AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-5331-2016A novel single-cavity three-wavelength photoacoustic spectrometer for
atmospheric aerosol researchLinkeClaudiaclaudia.linke@kit.eduIbrahimInasSchleicherNinaHitzenbergerReginaAndreaeMeinrat O.https://orcid.org/0000-0003-1968-7925LeisnerThomasSchnaiterMartinhttps://orcid.org/0000-0002-9560-8072Institute of Meteorology and Climate Research, Atmospheric Aerosol
Research, KIT, Karlsruhe, GermanyInstitute of Geography and Geoecology, KIT, Karlsruhe, GermanyUniversity of Vienna, Faculty of Physics, Vienna, AustriaBiogeochemistry Department, Max Planck Institute for Chemistry, Mainz,
GermanyClaudia Linke (claudia.linke@kit.edu)7November2016911533153468March201615March201620September20163October2016This 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://amt.copernicus.org/articles/9/5331/2016/amt-9-5331-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/5331/2016/amt-9-5331-2016.pdf
The spectral light-absorbing behavior of carbonaceous
aerosols varies depending on the chemical composition and structure of the
particles. A new single-cavity three-wavelength photoacoustic spectrometer
was developed and characterized for measuring absorption coefficients at
three wavelengths across the visible spectral range. In laboratory studies,
several types of soot with different organic content were generated by a
diffusion flame burner and were investigated for changes in mass-specific
absorption cross section (MAC) values, absorption and scattering Ångström
exponents (αabs and αsca), and single scattering
albedo (ω). By increasing the organic carbonaceous (OC) content of
the aerosol from 50 to 90 % of the total carbonaceous mass, for 660 nm
nearly no change of MAC was found with increasing OC content. In contrast,
for 532 nm a significant increase, and for 445 nm a strong increase of MAC was found
with increasing OC content of the aerosol. Depending on the OC content, the
Ångström exponents of absorption and scattering as well as the
single scattering albedo increased. These laboratory results were compared
to a field study at a traffic-dominated urban site, which was also
influenced by residential wood combustion. For this site a daily average
value of αabs(445–660) of 1.9 was found.
Introduction
Carbonaceous particulate matter is a unique component of the atmospheric
aerosol because of its absorption ability and large chemical variability
(IPCC, 2013). Its presence in the atmosphere causes direct and indirect
climatic effects. The ability to absorb and scatter sunlight over a wide
spectral range directly affects Earth's radiative balance. Indirectly, the
presence of aerosol particles influences the lifetime of clouds and their
precipitation properties (Rosenfeld et al., 2008; Sun et al., 2007).
In climate modeling, carbonaceous aerosols are represented as black carbon
(BC) and organic carbon (OC; ICCP, 2013). In contrast to BC, which is a
strongly light-absorbing material, most models assume OC to be a
nonabsorbing material (Kameel et al., 2014; Andreae and Ramanathan, 2013;
Chung et al., 2012). Based on these assumptions, BC contributes by a rather
strong positive forcing and OC by a negative forcing to the overall
radiative forcing by carbonaceous aerosols, which is then estimated to be
close to zero (IPCC, 2013).
While BC is of deep black appearance, there are further carbonaceous
compounds that contribute to atmospheric aerosol absorption. Humic-like
substances (HULIS) or brown carbon (brC) are of yellow or brownish color.
Even though these compounds are only weak absorbers in comparison to BC,
their steeply increasing light absorption towards the ultraviolet (UV) spectrum, and their
atmospheric abundance might result in a significant contribution to the
shortwave absorption by aerosols in the atmosphere (Kirchstetter et al.,
2012; Chakrabarty et al., 2010; Hoffer et al., 2006; Andreae and
Gelencsér, 2006).
The formation of absorbing organic aerosol particles can occur through
different pathways. Chemical and photochemical processes as well as
coagulation and oligomerization potentially end up in macromolecular
structures. Such chemical systems will form multiple conjugated double and
aromatic bonds as well as polyfunctional groups, which build light-absorbing
chromophoric structures. These chromophoric structures are responsible for
the wavelength-dependent light absorption by organic species in the visible
(VIS) and near-UV spectral range (Rincón et al., 2009; Prévôt et al.,
2009; Gelencsér et al., 2002).
The transition from volatile organic carbon (VOC) to semivolatile (SVOC),
low volatile (LVOC), and extremely low volatile (ELVOC) carbon is
continuous. Saleh et al. (2014) showed that for biomass burning emissions, an
increase in light absorptivity is linked to a decrease in volatility of the
organic compounds.
Emissions from biomass burning and to a large extent from fossil fuel and
biofuel combustion contain both black carbon and organic carbon. During
burning processes, carbonaceous aerosols are generated by incomplete
combustion of these fuels. The chemical composition of the resulting
particles is strongly dependent on the combustion conditions (Andreae and
Gelencsér, 2006). Particularly, biomass and wood burning is often
performed at low temperatures, resulting in high emissions of light-absorbing particulate matter (Kirchstetter et al., 2004; Sandradewi et al., 2008a). Moreover,
co-emitted primary organic matter of biomass burning emissions might
be weakly absorbing or even nonabsorbing, but it enhances the absorbing properties when
forming an internal mixture with BC. Therefore, the spectral light
absorption properties of such internally mixed carbonaceous particles may be
shifted toward the UV wavelength range (Lack et al., 2012).
Considering the diversity of possible sources for carbonaceous particulate
matter in the atmosphere, which covers the whole range from biogenic
emissions over wildfires to residential heating, it is clear that the
contribution of carbonaceous material to climate forcing on a global scale
still remains difficult to estimate (Kirchstetter et al., 2012; Bond et al.,
2013; Andreae and Ramanathan, 2013).
For BC, the imaginary part of the refractive index is nearly wavelength-independent over the visible and near-UV spectral range. In contrast, the
imaginary part of the refractive index of brown carbon (brC) continuously
increases from the red over the blue to at least the near-UV spectral range.
(Moosmüller et al., 2009; Schnaiter et al., 2006). The small size of the
particles and their spectral refractive index induce a wavelength dependence
of the aerosol absorption coefficient, which is characterized by the
absorption Ångström exponent, αabs. While, in the case
of BC, the absorption coefficient is only slightly increasing with
decreasing wavelength, which results in αabs close to 1.0
(Lewis et al., 2008; Kirchstetter et al., 2004; Schnaiter et al., 2003), brC
can have a significantly higher absorption coefficient in the blue and
near-UV spectral range, resulting in an αabs higher than 1. From
laboratory (McMeeking et al., 2014; Schnaiter et al., 2006) and field studies
(Kirchstetter and Thatcher, 2012; Rizzo et al., 2011; Sandradevi et al., 2008a), there is evidence that for brC, the αabs is much larger than
1.0.
McMeeking et al. (2014) reported values for the absorption Ångström
exponent αabs between 1.5 and 7 from laboratory biomass
burning experiments using a variety of different fuels. For carbon particles
generated in a diffusion flame under different burning conditions, Schnaiter
et al. (2006) determined αabs values in the range of 1 to 7.
Kirchstetter and Thatcher (2012) sampled particulate matter during wintertime in a rural region in
California, where residential heating was mainly done by wood burning. From their samples, which they collected during
evening and nighttime, they derived αabs values between 3.0 and
7.4. Sandradevi et al. (2008a) investigated the spectral aerosol absorption
in a small village in Switzerland, where the majority of the households also
used wood burning for heating in winter. For this village, they found values
for αabs of 1.8–1.9. Nakayama et al. (2014) showed temporal variations for αabs between
summer and winter of 1.21 and 1.43, respectively, for the city of Nagoya, Japan. For measurements in the
Amazon basin during the dry season, Rizzo et al. (2011) reported αabs values between 1.5 and 2.5 resulting from light-absorbing
organic carbon released from biomass burning.
Despite their significance, the light absorption properties of carbonaceous
aerosol are poorly investigated, mostly due to lack of appropriate
instrumentation (Schmid et al., 2006). Considering the organic mass fraction,
which is the dominating component of carbonaceous aerosol in the atmosphere,
even a small contribution of organic matter to light absorption may result
in a significant absorption of solar radiation (Kirchstetter and Thatcher,
2012; Hoffer et al., 2006).
For in situ absorption measurements, different kinds of instruments are
used. Most in situ measurements are performed using filter-based methods,
where the aerosol particles are deposited on a filter. This works well for
strongly absorbing aerosols. For aerosols with higher organic content and,
therefore, with stronger scattering properties, filter-based absorption
methods raise experimental uncertainties (Lack et al., 2008; Schnaiter et al.,
2005; Weingartner et al., 2003; Bond et al., 1999; Hitzenberger, 1993).
The deposition of the aerosol on a filter may change the physical properties
of the particles and the combined optical properties of the deposit and
filter substrate (Collaud Coen et al., 2010; Lack et al., 2008). Additionally,
filter-based methods require various corrections for multiple scattering on
the filter substrate, for backscattering from the deposited particles, and
for the reduction of scattering with increasing particle load (Chow et al.,
2009). This leads to large uncertainties, especially for atmospheric
measurements where the aerosol optical properties are dominated by
scattering. Under these conditions, an accurate absorption measurement using
filter-based methods is limited by the strong cross-sensitivity of the
method to the aerosol light scattering.
Unlike these filter-based instruments, the photoacoustic method directly
determines the light absorption of aerosol particles in the airborne state.
Due to the fact that the photoacoustic method is specific to the absorption
properties of the particles there is no cross-sensitivity to particle light
scattering, which makes this method ideally suited for atmospheric
measurements (Rosencwaig, 1975; Miklós et al., 2001). However, despite
these advantages of a direct absorption measurement, during field
applications the presence of ambient trace gases, like NO2, ozone, or
water vapor can influence the photoacoustic measurement (Arnott et al.,
1999). Especially the impact of high relative humidities on the
photoacoustic measurement has to be considered (Arnott et al., 2003; Raspet et al., 2003).
Setup of the three-wavelength photoacoustic spectrometer.
In this study, we present a novel single-cavity photoacoustic spectrometer
that measures the absorption coefficient of atmospheric aerosols at three
wavelengths in the visible spectral range. In Sect. 2, we describe the
instrument setup and discuss the instrument characterization in terms of
laser power stability, linearity of the electronic setup, and detection
limit together with calibration results. A laboratory study is presented in
Sect. 3, where combustion emissions from a propane flame were used as
surrogates for atmospheric combustion particles with different organic
content. Finally, the instrument was applied in an urban field study in
Karlsruhe, Germany. The wavelength-dependent absorption properties measured
in this study are presented in Sect. 4. The laboratory and field studies
have shown a reliable application of the novel photoacoustic spectrometer
for ambient aerosol measurements.
Instrument setup
The absorption measurement by the photoacoustic method is initiated by a
modulated laser. Due to the absorption of light by the particles within the
laser beam, the carbonaceous material gets energetically excited and,
consequently, heats up. The deposited thermal energy is subsequently
released into the surrounding gas. Due to the periodic thermal expansion
caused by the laser modulation, a pressure or sound wave is generated in the
gas. This sound wave is amplified in an acoustic resonator and is detected
by a microphone. The acoustic resonator and the microphone form the
so-called photoacoustic cell (Rosencwaig, 1975). The photoacoustic signal,
PA, generated by the photoacoustic cell, can be expressed as follows:
PA=babs×Plaser×ccell.
According to Eq. (1), PA (V) at the resonance frequency of the cell
is determined by the aerosol light absorption coefficient babs
(m-1), the laser power Plaser (W), and a cell constant ccell
(V W-1 m).
The cell constant ccell is a function of the overall geometric setup of
the laser, the microphone, and the resonance cell (Miklós et al., 2001).
The value of ccell must be determined by calibrating the system with a
material of known absorption cross section.
The setup of the newly developed photoacoustic instrument is shown in Fig. 1. Three-wavelength light absorption and heating of the sample in the
photoacoustic cell is induced by three successively irradiating laser beams.
Three diode laser modules (Qioptiq Nano 250 series), emitting at 445, 532, and 660 nm, are tuned to the resonance frequency of the cell. During
operation, the laser powers are 150, 100, and 50 mW, respectively. The
three laser beams are merged with the optical axis of the photoacoustic cell
by beam splitters (DLMP 425, DLMP 505, Thorlabs). The laser power is
continuously monitored by a power meter (PowerMax, Coherent) at the exit of
the resonance cell. The photoacoustic cell was manufactured by HiLase Ltd.,
Hungary, according to our specifications, and was modified by replacing the
original windows by windows with specific broadband anti-reflection
coatings. A resonance frequency of 1930 Hz was determined for the cell at a
temperature of 25 ∘C. The actual acoustic resonator is a pipe with
open ends, with a length of 90 mm, a diameter of 8.5 mm, and a volume of
5.1 cm3. The length of the pipe corresponds to λ/2 of the
resonance frequency of the cell. With these geometric specifications, the
pipe works as an open acoustic resonator for the generation of longitudinal
acoustic modes. Acoustic buffers are implemented on both sides of the
resonator, each with a length of λ/4 and a diameter of 60 mm,
resulting in a buffer volume of about 130 cm3. The length of the
buffers and the sharp enlargement of the diameter between the resonator pipe
and the buffer volume ensure (i) optimal back reflection of sound from
inside the acoustic resonator, and (ii) resonance frequency filtering of
noise from outside the resonator by destructive interference in the buffers.
The buffers are terminated by optical windows, which seal the cell against
the surrounding gas. Windows with broadband anti-reflection coatings are
used to minimize residual reflections of the laser beam when entering and
exiting the resonance cell.
The aerosol enters and exits the cell through the buffers, which results in a
90∘ bend in the aerosol flow at the transition points from the
buffer volumes to the acoustic resonator. A continuous aerosol flow of 1 std. liter per minute through the cell is maintained by a mass flow
controller (Mykrolis, Tylon 2900 series). At this flow rate, the calculated
cell volume of about 265 cm3 is exchanged around three to four times a minute.
Additionally, λ/4 notch filters are installed in line of the
aerosol tube, to prevent flow noise from entering the resonator. The
calculated aerosol transmission efficiency through the cell including the
acoustic notch filters is 97 % for particles with a size of 1 µm and
a density of 1.8 g cm-3.
Test of the linearity response of the photoacoustic detection
system with laser power.
The microphone is connected to the resonator pipe through a small hole of
5 mm diameter close to the amplitude node of the fundamental acoustic mode
of the cell, i.e., at the center of the pipe. In this way, the
photoacoustically generated wave is detected with high sensitivity by a
commercial hearing aid microphone (EK-3029, Knowles). This specific
microphone was chosen for effective signal detection at resonance
frequencies above 1000 Hz.
The microphone signal is fed to a lock-in amplifier (LIA-BVD-150-L, Femto).
This lock-in amplifier is a two-phase instrument, for phase sensitive
detection, which provides signal X, magnitude R, and phase-shift Y with
respect to the phase of the incident laser modulation frequency.
Suitable lock-in parameters for the amplification and filter characteristics
can be set for the input and output amplifier. The signal-to-noise ratio can
be optimized by an additional low pass filter with adjustable time constant.
To determine the absorption by the aerosol particles, periodic background
measurements are performed by passing the aerosol through a particle filter,
which effectively removes the particles. The complete exchange of the gas in
the cell volume between the background and aerosol measurements defines the
minimum time that is required for the absorption measurement.
Cell constant determined for the lock-in sensitivity 6 from
calibration measurement with NO2 at 445 and 532 nm. Note that some
of the 532 nm measurements were performed at sensitivity 7 and were
subsequently rescaled to sensitivity 6.
From Eq. (1) it is obvious that for accurate measurements a reliable
control of the laser power during electronic recording is essential. The
laser power stability for an operation period of 8 h at room temperature
was tested and showed maximum deviations from the specified power of less
than 2 % for all lasers. In detail, at 445 nm a laser power stability of
152 mW ± 0.2 mW, at 532 nm of 105 mW ± 0.4 mW, and at 660 nm of
52 mW ± 0.6 mW was found over this operation period. The linearity of
the electronic detection and amplification system was checked by gradually
decreasing the laser power, while the cell was continuously flushed with
nitrogen dioxide (NO2) gas (10 ppm, Air Liquide). Figure 2 shows the
result for the 445, 532, and 660 nm wavelengths. The tested power
range was up to 150 mW for 445 nm, 100 mW for 532 nm, and 50 mW for 660 nm.
The power measurements for the three lasers exhibit different slopes for
each wavelength. This is attributed to the wavelength-dependent
absorption coefficient of NO2, resulting in specific photoacoustic
signals, which increase from 660 to 445 nm as shown in Fig. 2.
Furthermore, the photoacoustic response behavior shows high linearity with
laser power for all three laser wavelengths. However, variations in the
laser power during long-term measurements can directly affect the
photoacoustic signal. These errors are reduced by simultaneously measuring
the photoacoustic signal and the laser power.
The instrument setup has to be calibrated by a substance of known absorption
cross section to determine the correlation between a given absorption
coefficient and the measured photoacoustic signal in order to specify the
cell constant of the setup (cf. Eq. 1). For this purpose, light-absorbing
nitrogen dioxide (NO2) was used as calibration gas. Absorption cross
sections for nitrogen dioxide are published by Voigt et al. (2002) over a
broad wavelength range and for 293 K and 1000 mbar temperature and total
pressure, respectively. The derived absorption cross sections of NO2
are 1.65 × 10-6 for 445 nm, 3.56 × 10-7 for
532 nm, and 1.75 × 10-8 m-1 ppb-1 for 660 nm. The measuring range of
the lock-in amplifier can be regulated by adjustment of the gain settings.
To determine cell constants at different gains, certified gas mixtures of
NO2 in synthetic air (Crystal-Mix, Air Liquide) of 5, 1, or 0.2 ppm were used. Based on the certified gas mixtures, different NO2 gas
concentrations are generated by diluting and mixing NO2 standards with
synthetic air. For each gain setting a distinct cell constant is determined.
In this study, the cell constants for the amplifier gain settings, i.e.,
lock-in sensitivities 6 and 7, were determined. In Fig. 3, the cell
constant for the lock-in sensitivity 6 of 78 518 ± 1123 V W-1 m is
derived from calibration measurements at 532 (green squares) and 445 nm
(blue squares). The concentrations of the deployed NO2 calibration gas
mixtures, indicated at the top axis of Fig. 3, vary from 80 up to 1000 ppb NO2. On the bottom axis the corresponding absorption coefficients
for NO2 are plotted, calculated from the absorption cross sections for
NO2 (Voigt et al., 2002). The cell constant is then calculated from the
correlation slope between photoacoustic signal, laser power, and absorption
coefficient according to Eq. (1). For the calibration at 532 nm, about one-third of the measurements were originally done at sensitivity 7 and were
subsequently rescaled to sensitivity 6 by applying the gain specifications
given by the manufacturer.
Comparison of two photoacoustic instruments (DRI, PAS KIT) for
CAST soot C / O 0.29 at 532 nm.
We compared our novel three-wavelength instrument with a 532 nm
photoacoustic instrument of the Max Planck Institute for Chemistry, Mainz,
Germany, which had been originally developed by Desert Research Institute,
Reno, USA (Arnott et al., 1999). In Fig. 4, absorption coefficients at 532 nm measured with both instruments are compared for an aerosol chamber
experiment with BC aerosol from the Combustion Aerosol Standard (CAST)
generator (see Sect. 3 for details on the CAST generator and the chamber
experiments).
During the experiment both instruments measured absorption coefficients
between 2 × 10-5 and 1.7 × 10-4 m-1. The slope of the
regression line is 1.03 ± 0.02, and the coefficient of determination,
R2= 0.98, indicates good agreement of both instruments at 532 nm over
the measured range. Given that two different instruments with different
laser and detection systems were compared, this is a very convincing result.
During the laboratory chamber studies the RH was always below 30 %.
During field measurements there was a silica gel dryer installed upstream of
the PA and nephelometer sampling line, which confined the RH to below 60 % throughout the campaign.
The limit of detection (LOD) of the photoacoustic instrument was determined
from 4 different days of field measurements by analyzing 10 background
(particle-free ambient air) measurements for each wavelength. The LOD was
then calculated according to the German standard DIN 32645:
LOD=baBG‾+3×σBG,
with bBG the mean background absorption coefficient and σBG the corresponding standard deviation. LOD values
of 5.6 × 10-6, 6.6 × 10-6, and 1.8 × 10-5 m-1 were deduced
for the wavelengths 445, 532, and 660 nm, respectively.
Comparison of mass concentrations determined for two experiments
in the SOOT11 campaign by SP-2, filter sampling, and 1-λ-PAS at 532 nm measurements.
Laboratory studies on CAST soot of different C / O fuel ratios
Two sets of experiments were performed at the aerosol and cloud chamber
facility AIDA. One set was conducted during the SOOT11 campaign in 2010. The
second set of experiments was done during the SOOT15 campaign in June 2013.
While the SOOT11 campaign took place at the 84 m3 sized AIDA chamber
(Wagner et al., 2009) SOOT15 was performed at the smaller stainless steel
chamber NAUA with a volume of 3.7 m3 (Schnaiter et al., 2006; Linke et al., 2006).
Before each experiment, the chamber was evacuated, flushed, and refilled with
particle-free synthetic air. Combustion Aerosol Standard (CAST) aerosol,
generated by a co-flow diffusion burner (miniCAST Series 5200, Jing Ltd.,
Switzerland), was filled into the clean chamber. The miniCAST generator is a
compact version of the first instrument series that had been used in the
soot absorption study by Schnaiter et al. (2006). Further details on the
operation of the burner together with a characterization of the particle
emission can be found there. Note that due to mechanical design changes, the
composition characteristics of the emitted particles found by Schnaiter et al. (2006) changed for the miniCAST version. After the aerosol addition
to the NAUA chamber, the aerosol was continuously stirred by a mixing fan to
ensure homogeneous gas and particle conditions throughout the chamber
volume. Stainless steel tubes of 6 mm diameter were used for aerosol
sampling. The aerosol flow through the connected photoacoustic instruments,
an integrating nephelometer (TSI, model 3563), and a condensation particle
counter (CPC; TSI, model 3022A) was 1, 5, and
1.5 L min-1, respectively.
The miniCAST burner was operated with a mixture of propane and synthetic
air. Different mass ratios of fuel to air were adjusted to operate the
burner at different burning conditions. In this way, the ratio of elemental
carbon (EC) to organic carbon (OC) of the generated carbonaceous aerosol
could be altered over a broad range (Schnaiter et al., 2006). At low C / O
ratios the fraction of EC is dominating in the aerosol generated by the
burner. When the C / O ratios increase, the fraction of EC is reduced while
the OC content of the generated aerosol increases. In our studies the EC
content varied between 10 and 50 % of the total carbon (TC) mass.
Comparison of MAC determined at 532nm during two campaigns – the
method-specific carbonaceous mass that the MAC is related to is given by the
abbreviations EC and TC (Sunset) as well as rBC (SP-2).
SOOT 11 SOOT 15 DRI-1-λ-PAS KIT-3-λ-PAS CAST sootC / O 0.29C / O 0.29C / O 0.29C / O 0.29C / O 0.29m2 g-1m2 g-1m2 g-1m2 g-1m2 g-114.5 ± 1.65.4 ± 0.812.5–16.4 ± 0.4 MAC-ECMAC-TCMAC-rBCMAC-rBCLaborde et al. (2012)CAST sootC / O 0.4C / O 0.4C / O 0.4C / O 0.4C / O 0.418.6 ± 4.21.6 ± 0.1––21.0 ± 2.5MAC-ECMAC-TCMAC-rBC
The TC, EC, and OC concentrations of the aerosol samples were determined
from thermal analyses of particle-laden quartz fiber filters. For the filter
sampling, 47 mm diameter quartz fiber filters (Munktell MK360) were inserted
into a stainless steel filter holder (Sartorius) and connected directly to
the aerosol chamber. With the aid of a mass flow controller with a set flow
of 10 std. L min-1, a defined volume of aerosol was sampled from the
chamber through the filter. TC, EC, and OC analysis of the particle-laden
filters were performed using a Sunset OC / EC thermal analyzer (Sunset
Laboratory Inc., USA), applying the EUSAAR-2 temperature protocol (Cavalli
et al., 2010). During the thermo-optical measurement, the filter medium is
first heated stepwise to 600 ∘C in an inert helium atmosphere to
desorb OC, then cooled to 500 ∘C, and reheated in several
temperature steps to 800 ∘C under a helium/oxygen atmosphere to
oxidize EC. To correct for pyrolysis of OC in the inert atmosphere, which
would lead to a positive EC artifact, a laser signal is used as optical
control to determine the OC–EC split.
The mass concentration of refractory black carbon (rBC) in the carbonaceous
aerosol was determined by a single particle soot photometer SP-2 (DMT, USA).
The SP-2 utilizes incandescence of single rBC particles in a laser cavity to
quantify the particle mass down to the sub-femtogram level. The incandescence
signal of the instrument was calibrated with size-selected fullerene soot
particles (Alfa Aesar). For analysis of the SP-2 data we used the software
toolkit developed by Martin Gysel from the Paul Scherrer Institute,
Switzerland (http://aerosolsoftware.web.psi.ch/). Details on
the instrument characteristics, accuracy, and reproducibility as well as the
operation at the AIDA facility were described by Laborde et al. (2012).
Photoacoustic measurements during the SOOT11 campaign
During the SOOT11 campaign, miniCAST experiments with two different C / O fuel
ratios were performed. One experiment was done with a C / O ratio of 0.29,
which corresponds to an OC / TC ratio of about 60 %, and the second
experiment was conducted at a C / O ratio of 0.4, corresponding to an OC
fraction of about 90 %. At the beginning and the end of each experiment,
filter samples were taken for offline OC / EC analysis. Figure 5 shows mass
concentrations for both experiments determined by SP-2, filter sampling, and
1-λ-PAS measurements at 532 nm. Details of the instrumental setup
are given by Laborde et al. (2012). In SOOT11, only the single-wavelength
photoacoustic instrument from the Max Planck Institute for Chemistry was
available, which measured the absorption coefficient at 532 nm.
Mass-specific absorption cross section (MAC) values at 532 nm were deduced from
the photoacoustic absorption coefficients in conjunction with the EC values
obtained from the thermo-optical analysis. The MAC values (532) found for C / O
ratios of 0.29 and 0.4 were 14.5 ± 1.6 and 18.6 ± 4.2 m2 g-1, respectively (Table 1).
Optical properties resulting from different C / O ratios of miniCAST
soot during SOOT15.
* Filter samples were taken before optical measurement.
FF: incorrect filter sampling.
Alternatively, the MAC (532) can be determined by using the rBC mass
concentrations measured by the SP-2 instrument resulting in a MAC (532) for
the C / O ratio 0.29 of 12.5 m2 g-1 (Laborde et al., 2012). It was
not possible to determine the corresponding MAC (532) for the C / O ratio of
0.4 in the same way as the rBC concentration during this experiment was too
low and the SP-2 did not provide incandescence signals for this aerosol type
(Gysel et al., 2012).
Photoacoustic measurements during the SOOT15 campaign
During the SOOT15 campaign, miniCAST soot experiments at four different
burning conditions were conducted. The corresponding C / O ratios of 0.25,
0.29, 0.33, and 0.38 resulted in OC / TC ratios of the combustion aerosols
between 50 and 90 % (Table 2). At the beginning of each experiment, a
filter sample was taken for offline OC / EC analysis. The experimental setup
is shown in Fig. 6.
Particle number concentrations were measured (i) directly from the chamber
and (ii) after a dilution stage (PALAS, 3xVKL10). High particle number
concentrations in the chamber caused strong coagulation of the aerosol
during the first part of the experiment. Initial particle number
concentrations in the chamber varied between 3 × 104 and
8 × 104 cm-3.
The particle size distribution was measured with a scanning mobility
particle spectrometer (SMPS), composed of a differential mobility analyzer
(DMA 3071, TSI) and a condensation particle counter (CPC 3010, TSI). Due to
the aerosol preparation and coagulation in the chamber, the median particle
sizes measured in these experiments ranged from 80 to 250 nm.
For the optical characterization of the chamber aerosol, the absorption and
scattering coefficients were determined with our new three-wavelength
photoacoustic spectrometer in combination with a three-wavelength
integrating nephelometer (model 3563, TSI). Before the experiments, the
nephelometer was calibrated with CO2 and air. Details on nephelometer
operation at the AIDA facility, including a discussion of the error
corrections, are given by Schnaiter et al. (2005). Both instruments were
connected directly to the aerosol chamber. The scattering coefficients were
measured continuously. An automated pneumatic valve (Swagelok, Germany) was
used upstream of the inlet of the photoacoustic instrument to change between
particle and particle-free background measurements in periodic cycles of
about 7 min.
Optical properties of CAST soot
From the photoacoustic and integrating nephelometer measurements, spectrally
resolved absorption and scattering coefficients babs(λ) and
bsca(λ) were determined, and the following optical parameters
were derived:
Ångström exponents of absorption, αabs, and scattering,
αsca;
wavelength resolved single scattering albedo, ω(λ).
These parameters are summarized in Table 2 for the different miniCAST
aerosol types, and are discussed in more detail in the following sections.
Setup of instruments during the laboratory study SOOT15 and filter
samples of CAST soot C / O 0.29 (left) and C / O 0.38 (right).
Specific absorption cross sections
In this study, time-resolved mass-specific absorption cross section
(MAC) values (λ) were derived from the concurrent measurements of the
photoacoustic absorption coefficients and the rBC mass from the SP-2. The
filter sampling for EC / OC analysis could only be done at the beginning of
each experiment to determine the OC / TC fraction of the aerosol at each C / O
ratio of the miniCAST. In order to avoid perturbation of the aerosol
sampling during the optical measurements, no filter sampling was possible in
parallel with the experiments; therefore no comparisons of BC mass from SP-2
to EC mass from filter measurements are available for the SOOT15
experiments.
With increasing C / O fuel ratio in the flame of the miniCAST generator, the
organic content of the emitted carbonaceous aerosol rises. The OC fraction
in the total carbon aerosol mass at C / O ratio of 0.25 is about 50 % and
increases towards higher C / O ratio up to 90 % or more.
The color of the aerosol filter samples taken from the chamber already
indicated a change in the optical behavior of the emitted aerosol. While the
quartz filters were deep black at C / O ratios of 0.25 and 0.29, the filter
sample at 0.38 was of brownish appearance (Fig. 6).
Setup of instruments during field study.
The MAC at 660 nm, which was the longest wavelength measured here,
remained almost constant with increasing organic content of the aerosol
(Table 2). In contrast, the MAC at 532 nm clearly increased with increasing
C / O ratio from 10.4 ± 2.3 to 21.0 ± 2.5 m2 g-1. An
even stronger functional dependence was observed for the MAC determined for
the shortest investigated wavelength of 445 nm. For this wavelength, the MAC
increased from 12.5 ± 1.7 m2 g-1 at a C / O ratio of 0.25 to
31.0 ± 5.9 m2 g-1 at a C / O ratio of 0.38. Here, the MAC(445
nm) of the carbonaceous aerosol with a OC content of about 90 % was 2.5
times higher than the corresponding MAC value of the aerosol with an OC
content of only 50 %. As the MAC in this case is based on the SP-2 rBC
mass measurement, this result clearly shows that the OC material that was
co-emitted with the BC must have a non-negligible absorption cross section
with a strong wavelength dependence.
The results of the SOOT15 campaign were in agreement with the results of the
SOOT11 study, where the absorption coefficients as well as the carbonaceous
content of the samples were determined with partly different instrumentation
at C / O ratios of 0.29 and 0.4, but only at 532 nm.
In SOOT15, the determined value of the MAC (532) at a C / O ratio of 0.29 was
16.4 ± 0.4 m2 g-1 (Table 2). This fits quite well with measurements
done during the SOOT11 campaign, where we determined a MAC (532) of 14.5 ± 1.6 m2 g-1 with the 1 λ-PAS (DRI) and
thermo-optical EC (Table 1). When comparing the MAC (532) value for C / O 0.29
of SOOT15 with the corresponding MAC (532) value of 12.5 m2 g-1
from SOOT11 that was based on the SP-2 incandescence measurement (Laborde et al., 2012), both values agree within a range of about 30 %. Due to the high
organic carbon content of around 90 %, the CAST aerosol produced at a C / O
ratio of 0.38 in SOOT15 should be comparable to the corresponding aerosol in
SOOT11, which was produced at a C / O ratio of 0.4. For SOOT15 we derived a
MAC (532) value of 21.0 ± 2.5 m2 g-1 for this aerosol
type. For SOOT11 the corresponding value was 18.6 ± 4.2 m2 g-1. Both values again agree nicely, given the fact that they were
measured with different instruments. A reliable SP-2 incandescence
measurement at these high C / O ratios was found to be impossible, and for this
reason no rBC mass-specific absorption cross section could be specified.
SP-2 incandescence mass concentrations (black line) and
photoacoustic absorption coefficients (blue, green, red squares) for 1 day
in fall 2012, Karlsruhe, Germany.
Ångström exponents
The Ångström exponents of the absorption and scattering coefficients
were deduced by fitting a power law function to the three measured
coefficients:
logbabs,sca(λ2)=αabs,sca×log(λ2/λ1)+logbabs,sca(λ1).
Carbonaceous material with αabs of unity only shows a slight
spectral behavior as described for BC in the UV–VIS spectral wavelength
range by Schnaiter et al. (2003) and Moosmüller (2009). For the miniCAST
burner, a fuel-to-air ratio with C / O of 0.29 is close to the conditions for
the stoichiometric combustion. For this C / O ratio, we derived the lowest
values for the Ångström exponents αabs and αsca of 1.3 ± 0.2 and 2.2 ± 0.02, respectively (Table 2).
The αabs value of 1.3 is rather low and represents a weak
wavelength dependence. With increasing organic content in the soot samples,
both Ångström exponents clearly increase, reflecting a steeper
wavelength dependence of both coefficients, babs(λ) and
bsca(λ). At a C / O ratio of 0.38 with an organic content of
almost 90 %, the Ångström exponent αabs reached 3.1 ± 1.0, while αsca increased to 3.4 ± 0.1.
Single scattering albedo
The single scattering albedo ω is defined as the ratio of the light
scattering coefficient to the extinction coefficient, i.e., the sum of the
scattering and absorption coefficients. ω(λs) was
deduced from the measured scattering and absorption coefficients at the
wavelength positions λs of the integrating nephelometer. For
that, the absorption Ångström exponents αabs of Table 2
were used to inter- and extrapolate the photoacoustic absorption
coefficients measured at 445, 532, and 660 nm to absorption
coefficients at the nephelometer wavelengths, i.e., at 450, 550, and
700 nm.
After adjusting the absorption coefficients to the corresponding
nephelometer wavelengths the single scattering albedo of the aerosol at 450, 550, and 700 nm was calculated and is presented in Table 2.
The single scattering albedo ω(λ) increases with increasing
C / O ratio of the carbonaceous aerosol.
Number concentrations measured by CPC (purple line), SP-2 for all
scattering particles (gray line), and scattering particles without
incandescence (light blue line) compared to scattering coefficients from the nephelometer at three wavelengths.
Discussion of the chamber results
In the chamber experiments presented, we determined MAC values of combustion
aerosols with increasing organic content. It is important to note that the
CAST propane diffusion flame generator was used as a reproducible source for
a combustion aerosol analog. By changing the fuel-to-oxygen ratio, the
influence of incomplete combustion – manifested by increasing organic
carbon (OC) content – on the absorbing properties of the soot aerosol could
be systematically investigated. However, combustion OC material does not
necessarily represent all atmospheric OC compounds as most of the latter
material stems from secondary processes like the oxidation of volatile
organic compounds. This should be kept in mind when comparing our
laboratory results with results from field measurements. The rBC mass
measured by the SP-2 incandescence method was compared to the offline
elemental carbon (EC) and total carbon (TC) analysis results that were
obtained by the thermo-optical method. The experiments show that for
aerosols with higher organic content, the MAC of rBC (MAC-rBC) and EC
(MAC-EC) increases.
On the other hand, due to the increase in the OC mass, the MAC of TC
(MAC-TC) decreases with increasing C / O ratio. Schnaiter et al. (2006) found
MAC-TC with the predecessor version of the CAST burner of 5.5 ± 0.7 and 3.8 ± 0.5 m2 g-1 for combustion
aerosol produced with a C / O ratio of 0.29 and 0.4, respectively. Relating
the absorption coefficients measured at 532 nm during SOOT11 to the TC mass
concentration, MAC-TC values of 5.4 ± 0.8 and 1.6 ± 0.2 m2 g-1 are deduced for the C / O 0.29 and C / O 0.4
combustion aerosol, respectively. Comparing the MAC from both studies, the
values nicely agree for the C / O ratio of 0.29 but differ significantly for
the C / O ratio of 0.4. The latter discrepancy is due to the fact that
different CAST burner models were used. Schnaiter et al. (2006) used the
predecessor version of the CAST burner, which significantly differs in the
dimensions of the combustion chamber compared to the miniCAST 5200 burner
that was used in the SOOT11 and SOOT15 studies. Due to these differences,
the flaming conditions in both burner models are different, resulting in
different EC / OC vs. C / O characteristics for the emitted combustion
aerosol (see a comparison of both burners in Crawford et al., 2011), which
limits direct comparisons of the EC / OC ratios of the aerosol emitted for
similar C / O ratios. As shown by Crawford et al. (2011), combustion at a C / O
ratio of 0.4 in the miniCAST model produces aerosol with an EC / OC ratio that
corresponds to the aerosol emission for flaming conditions at C / O ratios
around 0.8 in the predecessor version of the CAST burner. Note that the
thermographic analysis of EC, TC, and OC in Schaiter et al. (2006) was done
according to the VDI method (Ulrich et al., 1990; Watson et al., 2005).
In a laboratory study by Kirchstetter and Novakov (2007), the MAC values of
BC aerosol generated with a diffusion flame were determined. The generated
BC particles, which were analyzed by a thermal optical analysis (TOA) method
described by Novakov (1981), contained no significant amounts of OC.
Kirchstetter and Novac used a Particle Soot Absorption Photometer (PSAP) to
measure the absorption coefficient at a wavelength of 530 nm. This
measurement was then related to the “BC” mass of the TOA method, or in
this case, because OC was negligible, the EC mass, to deduce a MAC of 8.5 m2 g-1 for the combustion aerosol. A single scattering albedo,
ω(550 nm), of 0.15 was determined from the PSAP data and
simultaneous measurements of the scattering coefficient with a single-wavelength integrating nephelometer. In our study, the BC aerosol with the
lowest OC content of 50 % (C / O = 0.25) resulted in a MAC-rBC at 532 nm of
10.4 ± 2.3 m2 g-1 and a ω at 532 nm of 0.12.
Comparing both results, it can be concluded that for 532 nm even a
significant increase in the OC content of the aerosol only results in a
moderate increase of the MAC. Even though these laboratory studies are not
directly comparable to atmospherically processed combustion emissions and,
furthermore, different methods were used in the laboratory and field
studies, it should be mentioned that comparable MAC values of BC have also
been obtained from atmospheric measurements. Kondo et al. (2009)
analyzed ambient aerosols at six rural and urban sites in Asia, which are
strongly impacted by vehicle and/or biomass burning emissions. For their
measurements they used two different filter-based photometers, a Particle
Soot Absorption Photometer (PSAP) and a Continuous Soot Monitoring System
(COSMOS). To remove the volatile aerosol components they used a heated inlet
system at temperatures of 400 ∘C before measuring BC on the
filter. Together with mass measurements (EC / OC analyzer, Sunset Laboratory)
they found MAC values at 565 nm of 10.5 ± 0.7 m2 g-1 for the remaining BC for samples of widely different BC sources.
The light absorption by organic species increases towards shorter
wavelengths. Lewis et al. (2008) investigated the optical properties of
different biomass burning aerosols during laboratory measurements in the
Fire Laboratory Missoula Experiment (FLAME). For different biomass types,
they determined the Ångström exponent, αabs, in the
wavelength range covered by their measurements (405, 532, and 870 nm).
From these experiments, they found that for several biomass fuels the span
of Ångström exponents is greater in the wavelength range between 405 and 870 nm than between 532 and 870 nm, and concluded that the particle
emissions from these fuels absorb near-UV radiation much more efficiently.
Similarly, in our measurements the αabs, determined between 445 and 660 nm, increased significantly with increasing organic content of
the aerosol (Table 1). While the carbonaceous aerosol with an organic content
of about 60 % has a moderate αabs of 1.3, the carbonaceous
aerosol with almost 90 % organics reaches αabs= 3.1. This increase reflects the stronger MAC increase at shorter
wavelengths in case of combustion aerosol with a high organic content (Table 1).
To estimate the contribution of brC to the total absorption at 445 nm for
the different C / O fuel ratios, we calculated the hypothetical fraction of
absorption by BC at 445 nm assuming (i) a αabs of 1.0 for BC in
all four experiments listed in Table 2 and (ii) the MAC-rBC at 660 nm to be only
due to BC. From the difference between the measured MAC-rBC at 445 nm
and MAC-rBC at 445 nm only calculated for the hypothetical BC fraction, the
fraction of absorption by brC in the light-absorbing mass can be estimated.
The fraction of absorption by brC during these experiments changes from
5.1 at C / O 0.25 to 55 % at C / O 0.38.
Urban field study
The three-wavelength photoacoustic spectrometer was operated for the first
time in the field during a campaign in October/November 2012. The measuring
site at Durlacher Tor is a central traffic junction in the urban area of
Karlsruhe, Germany. This site is generally characterized by high traffic
volume, but at the time of the campaign, intense construction work for a new
subway tunnel was taking place.
The measuring site was located next to a three-lane road and close to a
street crossing. The aerosol was sampled at a distance of 5 m from the road
and a height of 3 m above ground using a particulate matter head (PM2.5) (DPM2.5/1/00,
Fa.Digitel) with a sampling volume of 1 m3 h-1. The sampled
aerosol was led down to the instruments, which were located beneath the
sampling head inside a simple cabin with a power supply connection. The
aerosol inlet flow enters the cabin via a 14 mm diameter stainless steel
tubing. Within a flow splitter, 5 and 2.6 L min-1 were
taken isokinetically from the aerosol inlet flow. From the splitter, 5 L min-1 was led through the nephelometer and 2.6 L min-1 through
the joint flow line for the 3-λ-photoacoustic spectrometer with a
sample flow of 1 L min-1, the SP-2 with a sample flow of 0.12 L min-1, and the CPC (model 3775, TSI)
with a sample flow of 1.5 L min-1. Upstream of the 3-λ-photoacoustic spectrometer, a
diffusion dryer (filled with silica gel) was installed. The setup of the
instruments during the campaign is shown in Fig. 7.
Additionally, filter samples were taken with a second sampler unit using a
PM2.5 head and a sampling volume of 2.3 m3 h-1 on quartz filters
for offline thermo-optical EC / OC analysis. Each day the filter sampling
took place for 23.5 h from 14:00 to 13:30 local time of the next day.
The campaign started on 16 October and ended on 6 November 2012. During the
first 2 weeks the weather was sunny, with daytime temperatures around
15 ∘C and without any rain. On 27 October, the weather changed
considerably from warm and sunny fall weather to cold nights and cold but
sunny days, which also initiated the start of the home heating season.
Specific absorption cross section (MAC) values determined from
photoacoustic, SP-2 incandescence, and thermo-optical measurements for 31
October 2012 (24 h average) at Durlacher Tor, Karlsruhe.
λ (nm)445532660MAC (m2 g-1)12.9 ± 2.88.4 ± 3.17.5 ± 4.9(rBC SP-2)MAC (m2 g-1)11.6 ± 6.27.7 ± 3.56.9 ± 2.7(EC Sunset)Results from the urban field campaign
Here, we exemplarily show and discuss the measurement results for 31
October. In Fig. 8, the SP-2 mass measurements are shown (black line; left
axis) as refractory rBC mass concentration. Furthermore, the absorption
coefficients from the photoacoustic measurements are shown for the three
instrument wavelengths (blue, green, and red squares; right axis). The figure
shows a good correlation between rBC mass concentration and the absorption
coefficients. Typical diurnal variations in rBC mass concentration due to
rush hour traffic in the morning and traffic and domestic heating in the
evening can be observed.
Figure 9 presents the particle number concentration determined by CPC and
the number concentration of scattering particles deduced from the SP-2
measurements (left axis). While the gray line represents the number
concentration of all scattering particles, the light blue line only shows
the number concentration of those scattering particles that have no
incandescence signal, i.e., contain no rBC mass. The scattering coefficients
obtained from the nephelometer are shown for the three nephelometer
wavelengths (red, green, and blue; right axis). The temporal evolution of
the nephelometer data matches the measured number concentration of all
scattering particles (gray line), while there is only a weak correlation with
the number concentration of rBC-free scattering particles. This clearly
indicates that the light scattering coefficient was dominated by particles
that are linked to combustion processes (traffic or heating emissions).
We derived MAC-rBC(λ) values from the photoacoustic absorption
coefficients and SP-2 mass measurements. These values were averaged over a
time period of 24 h, starting at midnight. The absorption coefficients were
derived with a time resolution of almost 14 min. The time resolution of the
SP-2 incandescence measurement was matched to the time resolution of the
photoacoustic measurement. The absorption coefficients were then related to
the associated SP-2 rBC mass concentration in the corresponding time window.
Over one 24 h cycle, about 100 individual values for MAC-rBC(λ) were determined. As shown in Table 3, we obtained a MAC-rBC(660)
value of 7.5 ± 4.9 m2 g-1, a MAC-rBC(532) value of 8.4 ± 3.1 m2 g-1, and a MAC-rBC(445) value of
12.9 ± 2.8 m2 g-1.
Angström exponents of absorption, αabs, derived
for 31 October 2012 at Durlacher Tor, Karlsruhe.
Range λ1 to λ2445–532 nm532–660 nm445–660 nmαabs (24 h ave)2.6 ± 0.81.3 ± 0.61.9 ± 0.6αabs (20:00–22:00)2.9 ± 0.91.7 ± 0.52.3 ± 0.4
These data can be compared to the MAC-EC(λ) values derived by
relating the photoacoustic measurement to the EC mass concentration
determined by the thermo-optical method. For this, the absorption
coefficients were averaged over the 24 h time period of the filter sampling
and then related to the EC mass concentration obtained from offline filter
analysis. We obtained a MAC-EC(660) value of 6.9 ± 2.7 m2 g-1, a MAC-EC (532) value of 7.7 ± 3.5 m2 g-1, and a
MAC-EC (445) value of 11.6 ± 6.2 m2 g-1. The MAC(λ)
values determined from the SP-2 incandescence and the Sunset thermo-optical
analysis are in very good agreement, keeping in mind that two different
techniques were used to obtain the rBC and EC mass concentrations (Table 3).
To provide evidence that residential wood burning in the evening hours
increases the OC fraction in the carbonaceous aerosol and, consequently,
increases the shortwave MAC at this urban site, we compared average MAC
values for two time windows. For the traffic-dominated period, we chose the
time between 07:00 and 09:00 local time. For the period that was likely to
be influenced by residential wood burning, we selected the time between
20:00 and 22:00. The MAC values for the traffic-dominated period that we
found were MAC-rBC(445) = 10.8 ± 1.0, MAC-rBC(532)=7.0 ± 0.8, and MAC-rBC(660) = 5.9 ± 1.5 m2 g-1.
The averaged MAC values for evening hours were MAC-rBC(445) = 14.8 ± 4.2, MAC-rBC(532) = 8.6 ± 2.0, and
MAC-rBC(660) = 6.0 ± 1.7 m2 g-1, thus showing a clear
increase in MAC in the blue spectral range during the period influenced by
residential wood burning.
The Ångström exponents of absorption derived for 31 October are
shown in Table 4. Averaging the measurements over the 24 h period results in
an αabs value of 1.9 for the 445 to 660 nm spectral range.
When αabs was specifically calculated for the shortwave range
between 445 and 532 nm and the longwave range between 532 and 660 nm,
values for αabs(445–532) and αabs(532–660) of 2.6
and 1.3 were derived, respectively.
Single scattering albedo ω(λ) (nephelometer
wavelengths) and Scattering Angström Exponents, αsca,
derived for 31 October 2012 (24 h average) at Durlacher Tor, Karlsruhe.
In Table 5, the single scattering albedo, ω(λ), at the
three nephelometer wavelengths, and the scattering Ångström exponent
αsca are given for the measurements at Durlacher Tor. The values
for ω(λ) are in the range typical for atmospheric aerosols,
but are significantly higher than the values given in Table 2 for the
chamber experiments. The value of the scattering Ångström exponent,
αsca, is 1.7 ± 0.2, and is significantly lower than the
corresponding value deduced for all four chamber experiments (Table 2). A
possible explanation for these observations is that in contrast to the
laboratory-generated combustion aerosol, the aerosol at Durlacher Tor is
likely burdened with additional dust particles such as those released by the large
construction area.
Comparison of the field results with literature data
The MAC determined for freshly generated BC at 550 nm is in the range of 7.5 ± 1.2 m2 g-1 (Bond et al., 2013; Bond and
Bergström,
2006). These authors state that the MAC(3) will increase by up to 50 %
when the BC aerosol is internally mixed with other aerosol compounds. For
brown carbon, a weak light absorption with a MAC(3) of about 1 m2 g-1 is usually given in the literature (Bond et al., 2013).
From field studies, a range of different MAC(λ) was published
(Kondo et al., 2009; Knox et al., 2009; Laborde et al., 2013; Petzold et al.,
2002; Snyder and Schauer, 2007; Thompson et al., 2012). Additionally,
different protocols for the EC / OC mass analysis hamper the comparison of MAC
values given in the literature. Chan et al. (2011) reported MAC(781) values
for rural, suburban, and urban locations in Canada, which differ between 9 and
43 m2 g-1 when using the NIOSH 5040 protocol and between 6 and 27 m2 g-1 when using the IMPROVE protocol for the EC / OC mass
analysis. In our study, we used the EUSAAR 2 protocol, which was set up to
optimize charring corrections and to minimize biases in EC and split point
determination (Cavalli et al., 2010).
At 532 nm we derived MAC(532) values of 8.4 and 7.7 m2 g-1 when relating to SP-2 and filter-based mass measurements,
respectively (Table 3). Interpolating these values to 550 nm (with αabs(445–660) = 1.9) results in MAC(3) values of 7.9
and 7.2 m2 g-1, which is in very good agreement with the average
value of 7.5 ± 1.2 m2 g-1 derived by Bond and Bergström (2006) for freshly emitted BC.
Especially in the blue range of the visible spectrum there are fewer
literature data available for the MAC. From biomass burning experiments
during FLAME 3, McMeeking et al. (2014) reported an average MAC(405) of 9.8 ± 1.5 m2 g-1 determined from combined photoacoustic and
single-particle mass spectrometric measurements for heated and unheated
aerosol samples with minimal coating.
Healy et al. (2015) reported an average MAC(405) of 8.4 m2 g-1
for a traffic site in Toronto, Canada, from combined photoacoustic and
single-particle mass spectrometric measurements. Ueda et al. (2015) measured
soot-containing aerosols at the NOTO site in the city of Suzu, Japan. They reported
values of photoacoustic absorption coefficients and SP-2 mass concentrations,
which result in MAC(405) values between 11.1 and 15.8 m2 g-1.
Olson et al. (2015) reported MAC data in the wavelength range between 880
and 370 nm from various source emissions measured by an aethalometer, a
photoacoustic extinctiometer, and thermo-optical EC / OC mass analysis.
Three-wavelength absorption was measured for aerosol produced by different
fuels including wood, agricultural biomass, coals, plant matter, and
petroleum distillates in controlled combustion settings. They reported bulk
MAC (660/520/470/370) values for Diesel aerosol of 6.61/8.3/9.28/11.07 m2 g-1, for wood burning aerosol of
2.66/3.34/3.73/4.52 m2 g-1, and for pellets burning of 2.69/7.15/14/49.91 m2 g-1.
For our traffic-dominated, but residential-heating-influenced measurements,
we found MAC(445) between 11.6 and 12.9 m2 g-1
(Table 3), which is in the range of the literature data for fresh traffic
emissions. Calculating the difference between the determined MAC at 445 nm
of the whole aerosol and the MAC(445) calculated only for BC using an
αabs of 1.0 accounts for the fraction of brC. In this case the
calculated absorption fraction of brC is in the range between 11.8 and
13.8 %, which indicates that the aerosol absorption was dominated by
traffic emissions.
During biomass burning experiments, McMeeking et al. (2014) and Liu et al. (2014) showed that αabs is strongly related to ω(λ). McMeeking et al., found αabs of 1.5 to 7 and
ω(781) values of 0.4 to 1.0. For αabs around 2 they
found ω(781) below 0.8, while when αabs rapidly
increases up to 7, the ω(781) value approaches 1.0. These results
are in good agreement with our data given in Tables 4 and 5.
Chakrabarty et al. (2010) determined αabs and ω(λ) of tar balls from agricultural biomass combustion of duff. The
measurements result in αabs(405–532) of 6.4 and ω(405)
of 0.95, ω(532) of 0.98 and ω(780) of 0.98. As a possible
criterion for identifying brC, Chakrabarty et al. (2010) proposed a negative
Ångström exponent of ω(λ).
While Schnaiter et al. (2006) reported negative Ångström exponents
of ω(λ) for CAST soot experiments at high OC content,
neither during our current laboratory study nor at the field measurements
were negative Ångström exponents of ω(λ) found.
Conclusions
For an effective characterization of the optical properties of carbonaceous
aerosols, the accurate determination of spectrally resolved absorption
coefficients is essential. The discussion of light-absorbing organic species
makes a wavelength-dependent determination of the absorption coefficients
indispensable. Ideal measurements should cover the whole wavelength range
from the near-UV to visible to the near-infrared spectrum.
During chamber and field experiments, our novel single-cavity photoacoustic
spectrometer was found to accurately measure spectrally resolved absorption
coefficients at three wavelengths. The advantage of a single-cavity
instrument is that the cell constant can be successfully determined at
different wavelengths.
Different artificially generated carbonaceous aerosols with increasing
organic content were investigated. From these measurements, increasing
mass-specific absorption cross section (MAC) values were derived towards shorter
visible wavelengths. It was shown that the shortwave MAC strongly increases
with organic content of the aerosol. This sensitivity to the OC content of
the combustion aerosol was also reflected by the single scattering albedo,
ω(λ), and the absorption Ångström exponent, αabs, which both increase towards higher organic content of the
aerosol.
Comparing the laboratory-deduced MAC values with the values derived for
ambient carbonaceous aerosol measured at an urban field site, it became
obvious that the aerosol at the site was dominated by traffic emissions.
However, a specific investigation of the diurnal variations in the spectral
absorption measurements showed that the aerosol was influenced by
residential wood burning during the evening hours. These first field results
with our photoacoustic spectrometer are encouraging and strengthen the
necessity for long-term measurements of the spectral aerosol absorption in
urban environments.
Data availability
All the data presented in this study are available from the authors upon request.
Acknowledgements
This work was funded by the Helmholtz-Gemeinschaft Deutscher Forschungszentren as part of the
program “Atmosphere and Climate” and a start-up budget of the “KIT Kompetenzbereich Erde und Umwelt”. The SP-2 and the
development of the PAS were funded by the HGF Ausbauinvestition ATMONSYS. We thank the AIDA team and especially Georg Scheurig,
Thomasz Chudy, and Steffen Vogt for their technical support. We would like to thank Reiner Gebhardt (IfGG, KIT) who installed the
cabin and facilities at Durlacher Tor. We gratefully acknowledge Otmar Schmid (formerly Max Planck Institute for Chemistry),
Uli Pöschl (Max Planck Institute for Chemistry), and the Max Planck Society for their support.
The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
Edited by: A. Wiedensohler
Reviewed by: three anonymous referees
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