Please see attached PDF for the review of this paper.

General Comments

Overall this paper provides a good overview of the QUANT study and some salient results. A few clarifications are needed, as outlined below.

Section 2.3 should describe any harmonization of the data from the sensors’ reporting frequencies to a standard frequency, i.e., what was the common time frequency for which the measurements were averaged for analysis and comparison with the reference? Or was this done differently for the native reporting frequencies of each instrument? Finally, in the available QUANT dataset, are the measurements reported at the initial sampling frequency or at the down-averaged frequency (or both)?

In Section 3.1, results are only presented for the PM2.5 data. I would suggest that information on the inter-sensor precision for all measurands should be provided, maybe as part of the supplemental materials, since this is a basic feature of the different sensors which can inform all the other results presented later.

Since one of the goals of this paper is to introduce the QUANT dataset as a public resource for long-term performance assessment, it may be worth adding a section which details the dataset itself, or expanding the “Data Availability” section to do this. Some points to consider for this section would be the size of the dataset, the parameters included, what quality controls are applied (especially to the reference data), and any licensing of the dataset or policies associated with its use. Currently, the link provided in the “Data Availability” section does not seem to be working; presumably this will be active by the time of publication.

Specific Comments

Line 19: suggest clarification that this technology is providing the first steps for regions without pre-existing monitoring.

Line 34: “end-users” should be “end-user”.

Line 35: “capabilities the” should be “capabilities, the”.

Line 54: “helping mitigating” should be “helping to mitigate”.

Line 61: suggest removing “of”.

Line 90: “extensive” is repeated.

Line 118: “inlfuenced” should be “influenced”.

Line 128: “Quant” should be “QUANT”.

Line 142: “Polludrone: Poll” should be “Poll: Polludrone”.

Figure 2: Suggest using the same colors for the different sensors between the left and right panels.

Figure 3: Suggest moving this figure and associated discussion to the next section, since it is an assessment of performance against a reference rather than an assessment of inter-sensor consistency.

Figure 8: These seems to be a switch between the use of uncalibrated and calibrated data between the left and right panels as well. It is not clear what these calibrations are based on, and the application of the calibration might be a contributing factor to the difference in performance, together with the move between sites. It may be more illustrative to present a comparison at both sites with either the calibrated or the uncalibrated data only.

Lines 329-331: Sentence may be incomplete.

Lines 373-374: The meaning of this is unclear; does this mean that results from 2 systems were combined (e.g., to increase coverage)? Or were coverage and REU assessed separately for each device and then data from both devices combined to create the density plots of Figure 10?

Line 374: “systems, not” should be “systems not”.

Line 423-424: Please explain further the use of the reference data as a prior in this method.

Lines 435-436: Consider changing one of “developments” or “developing”.

Lines 460-469: I would suggest adding a sentence earlier in the document (and perhaps in the abstract) noting that further analysis will be left for future publications. I was expecting at several points a more comprehensive presentation of results across all pollutants and for all phases of the study, while only particular aspects of the results were highlighted. This is alright, but I think it needs to be more clearly stated up-front that this is not a comprehensive presentation of the study results. I would also suggest, as a topic for future work, examining the manufacturer-suppled calibrations in more detail, seeing where these improved upon the raw and where they perhaps did not, and how robust the calibrations are to environment changes and movement of the sensors to new sites. This is briefly presented in several figures, e.g., Figure 8, but a more comprehensive assessment across all sensors and pollutants could be made.

Supplemental Information, Lines 4-6: Indicate which of these channels and/or data products were considered for this study. Also report the sampling frequency for this sensor.

Supplemental Information, Line 55: “y” should be “and”.

I recommend this publication. However, as this is an important study that can be reused or used as a basis for others, I think it is important to go into more detail, especially methodologically, so that it can be continued and used, as the authors suggest at the end.