This study provides a comprehensive characterization of the modified airborne dual-wavelength CAPS monitor ("A2S2") based the commercial version of Aerodyne CAPS-PM_SSA. Through both laboratory investigations and field deployment, the modified instrument's performance has been validated. The results also reveal limitations in accurately characterizing the optical properties of larger particles, which could have implications for future studies employing CAPS-PM_SSA. While overall this study fits the scope of AMT, the current manuscript appears to resemble a measurement report, lacking in-depth discussions regarding the obtained results:

Major comments:

1. As this study includes some field deployment of their new instrument. However, it lacks some in-depth discussions. From a scientific point of view, what are the key findings from the observation results? According to a technical standpoint, how do the authors anticipate that their findings from the airborne measurements will implicate the future deployment of CAPS instruments?
2. I am concerning about the baseline issue as described by Pfeifer et al. (2020). As the aircraft travels over the different backgrounds rapidly, will this issue persist in the airborne deployment of CAPS, even with more frequent baseline characterizations? The authors should give some more details regarding to this.
3. Page 10, Section 3.1.3, there are some other intercomparison studies for the CAPS-PM_SSA (for example Perim de Faria et al., (2021)). The authors may improve their discussions by comparing their results with the previous studies.

Minor comments:

1. Font size is too small in some of the figures. Especially Figure 12 and Figure 13.
2. Figure 11, it is difficult to distinguish the symbols in the graph, suggest improving the graph.

References

Perim de Faria, J., Bundke, U., Freedman, A., Onasch, T. B., and Petzold, A.: Laboratory validation of a compact single-scattering albedo (SSA) monitor, Atmos. Meas. Tech., 14, 1635–1653, https://doi.org/10.5194/amt-14-1635-2021, 2021.

Pfeifer, S., Müller, T., Freedman, A., and Wiedensohler, A.: The influence of the baseline drift on the resulting extinction values of a cavity attenuated phase shift-based extinction monitor (CAPS PMex), Atmos. Meas. Tech., 13, 2161-2167, 10.5194/amt-13-2161-2020, 2020.

Review for :”Characterisation of particle single scattering albedo with a modified airborne dual-wavelengths CAPS monitor” by Chenjie Yu et al.

This study uses a combined measurement package consisting of two commercially available CAPS PMSSA instruments. Laboratory inter-comparison results with several reference instruments were presented, and a flight analysis was performed using a nephelometer and remote sensing techniques for validation. The topic is within the scope of AMT and will attract many readers, but the current script lacks new insights and clarity.

Specific Questions and Comments

Please harmonize the script. Especially during introduction some statements were unreasonable

uncertainties were given as an absolute Value. Please give context for a relative value as well. There is no error analysis of the techniques being compared. Instead, variance the measurements is used for uncertainty. Please add this detail.

Please add an error analysis for the Angström exponent.

Why is there no approach for the Extinction angstrom exponent or absorption angstrom exponent, since the combined instrument is capable of deriving those (Weber et al 2022)

Please add a statement or error analysis for using the Anderson and Ogren (1998) or Massoli (2009) algorithm on the nephelometer correction, since measurements were done at high polluted conditions.

“Aerosol Absorption Spectral Sizer” A2S2 seems like a great acronym (Popculture), Please explain why you use Sizer, since I would associate the term with a diameter.

Please discuss if the combined instrument is within the requirements for modelling studies. Laj et al. 2020

Please overlook your graphs again!

Major Comments

Line 21 “550 hPa […] 315 K” Please add flight height and where the 315 was measured (inside the Instrument or Cabin…)

Line 29 The conclusion to use the A2S2 to replace the nephelometer seem far fetched. At least three CAPS with different wavelengths would be needed to provide the same spectral information as a nephelometer.  Secondly, the CAPS PMssa provide no backscattering coefficient.  The Ecotech Aurora 3000 nephelometer seems to be a more reasonable TSI nephelometer replacement.

Line 76 “theoretically smaller truncation effect” Please add precise information. Additionally, impacts on ambient measurements, and refractive index on the truncation error were done for the nephelometer by Massoli et al. 2009. This seems to be a white spot for the CAPS PMssa scattering truncation correction.

Line 85 “fast response” the CAPS PMssa needs about 7 seconds to flush the cavity. Is this still suitable for aircraft operation?

Line 94-95 “These Measurements […] “Please step down a notch. This statement is daring.

Line 104 “act as a nephelometer” No, integrating sphere and integrating nephelometer are not the same. Please rephrase

Line 122 Critical Orifices need a sufficient delta P to have a constant flow. Is this given at all Pressure ranges you operate?

Line 142 The Allan variance gives you the lower detection limit with the highest precision at an certain integration time. Why is this not used for the measured flight data? (around 30 sec)

Line 198 Were the CAPS PMssa calibrated with CO2 as well to check for the geometry correction factor? Modini et al. 2021

Line 249 How was the uncertainty determined?

Line 262 At lower pressures, the ratio of the flow between purge flow and sample flow may vary because critical orifices were used, causing disturbances

Line 300 A correction function based on the SAE (scattering angstrom exponent) would be suitable instead of applying a factor as soon a certain threshold is passed

Line 323ff Be more precise and the uncertainty of the EMS (extinction minus scattering); may fit well even for pure aquadag aerosols at low aerosol load. Discuss with Weber et al 2022

Line 356 High uncertainty of the angstrom exponent is most likely due the uncertainty of the CAPS scattering signal.

Line 364 mineral dust should have a different impact on the SSA for the different wavelengths since it is transparent at 630 nm but can absorb light at 450 nm. Please discuss. Can this be used for mineral dust determination?

Line 383 a SAE based function might decrease the discrepancies even more.

Line 396 “70%” Please show this reduction, Please Discuss why 30 sec was not used (Allan variance determined) as integration time (10 sec might be the flush time of CAPS and NEPH)

Line 398 “clean background environment” might be different because of sea salt aerosols

Line 403 Please mention your truncation factor here as well

Line 406 Please discuss the refractive index of mineral dust as well (wavelength dependence of the complex refractive index)

Line 414 “replacement for the NEPH on aircraft platforms” The nephelometer even works at lower pressure levels. Please rephrase!

Figure 1: Please redo the figure. Arrows do not point at the mirrors, the glass tube is missing

Figures: Please add grid-lines to all your graphs

Figure 9 Unit for dn/dlogdp is 1/cm^3, not nm!

Minor Comments

Line 16 “New pressure and temperature sensors” – Change to Additional or replaced

Line 24 Truncation error. Please add your truncation factor up here

Line 35 “ and this is also known” please rephrase

Line 57 Please a statement for the correction algorithm; Virkkula 2010

Line 57 “relative slow” please rephrase and be more specific: not suitable for measurements on an aircraft due to the time resolution

Line 61 “limited range of scattering angles” please rephrase by adding the precise angles

Line 62 “truncation issue” change to truncation error

Line 63 “recent advancement” by citing a 20 year old paper…. This is not recent

Line 87 Please explain the purge flow

Line 113 “into a single measurement monitor” into a combined package?

Line 123 “new […] sensors have been integrated” change to: existing sensors were replaced with

Line 126 please add the uncertainty of the sensors

Line 129 Please add the explanation of the short duty cycle here

Line 173 “AAC can generate truly monodisperse” do you mean, without multi charged particles? Or is the GSD (global standard deviation) at a value of 1?

Line 310-315 I suggest to add an table for all values for clarity

Line 330 Please at the wavelength you are comparing with. This paper mentions only the 630nm version

Line 369 Please add the value of the SAE

Line 377ff Please clarify “relative higher… relative poorer

Line 388 “adequantely replace the NEPH” Please rephrase, so it become clear, that this is true, when only the scattering coefficient at two wavelengths is needed.

References

Weber, P., Petzold, A., Bischof, O. F., Fischer, B., Berg, M., Freedman, A., Onasch, T. B., and Bundke, U.: Relative errors in derived multi-wavelength intensive aerosol optical properties using cavity attenuated phase shift single-scattering albedo monitors, a nephelometer, and tricolour absorption photometer measurements, Atmos. Meas. Tech., 15, 3279–3296, https://doi.org/10.5194/amt-15-3279-2022, 2022.

Virkkula, A.: Correction of the Calibration of the 3-wavelength Particle Soot Absorption Photometer (3 PSAP), Aerosol Science and Technology, 44, 706-712, 10.1080/02786826.2010.482110, 2010.

Massoli, P., Murphy, D. M., Lack, D. A., Baynard, T., Brock, C. A., and Lovejoy, E. R.: Uncertainty in Light Scattering Measurements by TSI Nephelometer: Results from Laboratory Studies and Implications for Ambient Measurements, Aerosol Science and Technology, 42, 1064-1074, 10.1080/02786820903156542, 2009.

Anderson , T. L. and Ogren , J. A. 1998 . Determining Aerosol Radiative Properties Using the TSI 3563 Integrating Nephelometer . Aerosol Sci. Tech. , 29 : 57 – 69

Modini, R. L., Corbin, J. C., Brem, B. T., Irwin, M., Bertò, M., Pileci, R. E., Fetfatzis, P., Eleftheriadis, K., Henzing, B., Moerman, M. M., Liu, F., Müller, T., and Gysel-Beer, M.: Detailed characterization of the CAPS single-scattering albedo monitor (CAPS PMssa) as a field-deployable instrument for measuring aerosol light absorption with the extinction-minus-scattering method, Atmos. Meas. Tech., 14, 819-851, 10.5194/amt-14-819-2021, 2021

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