Introduction
Aerosol particles affect many atmospheric processes. Globally, aerosol
particles influence Earth's radiative balance both directly by scattering
and absorbing solar radiation and indirectly by acting as cloud nuclei. At
the urban and regional scale, aerosol particles contribute to the
degradation of visibility (Chen et al., 2012) and adversely influence human health (Dockery
et al., 1993). A major fraction of the ambient particle population is
secondary organic material (SOM) produced by oxidation of anthropogenic and
biogenic gaseous precursors (Kanakidou et al., 2005; Hallquist et al.,
2009). The magnitude and properties of SOM, however, are still poorly
represented in models (Heald et al., 2005; Volkamer et al., 2006).
Ultraviolet-absorbing components in atmospheric particles can significantly
reduce ultraviolet (UV) irradiation, thus affecting photochemistry in the
atmospheric boundary layer (Jacobson, 1999; Barnard et al., 2008).
Modeling the optical properties and radiative effects, however, requires
spectral data of complex refractive indices (m=n-ik), including both the
refractory index n and the absorptive index k. Lack of this information,
especially in the UV region (300–400 nm), hampers an understanding of the
photochemical effect of anthropogenic SOM.
In highly polluted urban regions, anthropogenic aromatic compounds constitute
up to 70 % of the non-methane hydrocarbons (Ran et al., 2012). Among the
anthropogenic aromatics, toluene and xylenes are the most abundant compounds,
and the yields for the production of SOM from these compounds are high (Odum
et al., 1996, 1997; Ng et al., 2007; Hildebrandt et al., 2009; Zhang et al.,
2014). Quantitative estimates of the climate effects of anthropogenic SOM,
however, remain limited for several different reasons, including the
incomplete knowledge of chemical composition and optical properties.
The gas- and particle-phase chemistry of aromatic oxidation is complex, and
a broad spectrum of products is produced from the oxidation of a single
precursor (Forstner et al., 1997; Jang and Kamens, 2001a). The
concentration of nitrogen oxides (NOx) further influences the product
distribution (Sato et al., 2007). Some of the molecular products
resulting from the oxidation of aromatic precursors have been identified and
quantified (Forstner et al., 1997; Cocker III et al., 2001; Jang and
Kamens, 2001a; Hamilton et al., 2005; Sato et al., 2007). These products,
however, typically constitute less than 50 % of the reacted carbon
(Forstner et al., 1997; Hamilton et al., 2005; Sato et al., 2007).
Particle-phase reactions have been proposed as an important mechanism of SOM
production from aromatic precursors, which may explain the uncharacterized
components of aromatic-derived SOM (Jang and Kamens, 2001a; Jang et al.,
2002; Kalberer et al., 2004).
The chemical complexity of SOM propagates into the optical properties.
Recent studies have shown that different types of SOM have varying
light-absorbing properties in the ultraviolet (UV) and visible regions of
the electromagnetic spectrum (Nakayama et al., 2010; Lambe et al.,
2013; Liu et al., 2013). Some types of SOM absorb light significantly in the
UV–visible region and are then considered constituents of so-called “brown
carbon” (Shapiro et al., 2009; Laskin et al., 2010; Updyke et al.,
2012; Zarzana et al., 2012). In particular, several studies have pointed out
that aromatic-derived SOM is an important class of UV absorbers. The
investigated precursors have included phenolic catechol and guaiacol
(Ofner et al., 2011; Lambe et al., 2013; Liu et al., 2013), polycyclic
aromatic naphthalene (Lambe et al., 2013), and
monocyclic toluene (Nakayama et al., 2010, 2013; Zhong and Jang, 2011; Zhong et
al., 2012; Li et al., 2014).
In the present work, the spectroscopic complex refractive indices of several
types of aromatic-derived SOM are measured using spectroscopic ellipsometry
for n (Liu et al., 2013) and UV–visible spectroscopy for k. The
SOMs were produced from the photooxidation of toluene and m-xylene in an
oxidation flow reactor, both in the absence of NOx (hereafter “low
NOx”; < 70 ppt) and in the presence of NOx at different
levels (hereafter “high NOx”). For the high-NOx experiments, the
initial NOx concentration varied from 2.5 to 10 ppm, corresponding to
initial hydrocarbon: NOx ratios of 3.5 to 16 ppbC ppbN-1. In
complement to the optical measurements, the chemical composition was
characterized by the infrared spectroscopy. The paper concludes with a case
study of light-absorbing particles in an urban plume. Based on the
calculations using a Mie-theory-based optical model, possible radiative
effects of brown carbon from anthropogenically derived SOMs are discussed.
Experimental
Production of secondary organic material
Toluene (EMD Chemicals, ≥ 99.8 %) and m-xylene (Sigma Aldrich,
≥ 99 %) were continuously injected by a syringe pump (Chemyx, Fusion 200)
into a glass round-bottom flask (Ace glass, 100 mL). The flask was held at
310 K, and the organic liquids vaporized at the tip of a syringe prior to
the formation of a falling drop. The resulting gas-phase molecules were
swept in a flow of pure air (Aadco 737 Pure Air Generator; 1.0 L min-1) into an oxidation flow reactor (OFR). The injected
precursor concentration was 5.0 ± 0.5 ppm, which was calculated from
the injection rate of the liquid into the flow rate of the reactor.
Experimental conditions, infrared band areas of organonitrogen
groups normalized by the area of alkane bands, and complex refractive indices
of the studied SOMs.
Precursor
HC0
NO0
HC0 / NO0
Particle mass
Infrared band
320 nm
405 nm
(ppm)
(ppm)
(ppbC ppbN-1)
concentration
area ratiob
(µg m-3)a
A-NO2 /
A-ONO2 /
n
k (×103)
n
k (×103)
AC-H
AC-H
A1
toluene
5.0
0.0
n/a
(2.08 ± 0.17)× 103
0.00
0.00
1.567 ± 0.008
11 ± 1
1.546 ± 0.004
1.7 ± 0.2
A2
toluene
5.0
2.5
14
(1.84 ± 0.16)× 103
0.46
0.79
1.570 ± 0.007
22 ± 2
1.552 ± 0.004
4.1 ± 0.5
A3
toluene
5.0
5.0
7.0
(1.37 ± 0.15)× 103
0.94
2.11
1.585 ± 0.007
26 ± 2
1.562 ± 0.005
6.6 ± 0.6
A4
toluene
5.0
10.0
3.5
(0.77 ± 0.13)× 103
1.47
2.68
1.591 ± 0.009
33 ± 4
1.571 ± 0.005
15.3 ± 1.6
B1
m-xylene
5.0
0.0
n/a
(2.75 ± 0.10)× 103
0.00
0.00
1.554 ± 0.011
7 ± 1
1.531 ± 0.006
0.8 ± 0.1
B2
m-xylene
5.0
2.5
16
(2.81 ± 0.13)× 103
0.22
0.78
1.558 ± 0.006
10 ± 1
1.535 ± 0.004
1.2 ± 0.3
B3
m-xylene
5.0
5.0
8.0
(1.67 ± 0.10)× 103
0.46
1.65
1.578 ± 0.012
11 ± 1
1.549 ± 0.006
1.6 ± 0.2
B4
m-xylene
5.0
10.0
4.0
(0.84 ± 0.10)× 103
0.94
2.66
1.589 ± 0.008
15 ± 1
1.565 ± 0.007
3.0 ± 0.3
aValues are shown as (mean ± 1 standard deviation)
during the sampling periods. bA-NO2 represents the area of -NO2 band at 1558 cm-1;
A-ONO2 represents the area of -ONO2 band at 1647 cm-1; and
AC-H represents the area of C-H bands from 2790 to
2980 cm-1.
Aerosol particles composed of secondary organic material were produced in
the oxidation flow reactor (Kang et al., 2007; Lambe
et al., 2011). Aromatic precursors were oxidized by hydroxyl radicals (OH) (Fig. 1),
and some of the resulting low-volatility products contributed to
new particle production in the OFR. The experimental conditions are listed
in Table 1. The OFR was operated at a temperature of 293 ± 2 K, a flow
rate of 7.0 ± 0.1 L min-1, and a residence time of 110 ± 2 s.
Hydroxyl radicals were produced by photochemical reactions involving
ozone and water inside the OFR as follows: (i) O3+hν (254 nm)
→ O2+ O(1D) followed by (ii) O(1D) + H2O →2 OH. Ozone was generated outside the reactor by irradiating pure air with
the ultraviolet emissions of a mercury lamp (λ=185 nm). The
injected ozone concentration was 13 ± 2 ppm. Water vapor was
introduced by bubbling ultrapure water (18.2 MΩ-cm) with air. The
relative humidity inside the reactor was 13 ± 3 %.
Schematic diagram of experimental apparatus. (I) Production
of secondary organic material in aerosol form. (II) Sampling of
particles onto a Teflon filter and growth of a thin film by electrostatic
precipitation of particles onto a silicon substrate. (III) Fourier
transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy
(UV–VIS), and spectroscopic ellipsometry of collected secondary organic
material.
The experiments were conducted for several different concentrations of
NOx. For “low NOx” experiments, no NOx was added, and the
nitric oxide (NO) concentration in the pure air was below the detection
limit of the NOx analyzer (Eco Physics CLD 899 Y; < 70 ppt
NOx). For “high NOx” experiments, NO of 2.5 to 10 ppm was
injected using a mass flow controller (Table 1). In a control experiment to
assess the possible importance of ozonolysis, in the dark the produced
particle mass concentration was 0.1 % of that obtained when the
ultraviolet lamps were illuminated, indicating that photooxidation was the
major pathway of SOM production.
Spectroscopic ellipsometry
Spectroscopic ellipsometry of SOM films followed the procedures introduced
in Liu et al. (2013). Thin, continuous, mirror-like films of SOM
were synthesized by electrostatic deposition of aerosol particles onto
silicon substrates. Information about film preparation and characterization
was provided in Liu et al. (2013). Spectroscopic ellipsometry was carried
out across 280 to 1200 nm at different incident angles using a
variable-angle spectroscopic ellipsometer (J.A. Woollam VASE). From the data
sets, film thickness, film non-uniformity, and wavelength-dependent real
refractive indices, represented by n, were retrieved using the WVASE32 software package (J.A.
Woollam VASE). Individual film thickness ranged from 100 to 350 nm. The
non-uniformity in film thickness was 5 to 10 % over the ellipsometric
sampling spot. An absence of systematic error in retrieved refractive
indices, as related to film thickness or film non-uniformity, was confirmed
previously (Liu et al., 2013).
For each thin film sample, duplicate measurements were conducted at two or
more different spots. The retrieved n values agreed with each other within an
absolute difference of 0.015. An overall uncertainty of n, taking both the
reproducibility and the fitting error into account, was within ±0.015
(10 and 90 % confidence interval) across the studied wavelength range from
280 to 1200 nm. As a confirmation of the overall approach, the n values
retrieved for squalane, a non-absorbing standard, and nigrosin dye, a
light-absorbing standard, were consistent with literature values (cf.
supporting information in Liu et al., 2013).
Ultraviolet–visible spectroscopy
Particles were collected onto Teflon filters (Millipore FGLP, 0.2 µm
pore size). The collected SOM mass was determined by weighing the filter
before and after sample collection using an analytical balance (Sartorius,
LA120S; 2.0 to 10.0 mg, with an uncertainty of 0.1 mg). The filters were
extracted in methanol (40.0 to 100.0 mL) while ultrasonicating (Branson
2510; 20 min). The concentration c was calculated from the measured SOM mass
and the volume of the solvent. An extraction efficiency of 100 % was assumed
(Updyke et al., 2012). The extract was pipetted into a quartz cuvette having
an optical length of 10 mm. Absorbance spectra were recording using an
ultraviolet–visible spectrometer (Agilent Model 8453). The spectrum of neat
methanol was used as baseline. Analysis of blank filters showed no
absorbance across the studied region of 240 to 800 nm.
The imaginary refractive indices k were calculated from the data sets as
follows (Sun et al., 2007):
k=ln(10)4πρλcLA(λ)
for an absorbance A, an optical path length L (m), a material density
ρ (kg m-3), and a concentration c (kg m-3). A material
density of (1.4 ± 0.1) × 103 kg m-3 was used in the
analysis for toluene- and m-xylene-derived SOMs (Ng et al., 2007). For
Suwannee River fulvic acid (International Humic Substances Society, 2S101F),
a material density of 1.3 × 103 kg m-3 was used. For the
k values derived from UV–visible spectroscopy, propagation of uncertainties
in ρ, c, and A(λ) leads to an overall uncertainty of
±15 % (10 and 90 % confidence interval) for λ < 420 nm.
In an alternative to the UV–visible spectroscopy, the k values are also
retrieved by ellipsometry (Liu et al., 2013). A comparison of k values
derived from UV–visible spectroscopy and to those from ellipsometry is
provided in the supporting information (cf. Fig. S1 in the Supplement). Overall agreement is
good. For the smallest k values (< 0.005), the ellipsometry
retrievals have uncertainties approaching > 50 %. The
uncertainties for UV–visible spectroscopy are considerably smaller
(< 15 %). Even so, this method requires sample extraction, and
artifacts associated with extraction efficiency, material density, and
solvent effect can be introduced. All factors considered, the k values
derived from UV–visible spectroscopy were adopted in this study for further
analysis.
Absorptive component k of the refractive indices of (column 1)
toluene-derived SOMs and (column 2) m-xylene-derived SOMs for several
different initial NO concentrations. Row 1 shows the k values, and row 2
shows Δk values (i.e., Δk=k-kNO0=0). The k values
are derived from UV–Vis measurements (see main text). The shaded regions show
confidence intervals of 10 to 90 %, as calculated by propagation of
individual uncertainties for the parameters of Eq. (1).
Infrared spectroscopy
Aerosol particles were collected on Teflon filters (Sartorius Stem,
0.2 µm) at a flow rate of 2 L min-1 for up to 24 h. The
collected mass on the filters ranged from 0.8 to 2.0 mg. The Teflon filter
was cut to the shape of the germanium element of an attenuated total
reflectance (ATR) accessory (Pike Technologies). The assembly was then
screw-pressed to the crystal surface, the holder was opened, and the filter
was peeled off. A thin layer of secondary organic material remained on the
surface of the crystal (Hung et al., 2012). An experiment using a blank
filter showed no residual signal from the Teflon filter after peeling. This
preparation method avoided interference from Teflon filters across
900–1250 cm-1 (Russell et al., 2009b, 2011; Takahama et al., 2012),
which otherwise obscured the absorption bands of C–O stretching.
After preparation, the filter samples were taken for spectroscopy analysis.
Infrared spectra were recorded using the ATR accessory in a Fourier
transform infrared spectrometer (FTIR, Nicolet 670). The spectral resolution
was 0.5 cm-1. The number of scans was 16. Additional information about
the ATR-FTIR protocols is provided in Hung et al. (2012). A
band fitting algorithm, implemented in MATLAB, was used to analyze the
infrared spectra. The algorithm was adopted from Russell et al. (2009a) and Takahama et al. (2012). The
absorption bands of alkanes (C–H), carboxylic hydroxyls (O–H), alcoholic
hydroxyls (O–H), and carbonyls (C = O) were identified using the
literature-described algorithm. The algorithm was further developed in the
present study to characterize nitrate (-ONO2), nitro (-NO2), and
ether (C–O–C) groups.
Results and discussion
Optical properties of aromatic-derived SOMs and the effect of
NOx
The wavelength-dependent absorptive component k of the refractive index is
plotted in Fig. 2a for toluene- and m-xylene-derived SOMs prepared at several
different initial NO concentrations (cf. Table S1 in the Supplement for tabulated data). The
k values increase with increasing initial NO concentration. For
toluene-derived SOMs, the k values at 405 nm range from 0.0017 to 0.0153.
These values compare to a range of 0.0018 to 0.0072 reported by Nakayama et
al. (2013) for SOMs produced by toluene photooxidation
in an environmental chamber.
The increase of k for high NOx can in part be explained by the
production of light-absorptive organonitrogen compounds, mostly
nitro-aromatic compounds, such as nitrophenols, nitrocatechols, and
dinitrophenols (cf. Sect. 3.2). These compounds have been identified as
products from toluene photooxidation under high-NOx conditions
(Forstner et al., 1997; Jang and Kamens, 2001b; Sato et al., 2007; Zhong et
al., 2012; Nakayama et al., 2013). Nitro-aromatic compounds have also been
identified in brown carbon sampled in urban plumes dominated by
anthropogenic SOM (Zhang et al., 2011, 2013). The spectra
for several methyl-nitrophenol isomers were measured (cf. Sect. S1 in the Supplement), and
the results confirm that these compounds are strong UV absorbers. In
particular, the aromatic compounds having hydroxyl and nitro groups in
para substitution, such as 2-methyl-4-nitrophenol (a major product of aromatic
photooxidation), have a strong absorption band at 320 nm. Compounds having
this configuration are good candidates for contribution to the main peak in
the difference spectra Δk (i.e., Δk=k-kNO0=0) (Fig. 2b).
For similar reaction conditions, the m-xylene-derived SOMs are less
absorptive than the toluene-derived SOMs (Fig. 3a; Table 1). The filter
samples of toluene-derived SOMs have a yellowish to light brownish color,
even for samples collected in low-NOx experiments. For comparison,
m-xylene samples have a light yellowish color only for high-NOx
experiments and are visually white otherwise.
(a) Imaginary refractive indices k at 320 nm,
(b) real refractive indices n at 320 and 550 nm, (c) the
areas of nitro and nitrate absorption bands normalized by the area of alkane
bands, all as a function of initial NO concentration. Shown in (d)
is the ratio of k / kNO0=0 as a function of the
normalized area of nitro band
(A-NO2 / AC-H), as drawn from
(a) and (c). See also note b for Table 1.
Comparison of wavelength-dependent k values for different types of
atmospherically relevant light-absorbing materials. This study: anthropogenic
SOMs (A-SOM) derived from reacting toluene or m-xylene at low
NOx (NO0=0 ppm) and high NOx
(NO0 = 10 ppm; HC0 / NO0=3.5 and
4.0 ppbC ppbN-1 for toluene and m-xylene, respectively), and a brown
carbon (BrC) surrogate (Suwannee River fulvic acid). Literature: α-pinene- and limonene-derived biogenic SOMs (B-SOM), various atmospheric BrC
in both biomass burning (BB) and urban plumes, and black carbon (BC).
The k values of the aromatic-derived SOMs can be compared to those of other
light-absorbing material relevant to atmospheric aerosol particles (Fig. 4).
The k values decrease for increasing wavelength for the aromatic-derived
SOMs. Similar wavelength-dependent behavior is observed for light-absorbing
carbonaceous materials referred to as “brown carbon” in literature
(Kirchstetter et al., 2004; Andreae and Gelencser, 2006; Hoffer et al.,
2006; Alexander et al., 2008; Dinar et al., 2008; Chakrabarty et al.,
2010; Cappa et al., 2012; Lack et al., 2013). In contradistinction, the value
of k for black carbon is independent of wavelength
(Kirchstetter et al., 2004). Compared to the k values of SOMs
derived from examples of biogenic precursors (B-SOM), such as α-pinene and limonene SOM (Liu et al., 2013), the k values of the studied
anthropogenic SOMs (A-SOM) are 1 order of magnitude more absorptive in the
UV–visible region, even for those produced at low NOx. These higher
values suggest that conjugated double bonds are retained in some oxidation
products, which have absorption transitions in the ultraviolet to near
visible (Lambe et al., 2013). Even so, the k values in the
low-NOx experiments are smaller than those of a reference compound like
Suwannee River fulvic acid, which is often cited as a surrogate of
atmospheric humic-like substances (HULIS)
(Gelencsér et al., 2003; Dinar et al., 2006).
In the high-NOx experiments, however, the k values are within the range
of atmospheric brown carbon (cf. shaded region in Fig. 4).
Real refractive indices n of (a) toluene-derived SOMs and
(b) m-xylene-derived SOMs for several different initial NO
concentrations. The shaded regions represent confidence intervals of 10 to
90 % for the ellipsometry analysis.
The real refractive indices n of toluene- and m-xylene-derived SOMs are shown
in Fig. 5 for several different initial NO concentrations. The n values
decrease for increasing wavelength. The curves can be parameterized by the
three-term form of Cauchy's equation (cf. Table S2). The Cauchy-form of the
curves for the studied anthropogenic SOMs also holds for the biogenic SOMs
reported previously (Liu et al., 2013).
The refractive indices n shift +0.02 for both toluene- and
m-xylene-derived SOMs for an increase of the initial NO concentration from 0
to 10 ppm (Fig. 5). This upward shift of n for increasing initial NO
concentration is possibly attributed to an increasing abundance of nitrogen
in the SOM produced at higher initial NO concentrations. Nitrogen has a
higher atomic polarizability (1.03 Å3) than both oxygen
(0.57 Å3) and hydrogen (0.17 Å3) (Bosque and
Sales, 2002). The n value of a material is related to its polarizability by
the Lorentz–Lorenz equation (Bosque and Sales, 2002). Within
the tolerance of the measurement uncertainty, the n values do not differ
between SOMs derived from the two different aromatic precursors at the same
initial NOx concentration. The implication could be that the n values of
SOMs are mainly determined by bulk chemical properties, such as the
elemental ratios or functional groups (cf. Sects. 3.2 and 3.3). Detailed
chemical properties, such as the molecular structure, might play a minor
role in determining the value of n. In this case, upscaling of the laboratory
parameterizations to large-scale models of the effects of different types of
SOMs on radiative forcing and climate is simplified (Lambe et al.,
2013; Flores et al., 2014; Kim et al., 2014). As a caveat, the SOMs of this
study were produced at mass concentrations much higher than typical
atmospheric concentrations. Both elemental composition and refractive index
can depend on mass concentration (Shilling et al., 2009; Kim et al., 2012;
Kim and Paulson, 2013), and further investigations are needed to quantify
this possible effect.
Production of organonitrogen compounds and light absorption
The infrared spectra in the presence and absence of NOx are similar,
except for organonitrogen groups, such as -NO2 and -ONO2
(Fig. 6). This similarity suggests that the oxygen-containing functional groups,
excluding nitrogen-containing groups, are substantially similar for SOMs
produced at the different NOx concentrations. SOMs derived from toluene
and m-xylene also have similar overall compositions.
Organonitrogen compounds are detected in SOM collected in high-NOx
experiments. The bands at 846, 1281, and 1647 cm-1, corresponding to
organonitrate groups (-ONO2), are present in both toluene- and
m-xylene-derived SOMs (Roberts, 1990; Liu et al., 2012). The area of the
-ONO2 band from 1610 to 1690 cm-1, when normalized by the alkane
C–H bands from 2790 to 2980 cm-1 to account for different masses on
the filters, increases for greater initial NO concentrations (Fig. 3c).
Comparison of the spectrum of the toluene-derived SOM to that of
m-xylene-derived SOM for fixed initial NOx concentration shows that the
-ONO2 fractions are approximately equal for both types of SOMs
produced. The dominant mechanism for -ONO2 production, which is the
reaction of peroxy radicals (RO2) with NO (Roberts, 1990),
can explain why the fraction of -ONO2 increases for greater NO
concentrations.
Infrared spectra of (a) toluene-derived SOMs and
(b) m-xylene-derived SOMs for several different initial NO
concentrations. Individual curves are offset from each other so that
differences can be seen. Spectra are normalized to the peak height at
1100 cm-1, corresponding to a C–O stretch, to compensate for different
masses on the filter samples.
Ternary diagram representing the relative areas of O–H, C–O, and
C = O bands for aromatic-derived SOMs. Data for reference compounds in
the NIST database, as well as of individual products reported in the
literature for toluene-derived SOM, are also shown for comparison. The arrow
illustrates the direction of particle-phase reactions. See Sect. S2 in
the Supplement for further explanation of this diagram.
In the high-NOx experiments, -NO2 groups are produced. For
the toluene-derived SOM, the production of -NO2 groups is indicated by
a strong band at 1558 cm-1 and a weak band at 1342 cm-1 (Fig. 6a).
For m-xylene-derived SOM, only the strong band at 1558 cm-1 is
observed (Fig. 6b). Based on an analysis of area ratios, the mole fraction of
-NO2 groups in the m-xylene-derived SOM is 35 to 50 % lower than
that in the toluene-derived SOM for fixed initial NOx
concentration (Fig. 3c). The production mechanism of -NO2 group has
been proposed as the adduction of -NO2 to phenoxy radicals to produce
nitrophenols (Forstner et al., 1997; Jang and Kamens, 2001b; Nakayama et al.,
2013). Compared to toluene, the alkyl substitution at a metasite of
m-xylene can inhibit the production of stable -NO2 adducts of
phenoxy radicals (Nakayama et al., 2013), which can explain the lower
-NO2 fraction observed for m-xylene-derived SOM.
The difference in -NO2 fraction explains in part but not entirely the
differences in k values for toluene- compared to m-xylene-derived SOMs (cf.
Sect. 3.1). When normalized by -NO2 fractions and for similar
reaction conditions, Δk at 320 nm for toluene-derived SOM is 50 %
higher than that of m-xylene-derived SOM. The implication is that the
compounds in toluene-derived SOM are more UV-absorptive than those in
m-xylene-derived SOM. Differences in the extent of conjugation of the
oxygenated products can be important. The k values in the UV region at low
NOx (kNO0=0) provide a baseline to quantify this influence. When
normalized by both kNO0=0 and -NO2 fraction, the two types of SOM
are similarly absorptive (Fig. 3d). Organonitrogen groups attached to a
conjugated chain can have increased light absorption as well as shifts in
absorption to longer wavelengths.
Oxygenated groups and the importance of particle-phase reactions
The infrared spectra show that several different types of oxygenated
functional groups are present in the SOMs (Fig. 6). The groups include
alcoholic hydroxyls (O–H) at 3100–3700 cm-1, carboxylic hydroxyls
(O–H) at 2400–3300 cm-1, carboxylic carbonyls (C = O), and mixed
ketones / aldehydes (C = O) at 1640–1850 cm-1. The spectra of
aromatic-derived SOMs are similar to those reported in the literature for
related SOMs (Jang and Kamens, 2001a; Liu et al., 2012). The new finding of
the present study is the presence of a strong C–O stretch at
1000–1260 cm-1 (cf. Table S3). This band was obscured in previous
studies by ammonium sulfate or Teflon filter. As explained for Fig. 7, this
C–O band cannot be explained by alcohols, phenols, cyclic anhydrides,
carboxylic acids, or other carbonyls that have been identified as major
products from oxidation reactions of aromatic precursors.
Parameters describing the different particle populations used in the
case study of Sect. 3.4.
Particle number-
Material
Type of particles
diameter distributiona
density
Complex refractive indices
gmd (nm)
gsd
(kg m-3)
Brown carbon
toluene + OH + NOx
100
1.8
1400
wavelength dependent
(this study)b
m-xylene + OH + NOx
100
1.8
1400
wavelength dependent
(this study)c
Black carbon
50
1.8
1800
1.85–0.71 id
Sulfate
100
1.8
1770
1.53–0.00 ie
a Parameters describing a single-mode log-normal
number–diameter distribution for the governing equation:
dNdlogdp=NT12πgsdexp[-(logdp-loggmd)22gsd2]. NT
represents the total number concentration (m-3) and dp represents
the particle diameter (nm). b Values taken from Exp. A4 (cf.
Table 1). c Values taken from Exp. B4 (cf.
Table 1). d Value recommended by Bond and
Bergstrom (2006). e Value taken from Toon et al. (1976).
Figure 7 shows a ternary diagram representing the relative areas of O–H,
C–O, and C = O bands for toluene- and m-xylene-derived SOMs (cf. Sect. S2). Reference compounds having different types of oxygenated functional
group are also plotted. The ternary diagram groups different types of
oxygenated compounds into clusters. The individual products from
photooxidation of toluene identified by gas chromatography/mass spectrometry
(GC/MS) are plotted for comparison (Forstner et al., 1997).
The cluster representing the toluene- and m-xylene-derived SOMs is uniquely
situated and differentiated from the reference compounds because of the C–O
stretch at 1000–1260 cm-1.
This absorption band at 1000–1260 cm-1 is plausibly contributed by an
ether group (C–O–C) of acetals and hemiacetals produced via particle-phase
reactions (Jang et al., 2002; Kroll and Seinfeld, 2008; Lim et al., 2010).
These reactions tend to drive product distribution toward the C–O vertex of
the composition diagram (cf. arrow in Fig. 7). The gas-phase oxidation of
aromatic precursors produces dialdehydes in high yields, including glyoxal
and methylglyoxal. These dialdehydes readily oligomerize along hemiacetal and
acetal pathways, with associated changes in the C–O / C = O stretch
band ratio (Loeffler et al., 2006). Hemiacetal / acetal production
reactions leading to oligomerization can occur in SOM produced by
photooxidation of trimethylbenzene (TMB), even in the absence of catalysis by
sulfuric acid (Kalberer et al., 2004).
The mole fraction of each functional group is estimated using the
absorptivity of Russell et al. (2009b) and Takahama et
al. (2012), along with the area ratios of ether (C–O–C) to
alkane (C–H) bands for 19 ether and acetal compounds appearing in the
NIST (National Institute of Standards and Technology)
database. The analysis concludes that ether groups constitute up to 50 %
of the SOM mass. This result agrees with a modeling study suggesting that
20–80 % of the SOM derived from toluene is produced by particle-phase
reactions (Cao and Jang, 2009). These particle-phase reactions can
produce oligomers having conjugated structures that contribute to the light
absorption even in the absence of nitrogen moieties (Zhong et
al., 2012).
Atmospheric implications
For the obtained spectral data sets of n and k (Sect. 3.1), the optical
effects of brown carbon (BrC) from anthropogenic SOM can be assessed. A
model case study is formulated to represent light-absorbing particles in a
fresh urban plume close to the anthropogenic sources. Parameters defining
the case study are listed in Table 2. The case study considers a population
of brown carbon particles produced by photooxidation of anthropogenic
aromatic precursors in the presence of NOx. This population is compared
with populations of black carbon (BC) particles (representing emissions from
fossil fuel combustion) and externally mixed at variable ratios with
ammonium sulfate particles (representing the regional background atmospheric
aerosol). The number–diameter distributions of the BrC and sulfate particle
populations are representative of polluted urban regions (Wu et al., 2008).
The BC particle population having a relatively smaller mode diameter is
typical for fresh soot particles emitted from motor vehicles (Kleeman et
al., 2000). The investigated ratios of BC and ammonium sulfate are
representative of Asian outflows (Ramana et al., 2010).
The external mixing assumption is consistent with the small absorption
enhancement of BC in urban regions (Cappa et al.,
2012). The single-scattering albedo ω, defined as the ratio of
scattering to total extinction, is calculated for each population and their
mixtures using a Mie-theory-based optical model (Bohren and Huffman, 1983;
Liu et al., 2013) (Fig. 8a). The relative contribution of BrC absorption to
total light absorption (i.e., BrC / (BrC + BC)) is calculated as a function
of the mass ratio of organic matter to BC. The calculated results for
λ=320, 405, and 550 nm are plotted in Fig. 8b. These three
wavelengths are selected because solar radiation in these bands respectively
regulates O3 photolysis, NO2 photolysis, and energy balance (cf.
Fig. 8a).
(a) Calculated single-scattering albedo of light absorbing
particles for a scenario of an urban plume. Cumulative distributions for
solar irradiance (orange) and photolysis rate coefficients (light blue) are
also shown. The k values of BrC were bounded by the cases for m-xylene + OH + NOx
(HC0 / NO0=4.0 ppbC ppbN-1) and the
toluene + OH + NOx (HC0 / NO0=3.5 ppbC ppbN-1),
as shown in Fig. 4. The cases for “BC + sulfate” represent
a BC population externally mixed with an ammonium sulfate population at
variable mixing ratios representative of typical ambient values in a
pollution plume (Ramana et al., 2010). Table 2 lists the physical parameters
used for modeling the several different particle populations. The cumulative
distributions were calculated for a standard solar spectrum of Air Mass 1.5 (http://rredc.nrel.gov/solar/spectra/am1.5/). (b)
Contribution of BrC absorption to total absorption as a function of the mass
ratio of organic matter to black carbon. The BrC cases were the same as those
shown in (a). External mixing of BrC and BC populations was
assumed in the calculation.
Results of the case study have several implications for climate and
atmospheric chemistry modeling. The ω values of the BrC particle
populations are close to unity for λ > 500 nm
(Fig. 8a). When externally mixed with BC, the studied BrC has a negligible
contribution to light absorption at 550 nm (Fig. 8b). These results indicate
that these BrC populations have a net cooling effect. The ω values,
however, decrease below unity for λ < 400 nm, meaning
that the particle population becomes absorptive in the UV region. Although
the solar irradiance in the UV region contributes only 10 % of the total
solar irradiation, meaning a small heating effect by brown carbon for the
conditions of the case study, the effect can still be important because UV
irradiance determines the photolysis rates of many chemical species
(Fig. 8a). For example, reduced UV irradiance for λ < 320 nm slows ozone photolysis, thus suppressing the production
of OH radicals (Martin et al., 2003; Tie et al., 2003). The case study
suggests that BrC populations can have a substantial contribution for light
absorption in this band (Fig. 8b). For a mass ratio of organic matter to BC
in a range of 2 to 20, which is typical for urban atmosphere (Turpin et al.,
1991), BrC accounts for 15–80 % of the UV absorption at 320 nm. The
photolysis of NO2 is similarly suppressed by reduced UV irradiance for
λ < 405 nm, thus inhibiting the production of ozone
(Dickerson et al., 1997; Martin et al., 2003). The implication is that, as an
effective UV absorber, BrC influences the production of O3 and OH by
reducing UV irradiance and consequently affects the oxidation capacity of the
regional atmosphere.
In conclusion, photooxidation of toluene and m-xylene in the presence of
NOx can produce SOMs having k values similar to those reported
for brown carbon in biomass burning and urban plumes (Kirchstetter et al.,
2004; Hoffer et al., 2006; Alexander et al., 2008; Dinar et al., 2008;
Chakrabarty et al., 2010; Cappa et al., 2012; Lack et al., 2013). The
implication is that the photooxidation of anthropogenic precursors can be a
significant source of atmospheric brown carbon. These findings are consistent
with atmospheric observations in urban regions, such as the Los Angeles basin
(Zhang et al., 2011, 2013; Cappa et al., 2012), Mexico City (Barnard et al.,
2008), and Beijing (Cheng et al., 2011). The case studies considered in the
present study suggest that anthropogenic brown carbon, along with brown
carbon from biomass burning, can have a major influence on light absorption
at wavelengths that drive photochemical reactions. This effect needs to be
evaluated in the future modeling studies of atmospheric chemistry.