of functional
groups are obviously suppressed at high RH, but the extents of the
suppression for different functional groups are basically divided into two
types. The ratios of O-H, C-H, C=O and C-C-OH groups are 0.29 to 0.34, which
are close to the ratio of the SOA mass at 74 % RH to that at 14 % RH
collected on the ZnSe disk, whereas the ratios of CO-H, C-O-C and C-O-H in
COOH are above 0.48. The relative intensity of the C-O-C group is
significantly higher than the C=O group, which can be explained by more
oligomerization with the formation of C-O-C than other reactions at high RH.
Nevertheless, the FTIR results cannot provide further information to well
explain the differences of SOA yields between low and high RH, which will be
further discussed in terms of mass spectra of SOA in the next section.
Sum of peak abundance based on peaks selected in Fig. 4 as a
function of the number of carbon atoms under the positive ion mode and
negative ion mode (note that the y axis scale at low and high RH are
largely different, with a label step of 4.0×106 at low RH and
4.0×105 at high RH in the positive ion mode, and 5.0×106 at low RH and 1.0×105 at high RH in the negative ion
mode).
RH effects on mass spectra of SOA
The blank-corrected mass spectra of the SOA sample formed from m-xylene–OH
photooxidation under 14 % and 74 % RH conditions in positive and
negative ion modes are presented in Fig. 4, which is plotted as a function of
the mass-to-charge ratio. It should be noted that the y axis scales for low
and high RH are largely different: 106 at low RH and 105 at high
RH. In Fig. 4, a visible decrease in the overall peak abundance for both
positive and negative ion modes can be clearly observed as the RH elevates,
which is consistent with the result that the SOA mass concentration is lower
at high RH. In addition, it is obvious that the number of peaks is less under
high-RH conditions. As shown in Fig. 4, where the m/z values of SOA samples
are close for both low- and high-RH conditions, the absolute and relative
peak abundance is much different, indicating that RH significantly affects
the concentration of SOA components.
The route of OH-initiated m-xylene oxidation. The red number below
the molecular formula is its molecular weight, which is determined by HRMS
to exist in the particle phase.
For rough quantification of the RH effect, the blank-corrected mass peaks of
SOA samples were selected according to whose abundance is larger than
105 under low-RH conditions and corresponding mass spectra under high-RH
conditions, and then assigned with the number of carbon atoms. The peak
abundance with the same number of carbon atoms (nC) is summed, which is
presented in Fig. 5. It should be noted that the y axis scales at 14 %
and 74 % RHs are largely different, with a label step of 4.0×106 at 14 % RH and 4.0×105 at 74 % RH in the positive
ion mode, and 5.0×106 at 14 % RH and 1.0×105 at
74 % RH in the negative ion mode. The compounds with nC >8, a larger
number of carbon atoms than m-xylene, are proposed to be oligomers that
account for a large mass fraction of SOA due to their large molecular weights
and lower volatilities, though their peak abundance is lower. As a result,
the processes for formation of such compounds play an important role in the
formation of SOA. It can be clearly observed that the peak abundance is much
lower at high RH in the negative ion mode than that in the positive mode,
indicating that the decrease of the compounds obtained in the negative ion
mode account for a larger decrease at high RH.
Proposed mechanism of RH effects on SOA formation
The large difference of SOA yields and composition between low and high RHs
suggests that water is directly involved in the chemical mechanism and
further affects the SOA growth. In the particle-phase accretion equilibrium
reactions, where water is involved as a by-product, the elevated RH alters
the equilibrium of the reaction by moving toward reducing the fraction of
oligomers with low volatility and increasing the fraction of monomers (Nguyen
et al., 2011; Hinks et al., 2018). In this study and the previous study on
toluene SOA formation, C2H2O was one of the most frequent mass
differences at low and high RHs, but the peak abundance of its related
compounds was much lower under elevated RH conditions (Hinks et al., 2018).
C2H2O was proposed to be from the oligomerization reaction of
glycolaldehyde (C2H4O2), which can react with carbonyl
compounds by aldol condensation reactions with water as the by-product. This
chemistry may dominantly affect the negative RH effect on the whole process
of SOA formation.
Moreover, there may exist other processes that enlarge the difference of SOA
formation under various RH conditions. Before we discuss the possible
processes, the reaction pathway between m-xylene and OH radicals need to go
through first. Reactions between m-xylene (C8H10) and OH
radicals have two pathways, the H-abstraction from the methyl group and OH
addition to the aromatic ring, which generates products such as
methylbenzaldehyde (C8H8O) and methylbenzyl alcohol
(C8H10O), as shown in Scheme 1. OH addition is the dominant
pathway, as the branching ratio of H-abstraction only accounts for 4 %
based on MCM. OH addition to the aromatic ring is followed by O2
adduct and isomerization to form a carbon-centered radical, which can form
dimethylphenol (C8H10O) or is adducted by an O2 molecule
forming a bicyclic peroxy radical (BPR, C8H11O5; Calvert et
al., 2002; Birdsall et al., 2010; Wu et al., 2014). The BPR reacts with other
RO2 radicals or HO2, forming the bicyclic oxy radical
(C8H11O4). This RO radical can further react and finally form
carbonylic products, such as (methyl) glyoxal and other SOA precursors
(Jenkin et al., 2003; Hallquist et al., 2009; Carlton et al., 2010; Carter
and Heo, 2013), react with HO2 radicals, forming bicyclic
hydroxyhydroperoxides (ROOH, C8H12O5) or react with other
RO2 radicals, forming ROH (C8H12O4) and R-HO
(C8H10O4). The self- and cross-reactions of RO2
radicals also form ROOR (C16H22O10) or ROOR', which are the
accretion products (Berndt et al., 2018; Molteni et al., 2018). The further
O2 adduct of BPR can form highly oxygenated RO2 radicals and
further react and finally form highly oxygenated organic molecules (HOMs;
Types 1 and 2 in Scheme 1; Wang et al., 2017; Crounse et al., 2013; Ehn et
al., 2014; Jokinen et al., 2015; Berndt et al., 2016). Dimethylphenol
(C8H10O) and other products from the termination reaction with
the benzene ring or double bond can react with OH radicals and further react
to form HOMs as well.
Most HOMs can fall into the category of
extremely low or low volatility organic compounds, and a small number of HOMs
are semi-volatile organic compounds (SVOC; Bianchi et al., 2019). Extremely
low volatility organic compounds can condense onto particles but SVOCs exist
in significant fractions in the condensed and gas phases at equilibrium. As
SMPS measured, at the end of the experiment the number concentrations (not
corrected) of Exp. 1 (14 % RH) and Exp. 9 (79 % RH) were 1.9×103 and 5.8×102 particles cm-3, with a factor of 3,
while the mass concentrations (not corrected) of Exp. 1 (14 % RH) and
Exp. 9 (79 % RH) were 116.9 and 8.7 µg m-3, with a factor
of 13. This indicates that the size of particles at low RH are higher than
that at high RH. The O : C ratios in positive and negative ion modes
under low- and high-RH conditions were roughly calculated using the carbon
and oxygen atom numbers multiplied by the relative abundance obtained by
HRMS. The O : C ratio in the positive ion mode was close to each other,
0.56 and 0.58 at low and high RHs, respectively, while the O : C ratio in
the negative ion mode was different, 0.66 and 0.77 at low and high RHs,
respectively. Based on the gas-to-particle partitioning rule, more-volatile
compounds in the gas phase can condense to the particles of a larger size (Li
et al., 2018). It can be deduced that particles of a larger size in reduced
RH result in more SVOC in the gas phase condensing, leading to the difference
of SOA mass at various RHs. As shown in Fig. 5, more compounds with less nC
(nC < 8) are present under the low-RH experiment, also indicating that
more SVOCs in the gas phase condense onto the particles. SVOCs tend to escape
to the wet reactor wall as we discussed in Sect. 3.1, which interprets a
certain proportion of SOA reduction at high RH. The wall process of the
reactor enlarges the difference of SOA mass between low and high RH.
The higher O : C ratio in the negative ion mode demonstrates that the
compounds in the negative ion mode are much more oxygenated than those in the
positive ion mode. As shown in Fig. 5, the peak abundance at high RH is much
lower in the negative ion mode than in the positive mode, indicating that the
decrease of the more oxygenated compounds accounts for the larger fraction at
high RH. These high O : C ratios cannot be explained by any of the
formerly known oxidization pathways, except that the formation of HOMs from
RO2 autoxidation is taken into consideration (Crounse et al., 2013;
Barsanti et al., 2017). To our knowledge, RH does not directly impact the
formation of HOMs (Li et al., 2019). It is possible that HOMs undergo further
particle-phase reactions, as suggested in a previous study (Bianchi et al.,
2019), which may be influenced by RH, but this process need to be further
investigated in future studies.
Conclusion and atmospheric implication
The current study investigates the effect of RH on SOA formation from the
oxidation of m-xylene under low-NOx conditions in the
absence of seed particles. The elevated RH can significantly obstruct the SOA
formation from the m-xylene–OH system, so that the SOA yield decrease from
14.0 %–16.5 % at low RH to 0.8 %–0.8 % at high RH, with a significant discrepancy
of nearly 1 order of magnitude. Some of the reduction in the apparent
yield may be due to the faster wall loss of semi-volatile products of
oxidation at higher RH. The FTIR analysis shows that the proportion of
oligomers with C-O-C groups from carbonyl compounds in SOA at high RH is
higher than that at low RH, but the negative RH effect on SOA formation
cannot be well explained as the FTIR results cannot provide further
information. From the analysis of the HRMS spectra, it is found that
C2H2O is one of the most common mass differences at low and high
RHs, that the compounds with a lower carbon number in the formula at low RH
account for a larger proportion than those at high RH, and that the compounds
at high RH have higher O : C ratios than those at low RH. The HRMS
results suggest that the RH may suppress oligomerization where water is
involved as a by-product and may influence the further particle-phase
reaction of highly oxygenated organic molecules (HOMs) formed in the gas
phase. In addition to the chemical processes, the negative RH effect on SOA
formation is enlarged based on the gas-to-particle partitioning rule.
Together with the previous study on toluene SOA, it is conceivable that the
effect of RH on SOA yield is a common feature of SOA formation from
monocyclic aromatics oxidation under low-NOx conditions and
using H2O2 as the OH radical source. Our results indicate that
the production of SOA from aromatics in low-NOx environments
can be modulated by the ambient RH. Our study highlights the role of water in
SOA formation, which is particularly related to chemical mechanisms used to
explain observed air quality and to predict chemistry in air quality models
and climate models. The clear pathway of the influence of H2O on
the particle-phase reaction of HOMs formed in the gas phase needs to be
further studied in the future.
Data availability
Data are available by contacting the corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-15007-2019-supplement.
Author contributions
QZ and YX designed the research. QZ carried out the
experiments and analyzed the data. LJ provided valuable advice on the
experiment operations. YX and LJ provided advice on the analysis of results.
QZ prepared the manuscript with contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors are very grateful to the four anonymous reviewers for useful discussions.
Financial support
This research has been supported by the National Key R&D Program of China (2017YFC0210005) and National Natural Science Foundation of China (no. 41375129).
Review statement
This paper was edited by Sergey A. Nizkorodov and reviewed by four anonymous referees.
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