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Comparing black-carbon-and aerosol-absorption-measuring instruments -a new system using lab-generated soot coated with controlled amounts of secondary organic matter

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We report on an inter-comparison of black-carbon- and aerosol-absorption-measuring instruments with laboratory-generated soot particles coated with controlled amounts of secondary organic matter (SOM). The aerosol generation setup consisted of a miniCAST 5201 Type BC burner for the generation of soot particles and a new automated oxidation flow reactor based on the micro smog chamber (MSC) for the generation of SOM from the ozonolysis of α-pinene. A series of test aerosols was generated with elemental to total carbon (EC / TC) mass fraction ranging from about 90 % down to 10 % and single-scattering albedo (SSA at 637 nm) from almost 0 to about 0.7. A dual-spot Aethalometer AE33, a photoacoustic extinctiometer (PAX, 870 nm), a multi-angle absorption photometer (MAAP), a prototype photoacoustic instrument, and two prototype photo-thermal interferometers (PTAAM-2λ and MSPTI) were exposed to the test aerosols in parallel. Significant deviations in the response of the instruments were observed depending on the amount of secondary organic coating. We believe that the setup and methodology described in this study can easily be standardised and provide a straightforward and reproducible procedure for the inter-comparison and characterisation of both filter-based and in situ black-carbon-measuring (BC-measuring) instruments based on realistic test aerosols.
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Atmos. Meas. Tech., 15, 561–572, 2022
https://doi.org/10.5194/amt-15-561-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
Comparing black-carbon- and aerosol-absorption-measuring
instruments – a new system using lab-generated soot coated
with controlled amounts of secondary organic matter
Daniel M. Kalbermatter1, Griša Moˇ
cnik2,3,4, Luka Drinovec2,3,4, Bradley Visser5, Jannis Röhrbein5, Matthias Oscity5,
Ernest Weingartner5, Antti-Pekka Hyvärinen6, and Konstantina Vasilatou1
1Laboratory Particles and Aerosols, Federal Institute of Metrology METAS, Bern–Wabern, 3003, Switzerland
2Center for Atmospheric Research, University of Nova Gorica, Nova Gorica, 5270, Slovenia
3Haze Instruments d.o.o., Ljubljana, 1000, Slovenia
4Department of Condensed Matter Physics, Jozef Stefan Institute, Ljubljana, 1000, Slovenia
5Institute for Sensors and Electronics, University of Applied Sciences Northwestern Switzerland FHNW,
Windisch, 5210, Switzerland
6Atmospheric Composition Research Unit, Finnish Meteorological Institute, Helsinki, 00560, Finland
Correspondence: Konstantina Vasilatou (konstantina.vasilatou@metas.ch)
Received: 16 July 2021 – Discussion started: 21 July 2021
Revised: 29 November 2021 – Accepted: 13 December 2021 – Published: 1 February 2022
Abstract. We report on an inter-comparison of black-
carbon- and aerosol-absorption-measuring instruments with
laboratory-generated soot particles coated with controlled
amounts of secondary organic matter (SOM). The aerosol
generation setup consisted of a miniCAST 5201 Type BC
burner for the generation of soot particles and a new au-
tomated oxidation flow reactor based on the micro smog
chamber (MSC) for the generation of SOM from the ozonol-
ysis of α-pinene. A series of test aerosols was generated
with elemental to total carbon (EC /TC) mass fraction rang-
ing from about 90 % down to 10 % and single-scattering
albedo (SSA at 637 nm) from almost 0 to about 0.7. A
dual-spot Aethalometer AE33, a photoacoustic extinctiome-
ter (PAX, 870 nm), a multi-angle absorption photometer
(MAAP), a prototype photoacoustic instrument, and two
prototype photo-thermal interferometers (PTAAM-2λand
MSPTI) were exposed to the test aerosols in parallel. Signif-
icant deviations in the response of the instruments were ob-
served depending on the amount of secondary organic coat-
ing. We believe that the setup and methodology described in
this study can easily be standardised and provide a straight-
forward and reproducible procedure for the inter-comparison
and characterisation of both filter-based and in situ black-
carbon-measuring (BC-measuring) instruments based on re-
alistic test aerosols.
1 Introduction
Black-carbon-containing (BC-containing) particles are pro-
duced from incomplete combustion of fossil fuels or
biomass. BC is believed to be the second most significant
radiative forcing agent after carbon dioxide (Bond et al.,
2013; Ramanathan and Carmichael, 2008). However, its in-
fluence on the radiative balance of the Earth cannot be eas-
ily quantified because BC particles in ambient air are usu-
ally internally mixed with organic and/or inorganic species,
which may cause absorption enhancement through the so-
called “lensing effect” (Cappa et al., 2012; Liu et al., 2015).
Despite a plethora of commercially available BC-
monitoring instruments based on different measurement
techniques, quantification of BC mass concentration remains
a challenge to this day . Deviations between 15% and 30 %
among instruments of the same type (Cuesta-Mosquera et al.,
2021; Müller et al., 2011a) and up to 50 %–60 % for instru-
ments of different measurement principles (Chirico et al.,
2010; Slowik et al., 2007) have been reported. Among all
Published by Copernicus Publications on behalf of the European Geosciences Union.
562 D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments
commercial BC monitors, filter-based absorption photome-
ters, such as the Aethalometer, are the most widely used at air
quality monitoring stations thanks to their robust design. At
the same time, these instruments are the most prone to mea-
surement artefacts due to the use of filters for collecting the
particles. Even though correction algorithms have been pro-
posed for minimising measurement biases (see Collaud Coen
et al., 2010, and references therein; Drinovec et al., 2015, for
a measurement of the loading bias), no satisfactory solution
has been found for quantifying the absorption coefficient or
for determining site-independent equivalent BC (eBC) mass
concentrations.
To compare the performance of different instruments
or to investigate unit-to-unit variability, several field and
laboratory-based inter-comparisons of BC-monitoring in-
struments have been conducted in the past. Slowik et al.
compared a single-particle soot photometer (SP2), a multi-
angle absorption photometer (MAAP), and a photoacous-
tic spectrometer (PAS) with uncoated soot generated by a
McKenna burner and soot coated with organic material, such
as oleic acid and anthracene (Slowik et al., 2007). In an-
other study, soot generated by a McKenna burner was coated
with sulfuric acid and dioctyl sebacate (DOS), and the ef-
fect of non-absorbing coatings on the response of filter-
based and in situ BC-measuring instruments was determined
(Cross et al., 2010). Holder et al. compared an SP2, a three-
wavelength photoacoustic soot spectrometer (PASS-3), and
an Aethalometer (AE-42) during on-road and near-road mea-
surements (Holder et al., 2014), while Tasoglou et al. com-
pared six commercially available BC-measuring instruments
using aerosols from biomass burning (Tasoglou et al., 2018).
Moreover, two workshops with a large set of aerosol absorp-
tion photometers were conducted in 2005 and 2007, reveal-
ing a large variation in the response to absorbing aerosol par-
ticles for different types of instruments (Müller et al., 2011a).
More recently, an inter-comparison of 23 Aethalometers was
carried out with synthetic particles (soot generated by a mini-
CAST burner, nigrosin particles) and ambient air to investi-
gate the individual performance of the instruments and their
comparability (Cuesta-Mosquera et al., 2021).
Experiments in large-scale smog chambers are also con-
ducted to simulate atmospheric ageing of soot particles and
investigate the response of the instruments to secondary or-
ganic coating (Cappa et al., 2008; Chirico et al., 2010; Wein-
gartner et al., 2003). Whilst smog-chamber studies allow for
controlled laboratory experiments with realistic test aerosols,
they are time-consuming, with each measurement ranging
up to a few days (Weingartner et al., 2003). Consequently,
such experiments are typically restricted to the generation of
a single or a limited number of test aerosol types. A reliable
inter-comparison of BC-measuring instruments with a series
of different ambient-like aerosols would not be possible in a
reasonable timeframe.
Recently, a compact and user-friendly setup based on a
miniCAST combustion generator and an oxidation flow re-
actor (OFR) known as a micro smog chamber (MSC) was
proposed for the controlled generation of fresh and aged soot
particles in the laboratory (Ess et al., 2021a). A series of
test aerosols simulating a wide range of optical properties
and elemental to total carbon (EC /TC) mass fraction could
be generated within a few hours as opposed to a few days
with conventional smog chambers. Compared to other OFRs
reported in the literature (George et al., 2007; Kang et al.,
2007), the MSC is designed to operate at much higher aerosol
loads, which can subsequently be diluted, thus generating
aerosols at high flow rates but still sufficiently high number
concentrations to simultaneously feed multiple devices.
This study moves beyond the work by Ess et al. (2021a) by
demonstrating in practice how soot particles coated with con-
trolled amounts of secondary organic matter (SOM) from the
ozonolysis of α-pinene can be used to challenge a large num-
ber of BC-measuring instruments in parallel. More specifi-
cally, a dual-spot Aethalometer, a photoacoustic extinctiome-
ter (PAX, 870 nm), a MAAP, a prototype photoacoustic in-
strument (PAS), and two prototype photo-thermal interfer-
ometers (PTAAM-2λand MSPTI) were exposed to a series
of aerosols with EC /TC mass fraction ranging from >90 %
down to 10 % and single-scattering albedo (SSA) from al-
most 0 to about 0.7. The PTAAM-2λis now commercially
available (Haze Instruments, 2021), and this is the first time
it has been compared to a range of established instruments.
We believe that the setup and methodology described in this
study can easily be standardised and provide a straightfor-
ward and reproducible procedure for the inter-comparison
and characterisation of both filter-based and in situ BC-
measuring instruments based on realistic test aerosols.
2 Methods
2.1 Aerosol generation
Soot particles were generated by a miniCAST 5201 Type BC
(Jing Ltd., Switzerland), hereafter referred to simply as mini-
CAST BC, as previously described in Ess et al. (2021b; Ess
and Vasilatou, 2019). Two operation points in the “premixed
flame mode” were used, both resulting in particles of roughly
90 nm mobility diameter (see Table S1 in the Supplement for
gas flows of the operation points). The sample flow was dried
using a diffusion dryer (Silicagel orange Perlen, Dry & Safe
GmbH, Switzerland).
A novel “organic coating unit” (OCU, FHNW, Switzer-
land; Keller et al., 2021, 2022) was used to coat soot parti-
cles with secondary organic matter. The process is described
in Ess et al. (2021a); however, the OCU combines an op-
tional humidifier (not used in this study), a dosing system
for up to two volatile organic compounds (VOC1 and VOC2,
see Fig. 1), and an oxidation flow reactor in an integrated
unit. The soot was mixed in the OCU with α-pinene vapours
(VOC1), which were held at constant concentration using the
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D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments 563
Figure 1. Schematic of experimental setup. The numbers above the arrows indicate aerosol flows in litres per minute (L min1).
integrated dosing system and built-in photo-ionisation de-
tector (PID-A1 Rev 2, Alphasense Ltd, UK). The PID sen-
sor was regularly calibrated using a 100ppm isobutylene–air
mixture. The OCU was only used for the coated operation
points; for the uncoated operation points an identical setup
without the OCU was used.
Two variations of the setup were used in this study. For
setup 1 the soot aerosol generated by the miniCAST BC
was delivered undiluted to the OCU, while for setup 0.1 the
aerosol was diluted at a 1 : 10 ratio with dry air (VKL 10 di-
lution unit, Palas GmbH, Germany) as shown in Fig. 1. The
aerosol relative humidity before coating was about 25% and
<5 % for setups 1 and 0.1, respectively.
After generation, the aerosol was further diluted by a rota-
tional diluter (MD19, Matter Engineering AG, Switzerland)
using different dilutions depending on the experiment. As the
sample flow after the diluter (9.5 L min1) was not enough
for all the instruments under test, an additional dilution stage
was built using dry filtered air provided through a mass flow
controller (MFC) and a mixing volume. The aerosol was
then first split up to the high-volume instruments (MAAP
and nephelometer) before using a second 19-port flow split-
ter for all the other instruments (see Fig. 1 for a schematic
overview). The design of the custom-made flow splitter is
shown in Fig. S1 of the Supplement. The splitter bias was de-
termined as per ISO (2015) and found to be around 1 %. The
PAX, PTAAM, PAS, and AE33 have very similar sampling
flows (1–2 L min1), and the difference in diffusion losses
was compensated for by adapting the length of the connect-
ing tube to the flow of the instrument. For the MSPTI, which
has a flow of 0.25L min1, the connecting tube was kept as
short as possible. Possible differences in the internal path
length of the instruments (between the aerosol inlet and mea-
surement cell) were not taken into account. In the case of the
MAAP, which was operated at a flow of 12 L min1, it was
challenging to compensate for the difference in the diffusion
losses. We cannot rule out the possibility that the measure-
ments by the MAAP are biased because of lower diffusion
losses in the connecting tube, but we estimate that this bias
is <5 % and therefore much smaller than the systematic un-
certainties of this filter-based instrument.
2.2 BC- and aerosol-absorption-measuring
instruments
The dual-spot Aethalometer (AE33 Aethalometer, Magee
Scientific, Berkeley, USA) is a filter-based absorption pho-
tometer (Drinovec et al., 2015). It measures at seven differ-
ent wavelengths (370–950nm). To correct for filter-loading
artefacts, the device measures the change in light attenuation
at two distinct filter spots loaded at different flow rates. A
standard multiple-scattering parameter C=1.39 (provided
by the manufacturer) was applied to obtain the absorption
coefficient from the measured attenuation coefficient. The
Aethalometer was operated at a sample flow of 2L min1
and a temporal resolution of 1 min. Absorption Ångström
exponents (AAEs) were calculated using absorption coeffi-
cients measured by the Aethalometer over all wavelengths
(Drinovec et al., 2015). We estimate the measurement uncer-
tainties of the AE33 based on our measurements and cross-
sensitivity to scattering (Yus-Díez et al., 2021) to be around
20 % (for a coverage factor k=1), consistent with the 25%
value from the WMO (2016).
The Thermo Scientific model 5012 multi-angle absorption
photometer, MAAP, is a filter-based instrument that mea-
sures aerosol absorption at a nominal wavelength of 670nm.
The filter-loading-related artefacts affecting the determina-
tion of absorption coefficient are taken into account in the
design of the instrument. This is done by incorporating light
transmittance and reflectance measurements at multiple an-
gles and by implementing a radiative transfer calculation in
the internal programming of the instrument. The absorption
coefficient babs for MAAP has been derived throughout the
paper by
https://doi.org/10.5194/amt-15-561-2022 Atmos. Meas. Tech., 15, 561–572, 2022
564 D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments
babs =MBC ·QBC ·1.05,(1)
where MBC is the mass concentration of black carbon, QBC
is the specific absorption coefficient of 6.6m2g1of MAAP,
and 1.05 is a factor to correct the absorption coefficient to
the true wavelength of the instrument light source, 637nm
(Müller et al., 2011a). According to Petzold and Schönlin-
ner (2004), the measurement uncertainty in babs is estimated
to be 12 %.
Periodic variations in the babs measurements performed
with the MAAP were observed, especially at high black car-
bon concentrations, during the campaign (see Fig. S2 for an
example). While the MAAP is known to suffer from a mea-
surement artefact occurring at high concentrations (Hyväri-
nen et al., 2013), the observed variations were unrelated to it.
While the exact reason for the variations is not known, they
occur mid-range of the MAAP spot collection duration and
thus seem to be instrument-dependent and possibly related to
a non-disclosed internal averaging algorithm.
A photoacoustic extinctiometer, PAX (PAX 870nm,
Droplet Measurement Technologies Inc., Boulder, Colorado,
USA), was also used. The PAX measures absorption and ex-
tinction in parallel by combining a photoacoustic cell with
an integrating nephelometer (Arnott et al., 1999). The PAX
was operated at a sample flow of 1L min1and an averaging
time of 1 min with a 1 min zero measurement every 5min.
The estimated uncertainties for the absorption measurements
are 11 % according to Nakayama et al. (2015).
The prototype photoacoustic sensor (PAS) from the
FHNW group uses three different wavelengths (445, 520,
638 nm, 300 mW each) for in situ light absorption mea-
surements. The diode lasers are guided into a metallic res-
onator with elliptical cross-section along a focal point and
are modulated at ultrasonic frequencies (23.7 kHz) sequen-
tially every minute for each wavelength. The modulation fre-
quency adapts every 5 to 10min (thermal drift) to match
the resonance frequency of the resonator cell. The resulting
standing wave is measured with a digital microphone placed
in the middle of the resonator cell (long axis) at the other
focal point of the ellipse. The signal is then pre-amplified
and demodulated with a Stanford SR850 lock-in amplifier
(Stanford Research Systems, Sunnyvale, California, USA).
Section S4 describes the measurement principle, the device,
and the motivation behind it in more details. The instrument
was calibrated using nitrogen dioxide (NO2) and operated at
1 L min1. Due to technical issues during the campaign (dis-
cussed in Sect. 3), the uncertainty in the determination of the
absorption coefficient can be high, at times reaching 100%.
For this reason, the PAS measurements are presented in the
Supplement.
The photo-thermal interferometer PTAAM-2λis based on
a folded Mach–Zehnder interferometer design (similar to
Moosmüller and Arnott, 1996; Sedlacek, 2006; Visser et al.,
2020). The He–Ne probe laser beam is split into the sam-
ple chamber and reference beams. Pump lasers at 532 and
1064 nm are modulated at different frequencies and focused
in the sample chamber using an axicon for concurrent mea-
surement of the same sample. The quadrature point is main-
tained using a pressure cell. The interferometer signal is de-
tected by two photodiodes and resolved by a dual-channel
lock-in amplifier measuring at the two respective frequen-
cies. The green channel is calibrated using NO2. The cal-
ibration is transferred to the infrared (IR) channel using
aerosolised nigrosin and its relative green-to-infrared absorp-
tion ratio, determined using a Mie calculation based on size
distribution measurements. The verification at 532nm shows
a 6 % difference between the Mie calculation and the cali-
brated measurements of the absorption coefficient. The com-
bined measurement uncertainties (coverage factor k=1) for
the absorption coefficients at 532 and 1064 nm, as well as the
absorption Ångström exponents (AAEs), are 6 % (532 nm),
8 % (1064nm), and 9 % (AAE) (Drinovec et al., 2020, 2022).
The photo-thermal interferometer MSPTI is an improved
version of the instrument presented in Visser et al. (2020).
Briefly, the instrument design is similar to a folded Mach–
Zehnder interferometer (Moosmüller and Arnott, 1996; Sed-
lacek, 2006), with the optical elements in the interferome-
ter consisting of a combined beam splitter and mirror block
as well as a retroreflector. In contrast to the PTAAM-2
and other photo-thermal interferometers, the MSPTI oper-
ates with only a single modulated laser (Nd:YAG, 532 nm),
which is employed as both the pump and probe beam. This
beam is split 50–50, and one of the resulting beams is sent
through the sample chamber, whereas the other traverses the
reference chamber. The beams are recombined at the beam
splitter, resulting in interference patterns. In these experi-
ments the filtered sample (HEPA-grade absolute filter) is
employed as the “zero” sample in the reference arm of the
interferometer. Phase quadrature is maintained via an im-
proved version of the pressure cell from Visser et al. (2020).
The MSPTI is calibrated using NO2and was operated at a
flow rate of 0.25L min1. The combined measurement un-
certainty (k=1) for the absorption coefficient at 532 nm is
estimated to be about 13 %.
2.3 Additional aerosol characterisation
Mobility size distribution and number concentration were
measured using a scanning mobility particle sizer, SMPS
(Electrostatic Classifier Series 3080 with 85Kr radioactive
source, DMA column 3081, CPC 3776 low flow, TSI In-
corporated, USA). The DMA was operated with a sheath air
of 3 L min1and a sample flow of 0.3L min1. Geometric
mean mobility diameter (GMDmob) and total number con-
centrations were determined from the size distribution using
the software provided with the instrument (Aerosol Instru-
ment Manager, v 9.0.0.0, TSI Incorporated, USA). The ratio
of total number concentrations between the different oper-
Atmos. Meas. Tech., 15, 561–572, 2022 https://doi.org/10.5194/amt-15-561-2022
D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments 565
ation points was used to scale babs values reported by the
BC-measuring instruments in order to account for day-to-day
variability, with the concentration of the uncoated operation
points being the reference.
An integrating nephelometer (AirPhoton model IN101)
was used to measure light-scattering coefficients over the an-
gular range from 7 to 170. The AirPhoton IN101 utilises
LED light sources to make measurements at 450, 532, and
632 nm. According to the manufacturer, the truncation cor-
rection for the AirPhoton should be done identically to TSI
model 3563. However, as the single-scattering albedo in the
experiments was substantially below the specifications of the
correction scheme proposed by the manufacturer (Müller et
al., 2011b), no truncation correction was applied to the data.
A tapered element oscillating microbalance (TEOM 1405,
ambient particulate monitor, Thermo Fisher Scientific Inc.,
USA) was used to measure total aerosol mass concentrations.
The TEOM was operated at a flow of 1.2L min1at 30 C.
The frequency of the tapered element was recorded every
6 s and used to calculate the mass concentration over the
duration of the measurement. TEOM measurements agreed
within 1 %–4 % with the reference (manual) gravimetric
method.
Thermal–optical analysis was performed in order to estab-
lish the composition of the soot. Aerosols were sampled on
three sets of two superimposed filters for each measurement
point according to path b in Fig. 1. For the coated samples,
the aerosol was passed through an activated charcoal denuder
first. During sampling the filters (47 mm QR-100 quartz-
fibre filters, Advantec, Japan, prebaked at 500C for 1.5 h)
were placed in a metallic filter holder (Merck Millipore,
Germany). Punches of 1.5 cm2were later used for thermal–
optical analysis with a Lab OC–EC aerosol analyser (Sunset
Laboratory Inc., Hillsborough, USA). This instrument sepa-
rates carbonaceous material into EC and OC (elemental and
organic carbon) after being calibrated against solutions of
glucose at different concentrations. The EUSAAR2 protocol
(Cavalli et al., 2010) was slightly modified by extending the
last temperature step to ensure that the evolution of carbon
is complete (Ess and Vasilatou, 2019). OC and EC masses
were determined from the upper filter, with OC masses then
corrected by subtracting the mass of OC from the lower fil-
ter, consisting of the absorbed gas phase (Mader et al., 2003;
Moallemi et al., 2019). The results of the thermal–optical
analysis were then used to calculate EC /TC and OC /TC
ratios, where TC =OC +EC.
3 Results and discussion
Two series of test aerosols were generated as summarised
in Table 1. Each series consisted of four test aerosols: un-
coated soot and soot with three different amounts of SOM
coating. Note that the uncoated soot particles are “fresh” soot
particles generated by the miniCAST burner (and not soot
particles that have been coated and denuded). With setup 1,
i.e. no dilution unit between miniCAST and OCU, a high
concentration of about 4 ×107cm3of soot particles was
delivered to the OCU. The geometric mean mobility diame-
ter (GMDmob) of the soot particles gradually decreased from
92 nm (uncoated soot) to 83 nm (coated soot; coating 3) as
shown in Fig. 2a, while the EC/TC mass fraction dropped
from 90 % to 40 % and the SSA increased from about
0 to 0.2. The operation points of the miniCAST and MSC
are listed in Table S1 in the Supplement. The decrease in
GMDmob, despite the considerable amount of OC condensed
on the soot particles, is not surprising. As explained in Ess et
al. (2021a), this is due to (i) the decrease in dynamic shape
factor that dominates over the increase in volume equiva-
lent diameter and/or (ii) a restructuring of the soot core dur-
ing SOM condensation (see Ess et al., 2021a, and references
therein).
On the contrary, with setup 0.1, i.e., including a dilution
of factor 10 upstream of the OCU, the GMDmob of the soot
particles increased from 88 nm (uncoated soot) to 126 nm
(coated soot), while the EC /TC mass fraction dropped from
85 % to 10 % and the SSA increased up to 0.7. Due to
the lower concentration of soot particles by an order of mag-
nitude, the α-pinene /eBCPAX mass ratio rapidly increased
to 500 (see Table S1). As a result, the increase in volume
equivalent diameter due to the high amount of condensed
SOM dominated over the decrease in shape factor. The mo-
bility size distributions of the test aerosols are displayed in
Fig. 2b.
The response of the BC- and absorption-measuring in-
struments to the test aerosols generated by setup 0.1 is dis-
played in Fig. 3. In Fig. 3b, the absorption coefficient at
532 nm (babs,532 ) is plotted as a function of RBC.RBC =
(Mtotal MBC)/MBC is equal to the mass of organic coating
over the mass of uncoated soot as measured by the TEOM.
Note that our definition of RBC is similar but not identical
to the definition provided by Cappa et al. (2012) and Liu et
al. (2015), who calculate RBC as [NR-PMBC]/[BC], with
NR-PMBC being the fraction of non-refractory particulate
matter (NR-PM) exclusively associated with BC based on
measurements with soot particle–aerosol mass spectrometry
(Cappa et al., 2012; Liu et al., 2015). To facilitate compari-
son with previous literature, the total mass to BC mass ra-
tio (Mtotal/MBC) as measured by the TEOM is shown on
the secondary xaxis. The measurements by the TEOM do
not agree so well with the results from the thermal–optical
analysis. We believe that this is due to the high measurement
uncertainties of the thermal–optical analysis, particularly the
difficulty to define the split point. All babs values have been
converted to a wavelength of 532 nm using the absorption
Ångström exponents determined from the fit over the two
babs values from the PTAAM. The absorption enhancement
at 532 nm (Eabs,532) is shown in Fig. 3c and is equal to babs
of the coated soot divided by that of the uncoated soot. The
EC /TC mass fraction and SSA of the test aerosols are dis-
https://doi.org/10.5194/amt-15-561-2022 Atmos. Meas. Tech., 15, 561–572, 2022
566 D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments
Table 1. Physicochemical properties of the uncoated and coated soot particles generated in this study. The uncertainties for the GMDmob,
total concentration, SSA, and AAE correspond to 1 standard deviation of the mean (k=1; 68 % confidence interval; number of measurements
n=100–180 for SSA and AAE, n=29–35 for GMDmob and total concentration).
Operation GMDmob SSAPAX,870 SSAneph/MAAP,632 AAE1AAE2EC /TC Total
point (nm) (–) (–) (–) (–) mass fraction3concentration4
(%) (cm3)
1 – uncoated 91.7 ±0.1 0.027 ±0.001 0.0333 ±0.0002 1.14 ±0.01 0.875 ±0.014 91 ±7 25 900 ±300
1 – coating 1 86.1±0.1 0.052 ±0.001 0.0749±0.0003 1.20 ±0.01 0.984 ±0.009 65 ±5 36 500 ±100
1 – coating 2 83.4±0.1 0.12 ±0.01 0.148 ±0.001 1.28 ±0.01 1.05±0.01 48 ±3 35 000 ±100
1 – coating 3 83.0±0.1 0.18 ±0.01 0.220 ±0.001 1.29 ±0.01 1.06±0.01 39 ±3 35 500 ±100
0.1 – uncoated 88.3 ±0.1 0.0289 ±0.0002 0.0353±0.0002 1.17 ±0.01 0.844 ±0.016 84 ±8 26 200 ±100
0.1 – coating 1 90.2 ±0.1 0.130 ±0.001 0.156 ±0.001 1.30 ±0.01 1.15 ±0.02 37 ±4 26 700 ±200
0.1 – coating 2 111±1 0.497 ±0.001 0.439 ±0.002 1.46 ±0.01 1.26±0.02 13 ±1 29 300 ±100
0.1 – coating 3 126±1 0.677 ±0.001 0.646 ±0.002 1.48 ±0.01 1.36±0.02 10 ±1 24 600 ±400
1AAE determined from the fit over all babs values (370–950nm) from the Aethalometer. 2AAE determined from the fit over the babs values (532, 1064 nm) from the PTAAM.
3The uncertainty of the corrected OC and EC masses is based on the uncertainties given by the instrument’s software, calculated as the detection limit of 0.2µg C cm2plus 5 %
of the carbon mass determined in the analysis for each carbon fraction. The uncertainties due to the determination of the split point were not taken into account as they could
not be quantified. 4Measured right after the mixing volume.
Figure 2. Mobility size distributions as measured by SMPS (a) with setup 1 and (b) with setup 0.1.
played in Fig. 3a (main and secondary yaxis, respectively),
while GMDmob is shown as a label on the data points.
As shown in Fig. 3b, significant deviations in the response
of the different BC- and absorption-measuring instruments
are observed even for the uncoated soot aerosol. Instruments
based on photoacoustic spectroscopy and interferometry re-
port a babs in the range 20 to 50 Mm1, while the MAAP
and AE33 report 60 and 110 Mm1, respectively. The
largest deviation is observed between the PAX and the AE33,
with the AE33 overestimating babs by a factor of about 2
compared to the PAX. In general, babs increases with in-
creasing SOM coating, apart from the PAS, which shows
a rather erratic behaviour (see Table S2 for the PAS data).
The deviation between the AE33 and PAX increases with
increasing SOM coating up to a factor of 3 for the “thick-
est” coating (SSA 0.7, see Table 1 and Fig. 3a). Even when
taking into account the expanded measurement uncertainties
(k=2; 95 % confidence interval), the measurements by the
AE33 hardly agree with the measurements by the PAX and
PTAAM. This indicates that the 20 % measurement un-
certainty (k=1) assigned to the AE33 (see Sect. 2.2) might
be underestimated. Similar observations can be made for the
MAAP at high RBC ratios even though the deviations from
the PAX and PTAAM are less pronounced.
In the visible and near-UV region of the spectrum, the val-
ues of Eabs can include effects of both “lensing” and potential
absorption by SOM. Absorption by α-pinene-derived SOM
is very low with a MAC below 0.25 m2g1(Nakayama et al.,
2010) or even 0.01m2g1(Lambe et al., 2013) at 532 nm,
depending on the oxidation state and experimental details.
Instruments measuring in the wavelength region 520–637nm
all recorded an increase in Eabs,532 as a function of RBC
(Fig. 3c). At RBC 3.4, corresponding to an EC /TC mass
fraction of 10 % and an SSA of about 0.7, an absorption en-
hancement in the range 1.3 (PTAAM 532 nm) to 2 (MSPTI
532 nm) was observed.
A weak absorption enhancement of about 1.1–1.3 at
532 nm was calculated from the PAX data (Fig. 3c). We
therefore interpret the absorption enhancement shown in
Fig. 3c to be due to a transparent coating by SOM on the
absorbing BC core, as described by Lack and Cappa (2010).
Moreover, as biogenic SOM is only expected to absorb light
Atmos. Meas. Tech., 15, 561–572, 2022 https://doi.org/10.5194/amt-15-561-2022
D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments 567
Figure 3. Measurements obtained with setup 0.1. (a) EC /TC mass
fraction as a function of total mass to BC mass ratio and RBC. The
total mass to BC mass ratio and RBC are based on TEOM measure-
ments. The single-scattering albedo (SSA) at 637 nm, calculated
from babs measured by the MAAP and the scattering coefficient
measured by the nephelometer, is shown on the secondary yaxis.
The sample geometric mean diameter is shown as a label on each
data point. (b) Absorption coefficient (babs) as a function of total
mass to BC mass ratio and RBC. All babs values have been con-
verted to a wavelength of 532nm using the absorption Ångström ex-
ponents determined from the babs values of the PTAAM. The legend
below Fig. 3c indicates the wavelengths at which the measurements
were performed. All babs values have also been scaled by the num-
ber concentration. The values are listed in Table S2. (c) Absorption
enhancement factor Ebabs (532 nm) as a function of total mass to
BC mass ratio and RBC. The data points in panels (b) and (c) have
been slightly shifted along the xaxis to improve the readability of
the graph. The error bars designate measurement uncertainties for
k=1 (68 % confidence interval). The uncertainty (k=1) in RBC is
estimated to be about 5 % (not shown).
in the UV and near-UV region (Nakayama et al., 2010; Song
et al., 2013), it is surprising that the MAAP indicates such a
pronounced absorption enhancement at 637 nm. Apart from
the lensing effect, one additional reason could be coating of
BC in the filter by SOM or modification of the filter matrix
optical properties by SOM (Lack et al., 2008). The uncer-
tainties in Fig. 3c were calculated as the quadratic sum of the
uncertainties in babs for the uncoated and coated soot. Note
that this procedure is only a simplistic approximation. Ide-
ally, the uncertainty in babs should be partitioned into type A
(random) and type B (systematic) uncertainties, and correla-
tions between the different components should be taken into
account. A robust uncertainty calculation was, however, not
possible because the uncertainties of the instruments are not
so clearly understood, and, additionally, instruments such as
the PAS and the MSPTI at times suffered from unexpected
technical errors. In the case that babs is dominated by sys-
tematic uncertainties which remain the same when measur-
ing the uncoated and coated soot particles, such uncertainties
may cancel out, resulting in a much smaller combined uncer-
tainty in Ebabs than what is presented in Fig. 3c.
A limitation of our study is that the relative humidity of
the uncoated soot aerosols when entering the organic coat-
ing unit was low to very low (see Sect. 2.1). No experiments
were performed at high RH due to the presence of homoge-
neously nucleated secondary organic aerosol (SOA) particles
at RH above 40%–50 %. It is known that the absorption by
BC depends strongly on RH and may lead to a factor of 2
increase in absorption at high RH compared with dry condi-
tions (Fierce et al., 2016). This is one of the reasons why
GAW (Global Atmospheric Watch) recommends measure-
ments of light absorption at low RH (GAW, 2016). More-
over, soot-containing particles generated in the laboratory
under controlled conditions might have a more uniform com-
position compared to the aged soot particles in ambient air.
In general, the range of absorption enhancement Eabs =1.1–
1.3 calculated based on the results by the PAX and PTAAM
agrees very well with Fierce et al. (2016), who calculated
a limited absorption enhancement (Eabs =1–1.5) at low RH
when accounting for particle-level variation in the composi-
tion of soot-containing particles. The dry conditions during
and after ozonolysis of α-pinene may have also had an effect
on the phase state of the SOM, most probably leading to a
solid-state coating (Saukko et al., 2012).
Figure 4 shows the response of the BC- and absorption-
measuring instruments to the test aerosols generated by
setup 1. In this case, coating is more moderate, and, as ex-
plained above, the GMDmob of the soot particles decreases
slightly upon coating. AE33 overestimates babs by up to a
factor of 2 to 3 compared to the other instruments as shown
in Fig. 4b. The instruments report no enhancement or only
weak absorption enhancement as a function of SOM coating
(Fig. 4c). During the measurement campaign the PAS was af-
fected by a number of technical issues, which intermittently
caused high uncertainties in the measurements. As a result,
https://doi.org/10.5194/amt-15-561-2022 Atmos. Meas. Tech., 15, 561–572, 2022
568 D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments
the PAS data are not shown in Fig. 4 but can be found in
Table S3.
Two photo-thermal instruments based on different designs
(MSPTI and PTAAM) were operated in parallel to mea-
sure the aerosol absorption coefficient. Here, we compare
the measurements performed at 532 nm. The response of
the PTAAM was regularly tested during the campaign and
showed average variation of 3 % for the 532 nm channel
(Sect. S6 in the Supplement). Testing of the MSPTI response
showed larger variability at the end of the measurement cam-
paign as the laser became more unstable. This especially
affected the measurements of the uncoated particles, which
were performed at the end; due to this fact the MSPTI to
PTAAM ratio for the uncoated particles is more uncertain
(Fig. S6a). Two additional 1 d experiments were performed
comparing different coating treatments (Fig. S6b). These
measurements show the opposite behaviour for the uncoated
particles compared to Fig. S4a. Comparing the experiments,
one can conclude that the average response of both instru-
ments agrees well within the measurement uncertainty, thus
showing similar absorption enhancement at 532 nm for both
instruments.
To decouple a possible lensing effect from the light ab-
sorption by SOM, the absorption enhancement in the near-
infrared (NIR) region, Eabs,950, is plotted as a function of
RBC in Fig. 5 (for setup 0.1). Biogenic SOM does not ab-
sorb in the NIR region (Nakayama et al., 2010; Schnaiter et
al., 2003; Xie et al., 2017); thus, any absorption enhancement
would be due to the lensing effect. In this study, the only in-
struments measuring in the NIR were the AE33 (950 nm),
the PAX (870 nm) and the PTAAM (1064 nm). Prior to the
calculation of Eabs, all babs values had been converted to a
wavelength of 950nm using the absorption Ångström ex-
ponents determined from the pair of babs values at 880 and
950 nm reported by the Aethalometer. As shown in Fig. 5,
the measurements by the PTAAM and PAX agree very well,
and both instruments yield an Eabs,950 close to 1. On the con-
trary, the AE33 reports an absorption enhancement as a func-
tion of the organic coating, with Eabs,950 1.5 at RBC 3.4.
It is known that the multiple-scattering parameter Cof the
Aethalometer depends on the SSA and, possibly, on the size
of the aerosol particles (Yus-Díez et al., 2021). As mentioned
earlier, this variation was not taken into account, but, instead,
a fixed Cvalue of 1.39 (provided by the manufacturer) was
applied throughout this study to obtain the absorption coef-
ficient from the measured attenuation coefficient. We believe
that the absorption enhancement reported by the Aethalome-
ter is an artefact arising from keeping the Cvalue fixed. Un-
der this assumption, it is possible to calculate new values of
C(Bernardoni et al., 2021) as a function of SSA using the
mean babs value of the PTAAM and PAX as a reference:
CSSA =batn,AE
babs,ref .(2)
Figure 4. Measurements obtained with setup 1. (a) EC /TC mass
fraction as a function of total mass to BC mass ratio and RBC. The
total mass to BC mass ratio and RBC are based on TEOM measure-
ments. The single-scattering albedo (SSA) at 637 nm, calculated
from babs measured by the MAAP and the scattering coefficient
measured by the nephelometer, is shown on the secondary yaxis.
The sample geometric mean diameter is shown as a label on each
data point. (b) Absorption coefficient (babs) as a function of total
mass to BC mass ratio and RBC. All babs values have been con-
verted to a wavelength of 532nm using the absorption Ångström
exponents determined from the babs values from the PTAAM. The
legend below Fig. 4c indicates the wavelengths at which the mea-
surements were performed. All babs values have also been scaled by
the number concentration. The values are listed in Table S2. (c) Ab-
sorption enhancement factor Ebabs (532 nm) as a function of total
mass to BC mass ratio and RBC. The values are listed in Table S3.
The data points in panels (b) and (c) have been slightly shifted along
the xaxis to improve the readability of the graph. The error bars
designate measurement uncertainties for k=1 (68 % confidence in-
terval). The uncertainty (k=1) in RBC is estimated to be about 5%
(not shown).
Atmos. Meas. Tech., 15, 561–572, 2022 https://doi.org/10.5194/amt-15-561-2022
D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments 569
Figure 5. Measurements obtained with setup 0.1. Absorption en-
hancement factor Eabs (950 nm) as a function of total mass to BC
mass ratio and RBC. For the calculation of Eabs, all babs values
had been converted to a wavelength of 950 nm using the absorption
Ångström exponents determined from the 880 and 950 nm babs val-
ues from the Aethalometer. All babs values are also scaled by the
number concentration. The values are listed in Table S4. The data
points have been slightly shifted along the xaxis to improve the
readability of the graph. The error bars correspond to uncertainties
for k=1 (68 % confidence interval; see text for more details).
This results in calculated values of Cat 950nm of 3.4, 4.6,
4.9, and 5.3 for SSA870 values (measured by PAX) of 0.03,
0.13, 0.50, and 0.68, respectively.
If the SSA of the ambient aerosol is known, then the calcu-
lated CSSA values can be used to post-correct the data from
the Aethalometer. In well-equipped monitoring stations, this
can be performed using an integrating nephelometer and a
reference absorption measurement at a single wavelength
(Yus-Díez et al., 2021), and it can be extended using a multi-
wavelength reference absorption measurement and a multi-
wavelength scattering measurement in a representative cam-
paign. Multi-wavelength absorption corrections and the de-
termination of Ccan be derived from offline filter measure-
ments with a time resolution of hours to days (Bernardoni
et al., 2021). Moreover, a recent study suggests that “the
low-cost and widely used PA monitors can be used to mea-
sure and predict the aerosol light scattering coefficient in
the mid-visible nearly as well as integrating nephelometers”
(Ouimette et al., 2021). By letting a low-cost nephelometer
(temperature- and RH-controlled) run parallel to the AE33
at monitoring stations, an approximate SSA value (based on
babs by the AE33 and bscat by the low-cost sensor) can be
calculated, and the babs of the AE33 can be refined by imple-
menting a new CSSA. Both babs by the Aethalometer and SSA
can then be refined in multiple steps in an iterative procedure.
4 Conclusions
A series of test aerosols was produced using a miniCAST
BC generator and a novel organic coating unit comprised of
a micro smog chamber and an integrated dosing system for
VOCs. Both uncoated soot particles and soot particles coated
with varying amounts of α-pinene-derived SOM were gener-
ated, covering a wide range of particle sizes (83–126nm),
EC /TC mass fractions (10 %–91 %), and optical properties
(SSA almost 0 to 0.7 at 637 nm).
Several BC- and aerosol-absorption-measuring instru-
ments were compared using these aerosols: a dual-
spot Aethalometer, a photoacoustic extinctiometer (PAX,
870 nm), a MAAP, a prototype photoacoustic instrument,
and two prototype photo-thermal interferometers (PTAAM-
2λand MSPTI). This is the first time that the PTAAM-2λ,
which is now commercially available, has been compared
to other absorption-measuring instruments. In general, the
filter-based instruments (AE33 and MAAP) overestimated
babs compared to in situ measuring instruments. The bias is
systematic and increases with increasing SSA. The absorp-
tion enhancement is equally the highest for the filter-based
instruments. The PAX and the NIR channel of the PTAAM
measured almost no enhancement, though weak absorption
enhancement was observed at 532 nm.
The setup of miniCAST combined with the novel organic
coating unit and the methodology described in this study
provide a straightforward and reproducible procedure for
the inter-comparison and characterisation of both filter-based
and in situ BC-measuring instruments. The system is very ro-
bust, compact, and relatively inexpensive, and it allows gen-
erating realistic test aerosols in a reproducible and standard-
ised manner. Additionally, in comparison with smog cham-
bers, stability of the aerosols is reached within minutes af-
ter changing operation points, allowing for several measure-
ments within a day.
A limitation of this study was that the relative humidity of
the uncoated soot aerosols when entering the organic coat-
ing unit was below 30%. More studies are needed at higher
relative humidity in order to better simulate atmospheric pro-
cesses.
Data availability. Measurement data will be made available for
the final publication at https://doi.org/10.5281/zenodo.5926472
(Kalbermatter et al., 2022).
Supplement. The supplement related to this article is available on-
line at: https://doi.org/10.5194/amt-15-561-2022-supplement.
Author contributions. DMK and KV designed the study and wrote
the paper with contributions from all authors. DMK generated the
model aerosols and analysed the data from the AE33 and PAX; GM
https://doi.org/10.5194/amt-15-561-2022 Atmos. Meas. Tech., 15, 561–572, 2022
570 D. M. Kalbermatter et al.: Comparing black-carbon- and aerosol-absorption-measuring instruments
and LD operated and analysed the data from the PTAAM, and BV
and JR operated and analysed the data from the MSPTI. MO op-
erated and analysed the data from the PAS, and APH analysed the
data from the MAAP and nephelometer. EW supervised the mea-
surements with the MSPTI and the PAS. All authors contributed to
the interpretation of the results.
Competing interests. Luka Drinovec and Griša Moˇ
cnik are (in part)
employed by the manufacturer of the PTAAM-2λ.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. Daniel M. Kalbermatter and Konstantina Vasi-
latou thank Michaela Ess (previously at METAS) for valuable tech-
nical support during the preparation of the measurement campaign.
Financial support. This research has been supported by the Eu-
ropean Metrology Programme for Innovation and Research (grant
nos. 18HLT02 AeroTox and 16ENV02 Black Carbon), the Schweiz-
erischer Nationalfonds zur Förderung der Wissenschaftlichen
Forschung (grant no. 200021_172649), and Eurostars (IMALA,
grant no. 11386).
Review statement. This paper was edited by Charles Brock and re-
viewed by two anonymous referees.
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