Absorption instruments inter-comparison campaign at the Arctic Pallas station

Asmi, Eija; Backman, John; Servomaa, Henri; Virkkula, Aki; Gini, Maria I.; Eleftheriadis, Konstantinos; Müller, Thomas; Ohata, Sho; Kondo, Yutaka; Hyvärinen, Antti

doi:https://doi.org/10.5194/amt-14-5397-2021

Articles | Volume 14, issue 8

https://doi.org/10.5194/amt-14-5397-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-14-5397-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 8

Research article

|

06 Aug 2021

Research article |

| 06 Aug 2021

Absorption instruments inter-comparison campaign at the Arctic Pallas station

Eija Asmi, John Backman, Henri Servomaa, Aki Virkkula, Maria I. Gini, Konstantinos Eleftheriadis, Thomas Müller, Sho Ohata, Yutaka Kondo, and Antti Hyvärinen

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Final revised paper (published on 06 Aug 2021)
Supplement to the final revised paper
Preprint (discussion started on 28 Oct 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of Characterizing the Arctic absorbing aerosol with multi-instrument observations', Anonymous Referee #1, 24 Nov 2020
- AC1: 'Reply on RC1', Eija Asmi, 30 Apr 2021
RC2: 'Referee Review', Anonymous Referee #2, 15 Jan 2021
- AC2: 'Reply on RC2', Eija Asmi, 30 Apr 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Eija Asmi on behalf of the Authors (04 May 2021) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 May 2021) by Charles Brock

RR by Anonymous Referee #1 (12 May 2021)

RR by Anonymous Referee #2 (24 May 2021)

Suggestions for revision or reasons for rejection

The authors have made some attempt to reframe the manuscript toward focusing on intercomparing the absorption instruments, which is an improvement. However, the overall depth of analysis for this paper remains very shallow, and I'm left wondering what the main findings / recommendations of this work are for the scientific community as promised by the last line of the abstract: "provide some useful guidelines for instruments selection and uncertainty analysis". While the revision appears to be moving in the right direction, the manuscript is not yet at a level of scientific quality where I could recommend it for publication in AMT. Consequently, I recommend that the manuscript be returned to the authors for another major revision/rewrite that focuses on improving its scientific contribution and depth of analysis, conclusions, and recommendations.

General comments:

1) What is the new development, significant advance, or novel aspect of in situ aerosol measurements being communicated by this manuscript? Why are the results meaningful and important?

2) Based on the instrument measurement intercomparison presented in Section 3.3 and in Figure 4, it is clear that there are significant differences between the absorption measurement techniques. In particular, the AE31 significantly underperforms the AE33, while even the AE33 and COSMOS appear to have non-zero intercepts. While the PSAP-MAAP comparison looks pretty good, there are also some notable departures from the 1:1 line where the PSAP is seeing very low or negative values. What is going on with the instruments and air masses during these periods? This intercomparison is the only real meat of this study, and there needs to be some additional discussion about what is being observed here and why (not just simply campaign-average slopes and R^2 values). In addition to adding more discussion to Section 3.3, I'd like to see recommendations for how to use these results to make better, future measurements of aerosol absorption, scattering, and extinction at low-aerosol, Arctic sites like Pallas.

3) A lot is made of the extinction minus scattering (EMS) absorption estimates, which one would've expected to perform poorly under low aerosol loadings and high scattering:extinction ratios (i.e., where one is differencing two relatively large numbers to calculate a much smaller absorption coefficient). True to these expectations, the differencing technique does perform poorly. I don't see this as a real meaningful or important finding and suggest that Figure 3 is not necessary.

4) I understand the authors' hesitance to bring in new data and expand the scope of the study. Since there are previous long-term studies made a Pallas, it would be helpful to synthesize the results from those studies to contextualize the results being presented here. For example, what are the typical absorption, extinction, and scattering coefficients reported by these prior Pallas studies for June-July and throughout the year? How does the 2019 summer compare to "typical" conditions? Are there periods where aerosol absorption is higher than what was seen in 2019?

Specific Comments:

Line 125: What is ATN? What are the units of 60 ATN?

Line 142: What is insufficient about the manufacturer correction? This is not clear from this manuscript.

Line 171-172: Please add some additional discussion about why the absorption Angstrom exponent would be uncertain for aerosol with SSAs close to unity. Is this because the absorption signals are low or are there scattering interferences that are not well accounted for? This uncertainty is likely to be especially important for the Pallas site.

Line 225: I don't understand why is is noted here that EMS is traceable to SI units. This is just differencing the extinction and scattering measurements discussed in previous sections.

Lines 226: It's true that differencing two measurements avoids filter-based artifacts, but there can also be other sources of uncertainty that are quite large. In particular, when the aerosol SSA ~ 1, the EMS technique becomes very uncertain as it involves difference two numbers of similarly large magnitude relative to the absorption.

Line 239: How do the different geometry correction factors explain why the CAPSex and CAPSssa derived EMS calibrations would be different? Is this disagreement caused by the CAPS monitors or by the nephelometers?

Lines 239-241: I don't think that the instrument flow rates and/or inlet settings would have any impact on the correction factors. Hopefully, these calibrations were done with conductive tubing and at steady state conditions, and, ideally, with most of the actual ambient sampling setup. Are these sentences suggesting that there were significant particle transport losses that bias the calibration procedures?

Line 286-287: I don't think it can be stated that there is even qualitative agreement. This is particularly the case in July where the EMS inferred values are trending up even as the direct measurements show no obvious trend.

Line 288: Do you think that the high absorption values implied by EMS of ~ 3 Mm-1 are real or caused by taking the difference of two, large, noisy numbers?

Lines 312-313: I think it's fine to choose the MAAP as a common reference for comparison, but I'm confused about the rationale presented in the manuscript that there is "low demand of any artifact related post-correction in MAAP". What does this mean?

Lines 354-357: Please report averaging intervals for these limits of detection.

Line 357: What support is there for the statement that EMS methods are applicable above 0.1 Mm-1? Indeed, I would think that the applicability of these methods would be dependent on both the instrument signal-to-noise as well as the aerosol SSA.

Figure 2: Please make these panels full page width and add dashed lines to the lower panel that show the -0.2 to 0.6 Mm-1 range. Also, please include additional panels showing the time series for the scattering and extinction coefficient measurements as well as the SP2 BC timeseries.

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ED: Publish subject to minor revisions (review by editor) (26 May 2021) by Charles Brock

AR by Eija Asmi on behalf of the Authors (24 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (01 Jul 2021) by Charles Brock

AR by Eija Asmi on behalf of the Authors (07 Jul 2021) Author's response Manuscript

Short summary

Absorbing aerosols are warming the planet and accurate measurements of their concentrations in pristine environments are needed. We applied eight different absorbing-aerosol measurement methods in a field campaign at the Arctic Pallas station. The filter-based techniques were found to be the most sensitive to detect the minuscule amounts of black carbon present, showing a 40 % agreement between them. Our results help to reduce uncertainties in absorbing aerosol measurements.