Introduction
Climate forcing by various atmospheric components has been intensely
investigated over the last few decades but significant uncertainties still
exist (IPCC, 2013). One of the largest uncertainties comes from the role of
carbonaceous aerosols, including black carbon (BC) and organic carbon (OC).
Black carbon is formally defined as an ideally light-absorbing substance
composed of carbon (Petzold et al., 2013) with strong absorption across a
wide spectrum of visible wavelengths, which is caused by a significant,
wavelength-independent imaginary part k (i.e., ∼ 0.79; Bond et
al., 2013) of the refractive index. BC has long been known as the strongest
light-absorbing aerosol in the visible wavelengths (e.g., Bond et al.,
2013). On the other hand, OC has been treated as a scattering species, and
only a few recent global modeling studies have focused on the radiative
forcing by absorbing OC (G. Lin et al., 2014; Feng et al., 2013; Chung et al.,
2012). Light-absorbing organic aerosol (OA) are collectively called brown carbon (BrC) (Laskin
et al., 2015; Moise et al., 2015; Andreae and Gelencsér, 2006). In
contrast to BC, the imaginary refractive index of BrC has stronger
wavelength dependence (λ-2 - λ-6) that increases
towards shorter visible and ultraviolet (UV) wavelengths. This broad
absorption band in the blue/violet region of the spectrum gives BrC its
eponymous yellow or brown color (Alexander et al., 2008; Andreae and
Gelencsér, 2006; Lukács et al., 2007). BrC has been widely observed
in many environments, including urban environments largely impacted by
anthropogenic emissions (Zhang et al., 2013; Du et al., 2014), biomass burning
plumes (Lack et al., 2012, 2013; Forrister et al., 2015), over the
ocean (Bikkina and Sarin, 2013), rainwater (Kieber et al., 2006), and in the
troposphere (Liu et al., 2014; Alexander et al., 2008).
A variety of studies have investigated sources of BrC in both the laboratory
and in the field. Incomplete and smoldering combustion of hydrocarbons,
especially those associated with biomass burning, is known to directly
produce particulate BrC (Hoffer et al., 2006; Hecobian et al., 2010; Lack et
al., 2013; Desyaterik et al., 2013; Chakrabarty et al., 2010; Kirchstetter and
Thatcher, 2012; Mohr et al., 2013). There is also evidence based on ambient
studies of a secondary BrC source (Duarte et al., 2005) and laboratory
studies show formation of chromophores (components of molecules that absorb
light) through a variety of mechanisms, including photooxidation of
aromatics (Lambe et al., 2013; P. F. Liu et al., 2015), ozonolysis of terpenes
subsequently aged in the presence of ammonium ions and humidity (Bones et
al., 2010; Nguyen et al., 2013; Laskin et al., 2014; Updyke et al., 2012), and
a variety of additional aqueous phase reactions, such as lignin (Hoffer et
al., 2006) and isoprene oxidation (Limbeck et al., 2003), reactions of
carbonyls (e.g., glyoxal, methyglyoxal) in acidic solutions (Sareen et al.,
2010), with amino acids (De Haan et al., 2009), amines (De Haan et al.,
2009; Powelson et al., 2014; Zarzana et al., 2012), or ammonium salts (Sareen
et al., 2010; Lin et al., 2015a; Galloway et al., 2009; Kampf et al.,
2012; Shapiro et al., 2009). Among those studies, it is suggested that the
chemical and optical properties of laboratory-generated secondary organic aerosols (SOA) might be
influenced by a variety of factors, including the composition of the
volatile organic carbon (VOC) precursor, oxidation chemistry, relative
humidity (RH), and potentially “aging” at longer timescales (i.e.,
in-particle reactions and photobleaching). Particularly, SOA aged in the
presence of dissolved ammonium has been shown to produce BrC efficiently,
which may contribute to aerosol optical density in regions with elevated
concentrations of ammonium salts (i.e., Updyke et al., 2012).
This study focuses on measuring light absorption by laboratory-generated SOA
that simulate both urban and remote environments. Four VOCs representative
of biogenic and anthropogenic emission are chosen as SOA precursors in this
study. Biogenic VOCs selected include isoprene and α-pinene, of
which isoprene is the most abundant biogenic non-methane hydrocarbon emitted
into the atmosphere (Guenther et al., 2006), while α-pinene accounts
for approximately 40 % of global monoterpene (C10H16) emissions
(Guenther et al., 2012). For anthropogenic VOCs, we selected
trimethylbenzene (TMB) and toluene, the photooxidation of which in the
presence of NOx is a major source of anthropogenic SOA (Ng et al.,
2007; Kleindienst et al., 2004; Henze et al., 2008). Four different types of
experiments were conducted to investigate the effects of (1) NOx
levels, (2) VOC precursors, (3) photolysis time, and (4) RH on SOA light
absorption. We compare the UV–Vis absorption light absorption of these SOA samples
extracted in both water and methanol.
Experimental methods
Experiments were performed in the indoor 10.6 m3 Teflon chamber at the
Pacific Northwest National Laboratory (PNNL) operating in batch mode where a
discrete quantity of a VOC is introduced into the chamber and allowed to
react with the gas-phase oxidants (Liu et al., 2012). The Teflon chamber was
flushed continuously with dry purified air until particle concentrations
were less than 5 cm-3 prior to all experiments. For each experiment, a
measured amount of VOC was injected into a glass bulb with a syringe,
evaporated with gentle heating, and transferred to the chamber in a flow of
purified air. After the VOC injection, 0.5 mL of H2O2 solution
(Sigma-Aldrich, 50 wt % in H2O) was injected into the chamber in the
same manner. Humidity was controlled by passing pure air at a variable flow
rate through pure water (18.2 MΩ cm, < 5 ppbv TOC) with a
HEPA filter downstream of the bubbler to remove any contaminant particles.
In experiments in which NOx were present, NO was injected from a gas
cylinder containing a known NO concentration (500 ppm, Matheson
Tri-Gas®) with flows regulated by mass flow
controllers. After all components were injected and well mixed in the
chamber, UV lights were turned on to initiate photooxidation. The UV flux in
the chamber, averaged JNO2 = 0.16 min-1, was measured
continuously by a radiometer that is calibrated to an equivalent photolysis
rate of NO2 and suspended in the center of the chamber. Measurements of
JNO2 using the photostationary state method were in agreement
with the radiometer measurements (Leighton, 1961).
During the experiments, a suite of online instruments were used to
characterize the gas- and particle-phase composition. The mixing ratios of
the hydrocarbons were continuously monitored with an Ionicon proton-transfer-reaction mass spectrometry (PTR-MS). The mass loading of the
aerosol particles was measured using an Aerodyne high-resolution time-of-flight mass spectrometer (HR-ToF-AMS) (DeCarlo et al., 2006), while the
number and volume concentrations were measured with a TSI scanning mobility
particle sizer (SMPS). An NO/NO2/NOx analyzer (Thermo
Environmental Instruments model 42c) was used to measure the concentration
of NO and NOx. A UV absorption O3 analyzer (Thermo Environmental
Instruments model 49C) allowed for the measurement of O3 concentration.
SOA samples were collected on filters to measure their light absorption.
Photooxidation products were collected onto polytetrafluoroethylene (PTFE) filters (Pall Life
Sciences, 47 mm, 1 µm pore size) at a flow rate of 9 L min-1 for a
collection period of 60–120 min. Typically at least 20 µg of
organic mass is required for accurate measurement of light absorption. As
described in previous studies (Hecobian et al., 2010; Zhang et al., 2011),
filters were extracted in high purity water (> 18.2 MΩ cm),
filtered through a 25 mm diameter 0.45 µm pore syringe filter
(Fisher Scientific, FisherbrandTM Syringe Filters) and transferred into
a long-path (100 cm pathlength) UV–Visible spectrometer (Ocean Optics) to
determine the light-absorption spectra. After water extraction, filters were
also sonicated in methanol (VWR International, A.C.S. Grade) to extract
non-water soluble mass (Liu et al., 2013; J. Liu et al., 2015). Total
absorption due to BrC (Abs(λ)) is determined as the sum of
water-soluble and methanol-extracted absorption from the sequential
extraction processes. An extraction efficiency test was performed with six
filters, in which filters were cut in halves, one-half extracted with
methanol only and the other half processed with the sequential extraction.
Results show that the sum of light absorption from the sequential extraction
is comparable to methanol extraction alone, with a slope within 8 % of 1
(Fig. S1 in the Supplement). Studies have shown that the extraction efficiency of organic
mass is > 90 % using methanol as the solvent (Chen and Bond,
2010; Updyke et al., 2012). Thus, it is reasonable to assume that total light
absorption determined from the sequential extraction procedure closely
approximates the “true” optical properties of the SOA samples. The limit
of detection (LOD) was 0.081 Mm-1 in the 300–700 nm wavelength range
with an estimated uncertainty of 21 %. The mass absorption coefficient (MAC)
was then estimated using Eq. (1):
MAC(λ)=Abs(λ)OM
in which Abs(λ) is the light absorption from filter-collected
aerosol samples at a wavelength λ, and OM (organic materials) is the SOA mass
concentrations on the filter estimated from AMS measurements and the sampled
air volume. Wall-loss corrections were not applied to either measured SOA
mass concentrations or light absorption determined from filter-collected
aerosol samples for consistency. Based on lowest SOA mass concentrations
during all experiments, the LOD of the MAC is estimated as 0.004 m2 g-1.
Description of the SOA two-product model
Ambient studies have shown that SOA produced from urban emissions in
isoprene-rich environments tend to have much lower BrC absorption compared
to that in anthropogenic emission-dominant environments (Zhang et al.,
2011). In our study, two mixed-precursor experiments were conducted to
investigate the changes in aromatic BrC due to the addition of isoprene reaction
products. We employ a two-product model to describe the partitioning of
organic mass between aromatic- and isoprene-derived SOA (Pankow, 1994; Odum
et al., 1996). SOA yield parameters for pure compounds are determined by
fitting real-time batch-mode data as described in the literature (Presto and
Donahue, 2006). In the mixed-precursor experiments, the PTR-MS data are used
to determine the amount each precursor reacted during the filter
collection periods. Then, the pure compound yield parameterizations are used
to calculate the relative fractions of the isoprene- and aromatic-derived
SOA collected on the filter. The calculation assumes that all SOA components
are mutually miscible and reproduced the measured SOA mass with a
difference of less than 10 % (Table S2). These fractions are then used along
with the optical properties of the single-precursor SOA to predict the
optical properties of the mixed aerosol.
Results and discussion
Effects of VOC types and NO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mi>x</mml:mi></mml:msub></mml:math></inline-formula> levels
The wavelength-dependent MAC values for SOA derived from four selected
precursor VOCs are plotted in Fig. 1. In general, the shapes of the
spectra are characteristic of typical atmospheric BrC materials, with
relatively higher absorption in the UV range (i.e., Hecobian et al.,
2010; Chen and Bond, 2010). Figure 2 shows a comparison of the MAC at 365 nm
among four different SOA samples (isoprene, α-pinene, TMB and
toluene) produced under NOx-free and high-NOx conditions.
The MAC values of isoprene SOA are close to the LOD in the 300–700 nm
wavelength range and there is no significant difference in the UV–Vis
spectra of isoprene SOA formed under NOx-free and high-NOx
conditions. Quantum mechanical calculations suggest that electrons must be
delocalized over the equivalent of 7–8 bond lengths before an absorption
will occur at 360 nm (Kuhn, 1949). Therefore, our results suggest SOA
produced from isoprene photochemical oxidation does not contain products
that have extended carbon conjugated chains, consistent with current
understanding that isoprene photochemical oxidation products consist of
carbonyls, hydroxycarbonyls, diols, and organic peroxides (e.g., Nguyen et
al., 2011). On the other hand, Y.-H. Lin et al. (2014) has suggested that acidic
seeds may promote formation of oligomers through reactive uptake of IEPOX
and produced light-absorbing OA under certain conditions. In
our experiments, neither acidic seeds nor excess ammonia are present, which
could explain the difference between our observations and those of Y.-H. Lin et al. (2014).
MAC values for SOA formed under NOx-free and high-NOx
conditions, from isoprene, α-pinene, TMB, and toluene. Note the
10 × difference in scale between the terpene and aromatic
precursors. The MAC values shown in this figure are tabulated in the
Supplement (Table S1).
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Compared to isoprene SOA, SOA formed from photochemical oxidation of
α-pinene showed slightly higher absorption in the 300–350 nm wavelength
range, though the absolute MAC values are still small. We observe a slight
increase in the MAC values at wavelengths below 450 nm for the α-pinene
SOA formed under high-NOx conditions relative to that formed
in the absence of NOx. These observations are consistent with other
studies that have found minimal light absorption for α-pinene SOA,
again indicating that the compounds partitioning to the condensed phase do
not have extended conjugation (Henry and Donahue, 2012; Nakayama et al.,
2010; Laskin et al., 2014).
In contrast to the SOA produced from the terpene precursors, aromatic
precursors representative of anthropogenic VOCs produce SOA that
significantly absorbs light, particularly in the UV wavelength range.
Overall, the MAC values of the SOA produced from both TMB and toluene are
much higher than biogenic SOA, for both NOx-free and high-NOx
conditions (Fig. 2). Lambe et al. (2013) suggested that the conjugated
double bonds retained in oxidation products of aromatic precursors are
likely to contribute to absorption in the ultraviolet to near-visible range.
SOA formed from non-aromatic precursors, on the other hand, did not show
strong light absorption in the ultraviolet–visible range due to lack of
extended conjugated double bond networks.
Comparison of MAC from various types of SOA, at a wavelength of 365 nm.
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For both toluene and TMB SOA, high-NOx products show substantially
higher light absorption than low NOx. Shown in Figs. 1 and 2,
aromatic SOA formed under high-NOx conditions have much higher MAC
values, both in the UV and in the visible. Several studies, based upon both
chamber and field observations, have suggested that nitrogen-containing
molecules are strong light absorbers (i.e., Nakayama et al., 2013; P. F. Liu et
al., 2015; Zhang et al., 2011; Lin et al., 2015b). In a companion study, we
reported detailed characterization of the most prominent BrC chromophores in
toluene SOA formed under both NOx-free and high-NOx conditions by
deploying liquid chromatography combined with a UV–Vis detector and
high-resolution mass spectrometry (LC-UV/Vis-ESI/HRMS) (Lin et al., 2015b).
Samples of toluene SOA produced under high-NOx and NOx-free
conditions have substantially different chemical compositions. In
high-NOx SOA, we identified 15 nitro-aromatic compounds, including
nitrocatechol, dinitrocatechol, and nitrophenol, the total absorbance of
which accounts for 60 and 41 % of the overall absorbance in the
wavelength ranges of 300–400 and 400–500 nm, respectively (Lin et al.,
2015b). In contrast, photooxidation products observed in NOx-free SOA
are dominated by non-aromatic compounds with a high degree of saturation,
which did not show substantial light absorption in the UV–Vis range. Similar
to toluene SOA, TMB SOA produced under high-NOx conditions contains
nitrogen-containing compounds in contrast to NOx-free SOA, which
explains the difference in light-absorbing properties (Liu et al., 2012).
For similar reaction conditions, the TMB-derived SOA samples are less absorptive
than the toluene SOA. The difference in the light absorption properties
between toluene SOA and TMB SOA may be explained by the difference in the
production of nitrophenols. Sato et al. (2012) showed that nitrophenols were
not detected in the TMB SOA, possibly due to the fact that NO2 addition
to the phenoxy radical formed in reaction of TMB with OH is inhibited. Our
measurement is consistent with this hypothesis and infers that
nitro-aromatics such as nitrophenols are the main sources of light
absorption for the aromatic SOA.
(a) MAC values of Suwanee River fulvic acid (SRFA), and
toluene SOA formed at different high-NOx conditions. (b) Imaginary part
of the refractive index, k, derived from toluene high-NOx SOA
measurements through the 300–700 nm range, with SRFA and literature data as
references (Nakayama et al., 2010, 2013; Liu et al., 2015b; Zhong and Jang, 2011).
SRFA k values were estimated assuming a density of 1.47 g cm-3 (Dinar et al., 2006).
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The MAC values of SOA produced from aromatic VOCs are comparable to those of
other light-absorbing material relevant to atmospheric aerosol particles,
such as fulvic acid. Shown in Fig. 3a, the blue shaded area represents the
measured MAC range of SOA produced in the toluene + NOx experiments,
with the MAC of Suwannee River fulvic acid as a reference. Over the
wavelength range 380–480 nm, toluene SOA has higher MAC values than fulvic
acid. Since fulvic acid is often cited as a surrogate of strong
light-absorbing atmospheric BrC associated with biomass burning, this
comparison shows that light absorption by BrC produced from anthropogenic
VOCs can be significant under certain photochemical condition, consistent
with high MAC values measured previously in urban environments when biomass
burning impacts were low (e.g., Zhang et al., 2011, 2013; Liu et al., 2013).
Comparison of MAC values from single-precursor and mixed-precursor
experiments. Bars represent the MAC values at 365 nm from measurements, and
are color-coded by the mass fraction of aromatic SOA. The blue diamonds
represent the predicted MAC values based on the modeled fraction of isoprene
SOA and aromatic SOA, with error bars indicating the uncertainty.
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Mixed-precursor experiments
Results from laboratory studies have shown that the addition of isoprene
reduced the BrC absorption of aerosols formed from
toluene + α-pinene mixtures (Jaoui et al., 2008). The lower
absorption was attributed to decreased organic aerosol yields (e.g., lower
amounts of light-absorbing SOA were formed) (Jaoui et al., 2008). From
ambient observations, Zhang et al. (2011) reported contrasting light
absorption properties in two urban environments. Fresh SOA in Los Angeles
displayed much higher light-absorption presumably because of the
anthropogenic-dominated environment, whereas Atlanta aerosols formed from a
mix of anthropogenic and biogenic (isoprene) VOC precursors had a 4–6 times
lower MAC value. Hecobian et al. (2010) measured the light absorption of
water-soluble organic carbon (WSOC) in Atlanta in different seasons and found
that the winter WSOC has a ∼ 3 times higher MAC than summer, due to a
higher fraction of organic aerosols formed from biogenic VOCs in summer.
Using summertime samples collected in Atlanta, Liu et al. (2013) reported a
significantly higher BrC MAC value that was associated with primary
anthropogenic emissions, compared to the lower MAC value observed at sites
with local anthropogenic emissions on top of regional biogenic-dominant
emissions. To investigate whether isoprene photooxidation products enhance or
inhibit absorption of aromatic SOA, we conducted two mixed-precursor
experiments. Figure 4 shows the comparison of MAC values at 365 nm of SOA
formed from single precursor and from mixed isoprene and aromatic VOCs, under
high-NOx conditions. In both isoprene/toluene and isoprene/TMB
experiments, the SOA formed has lower MAC values than those formed from the
pure aromatics alone. Qualitatively, this is the behavior that one would
expect, since non-absorbing isoprene SOA will “dilute” the chromophores
from the aromatic-derived SOA. To determine whether the total aerosol
absorption can be described quantitatively, we first estimate the mass of
aromatic- and isoprene-derived SOA (Table S2) using a partitioning model
described in the “Description of the SOA two-product model” section. We
then calculate predicted aerosol MAC values as the mass-weighted average of
the MAC values measured for the pure isoprene- and aromatic-derived SOA
species. Figure 4 shows a comparison of the measured and predicted
mixed-precursor SOA optical properties. The predicted MAC values are
31–55 % lower than the measurements, a difference that is likely outside
of the measurement uncertainty. There are several potential explanations for
the difference between the predicted and observed MAC values. First, it is
possible that SOA formation is not well-described by partitioning theory. One
potential source of error in our calculation is that we assume isoprene and
aromatic SOA are fully miscible in one another; however, we note that the
total predicted SOA mass is within 10 % of the observed SOA mass and hence
the underprediction of the MAC values cannot be explained by this error. A
second possibility is that the partitioning model underestimates the mass of
aromatic SOA that has condensed into the mixed-phase particles. Studies have
shown that gas-phase wall loss of toluene reaction products can be
significant under certain conditions in batch-mode experiments (Zhang et al.,
2014). The SOA yield parameterizations are based on data collected in the
absence of seed particles, in which case gas-phase wall loss could be
significant. However, isoprene reacts much more quickly than toluene
(Fig. S2); therefore, isoprene SOA should form first and provide surface area
which should mitigate the gas-phase wall loss of the toluene reaction
products. Because no seed particles were present in the pure toluene
experiments, we would expect those yield values to be biased low relative to
the toluene yield in the mixed-precursor experiments, thus potentially
explaining the underprediction of MAC values. A third possibility is that
organic peroxides and alcohols, which were shown to be the dominant component
of isoprene SOA (Krechmer et al., 2015), may react with toluene SOA
components to produce oligomers capable of absorbing in the UV–VIS that are
not present in the single-precursor SOA particles. Examination of the AMS
spectra in the mixed experiments and comparison to the spectra of the pure
aromatic- and isoprene-SOA were inconclusive in providing evidence of this
hypothesis. Samples were not collected for detailed analysis by
LC-UV/Vis-ESI/HRMS. Therefore, at
this time we cannot conclusively explain the apparent absorption enhancements
we observe.
MAC spectra of TMB and toluene SOA formed at < 5, 30, 50, and 80 % RH.
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Effect of relative humidity on light absorption by aromatic SOA
In order to investigate the effect of RH on SOA light absorption, both
toluene and TMB photo-oxidation experiments were conducted at fixed VOC and
NOx values but variable RH levels (Table 1). Figure 5 illustrates the
light absorption spectra of toluene- and TMB-derived SOA as a function of
experimental RH. The data shown here were from samples collected at a
photolysis time of 200 min, which corresponds to the time when light
absorption reached its highest value. In both TMB and toluene experiments,
SOA generated under dry conditions (RH < 5 %) displayed
significantly lower MACs than SOA formed at RH > 30 %. SOA
formed at 30, 50, and 80 % RH have similar light absorption to one another.
Thus, moderate RH enhances the MAC values by a factor of 1.33 at 365 nm and
further increases in RH have no effect. An overview of toluene-SOA molecular
compositions was analyzed by nano-DESI/HRMS (Lin et al., 2015b), and showed
that a large number of nitrogen-containing compounds (CHON) were produced
under moderate RH condition (Fig. S3). The difference in molecular
compositions suggest that low RH inhibited the formation of
nitrogen-containing compounds, which have been shown to be major light
absorbers in toluene SOA formed in the presence of NOx (Nakayama et al.,
2013; P. F. Liu et al., 2015; Zhang et al., 2011; Lin et al., 2015b).
Summary of experiments and experimental conditions described in this
work.
Experiment
Experiment
VOC
Initial VOC
Initial NO (ppb)
RH
type
concentration
(%)
(ppb)
1
1
isoprene
359.37
< 1 (NOx free)
30
2
1
α-pinene
22.73
< 1 (NOx free)
30
3
1
TMB
316.30
< 1 (NOx free)
30
4
1
toluene
339.92
< 1 (NOx free)
30
5
2
isoprene
311.45
1754.67 (high NOx)
30
6
2
α-pinene
45.45
466.09 (high NOx)
30
7
2
TMB
289.94
1589.6 (high NOx)
30
8
2
toluene
317.26
1800 (high NOx)
30
9
2
Isoprene + TMB
178.51 + 123.71
1800 (high NOx)
30
10
2
Isoprene + toluene
158.09 + 106.43
1800 (high NOx)
30
11
3
TMB
263.58
1500 (high NOx)
30
12
3
toluene
339.92
1900 (high NOx)
30
13
4
TMB
263.58
1800 (high NOx)
< 5
14
4
TMB
263.58
1800 (high NOx)
50
15
4
TMB
263.58
1800 (high NOx)
80
16
4
Toluene
396
1800 (high NOx)
< 5
17
4
Toluene
300
1800 (high NOx)
50
18
4
Toluene
339.92
1800 (high NOx)
80
We are unable to identify any gas-phase reactions in the toluene photolysis
mechanism directly involving water vapor. Thus, we conclude that RH must be
affecting particle-phase reactions that enhance chromophore formation.
Several studies have investigated the effect of RH on various particle-phase
SOA chemistry and optical properties. Song et al. (2013) found that SOA
produced from α-pinene + NOx + O3 in the presence of
acidic seed aerosols at elevated RH was less light-absorbing than SOA formed
under dry conditions, which is opposite of our observations. They suggested
that the change in light-absorbing properties might be triggered by
evaporation of water, which may have enhanced the acidity of aerosol seeds
(Nguyen et al., 2012), thereby promoting oligomerization reactions. Zhong
and Jang (2014) investigated the light absorption of BrC formed from wood
burning and observed a faster decay of chromophores at higher RH, which they
attributed to the decomposition of chromophores by H2O2 that is
produced by aqueous-phase photooxidation in the presence of elevated water
content level. Moderate to high RH may promote heterogeneous reactions,
which aids in the reactive uptake of volatile compounds into aerosols. Cao
and Jang (2010) decoupled SOA mass into partitioning and heterogeneous
aerosol production in a toluene–NOx system, and suggested that moderate
RH results in a higher fraction of SOA formed via heterogeneous reactions
than low RH conditions. Similar effects might be also pertinent to the
toluene SOA. Another possible explanation is that SOA formed under low RH
conditions may exist in a viscous, semi-solid, or glassy state due to
particle-phase oligomerization reactions (Saukko et al., 2012; Shiraiwa et
al., 2013), whereas SOA formed at moderate/high RH would be less viscous. Since
only one experiment was conducted under dry condition for each compound it
is difficult to draw conclusions, but further investigations are warranted.
Measurements of the MAC values (at 365 nm) of toluene and TMB SOA
formed at 30 % RH in the presence of NOx as a function of photochemical
age (top panels). The bottom panels show the AMS-measured ON-to-OM ratio.
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Effect of photochemical aging on light absorption of aromatic SOA
Atmospheric aerosols have a wide range of lifetimes, ranging from hours to
days (i.e., Wagstrom and Pandis, 2009). Previous studies have observed a
decrease in aerosol absorption with aging in BrC from various sources
including biomass burning and SOA formed from aromatics (Forrister et al.,
2015; Zhong and Jang, 2011; Lee et al., 2014). We therefore performed several
experiments to study the effect of aging on BrC absorption. Figure 6 shows
the MAC values at 365 nm as a function of photolysis time for toluene and
TMB SOA produced in the presence of NOx at 30 % RH (complete spectra
in the wavelength range of 300–700 nm are provided as Fig. S4, with
values tabulated in Table S3). We observe a clear decrease in aerosol
absorption with aging with MAC values decreasing by ∼ 35 % after 400 min
and > 50 % after 800 min.
MAC values of aromatic SOA formed under high-NOx conditions and
aged in the chamber with the lights off at different RH levels.
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Laboratory studies have suggested that photo-bleaching was due to
degradation of BrC chromophores (Lee et al., 2014; Zhong and Jang, 2011, 2014).
In our observations, the decrease of MAC with aging is
accompanied by a decreasing ON-to-OM ratio, shown in Fig. 6. Here we
define the term ON as the sum of the NO, NO2, and
CxHyOzNw families measured by AMS, to represent organic
nitrates formed during the experiments. NO and NO2 come exclusively
from organic nitrates in these experiments. Ammonium is below the instrument
detection limit, and the ratio of m/z 30 : 46 (around 5–6) is indicative of
organic nitrate, thus ruling out formation of ammonium nitrate (Farmer et
al., 2010). Therefore, the decrease in the aerosol ON : OM with time indicates
the loss of ON groups (Fig. 6). ON groups have been identified as the
strong light absorbers in aromatic SOA formed under high-NOx
conditions; thus, the relative decrease in ON fraction relative to OM is
consistent with the observed evolution in OA light absorption.
This observed loss of ON could be caused by photolysis and/or hydrolysis of
ON groups. Lee et al. (2014) observed a substantial decline in the double
bond equivalent (DBE) values upon photolysis of aromatic SOA, and suggested
that the decrease in SOA light absorption and chemical composition was due to
photolysis. On the other hand, Liu et al. (2012) suggested that
particle-phase hydrolysis could substantially reduce ON group concentration,
which they also related to a decrease in BrC light absorption. To distinguish
between the effects from photolysis and hydrolysis, SOA was allowed to age in
the chamber with UV lights off but at elevated RH in several experiments.
Shown in Fig. 7, the MAC values of toluene and TMB SOA are approximately
constant with aging despite the elevated RH. Therefore, we conclude that
decrease in MAC values are driven primarily by photolysis (i.e.,
photobleaching), which is correlated with loss of ON groups that have been
shown in many studies, including our companion study, to be BrC chromophores
(Lin et al., 2015b; P. F. Liu et al., 2015; Zhang et al., 2013). The effect
of RH is less clear, with the dark experiments suggesting the net effect of
water-related processes, such as hydrolysis and oligomerization, is either
negligible or tends to slightly enhance BrC light absorption, while
comparison of experiments with and without RH (Sect. 3.3)
suggests moderate RH enhances the SOA MAC values.
Derived imaginary part of refractive index (k) of brown carbon formed
from various VOC precursors at 365 nm. Tabulated values are k × 103.
NOx free
High NOx
Isoprene
0.029
0.196
α-pinene
0
1.15
TMB
0.967
6.028–9.899
Toluene
0.461
19.48–46.87
Imaginary refractive indices
So far, our discussion focused on mass-normalized absorption based on
solution measurements, which is not directly relatable to light absorption
by aerosol particles. Therefore, we derive the imaginary refractive index,
k, from spectroscopic data, which can be incorporated into climate
models. The k value is derived using Eq. (2):
k=ρpλ⋅Abs(λ)4π⋅OM=ρpλ4πMAC(λ),
where Abs(λ) is the solution absorption at a
given wavelength, OM is the organic mass extracted in solution, and
ρp is the density of organic aerosols. The density of
organic aerosols was calculated by comparing the volume-weighted mobility
size measured by SMPS and the mass-weighted aerodynamic size distribution
measured by AMS (DeCarlo et al., 2004). A density of 1.25 ± 0.3 g cm-3
was obtained for SOA produced under NOx-free conditions,
while a density of 1.41 ± 0.2 g cm-3 was estimated for SOA
produced in high-NOx experiments. Those density values were employed in
Eq. (2) to estimate k values at 365 nm for various types of SOA,
which are summarized in Table 2 (k values for the 300–700 nm range are
listed in Table S4).
Although α-pinene and isoprene have large contributions to the
global SOA budget, they were shown to produce SOA with very small light
absorption coefficients under the photochemical conditions we investigated,
which agrees with literature data (i.e., Nakayama et al., 2010; Lang-Yona et
al., 2010). The SOA compounds produced are dominated by carbonyl, carboxyl,
and hydroxyl functional groups, which do not have strong electronic
transitions in the UV–Vis range. As a result, those biogenic SOA particles
are expected to have a mostly cooling effect on global radiative balance.
However, studies have shown that biogenic SOA can be converted into BrC via
reactions with dissolved ammonia (Updyke et al., 2012; Laskin et al., 2014),
or by monoterpene SOA formed from nighttime reactions with NO3 radical
(Washenfelder et al., 2015). Furthermore, it has been demonstrated that
reactive uptake of IEPOX into acidic aerosols produce BrC (Y.-H. Lin et al.,
2014), which may have substantial impacts on specific regions with elevated
ammonia levels and/or active IEPOX chemistry.
In the present study, the SOA generated from the photooxidation of aromatic
VOC precursors, particularly toluene, were found to have significant
absorption in the UV–Vis range when formed in the presence of NOx.
Toluene SOA formed under high-NOx conditions has a k value
ranging from 0.019 to 0.047 at 365 nm, and 0.011–0.033 at 405 nm. Shown in
Fig. 3b, the k values are in good agreement with the measurement by
Nakayama et al. (2010), where reported k values were 0.047 at 355 nm and
0.007 at 532 nm. The k values reported by Zhong and Jang (2011) and P. F. Liu et
al. (2015) are close to the lower limit from this work, the former reported
a k value of 0.0214 at 350 nm, and the latter reported a range of
0.022–0.033 at 320 nm. However, the k values derived in this work
are substantially higher than those in Nakayama et al. (2013), which
reported k values ranging from 0.0018 to 0.0072 at 405 nm. A
possible explanation is the difference in NOx levels among the
experiments; Zhong and Jang (2011) and Nakayama et al. (2013) studies were
conducted at NOx levels lower than 1 ppmv, which are lower than
employed in our study. Nakayama et al. (2013) has reported that light
absorption of SOA has a dependence on NOx, that MAC increases with
NOx, which likely also explains the higher k values reported by
earlier work from the same group (Nakayama et al., 2010). Another
potentially important difference among the experiments is the RH, with
Nakayama 2013 and the Liu studies conducted under dry conditions (Nakayama et
al., 2013; P. F. Liu et al., 2015). From what we have observed, moderate RH could
enhance the light absorption of BrC.
Conclusions and atmospheric implications
Among ambient studies reporting BrC light absorption, high MAC values are
almost exclusively reported for aerosols attributed to biomass burning
(Kirchstetter et al., 2004; Hoffer et al., 2006; Alexander et al., 2008; Dinar
et al., 2008; Chakrabarty et al., 2010; Lack et al., 2013), and the limited
number of models that include BrC generally use biomass burning aerosol
optical properties as high-absorption references (G. Lin et al., 2014; Feng et
al., 2013). Our results suggest that organic aerosols formed from certain
anthropogenic VOC precursors also display efficient light absorption.
Specifically, the MAC values obtained from the toluene + high-NOx
experiment were comparable to that of fulvic acid, which has been used as
model compounds for biomass burning HULIS (Dinar et al., 2006; Brooks et al.,
2004; Chan and Chan, 2003; Fuzzi et al., 2001; Samburova et al., 2005). The
results suggest that in addition to BrC from biomass burning, the
photooxidation of anthropogenic precursors can also have significant impacts
on light absorption at wavelengths that drive photochemical reactions.
BrC observed in urban environments has large variations in reported MAC
values, and our mixed-precursor experiments may provide some explanations
for the observed variation. From our measurements, SOA formed from mixtures
of isoprene + aromatic VOC have lower MAC values than those formed from the
pure aromatics, suggesting that isoprene photooxidation products dilute
light-absorbing compounds. Therefore, it is possible that some of the
variance in BrC properties between urban sites can be explained by the
presence or absence of biogenic emissions. In addition, our results
suggested that NOx concentration, RH level, and photolysis time have
considerable influences on the formation and decay of light-absorbing
compounds. Similar light-absorbing compounds have been identified in certain
SOA samples originating from biomass burning (Desyaterik et al., 2013; Iinuma
et al., 2010); since substantial variations in SOA formation in biomass
burning plumes have been observed both chemically and physically due to fuel
types and fire aging conditions (Hennigan et al., 2011), we cannot simply
assume similar effects of those parameters on SOA produced from biomass
burning emissions. Thus, the result suggests that we should revisit how SOA
is treated in climate models, especially in urban areas. Several current
regional and global models include NOx-dependent SOA yield (Lane et
al., 2008; Farina et al., 2010; Ahmadov et al., 2012); accurately
parameterizing BrC formation from SOA will require a similar strategy.