ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-13929-2016The effect of particle acidity on secondary organic aerosol formation from
α-pinene photooxidation under atmospherically relevant conditionsHanYuemeiyuemeihan@hotmail.comStroudCraig A.craig.stroud@canada.caLiggioJohnLiShao-Menghttps://orcid.org/0000-0002-7628-6581Air Quality Research Division, Atmospheric Science and Technology
Directorate, Environment and Climate Change Canada, Toronto, ON, M3H 5T4,
CanadaYuemei Han (yuemeihan@hotmail.com) and Craig A. Stroud (craig.stroud@canada.ca)11November2016162113929139446April201611April201619October201623October2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/13929/2016/acp-16-13929-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/13929/2016/acp-16-13929-2016.pdf
Secondary organic aerosol (SOA) formation from photooxidation of α-pinene has been investigated in a photochemical reaction chamber under
varied inorganic seed particle acidity levels at moderate relative humidity.
The effect of particle acidity on SOA yield and chemical composition was
examined under high- and low-NOx conditions. The SOA yield
(4.2–7.6 %) increased nearly linearly with the increase in particle
acidity under high-NOx conditions. In contrast, the SOA yield
(28.6–36.3 %) was substantially higher under low-NOx conditions,
but its dependency on particle acidity was insignificant. A relatively strong
increase in SOA yield (up to 220 %) was observed in the first hour of
α-pinene photooxidation under high-NOx conditions, suggesting
that SOA formation was more effective for early α-pinene oxidation
products in the presence of fresh acidic particles. The SOA yield decreased
gradually with the increase in organic mass in the initial stage
(approximately 0–1 h) under high-NOx conditions, which is likely due
to the inaccessibility to the acidity over time with the coating of α-pinene SOA, assuming a slow particle-phase diffusion of organic molecules
into the inorganic seeds. The formation of later-generation SOA was enhanced
by particle acidity even under low-NOx conditions when introducing
acidic seed particles after α-pinene photooxidation, suggesting a
different acidity effect exists for α-pinene SOA derived from later
oxidation stages. This effect could be important in the atmosphere under
conditions where α-pinene oxidation products in the gas-phase
originating in forested areas (with low NOx and SOx) are
transported to regions abundant in acidic aerosols such as power plant plumes
or urban regions. The fraction of oxygen-containing organic fragments
(CxHyO1+ 33–35 % and CxHyO2+
16–17 %) in the total organics and the O / C ratio (0.52–0.56) of
α-pinene SOA were lower under high-NOx conditions than those
under low-NOx conditions (39–40, 17–19, and 0.61–0.64 %),
suggesting that α-pinene SOA was less oxygenated in the studied
high-NOx conditions. The fraction of nitrogen-containing organic
fragments (CxHyNz+ and CxHyOzNp+) in
the total organics was enhanced with the increases in particle acidity under
high-NOx conditions, indicating that organic nitrates may be formed
heterogeneously through a mechanism catalyzed by particle acidity or that
acidic conditions facilitate the partitioning of gas-phase organic nitrates
into particle phase. The results of this study suggest that inorganic acidity
has a significant role to play in determining various organic aerosol
chemical properties such as mass yields, oxidation state, and organic nitrate
content. The acidity effect being further dependent on the timescale of SOA
formation is also an important parameter in the modeling of SOA.
Introduction
Secondary organic aerosols (SOA) formed by oxidation of biogenic and
anthropogenic volatile organic compounds (VOC) comprise a substantial portion
of submicron aerosol particles in the atmosphere (Kanakidou et al., 2005;
Zhang et al., 2007a). Understanding the physical and chemical properties
associated with SOA formation and transformation is important to adequately
assess aerosol impacts on climate and human health (Hallquist et al., 2009).
The effect of aerosol acidity on SOA formation is one of the scientific
questions currently under open debate, as described below. Acid-catalyzed
heterogeneous reactions such as hydration, hemiacetal/acetal formation,
polymerization, and aldol condensation have been proposed to form SOA (Jang
et al., 2002). The presence of acidic aerosol particles has been reported to
enhance the reactive uptake of gas-phase organic species and increase SOA
yields due to acid-catalyzed reactions (e.g., Garland et al., 2006; Jang et
al., 2004; Liggio and Li, 2006; Lin et al., 2012b; Northcross and Jang, 2007;
Surratt et al., 2010; Xu et al., 2015a). However, other studies have
suggested that those reactions may be thermodynamically or kinetically
unfavorable and are possibly insignificant in the real atmosphere (Barsanti
and Pankow, 2004; Casale et al., 2007; Kroll et al., 2005; Li et al., 2008).
In contrast, recent kinetics studies have demonstrated that particle acidity
strongly affects the reactive uptake of isoprene epoxydiols (Gaston et al.,
2014; Riedel et al., 2015). Furthermore, the enhanced formation of SOA and
organic sulfates has been reported from the acid-catalyzed reactive uptake of
VOC oxidation products in ambient aerosols that are acidic enough to promote
this multiphase chemistry (Hawkins et al., 2010; Lin et al., 2012a;
Rengarajan et al., 2011; Zhang et al., 2012; Zhou et al., 2012), which is
contrary to other field studies showing no apparent evidence of
acid-catalyzed SOA formation (Peltier et al., 2007; Takahama et al., 2006;
Tanner et al., 2009; Zhang et al., 2007b). The dependence of SOA formation on
aerosol acidity generally has not been incorporated in many atmospheric
chemistry models thus far due to the large uncertainties associated with the
quantification of acidity effects, with the exception of the acidity effect
for SOA via isoprene epoxydiol uptake (Marais et al., 2016; Pye et al.,
2013).
A number of laboratory studies have investigated the effect of particle
acidity on SOA formation from oxidation of various precursor hydrocarbons
such as isoprene, terpenes, toluene, m-xylene, and 1, 3-butadiene (e.g.,
Kristensen et al., 2014; Lewandowski et al., 2015; Ng et al., 2007a;
Offenberg et al., 2009; Song et al., 2013; Surratt et al., 2007b). α-Pinene is the most abundant biogenic monoterpene emitted from terrestrial
vegetation (Guenther et al., 2012). The oxidation of α-pinene by
hydroxyl radicals (OH), ozone (O3), and nitrate radicals produces a
variety of multifunctional organic compounds such as carboxylic acids,
carbonyls, peroxides, ester dimers, epoxides, alcohols, and organic nitrates
(Calogirou et al., 1999; Yasmeen et al., 2012; Zhang et al., 2015). Despite
the efforts of previous laboratory studies under various experimental
conditions, the effect of aerosol acidity on SOA formation from individual
hydrocarbons remains unclear due to the complexity of this scientific
question. In particular, the magnitude of the acidic effect on SOA yields for
α-pinene has been found to vary significantly. For instance, a nearly
40 % increase in organic carbon (OC) was observed for the ozonolysis of
α-pinene in the presence of acidic seed particles without NOx,
and aerosol acidity played an important role in the formation of high
molecular weight organic molecules in particles (Iinuma et al., 2004). A
linear increase of 0.04 % in OC mass per nmol H+ m-3 was
reported from the photooxidation of α-pinene with NOx, and this
effect was independent of initial hydrocarbon concentration or the generated
organic mass (Offenberg et al., 2009). In contrast, Eddingsaas et al. (2012)
reported a relatively small increase of SOA yield (approximately 22 %)
for OH photooxidation of α-pinene under high-NOx conditions, and
no effect of aerosol acidity on SOA yield under low-NOx conditions when
introducing acidic seeds before photooxidation. Kristensen et al. (2014)
similarly found that the increase of aerosol acidity has a negligible effect
on SOA formation from ozonolysis of α-pinene under low-NOx
conditions.
These inconsistent results reported previously are most likely attributed to
the varied experimental parameters such as particle acidity, initial
hydrocarbon concentration, oxidant type and level, NOx level,
temperature, and relative humidity (RH). Most previous studies were conducted
with different acidity levels, and therefore a quantitative comparison of the
acidity effect among various studies is difficult. Laboratory studies were
usually performed with relatively high loadings of hydrocarbons (e.g., from
tens of parts per billion to several parts per million), which would result
in higher yield and lower oxidation state of laboratory SOA compared to
ambient SOA (Ng et al., 2010; Odum et al., 1996; Pfaffenberger et al., 2013;
Shilling et al., 2009). The oxidant used in laboratory studies is also
possibly one of the important factors affecting SOA formation. For example, a
positive dependence of SOA yield on H2O2 level has been reported
for the photooxidation of isoprene (Liu et al., 2016). In addition, the
presence of NOx during α-pinene oxidation may change the
reaction chemistry and lead to the formation of relatively volatile oxidation
products, hence decreasing α-pinene SOA yields (Eddingsaas et al.,
2012; Ng et al., 2007a). Moreover, temperature is an important factor in SOA
formation; higher SOA yields may be obtained at lower temperature (Saathoff
et al., 2009; Takekawa et al., 2003). RH is another important factor, the
decrease of which may lead to an increase in α-pinene SOA yields
(Jonsson et al., 2006); however, many previous studies have been performed at
very low RH (e.g., less than 10 %) or even dry conditions. As a result of
the above issues, it is highly important for laboratory studies to
investigate the acidity effect on SOA formation under more realistic
conditions approaching those of the ambient atmosphere. This would facilitate
an accurate parameterization of the acidity effect for incorporation into air
quality models.
This study aims to improve our current understanding of the effect of
particle acidity on SOA formation from photooxidation of α-pinene.
Photochemical chamber experiments were performed under conditions with
relatively low α-pinene loadings and moderate RH, which are more
representative of the ambient atmosphere. The yield of α-pinene SOA
was obtained at various particle acidity levels under high- and low-NOx
conditions. The dependence of SOA yield on particle acidity and the timescale of the acidity effect are characterized and discussed. The effect of
particle acidity on the chemical composition of α-pinene SOA, the
fragment distributions of bulk organics, and the oxidation state of organics
are examined based on the high-resolution analysis of organic aerosol mass
spectra. The possible contribution of particle acidity to the formation of
particulate organic nitrates under high-NOx conditions is also
discussed. Finally, the potential significance of the observed acidity
effect in the ambient atmosphere is summarized.
Experimental methods
Photooxidation experiments were performed in a 2 m3 Teflon chamber
(Whelch Flurocarbon) enclosed in an aluminum support (Liggio and Li, 2006;
Liggio et al., 2005). Twelve black light lamps (model F32T8/350BL, Sylvania)
were used as the irradiation source with intensity peaking at approximately
350 nm. The chamber was flushed by zero air with the lamps turned on for
more than 20 h before each experiment to avoid contamination from previous
experiments. Hydrogen peroxide (H2O2) vapor was introduced into the
chamber to produce OH radicals during the first 6 h of flushing. Temperature
and RH inside the chamber were monitored continually using a temperature and
humidity probe (model HMP 60, Vaisala). Temperature was not controlled during
the experiments, but it was relatively constant at 25 ∘C before
experiments began and increased to a stable value (approximately
30–34 ∘C) after the lamps were turned on. RH was maintained
manually by adding water vapors generated from a bubbler with zero air as a
carrier gas (15 L min-1). Other chamber inputs (e.g., H2O2
vapor, NO, seed particles, and α-pinene) were conducted after the RH
reached approximately 60 %. RH inside the chamber stabilized at
approximately 29–43 % after the lamps were turned on for about 1 h due
to the increase in temperature.
H2O2 vapor, as the source of OH radical, was introduced into
chamber using a bubbler with a flow of zero air (0.09 L min-1) passing
through H2O2 aqueous solution (30 wt % in water,
Sigma-Aldrich) for 1 h. Ammonium sulfate (AS)/sulfuric acid (SA) solutions
with varied NH4/ SO4 molar ratios were used to provide various
acidities in seed particles. A complete list of the composition of the seed
particles and other initial conditions in all experiments is given in
Table 1. The seed particles were generated by atomizing AS/SA aqueous
solution using an aerosol generator (model 3706, TSI), dried in a silica gel
diffusion dryer, and then size-selected at 150 nm in mobility diameter using
a differential mobility analyzer (DMA, model 3081, TSI). Nitric oxide (NO)
was added into the chamber from a compressed gas cylinder (9.1 ppm NO in
nitrogen) in high-NOx experiments, in contrast to low-NOx
experiments where NO was not added. A micro-syringe was used to inject
approximately 0.25 µL liquid α-pinene (+99 %,
Sigma-Aldrich) into the chamber through a stainless steel tube with a zero
air at 3 L min-1. After achieving the desired experimental conditions
for a stable 30 min period, photooxidation reactions were initiated by
turning on the lamps. The typical photooxidation time was 6 and 15 h for
high- and low-NOx experiments, respectively.
Experimental conditions and SOA yields from OH-initiated
photooxidation of α-pinene under high- and
low-NOx conditions.
a NH4/ SO4 molar ratios of ammonium sulfate/sulfuric acid
aqueous solution used for atomizing seed particles. b Initial seed
composition was estimated using the E-AIM II. c Aerosol pH was
calculated with the E-AIM output. d Initial and final temperature
inside the chamber. e Initial and final RH inside the chamber.
Four experiments were also performed to investigate the effect of aerosol
acidity on α-pinene oxidation products at different photooxidation
stages (Exp. 9–12; Table 1). Photooxidation of α-pinene was
conducted without seed particles in the reaction chamber for 2 and 4 h under
high- and low-NOx conditions, respectively. This was followed by turning
off the lamps and adding neutral/acidic seed particles into the chamber
within 1 h. The experiments continued for another 6 h on the reactive
uptake of the α-pinene oxidation products by the newly introduced
seed particles in the dark.
The concentration of α-pinene in the chamber was measured in
real time using a proton-transfer-reaction time-of-flight mass spectrometer
(PTR-ToF-MS, Ionicon Analytik GmbH) (Hansel et al., 1999; Lindinger and
Jordan, 1998). The mixing ratios of NO and O3 were monitored using a NO
analyzer (model 42i-Y, Thermo Scientific) and an O3 monitor (model 202,
2B Technologies), respectively. The particle number size distribution was
measured using a scanning mobility particle sizer (SMPS) consisting of a DMA
(model 3081, TSI) and a condensation particle counter (model 3776, TSI). The
non-refractory chemical composition of the submicron aerosol particles,
including organics, sulfate, ammonium, nitrate, and chloride, was measured
using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS,
Aerodyne Research) (DeCarlo et al., 2006). The AMS instrument was operated in
a high-sensitivity mode (V-mode) with the data stored at 1 min intervals. The
AMS data were processed using the standard ToF-AMS data analysis software
(SQUIRREL v1.56D and PIKA v1.15D,
http://cires.colorado.edu/jimenez-group/ToFAMSResources/ToFSoftware/).
The mass concentrations of aerosol species were generated from the PIKA
analysis of raw mass spectral data. A collection efficiency value of 0.7 was
applied for the AMS data analysis based upon the comparison of the volume
concentrations derived from AMS and SMPS measurements, assuming that
particles are spherical and the densities of organics, sulfate, ammonium, and
nitrate are 1.4, 1.77, 1.77, and 1.725 g cm-3, respectively. Note that
aerosol particles were not dried upstream of the AMS and SMPS measurements,
and thus particle water content might have contributed to the SMPS-derived
volume concentrations. This was not taken into account for the AMS-derived
volume concentration. The detection limits of organics, sulfate, nitrate, and
ammonium, defined as 3 times the standard deviations of the mass
concentrations of individual species (1 min average) in particle-free air,
were 34, 4, 1, and 5 ng m-3, respectively.
SOA yield, which represents the aerosol formation potential of precursor
hydrocarbon, was calculated from the ratio of generated SOA mass (ΔM0) to the reacted α-pinene mass (ΔHC). The SOA yields
and ΔM0 presented in Table 1 correspond to the maximum values at
the end of each experiment. Organic mass concentrations derived from AMS
measurement were wall loss corrected according to the decay of sulfate
particles in the chamber, i.e., by multiplying the ratios of the initial
sulfate concentrations to the instantaneously measured sulfate
concentrations. This correction assumed that α-pinene oxidation
products condensed on the sulfate particles instead of their self-nucleation.
This assumption is appropriate given that less than 50 particles cm-3
were contributed by self-nucleation and that an obvious increase in organic
mass concentration was not observed from the AMS measurement in the
experiments without adding seed particles. The decay rate of particles coated
with organics was assumed to be same as that of pure sulfate particles,
although the latter could be slightly higher due to the larger Brownian
diffusion rate of smaller particles. The calculated SOA yield could have been
affected by the wall loss of vapors at low α-pinene loadings, in
particular for low- and semi-volatile gaseous species (Ehn et al., 2014);
however, such an effect was not taken into account herein.
The initial seed composition in each experiment was predicted using the
Extended Aerosol Inorganic Thermodynamic Model (E-AIM) II
(http://www.aim.env.uea.ac.uk/aim/aim.php) (Clegg et al., 1998). The
concentrations of inorganic sulfate, nitrate, and ammonium derived from the
AMS measurement as well as the temperature and RH in the chamber were input
parameters. The pH of aerosol particles was calculated by -log(γ× [H+]) using the model outputs, where γ and
[H+] are the activity coefficient of H+ and the molar concentration
of dissociated H+ (mol L-1) in the aqueous phase, respectively. OH
concentration in each experiment was estimated from a linear fitting of the
first order decay of gaseous α-pinene by OH radicals, i.e., the
difference between the total α-pinene decay and the α-pinene
consumed by O3, as described by Liu et al. (2015). The OH concentrations
were calculated to be approximately 4.3–5.9 × 106 and
0.8–1.1 × 106 molecules cm-3 for experiments under high-
and low-NOx conditions, respectively. Nitrate radical (NO3)
generated from the reactions such as NO2 with O3 might also affect
the α-pinene decay (and hence the estimated OH), whereas it was not
taken into account here because NO3 levels were likely to be small under
the studied irradiation conditions.
Results and discussionα-Pinene SOA formation under high- and low-NOx
conditions
An increase of α-pinene SOA mass concentration with the decay of
α-pinene mixing ratio in high- and low-NOx experiments using
(NH4)2SO4 seed particles (Exp. 1 and 5 in Table 1) is shown in
Fig. 1. Under the high-NOx condition, the increase of α-pinene
SOA mass was observed shortly after the irradiation started until the end of
the experiment (Fig. 1a). Gaseous α-pinene was mostly consumed within
approximately 1.5 h. NO (66 ppbv initially) was consumed in the first
30 min of the irradiation. The formation of O3 was not suppressed over
the experiment. O3 increased to more than 200 ppb at the end of the
experiments, and therefore ozonolysis reactions would have contributed to the
formation of α-pinene SOA. A rough estimation shows that α-pinene consumed by ozonolysis accounted for in the range of 0–28 % of
the total α-pinene decay, as seen from the difference between the
total α-pinene decay and OH-consumed α-pinene in Fig. 1a.
Nitrate radicals may also have been generated from NOx reactions and
have contributed to α-pinene SOA formation, whereas its direct
measurement was not available in this study.
Time series of the mass concentrations of generated SOA and the
mixing ratios of NO, O3, total α-pinene decay, and OH-consumed
α-pinene in (a) high- and (b) low-NOx experiments using
ammonium sulfate as seed particles. Time = 0 h is defined as α-pinene photooxidation initiated when lamps were turned on. The presented
SOA mass concentrations have been corrected for particle wall loss according
to the decay of sulfate mass.
In contrast, under the low-NOx condition, the increase of SOA mass
concentration and the decay of α-pinene were relatively slower
(Fig. 1b). This is most likely due to the lower production of OH radicals
from H2O2 photolysis under low-NOx condition, that is,
1.1 × 106 molecules cm-3 compared to that of
5.3 × 106 molecules cm-3 under high-NOx conditions.
A plateau of the generated SOA mass was observed after approximately 12 h of
irradiation, suggesting that SOA formation reached equilibrium after α-pinene was consumed completely, if the gas/particle partitioning was
reversible (Grieshop et al., 2007). NO was less than 0.3 ppbv through the
entire experiment. A slight increase of O3 (up to 30 ppb) was also
observed under low-NOx conditions, which might have resulted from the
photolysis of a small amount of NO2 released from the chamber walls.
Less than approximately 47 % of α-pinene was consumed by
ozonolysis.
The α-pinene SOA yield was 4.2 % when using
(NH4)2SO4 seed particles under high-NOx condition, which is
a factor of 8.4 lower than that under low-NOx condition (35.2 %)
(Table 1). The relatively lower SOA yield under higher NOx levels is
consistent with those reported previously for the photooxidation and
ozonolysis of α-pinene (Eddingsaas et al., 2012; Ng et al., 2007a;
Presto et al., 2005). Similar relationships are also observed in the
photooxidation of isoprene and aromatic hydrocarbons such as benzene,
toluene, and m-xylene (Kroll et al., 2006; Ng et al., 2007b). The
dependence of α-pinene SOA yield on NOx level is possibly due to
the different gas-phase chemical reactions of the intermediate organic peroxy
radicals (RO2) formed in the initial photooxidation stage. RO2
reacted primarily with NO under high-NOx conditions and generated
relatively more volatile products that reduced the overall SOA yield, whereas
the reactions of RO2 with other peroxy radicals (e.g., RO2 and
HO2) were dominant under low-NOx conditions (Kroll et al., 2006;
Presto et al., 2005; Xu et al., 2014). Approximately 62–99 % of RO2
radicals reacted with NO over the entire experimental time (in total 6 h)
under high-NOx conditions in this study, which was estimated based on
the Master Chemical Mechanism constrained by the initial experimental
conditions (S1 and Fig. S1 in the Supplement). The observed difference in SOA
yields might also be affected to some extent by other experimental conditions
such as the initial α-pinene concentration, seed loading, and
temperature, but NOx level was most likely the primary cause, given that
other factors did not vary as much as NOx in these experiments
(Table 1).
A comparison of SOA yields as a function of the generated SOA mass (ΔM0) in this study and previous studies of α-pinene
photooxidation is shown in Fig. 2. The experimental parameters and SOA yields
from previous studies are summarized in Table 2. SOA yields in these studies
varied in the range of approximately 1.3–24 % in the presence of
NOx, in contrast to those of 26–46 % without NOx. The SOA
yields observed in our study are generally comparable to those reported from
previous studies despite the different experimental parameters. The SOA yield
under high-NOx condition in our study is in particular closest to those
with lower α-pinene level (Ng et al., 2007a; Odum et al., 1996) and
at higher temperature (Takekawa et al., 2003). It has been established that
low α-pinene level and high temperature can lead to relatively low
SOA yields, which is possibly due to the changes of gas/particle partitioning
thermodynamics in the reaction system (Odum et al., 1996; Pankow, 2007;
Takekawa et al., 2003). The SOA yield under low-NOx conditions here is
similar to those reported by Eddingsaas et al. (2012) but lower than those
reported by Ng et al. (2007a). The varied SOA yields among these studies most
likely depend on different experimental conditions. Therefore, laboratory
studies performed under conditions relevant to the atmosphere are important
for an inter-comparison among studies and for ultimately using those results
in air quality models.
Comparison of final α-pinene SOA yields as a function of
organic mass concentration (ΔM0) under high- and low-NOx
conditions in this study with those reported in literature. The solid and
open symbols represent the SOA yields under high- and low-NOx
conditions, respectively. The black, pink, blue, and green cycles represent
the SOA yield for experiments in this study with NH4/ SO4 molar
ratios of 2.0, 1.0, 0.5, and 0.2, respectively. A factor of 1.6 was used to
convert SOC yield and OC mass concentration to SOA yield and OA mass
concentration in Kleindienst et al. (2006).
Comparison of experimental parameters and SOA yields
reported in literature for the photooxidation of α-pinene.
ReferenceTemp.RHOxidantSeedNOxα-PineneΔHCΔM0SOA yield(∘C)(%)(ppb)(ppb)(µg m-3)(µg m-3)(%)Chu et al. (2014)2812, 50HONOASn.a.8.1, 11.7n.a.5.9, 9.315.1, 23.42812, 50HONOFeSO4n.a.9.7, 10.0n.a.5.0, 2.910.9, 5.7Kim and Paulson (2013)33–42a15–25apropeneno seed47–230143–153n.a.9–1185.9–17Eddingsaas et al. (2012)20–25< 10H2O2ASn.a.45.0–48.5247–26563.5–76.625.7–28.920–23< 10HONO, CH3ONOAS∼ 80044.9–52.4249–25837.2–60.314.4–24.2Ng et al. (2007a)23–255.3–6.4H2O2AS0,1n.a.76.7, 264.129.3, 121.337.9–45.825–263.3–3.7H2O2+NO, HONOAS198–968n.a.69.8–259.14.5–40.86.6–21.2Kleindienst et al. (2006)26.329NOxno seed242, 54325501190, 815130, 67.3b10.9, 8.3c26.329NOxsulfate242, 54325501190, 81587–172b10.7–14.5cTakekawa et al. (2003)10∼ 60propeneNa2SO430–5355–100260–54036–8920–2330∼ 60propeneNa2SO454–10293–196500–100020–955.2–10Odum et al. (1996)35–40∼ 10propeneAS300∼ 19–143104–7691.3–96.01.25–12.5
a Final temperature and RH were presented. b OC mass was
reported. c SOC yield was reported.
Effect of particle acidity on α-pinene SOA yieldDependence of SOA yield on particle acidity
The initial pH value of aerosol particles calculated from the E-AIM was in
the range of -0.93 to -1.66 in the high- and low-NOx experiments
(see Table 1). An increase of the α-pinene SOA yield with an increase
of particle acidity was observed under high-NOx conditions. The final
SOA yields were 5.6, 6.6, and 7.6 % for acidic particles with the initial
NH4/ SO4 molar ratios of 1.0, 0.5, and 0.2, respectively
(Table 1). This corresponds to 1.3, 1.6, and 1.8 times the SOA yield for
neutral particles (i.e., 4.2 %). Conversely, the final SOA yields for
acidic particles varied from 28.6 to 36.3 % under low-NOx
conditions, from which a systematic increase in SOA yield with particle
acidity was not observed. Clearly, the presence of acidic particles promotes
the formation of α-pinene SOA under high-NOx conditions and
NOx is likely involved in the acid-catalyzed reactions during α-pinene photooxidation. The dependence of α-pinene SOA yield on
particle acidity under only high-NOx conditions in this study is similar
to those reported by Eddingsaas et al. (2012), whereas they observed a
smaller increase of SOA yield (approximately 22 % compared to
30–80 % here) when using acidic particles. The effect of particle
acidity on SOA formed from α-pinene has been reported to be much
lower than that for isoprene, e.g., the former one was 8 times lower than the
latter one (Offenberg et al., 2009).
In addition to the effect of particle acidity, the α-pinene SOA yield
was also possibly influenced by the liquid water content in the particles.
The initial water content in the seed particles estimated by the E-AIM was on
average 5.2, 6.3, and 10.3 µg m-3 for high-NOx
experiments with NH4/ SO4 molar ratios of 1.0, 0.5, and 0.2,
respectively. Therefore, more water was present in the particles with higher
acidity. The higher particle water content could prompt the partitioning of
gas-phase water-soluble organic species by providing a larger medium for
their dissolution and therefore potentially increase the SOA yield (Carlton
and Turpin, 2013). However, there was no apparent increase in the SOA yield
under low-NOx conditions, even though seed particles with similarly
varied water content were used (Exp. 5–8; Table 1) and despite the fact that
products with higher O / C (hence higher solubility) were formed
(Sect. 3.3). This suggests that the particle water content likely did not
contribute substantially to the observed increase in α-pinene SOA
yield with acidity under high-NOx conditions.
Timescale of acidity effect
SOA yield is found to be a strong function of the generated SOA mass (ΔM0) (Odum et al., 1996). The time-dependent SOA yield as a function of
ΔM0 for acidic and neutral particles under high- and
low-NOx conditions is shown in Fig. 3. Under high-NOx conditions,
the increase of SOA yield with particle acidity (black through green points
in Fig. 3a) was much stronger in the first hour of photooxidation than in the
later period, suggesting that the acidity effect was more significant in the
initial period of photooxidation in this reaction system. This is possibly
due to fresh acidic particles being more accessible for acid-catalyzed
reactions by early α-pinene oxidation products in the initial stage.
A slight decrease in the SOA yield for acidic particles was also observed
after the relatively higher SOA yields within the first 30 min. A possible
interpretation for such a decrease in yield is that acidic particles (i.e.,
the inorganic core) were gradually less accessible with increased organic
coating on acidic particles. This assumes that a phase separation of
particulate organic and inorganic components occurred, from which a
core-shell morphology is inferred (Drozd et al., 2013), and that the
diffusion of organic molecules into the inorganic core was considerably
slowed. This process was indeed possible at the studied final RH
(approximately 29–43 %), given that SOA could be in an amorphous solid
or semisolid state with high viscosity at low to moderate RH (e.g., ≤ 30 %) (Renbaum-Wolff et al., 2013; Virtanen et al., 2010). This
indicates that the acidity effect is particularly important in the initial
stages of α-pinene oxidation in the presence of acidic particles. It
is expected that further reactive uptake of α-pinene SOA to acidic
particles might have been suppressed due to a phase separation, as has been
reported by other studies (Drozd et al., 2013; Lin et al., 2014; Riva et al.,
2016). The SOA yields increased nearly linearly with ΔM0 after
2 h of irradiation, suggesting that the growth of SOA mass continued after
the complete consumption of the α-pinene. This is possibly due to the
further oxidation of early generation products such as carbonyls,
hydrocarbonyls, and organic nitrates, and/or the continued partitioning of
gas-phase oxidation products into particle-phase. In contrast, the growth
curves of SOA yields for acidic particles under low-NOx conditions were
quite similar to that for neutral particles over the irradiation time
(Fig. 3b), which again suggests that acidity effect is insignificant under
the studied low-NOx conditions.
SOA yields as a function of organic mass concentrations for
experiments using seed particles with varied acidity levels under (a) high-
and (b) low-NOx conditions. The dashed lines in (a) represent the
irradiation time at approximately 30 min, 1 h, and 2 h.
The acidity effect on α-pinene SOA yield was relatively strong in the
first hour of irradiation under high-NOx conditions, as illustrated
above. This effect was characterized more quantitatively as a function of
NH4/ SO4 molar ratio, a proxy of particle acidity, in Fig. 4.
Here, the SOA yields at several specific ΔM0 values from 0.7 to
1.9 µg m-3 (within the first hour in Fig. 3a) are used as it
represents the strongest acidity effect observed. As seen in Fig. 4, the SOA
yield increased nearly linearly with the decrease in the
NH4/ SO4 molar ratio. A maximum increase of 220 % in SOA
yield was observed for the most acidic particles (i.e.,
NH4/ SO4 molar ratio = 0.2) with the ΔM0 of
0.7 µg m-3 at the irradiation time of approximately 20 min
compared to those for neutral particles (NH4/ SO4 molar
ratio = 2.0) (Fig. 4). This increase is much higher than the increase in
the final SOA yield with particle acidity (i.e., 80 % at 6 h) in the
same experiments. Furthermore, the increase in the SOA yield gradually slowed
with the increase in organic mass, which is evident by the decreased trend of
the slope curve derived from the fitting of SOA yield with
NH4/ SO4 molar ratios (Fig. 4b). This could be again
explained, at least in part, by acidic particles being less accessible over
time with the coating of α-pinene SOA. Another possible cause is the
consumption of sulfate due to the formation of organic sulfates (Surratt et
al., 2007a, 2008). However, we cannot identify organic sulfates clearly based
upon the AMS measurement, since their fragmentation results mainly in
inorganic sulfate fragments (Farmer et al., 2010).
(a) SOA yield vs. NH4/ SO4 molar ratio in the initial
period of photooxidation (approximately 0–1 h) under high-NOx
conditions. The colored lines represent the linear fitting of the markers.
The SOA yields at specific ΔM0 values were retrieved from the
plotting of SOA yields vs. ΔM0 in Fig. 3a. (b) The
negative slope derived from the fitting of SOA yields with NH4/ SO4
molar ratios in (a) decreased with ΔM0.
Acidity effect on later-generation SOA
Due to organic coatings on acidic particles, the effect of particle acidity
on SOA formation in a later experimental stage may be underestimated when
introducing seed particles before α-pinene photooxidation, in
particular under low-NOx conditions with higher SOA yield. Figure 5
presents the growth curves of the generated organic aerosol mass for
experiments with seed particles injected after 2 and 4 h α-pinene
photooxidation under high- and low-NOx conditions, respectively. The
organic aerosol mass was normalized by the reacted α-pinene
concentration before adding seed particles. Aerosol particles from the
nucleation of gas molecules were not significant in these experiments (less
than 50 particles cm-3), and thus the oxidation products were likely
present mainly in the gas phase prior to adding seed particles.
(a) The increase of SOA mass with time for experiments injecting
ammonium sulfate and acidic seed particles after α-pinene
photooxidation for 2 and 4 h under high- and low-NOx conditions,
respectively (Exp. 9–12 in Table 1). Time = 0 h represents the
beginning of reactive uptake of oxidation products after seed particles were
added. The SOA mass was normalized by the reacted α-pinene
concentration before adding seed particles. (b) The ratio of SOA mass for
acidic particles to that of ammonium sulfate particles.
The generated organic aerosol mass increased immediately after adding seed
particles for all experiments. This can be explained by the reactive uptake
of the gas-phase α-pinene oxidation products formed in the early
stages onto the acidic and ammonium sulfate seed particles. A higher increase
in organic aerosol mass (up to 6 times) was observed for acidic particles
than that for neutral particles in the first 2 h under both high- and
low-NOx conditions (Fig. 5). This suggests that the formation and/or
partitioning of organic aerosols, possibly the mixture of early and
later-generation SOA (although their proportions are unknown based on the
available data), was enhanced in the presence of acidic particles even under
low-NOx conditions, where no discernable acidity effect was observed
previously (as seen in Fig. 3b). It is postulated that this effect is
apparent here because the acidic particles had not been coated previously with
early generation products of α-pinene photooxidation, which makes the
acidic particles accessible to further acid-catalyzed chemistry. Eddingsaas
et al. (2012) also reported that α-pinene photooxidation products
preferentially partition to highly acidic aerosols when introducing seed
particles after OH oxidation under low-NOx conditions. The results in
Fig. 5 also indicate that later products of α-pinene oxidation were
more likely to be acid-catalyzed than the early products under low-NOx
conditions. Therefore, acidity effects may be different for α-pinene
SOA products formed from multiple oxidation steps. A detailed analysis of
those products at the molecular level is essential to fully understand this
effect.
Chemical composition of SOA
The effect of particle acidity on the chemical composition of α-pinene SOA in high- and low-NOx experiments is examined from the
distribution of organic fragments in the high-resolution organic aerosol mass
spectra (see Fig. S2). The average fractions of organic fragment groups in
the organic aerosol mass spectra for particles of different acidity are shown
in Fig. 6. CxHy+ fragments (accounted for 41–44 % of total
signal) dominated the organic aerosol mass spectra, followed by
CxHyO1+ (33–35 %) and CxHyO2+
(16–17 %) fragments for experiments with varied particle acidity under
high-NOx conditions (Fig. 6a). In contrast, CxHyO1+
(39–40 %) was the most dominant organic fragment, followed by
CxHy+ (33–36 %) and CxHyO2+
(17–19 %) fragments under low-NOx conditions (Fig. 6b). An increase
in the fractions of oxygenated fragments (CxHyO1+ and
CxHyO2+) and a decrease in the fraction of hydrocarbon
fragments (CxHy+) were observed under low-NOx conditions
compared to those of high-NOx conditions. Also, lower O / C ratios
of α-pinene SOA were observed under high-NOx conditions
(0.52–0.56, averaged at the irradiation time of 1–6 h) compared to those
under low-NOx conditions (0.61–0.64, averaged at the irradiation time
of 2–12 h). This indicates that less oxygenated α-pinene SOA was
formed in the presence of high NOx despite the fact that oxidants (i.e.,
OH and O3) levels were higher during the high-NOx-containing
experiments (see Table 1 and Fig. 1).
The mass fractions of organic fragment groups in total organic
aerosols under (a) high- and (b) low-NOx conditions. The organic mass
spectra were averaged for the irradiation times of 1–6 and 2–12 h
under high- and low-NOx conditions, respectively. The bars represent
the standard deviations (±1σ) of the mean values for
individual fragment groups.
The dependence of chemical composition and oxidation state of α-pinene SOA on NOx level is most likely associated with the different
oxidation products from gas-phase chemistry of RO2. For instance,
peroxynitrates and organic nitrates formed from the chemical reaction of
RO2 and NOx are the dominant products under high-NOx
conditions, whereas organic peroxides and acids formed from RO2 with
HO2 are dominant under low-NOx conditions (Xia et al., 2008). Note
that the observed variations in organic fragments in Fig. 6 generally
represent those over the whole photooxidation period in each experiment,
since the individual mass spectrum of α-pinene SOA did not change
significantly with irradiation time, as illustrated by the small standard
deviations of individual fragment groups.
With the increase in particle acidity (i.e., NH4/ SO4 molar
ratio from 2.0 to 0.2) under high-NOx conditions (Fig. 6a), the
fractions of major fragment ions (CxHy+ and
CxHyO1+) decreased gradually while
CxHyO2+ fractions increased; a slight increase in the
O / C ratio from 0.52 to 0.56 was also observed. This suggests that more
oxygenated SOA was possibly formed in the presence of acidic particles under
high-NOx conditions. A possible interpretation is that particle acidity
enhances the formation of more oxygenated SOA in particles such as larger
oligomers via acid-catalyzed reactions (Gao et al., 2004) and/or promotes
the partitioning of those oxidation products into particle-phase (Healy et
al., 2008), or particle acidity may also help to hydrolyze unsaturated
organic molecules. Conversely, there is no systematic change in the chemical
composition of α-pinene SOA with particle acidity under low-NOx
conditions. Therefore, the effect of particle acidity on the chemical
composition of α-pinene SOA may be important only under the studied
high-NOx conditions when introducing acidic seed particles before
photooxidation, which is consistent with the acidity effect on the yield of
α-pinene SOA (Sect. 3.2). It is likely that acidic particles coated
rapidly by earlier-generation α-pinene SOA due to the higher SOA
yield under low-NOx conditions, or the reactions of RO2 with
HO2 and RO2 were dominated by termination products that were less
affected by particle acidity. In addition, some oxidation products such as
hydroperoxides might have reacted on the acidic particles and produced more
volatile products (Liu et al., 2016), which may manifest as a decrease in the
acidity effect (i.e., lower yield) for α-pinene SOA under
low-NOx conditions.
Acidity effect on organic nitrate formation
The formation of organic nitrates from α-pinene oxidation has been
reported previously in the presence of NOx (e.g., Atkinson et al., 2000;
Presto et al., 2005). Nitrogen (N)-containing organic fragments
(CxHyNp+ and CxHyOzNp+) accounted
for less than 10 % of total organic signal in our studied conditions.
These fragments were most likely contributed by organic nitrates generated
from the reactions of early α-pinene oxidation intermediate
(RO2) with NO and NO2. Organic nitrates likely account for an even
higher fraction of the total organic aerosols, since their fragmentation
would primarily contribute to inorganic nitrate fragments (NO+ and
NO2+) and other organic groups (Farmer et al., 2010). Assuming an
average molecular weight of organic nitrate molecules ranging from 200 to
300 g mol-1, where 62 g mol-1 is attributed to the -ONO2
group and the remaining from the organic mass (Boyd et al., 2015), the
organic nitrate mass was estimated to be approximately
0.6–1.4 µg m-3. This resulted in a contribution of
17.5–20.5 % to total α-pinene SOA and an overall organic nitrate
yield of 0.7–1.6 % under high-NOx conditions in this study. Organic
nitrates yield has been reported to be in the range of approximately 1 %
up to more than 20 % for α-pinene oxidation (Aschmann et al.,
2002; Nozière et al., 1999; Rindelaub et al., 2015).
Interestingly, both the fractions of CxHyNp+ and
CxHyOzNp+ fragments increased gradually with the
increase in particle acidity under high-NOx conditions (Fig. 6a), which
is distinct from those without an apparent change under low-NOx
conditions (Fig. 6b). The growth curves of the total N-containing organic
fragments (sum of CxHyNx+ and
CxHyOzNp+) for different acidic particles under
high-NOx conditions are shown in Fig. 7a. The absolute mass
concentrations of total N-containing organic fragments were also enhanced
with particle acidity over the irradiation period. These results indicate
that organic nitrates were formed heterogeneously through a mechanism
catalyzed by aerosol acidity or that acidic conditions facilitate the
partitioning of gas-phase nitrates into particle phase under high-NOx
conditions. One possible reaction is the acid-catalyzed formation of sulfated
organic nitrates through α-pinene oxidation products such as nitroxyl
alcohols and carbonyls reacting with sulfuric acid (Surratt et al., 2008).
Further investigations on the individual particle-phase organic nitrate
species at a molecular level combined with gas/particle kinetics are required
to elucidate the detailed reaction mechanisms. Moreover, it has been
demonstrated that acid-catalyzed hydrolysis is an important removal process
for organic nitrates in the particle phase, from which organic nitrates can
be converted to alcohols and nitric acid (Day et al., 2010; Hu et al., 2011;
Liu et al., 2012; Rindelaub et al., 2015). This process would also enhance
the partitioning of gaseous organic nitrates into the particle phase due to
the perturbation in gas/particle partitioning and therefore decrease the
organic nitrate yields both in the gas and particle phases (Rindelaub et al.,
2015). The observed increase in N-containing organic fragments with particle
acidity under high-NOx conditions suggests that the production of
organic nitrates generally exceeded their removal rates in this reaction
system.
The temporal variations of (a) total N-containing organic
fragments (the sum of CxHyNz+ and
CxHyOzNp+) and (b) NO+ and NO2+
fragments for experiments using ammonium sulfate and acidic particles under
high-NOx conditions.
The time-dependent mass concentrations of NO+ and NO2+ fragments
for various acidic particles under high-NOx conditions are shown in
Fig. 7b. The mass concentrations of the NO+ fragment for acidic
particles were higher than those of neutral particles, whereas no obvious
difference in the NO2+ fragment was observed for particles with
varied acidity. Therefore, the enhanced organic nitrates by particle acidity
might contribute mainly to the increase in the NO+ fragment. Large
relative contribution of organic nitrates to the nominal inorganic nitrate
fragments is demonstrated by a higher NO+/ NO2+ ratio than
those of pure ammonium nitrate (Bae et al., 2007; Boyd et al., 2015; Farmer
et al., 2010; Fry et al., 2009; Xu et al., 2015b). The average
NO+/ NO2+ ratio was 9.13 ± 4.24, 9.28 ± 5.19,
9.31 ± 4.27, and 10.44 ± 5.48 for particles with initial
NH4/ SO4 molar ratio of 2.0, 1.0, 0.5, and 0.2, respectively.
These values are significantly higher than 2.6 ± 0.2 from the current
AMS measurement of pure ammonium nitrate but close to that of 11 ± 8
reported for NO3 oxidation of α-pinene, from which organic
nitrates were likely the dominant aerosol component (Bruns et al., 2010). The
increase in NO+/ NO2+ ratio with particle acidity suggests
that the composition of organic nitrate species might be different under
various acidic conditions, which is possibly due to the varied effect of
particle acidity on the formation and/or partitioning of different organic
nitrate species.
It should be noted that a small amount of CxHyNp+ and
CxHyOzNp+ fragments were also observed under
low-NOx conditions, where NO was not added (Fig. 6b). This may be
contributed by the formation of minor amounts of organic nitrates from the
reactions of NO2 released from the chamber walls with α-pinene
oxidation products. The average NO+/ NO2+ ratio was in the
range of 6.92–7.91 for particles with different acidities under low-NOx
conditions, which indicates that some organic nitrate species different from
those under high-NOx conditions might be formed. No apparent changes are
observed in the mass fractions of CxHyNp+ and
CxHyOzNp+ fragments with particle acidity under
low-NOx conditions, suggesting that acid-catalyzed formation and
partitioning of those organic nitrate species were possibly insignificant.
Implications
This study investigated the effect of particle acidity on the yield and
chemical composition of α-pinene SOA from photooxidation in a
photochemical reaction chamber. A nearly linear increase of α-pinene
SOA yield with the increase in particle acidity was observed under
high-NOx conditions, which is contrary to the insignificant acidity
effect under low-NOx conditions. The potential mechanisms leading to
the different acidity effects between high- and low-NOx conditions
warrant further investigation. The acidity effect was relatively strong in
the early photooxidation stages under high-NOx conditions, and this
effect decreased gradually with the growth of SOA mass. This may be
explained by a reduced accessibility of the SOA partitioning species to the
acidic particles for acid-catalyzed chemistry, possibly as a result of the
SOA coating. Given that the α-pinene loading used in this study was
low and the generated organic aerosol mass was relevant to ambient levels
(e.g., the final ratio of organic/sulfate was 0.6–0.8 and 1.7–2.8 under
high- and low-NOx conditions, respectively), a similar process may also
occur in the atmosphere. Consequently, an ambient acidity effect is likely
stronger for newly formed particles and/or freshly formed sulfate coating.
Therefore, the timescale of SOA formation with respect to acidity effects
is expected to be an important factor for field studies measuring acidity
effect in the atmosphere.
More oxygenated SOA was formed with the increase of particle acidity under
high-NOx conditions. Since aerosol acidity could affect the oxidation
state of aerosol particles and alter their chemical composition and other
properties as demonstrated here, this may be an important process in the
atmosphere and deserve further investigation. The formation of SOA from
later-generation gas-phase products was enhanced by particle acidity even
under low-NOx conditions when introducing acidic seed particles after
α-pinene photooxidation. This suggests that the overall acidity
effect on the formation of SOA could be underestimated and that more
systematic studies are necessary to evaluate the acidity effect on SOA
generated from multiple oxidation steps. This effect could also be important
in the atmosphere under conditions where α-pinene oxidation products
in the gas-phase originating in forested areas (with low NOx and
SOx) are transported to regions abundant in acidic aerosols such as
power plant plumes or urban regions. Organic nitrates in these experiments
may be formed heterogeneously through a mechanism catalyzed by particle
acidity and/or the acidic conditions facilitate the partitioning of gas-phase nitrates into the particle phase under high-NOx conditions. This
implies that aerosol acidity could also be of importance in the atmosphere
by altering the deposition patterns and rates of gas-phase NOx via its
conversion to particle nitrates with differing atmospheric lifetimes.
Despite the initial pH value of aerosol particles investigated in this study
(-0.93 to -1.72) being in the higher acidity range relative to that
generally observed for ambient aerosols, pH values less than -2.0 have
been reported for atmospheric aerosol particles and haze droplets (Herrmann
et al., 2015). It is therefore expected that the effect of particle acidity
observed in this study is relevant to the ambient atmosphere, especially in
regions enriched with acidic aerosols, and possibly during initial particle
growth via sulfuric acid. Moreover, we have studied the acidity effect under
more realistic RH conditions. While RH is an important factor affecting the
concentrations of [H+], the kinetics of hydrolysis reactions, and the
physical properties of SOA such as viscosity, more investigation over a
broader RH range is essential to understand the acidity effect in the real
atmosphere. Finally, further studies on SOA formation from various other
hydrocarbons under conditions near ambient atmospheric levels will be
valuable to understand the complex physical and chemical interactions
facilitated by aerosol acidity and evaluating the acidity effect more
accurately and to ultimately incorporate such effects into regional air
quality model for improved SOA prediction.
Data availability
The experimental data reported in this study are available upon request to
the first author (yuemeihan@hotmail.com).
The Supplement related to this article is available online at doi:10.5194/acp-16-13929-2016-supplement.
Acknowledgements
This study was funded by the Joint Oil Sands Monitoring Program between
Alberta Environment and Sustainable Resource Development and Environment and
Climate Change Canada.
Edited by: S. A. Nizkorodov
Reviewed by: three anonymous referees
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