1. General comments

Article presents a new version of the CEPAS instrument and its deployment in the air quality measurement station. The study evaluates the instrument operation during a single measurement campaign with only a little more than 2 weeks of data. Instrument operation and the campaign are well described. There could be some improvement in data evaluation and presentation.

The article title positions the CEPAS as a field instrument for absorption measurement. For that the instrument is expected to be compared with other absorption instruments. Here the main comparison is done with the aethalometer which has high uncertainty for absorption measurement. More focus should be put on the comparison with MAAP which has lower uncertainty for absorption. The wavelength dependence of CEAPAS and AE33 should be compared by calculating the Angstrom exponent.

The article falls within the scope of the Aerosol Research journal and could be accepted with minor revisions.

2. Specific comments

Abstract:

- please state that instrument measures absorption of aerosol smaller than 1 µm.

- in my opinion the term “field instrument” would be more accurate than “portable instrument”.

Results:

- please include a correlation plot of the absorption coefficient measured using MAAP and CEPAS at 638 nm.

- please include a correlation plot of absorption Angstrom exponent measured with AE33 and CEPAS.

- what is the uncertainty of the instrument for determination of absorption coefficient, BC and NO2 concentration?

3. Technical corrections

Line 83: “The light source is an RGB laser module (μRGB module, OptLasers),”

               The change of laser diode spectra can affect measurement of NO2 concentration.

               Are the laser diodes temperature controlled? How stable are the laser diode emission spectra?

Line 115: “uncertainty in laser wavelengths imposes uncertainty in converting the NO2 absorption cross section to absorption”

               In the Appendix A it is stated that during the calibration the laser spectra were measured. In that case there should be no additional                uncertainty for the calibration.

               The aerosol absorption measurements are not affected by small wavelength shifts. More care should be taken when measuring NO2                in the field.

Line 143: “BC concentrations are relatively low”

               Concentrations are low compared to what? You can state average BC concentration.

Line 154: “The     standard deviations over the whole data set were 0.28, 0.18 and 0.19 Mm-1 for the red, green and blue channels, respectively.”

                What was the averaging interval?

Line 172: “Figure also shows BC concentration from two filter-based photometers (AE33 and MAAP) for comparison.”

                Which wavelength was used for AE33 BC data?

Line 229: “Since the NO2 absorption affects the background signals of the measurement …”

               Please clarify the term “background signal” as it contains “instrumental” and gas absorption components.

               How fast is the drift of the “instrumental background” (Mm-1 per day)?

Line 259: “portable instrument”

                Please change to “field instrument”.

Line 355: “The underlying data is available from the authors upon reasonable request.”

                Please correct the declaration in accordance with the journal’s Data policy.

GENERAL REMARKS

The manuscript introduces a further development of the cantilever-enhanced photoacoustic spectroscopy (CEPAS) instrument, suitable for ambient air measurements in the field. The instrument was evaluated with respect to noise and stability in the laboratory and then moved to the urban background station SMEAR III in Helsinki, Finland, for a time span of 18 days. On SMEAR III one multiwavelength Aethalometer AE33 and one MAAP are operated continuously and served as reference instruments for this study.

The study was carefully conducted, and the presentation of the results is well structured. Given the amount of available data, the article serves as a feasibility study of the CEPAS instrument for ambient air measurements in a moderately polluted environment. To fulfil the requirements of an in-depth evaluation of the CEPAS instrument, a more detailed intercomparison with other reference instruments is strongly recommended. In this respect, the title of the article should better reflect its nature of a feasibility study.

Besides this general remark, the article fits well into the scope of Aerosol Research and can be accepted after minor revisions listed below have been incorporated.

SPECIFIC COMMENTS

1. In Section 3.1 the Allan deviation of the absorption coefficient measured by CEPAS is shown. It might be of interest for the reader so see the Allan deviation plot not only for averaged data but with all available data points shown. This would allow the reader to judge the noise of the instrument.
2. In Section 3.2, the correlation between absorption coefficient measurements by CEPAS and AE33 are shown for all CEPAS wavelength. The correlation is very high, but the slopes differ significantly from unity. Since a MAAP instrument is available at the site, the correlation between MAAP and CEPAS should be shown, particularly because the MAAP reports more robust absorption coefficient data than the AE33.
3. Since both CEPAS and AE33 report absorption Angstroem exponents, a comparison of both AAE values would be of high interest. As was shown by Weber et al. (2022), the are large uncertainties associated to the measurement of AAE. The authors mention such uncertainties or differences, respectively, in lines 205 to 207, but without further discussion. This discussion is definitely needed, since the main purpose of multi-wavelength measurements is to derive Angstroem exponents for all aerosol optical parameters.

MINOR ISSUES

1. Line 142: Suggested rephrasing: “The instrument was moved to the air quality monitoring station …”.
2. Line 159: Please correct “by approximately one order of magnitude …”.
3. Line 178: Please correct “assuming an absorption Angstroem exponent of 1.0 .”
4. Line 289: The meaning of the phrase “will slowly flow pass the ... “ is not clear, please check.

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, doi: https://doi.org/10.5194/amt-15-3279-2022, 2022.