Responses to the Reviewers’ Comments

Title: Estimates of mass absorption cross sections of black carbon for filter-based
absorption photometers in the Arctic
MS No.: amt-2021-166
MS type: Research article

The co-authors and I very much appreciate the constructive comments on thismanuscript by the reviewers. The comments have been very thorough and useful inimproving the manuscript. We have taken them fully into account in revising it. Major
revisions made to the manuscript are described first, followed by responses to thecomments raised by each reviewer.

Corresponding author
Sho Ohata
Institute for Space-Earth Environmental Research, Nagoya University
Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601, Japan.
sho.ohata@isee.nagoya-u.ac.jp1

Major revisions
1. Addition of the COSMOS-MAAP comparison at Ny-Ålesund
In the revised manuscript, we have added analysis of COSMOS-MAAP comparison at
Ny-Ålesund during 2017–2020 in Sect. 3.2.3 to investigate the possibility of converting
babs (MAAP) to MBC at this site. Similar to the other comparisons at Arctic sites, thetime series data of babs (MAAP) were highly correlated with those for MBC (COSMOS),
and the MACcor (MAAP) values at Ny-Ålesund were estimated to be 10.6 and 10.9 m2
g–1 for the 1-h and 24-h data, respectively. These MACcor values are about 60 % higher
than the manufacturer’s default setting value (= 6.6 m2 g–1). Year-to-year variation of
MACcor (MAAP) has been also examined and yearly MACcor (MAAP) was stable towithin 10%. These results are shown in Fig. 7a and 9, Table 3, 8, and 11, and Fig. S6 in
the Supplement.

2

Responses to Anonymous Referee #1––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> This manuscript addresses observations of black carbon aerosol in the Arctic.
Knowing spatio-temporal variation of black carbon (BC) concentration and resulting
light absorption is of general atmospheric and climatic interest as motivated by theauthors. They present multi-annual and multi-site data sets of parallel measurementswith two or more methods. This is a great effort and of large value as starting point for
many studies. The methods and data treatment are sound and the manuscript is well
written and it provides ample and important information towards consistentinterpretation of observations made with different instrument types commonly deployed
to quantify black carbon mass concentrations, and also provides approximate values forthe conversion factor between light absorption coefficient and black carbon massconcentration, i.e. the MAC value, as far as achievable with filter-based methods.
Therefore matching the scope of this journal.
This manuscript was initially submitted to the journal “Atmospheric Chemistry andPhysics” (https://acp.copernicus.org/preprints/acp-2020-1190/) where it underwent a
first round of reviews, which resulted in transfer to the current journal (Atmos. Meas.
Techn.). I already provided a review at that stage(https://acp.copernicus.org/preprints/acp-2020-1190/acp-2020-1190-RC2.pdf). The
authors already implemented these comments in an appropriate manner. Therefore, I
only have few additional minor and technical comments listed below. Other than that, I
can only stress that this valuable manuscript warrants publication.
Answer 1-1: Thank you for the positive comments on our manuscript.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Minor and technical comments:
Equation 1: The value(s) applied for f_{fil} should be reported. And/or the ratio“f_{fil}/MAC(COSMOS,SP2)”, which is the final and only factor applied to measuredb_{0} for inferring M_{BC}.
Answer 1-2: In the revised manuscript, we have added an equation (Equation 2) in Sect.
2.2.2.1 to explain how ffil values are derived:
1𝑓!"# (𝑇𝑟) = [1.0796 Tr + 0.71] B with Tr ≥ 0.7,
3where Tr is the filter transmission and B (= 1.397) is a scaling factor.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Caption of Figure 2: I suggest to add something along the line: “D_{m} is the massequivalent diameter of bare BC or FeOx”.
Answer 1-3: We have added the statement “Dm is the mass equivalent diameter of bareBC or FeOx.” in the caption of Fig. 2, as the reviewer suggested.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Section 2.2.3.3: It seems that actual wavelength of the MAAP instrument at Fukue
was measured and found to be 639 nm, which is a very slight difference from 637 nm
reported in the literature. It might be worthwhile to mention that 639 nm is an actual
measured value.
Answer 1-4: In the revised manuscript, we have clarified that the peak wavelength ofthe light source of the MAAP at Fukue was measured to be 639 nm (Kanaya et al.,2013), which is very slightly different from the previously reported value (637 nm;
Müller et al., 2011) in Sect. 2.2.3.3.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Figure 3c: choosing logarithmic instead of linear axis scaling might provide better
visualization of performance in the lower concentration range, where the majority ofdata points appear. In the current variant it is not perfectly clear whether the value ofthe regression slope, which is driven by the high concentration data points due to fixing
the axis intercept at the origin, only matches data well at high concentrations or at low
concentrations too.
Choosing logarithmic instead of linear axis scaling might be preferable for several
figures for the reasons laid down in the previous graph.
Answer 1-5: Following the reviewer’s suggestion, we have examined the degree ofagreement between MBC (COSMOS) and MBC (SP2) at Alert on a logarithmic scale (Fig.
S1a in the Supplement). When MBC (SP2) is relatively low (MBC < 10 ng m–3), whichcorresponds to the monthly-averaged MBC ranges in summer at Arctic sites (Sinha et al.,2017), MBC (COSMOS) tended to be higher than MBC (SP2) by about 1–2 ng m–3. This
small absolute difference is consistent with the previously reported difference between
MBC (COSMOS) and MEC at Barrow (Sinha et al., 2017). We have added thesedescriptions in Sect. 3.1.1.
Furthermore, the same analysis has been made for COSMOS-SP2 comparison at Fukue
4

in Sect. 3.5.1 (Fig. S1b in the Supplement). At Fukue, when MBC (SP2) is lower than
~70 ng m–3, MBC (COSMOS) tended to be slightly higher than MBC (SP2). A similarfeature was previously reported at Cape Hedo in Japan (Ohata et al., 2019). The Cape
Hedo site is located near the coast (i.e., the distance of this site to the coast is ~0.2 km)
and the interference of submicron sea salt particles might contribute to this feature(Ohata et al., 2019). At Fukue, when maritime air mass is transported to the site, therelative abundance of sea salt particles to BC might be also enhanced possibly affecting
the COSMOS measurements, although the distance from the site to the coast (~1.5 km)
is slightly farther than for Cape Hedo. This feature was not clearly observed by aprevious study at Fukue (Miyakawa et al., 2017). We have added these descriptions in
Sect. 3.5.1.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Section 3.1.1: please put emphasis on the fact that quoted diameter values are BC
mass equivalent diameter.
Answer 1-6: We have added the explanation that the diameter is reported in massequivalent diameter in Sect. 3.1.1, as the reviewer suggested.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 358: “These results show that on average, the agreement between MBC(COSMOS) and MBC (SP2) at Alert was within 10 %.” – What exactly means “onaverage within 10%”?
Answer 1-7: The descriptions “on average” was not the statistically correct expression
here. In Sect. 3.1.1, we have modified this part as follows: “Based on the slope value ofthe regression for whole MBC ranges observed, the agreement between MBC (COSMOS)
and MBC (SP2) at Alert was generally within 10 %.”––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Figure 4: It is very good that the instrument comparison is presented in different
ways. Furthermore, distinction of performance in different concentration ranges as donein panels 4e and 4f is valuable. Having said so, the threshold is chosen at a very low
level (2 ng/m3), which separates the data set somewhere in the single digit percentile
range. I’d rather suggest to split somewhere between 15th and 25th percentile. In any
case, the histogram of the low concentration data only should also be added (besides alldata and high concentration data, which are already shown). This comment also applies
to several other figures).

5

Answer 1-8: The threshold value of 2 ng m–3 actually corresponds to ~20th percentile at
Alert (2018-2019) and at Ny-Ålesund for 1-h data. Instead of splitting somewhere
between 15th and 25th percentile (reviewer’s suggestion), we have split the histograms
at 10 ng m–3 to focus on the low-concentration data, as shown in Fig. S2 in the
Supplement. While the similar MACmed values were obtained for data with MBC(COSMOS) < 10 ng m–3 and for all dataset, the interquartile ranges of the babs (PSAP) /MBC (COSMOS) ratios are larger for MBC (COSMOS) < 10 ng m–3. This result showsthat the relative uncertainty becomes higher (lower) in summer (winter/spring) when the
MBC values tend to be low (high). We have added these descriptions in Sect. 3.1.2.
Furthermore, the same analysis was made for all the other comparisons at Arctic sites,
as shown in Figs. S3–8 in the supplement.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 393: “The babs, and therefore the MAC, for the PSAP and the two CLAP
instruments (CLAP1, CLAP2) agree to within 8%”. – Based on the values reported inTable 2, the difference appears to be somewhat larger?
Answer 1-9: In the revised manuscript, we have modified this part in Sect. 3.1.2 asfollows: “The babs, and therefore the MAC, for the PSAP and the two CLAP instruments(CLAP1, CLAP2) agree to within 13 % at λ = 550 nm, indicating a small difference inthe performance of these instruments.” Thank you for pointing it out.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Caption of Table 3: please explain the variable “V” in the caption (i.e. inter-quartile
range in relative terms). This comment may apply to other table captions too.
Answer 1-10: We have added the explanation of VMAC in the caption of Table 3.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 462: check consistency of quoted V-value with Table 3.
Answer 1-11: We have corrected the VMAC values in Sect. 3.2.1.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 475: PSAP derived absorption coefficients are around a factor of 1.5 larger thanaethalometer derived absorption coefficients at Ny Alesund. Zanatta et al. (2016)
reported a systematic difference the commercial and ITM variants of the PSAP, which
is approximately in this range (Sect. 2.3.2 and Table 3 in their manuscript). Whichvariant of the PSAP is operated at the Ny Alesund site?

6Answer 1-12: A custom-built PSAP (Krecl et al., 2007) was used at Ny-Ålesund andcommercial ones were used at Alert and Barrow. We have clarified this point in Sect.
2.2.3.1.
It seems that Zanatta et al. (2016) reported correction factors for the custom-built PSAP
through comparison with MAAP at 3 European background sites. In our revisedmanuscript, we have added analysis of COSMOS-MAAP comparison at Ny-Ålesund
during 2017–2020 in Sect. 3.2.3, as described above in the “Major revisions”. We havefound that MACcor (PSAP) values are 17 % and 20 % larger than MACcor (MAAP)
values for 1-h and 24-h data, respectively. The systematic difference of babs measured by
the custom-built PSAP and MAAP was also reported by Zanatta et al., 2016, although
their difference was much larger (more than 59 %) than that of our measurements at NyÅlesund. We have added these descriptions in Sect. 3.2.3.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 586: repetition of “at Alert”.
Answer 1-13: We have corrected it in Sect. 3.6.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> The authors discussion that BC mass from MAAP default instrument output
systematically differs from their direct measurements due to a difference in factory
default and actual MAC value. A brief discussion for BC mass from aethalometer
default instrument output compared to their direct measurements should also be
included, e.g. in Sect. 4.
Answer 1-14: In the revised manuscript, we have mentioned a manufacturer’s
suggested MAC value for Aethalometer in Sect. 2.2.3.2, which is given by 14625 / (λ[nm] ×C0) [m2 g–1]. The suggested MAC value is 7.1 m2 g–1 for λ = 590 nm and C0 =3.5. This MAC value is considerably lower than the MACcor (Aethalometer; λ = 590nm) values estimated in our study (12.5 m2 g–1 at Alert and 10.2 m2 g–1 at Ny-Ålesund
for 1-h data). We have added these descriptions in Sect. 4.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

7

Responses to Anonymous Referee #2––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> The presented article is an important contribution to the understanding of therelationship between black carbon and light absorption in Arctic regions. The methods
used are appropriate for the available data and the results are well supported with
figures and tables. In general, the manuscript is well written.
However, the reviewer disagrees with the authors in one particular point in theinterpretation of the results. The reviewer recommends the manuscript for publication
after considering the comments.
Answer 2-1: Thank you for the constructive comments. In response to the disagreed
point, we try to make our assertion clear in the following answers.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> General comment:
The authors use a filter-based absorption photometer to determine the massconcentration of black carbon. In line 136, the authors write: "We critically re-examine
the concepts underpinning the use of filter-based instruments to estimate MBC." Thisapproach should have been discussed more critically.
The COSMOS is basically a filter-based absorption photometer that evaporates thevolatile components by heating, so that only the absorption of a soot core is measured.
This device was "calibrated" (compare lines 641 to 644) using an SP2 with one type ofaerosol. This is critical because two different properties of the soot were compared.
However, the determination of a value for MAC(COSMOS) for this aerosol type iscorrect. In this study, an application to arctic aerosols was tested. Although comparisonsof MBC with COSMOS and SP2 for Alert and Fukue Island stations agreed within theuncertainties, the conclusions should not be that the COSMOS using a universal MAC
value is a "secondary reference device" for MBC. As with any filter-based absorption
photometer, the result should be labelled as "equivalent mass concentration" (seePetzold et al. 2013) using a specific MAC.
The comparison of two filter-based absorption photometers, where the volatile shell isremoved in one device, thus shows the light absorption enhancement factor, which is an
important component of the MAC. Furthermore, like all other filter-based absorption
photometers, the COSMOS is subject to certain errors such as uncertainty due to8particle penetration depth (see Nakayama et al. 2010).
In the view of the reviewer, in this study only the comparisons of a light absorption
photometer with an SP2 strictly fulfil the conditions to derive MAC values. The othercomparisons of COSMOS with other filter-based absorption photometers are rather
studies on the enhancement factor.
The reviewer does not want to criticise the quality and overall results of this study, but
to point out that soot in particular is a sensitive issue due to different metrics and thecommon nomenclature should therefore be followed very strictly. The reviewer
proposes to refer to the facts described above and to denote the BC mass concentrations
derived from COSMOS as "equivalent" mass concentrations.
Answer 2-2: We understand the importance of terminology recommended by Petzold etal. (2013) that clarifies the methods of BC measurements. However, considering thefundamental difference between COSMOS and the other filter-based absorption
photometers, we would like to avoid using the term “equivalent black carbon (EBC)”
for COSMOS measurements.
The COSMOS, which always uses a heated inlet and a PM1 cyclone, is designed tomeasure BC mass concentrations (MBC), while the other filter-based absorption
photometers are designed to measure aerosol absorption coefficient (babs).
The present and previous studies (e.g., Kondo et al., 2011; Ohata et al., 2019) show that
MBC measured by COSMOS and SP2 generally agree to within 15 % in various
environments including urban and remote atmospheres. These previous studies also
evaluated the effect of particle penetration depth within a filter on COSMOS
measurements and revealed that this effect is not significant under the typical variationsof ambient BC size distributions. Furthermore, the previous studies show that variationsof the mixing states of BC also have little effect on the COSMOS measurements. These
results demonstrate that MBC measured by COSMOS is generally traceable to MBC
measured by SP2 (i.e., “refractory BC mass concentrations” in the terminology by
Petzold et al. (2013)) in various environments (not only the environment in Tokyo,
where the standard COSMOS is “calibrated”).
These points are essentially different from the babs measurements by the other filter9

based absorption photometers. The babs values measured by the other filter-based
absorption photometers may be related to MBC, but the potential contributions frombrown carbon, mineral dust, and lensing effect of BC coatings are included in babs
values. Therefore, linearity of the conversion from babs to MBC can be much more
uncertain for these instruments. This also leads to a difficulty for simply interpreting babs
values reported by two filter-based absorption photometers with and without heated
inlets as “enhancement factor” from lensing effects. For example, the time variations ofyearly MACcor values for different filter-based instruments were not correlated at NyÅlesund (Fig. 7a), which means it would be difficult to discuss the enhancement factor
in our study. This result is attributed to the inherent uncertainties of babs measurements
for each instrument and different particle size cuts for each instrument (e.g., different
contributions possibly from mineral dust).
For these reasons, we would like to avoid using the term “EBC” for COSMOS
measurements, even though the COSMOS is also a filter-based absorption photometer.
Our focus is not on the enhancement factor and we believe that MAC values estimatedin this study can be used to obtain harmonized MBC values from babs measured at Arctic
sites.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Further comments:
Line 177: better use attenuation coefficient instead of extinction coefficient
Answer 2-3: We have modified it in Sect. 2.2.2.1, as the reviewer suggested.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 188: give values for ffil
Answer 2-4: Following the reviewer’s suggestion, we have modified this part in Sect.
2.2.2.1. Please see the Answer 1-2.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Lines 191 – 207: It is common for filter based instruments to report equivalent black
carbon assuming an MAC. But in this case it is unfortunate to have to different MAC
values when changing the filter type. The MAC is a property of the particle and not of
the instrument. The reviewer suggests to attribute the changes of the sensitivity to thevalue of ffil instead of MAC.
Answer 2-5: We agree with the reviewer’s suggestion. In the revised manuscript,
10

changes in the sensitivity due to filter types have been attributed to ffil (i.e., B value) in
Sect. 2.2.2.1.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 271: Magnetite is not a good proxy for iron oxides to the reviewer's knowledge.
The imaginary part of magnetite seems to be very high. This value cannot explain thereddish colour of many minerals. Alternatively, the refractive indices of hematite could
be considered (e.g. Sokolik et al. 1999).
Answer 2-6: Although hematite and magnetite are the major light-absorbing iron
species, the FeOx particles detected by SP2 in East Asia and in the Arctic were mainly
magnetite (Moteki et al., 2017; Ohata et al., 2018; Yoshida et al., 2018; 2020).
Therefore, we assume the refractive index of magnetite for Mie calculation in Sect.
2.2.2.2. We have clarified this point in the revised manuscript.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Lines 315–320: How large are the uncertainties of the correction value C0?
Answer 2-7: The uncertainty of C0 is approximately 25 % (WMO/GAW, 2016). We
have added this statement in Sect. 2.2.3.2.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 335: The MAAP was originally calibrated for the wavelength 670 nm. Was thecorrection factor of 1.05 taken into account to adjust the wavelength (see Müller et al.
2011)?
Answer 2-8: A correction factor of 1.05 due to the wavelength shift from the nominal
value (Müller et al., 2011) was applied in this study. We have added this statement in
Sect. 2.2.3.3.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––>> Line 877: Line break between Petzold (2004) und Petzold (2005)
Answer 2-9: We have corrected it. Thank you for pointing it out.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

11