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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

Atmospheric Measurement Techniques

February 2022
15(2):561-572

DOI:10.5194/amt-15-561-2022

License
CC BY 4.0

Authors:

Daniel Martin Kalbermatter

Aon - Impact Forecasting

Griša Močnik

University of Nova Gorica

Luka Drinovec

Jožef Stefan Institute

Show all 9 authorsHide

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.

Schematic of experimental setup. The numbers above the arrows indicate aerosol flows in litres per minute (L min-1).

…

Mobility size distributions as measured by SMPS (a) with setup 1 and (b) with setup 0.1.

…

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 measurements. 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 y axis. 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 converted to a wavelength of 532 nm using the absorption Ångström exponents 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 number 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 x axis 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).

…

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 measurements. 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 y axis. 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 converted to a wavelength of 532 nm using the absorption Ångström exponents determined from the babs values from the PTAAM. The legend below Fig. 4c indicates the wavelengths at which the measurements were performed. All babs values have also been scaled by the number 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 values are listed in Table S3. The data points in panels (b) and (c) have been slightly shifted along the x axis 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).

…

Measurements obtained with setup 0.1. Absorption enhancement 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 values 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 x axis to improve the readability of the graph. The error bars correspond to uncertainties for k=1 (68 % confidence interval; see text for more details).

…

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Atmos. Meas. Tech., 15, 561–572, 2022

https://doi.org/10.5194/amt-15-561-2022

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

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 563

Figure 1. Schematic of experimental setup. The numbers above the arrows indicate aerosol ﬂows in litres per minute (L min−1).

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 min−1) 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 min−1), 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 min−1, 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 min−1, 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 min−1

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 6.6m2g−1of 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 min−1and 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 min−1. 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 min−1. 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 min−1and a sample ﬂow of 0.3L min−1. 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 min−1at 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 500◦C 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 ×107cm−3of 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

(%) (cm−3)

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 cm−2plus 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 Mm−1, while the MAAP

and AE33 report ∼60 and ∼110 Mm−1, 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 m2g−1(Nakayama et al.,

2010) or even 0.01m2g−1(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