Dear Authors,

While I appreciate the work done for improving the manuscript based on reviewers suggestions/comments, I still feel some additional corrections are needed before publication.

Essentially, based on the evidence provided, I am not fully convinced that « the OPC-N2 instrument demonstrated reasonnable agreement with reference instrument for the measured mass concentration », which seems like a very positive statement considering the obvious limitations for its applicability. I believe you have to be much more cautious in your conclusions.

First-of-all, there is a clear need for RH correction and this is far from being obvious. Humidograms in Fig. 8 are biased with respect to usual hygroscopic growth curves showing an increase from RH85% instead of RH70% for natural aerosol – your statement that « at RH <85% the hygroscopic growth of real aerosol is small » is actually false and should be RH70%- . I believe this is because your are considering ambient RH while the RH inside the instrument after travelling through the inlet is already lower. This can be fixed for the specific case of this study but without internal RH measurement, the correction may be quite different. This means, the instrument cannot be used without a clear RH controlling process. Then, you apply a correction assuming a κ-value. In practice, given the short duration of your experiment, a constant κ is acceptable, but it is no longer the case when instrument is used for longer periods. I believe you actually can see different aerosol populations in fig8 at high RH, leading to huge differences in OPC/TEOM. Applicability of a constant RH/ κ correction scheme is to be questioned for longer measurement periods. This has to be made more clear.

I am also questioning Table 3 and the huge difference for TEOM in PM2.5 and PM10, which does not seem to be discussed. There is a loss mechanism somewhere that is not accounted for it seems.

Your paper is an important one and this is why you have to be very careful with the final statements. Essentially, what needs to be known is whether the instrument responds to PM quality objectives for air quality studies, and, if not, which kind of corrections are needed for its applicability. For me, your study shows that the sensor does not respond to DOQ as formulated for PM2.5 or PM10 standard application but, applying specific correction procedures, you can reach a reasonable agreement. Not vice versa. In the case of PM2.5 where the standard is a 1-yr average, I also tend to believe the correction procedures will be meaningless, given the intrinsic variability in RH/ κ . This has to be made very clear in the conclusions, otherwise, the risk is high that your paper will be simply referenced as blank check for applying this sensor to any kind of PM studies, without the proper limitations in mind.

In conclusion, I expect that you modify the paper in this direction before proceeding with publication.

Regards,

Paolo Laj

Dear Authors,

Many thanks for the revised manuscript. Actually, we still disagree on the following sentence: "Comparison of the OPC24N2 to the reference optical instruments demonstrated reasonable agreement, for a low-cost sensor, to the measured mass concentrations of PM1, PM2.5 and PM10."
I do not think that "reasonable agreement" has anything to do with the instrument cost. Would you agree to write the same sentence if the OPC24N2 would be twice its current price. What matters is how adequate the instrument is for measuring PM.
For me the abstract should rather read: "Comparison of the OPC24N2 to the reference optical instruments shows some limitations for measuring mass concentrations of PM1, PM2.5 and PM10". The next sentence does not then start with "however" but directly explain how correction procedures can be implemented.
Hope you would agree with that.

The corrected conclusion section is fine for me together with the other corrections you have done as respect to the original manuscript.


Regards,

Paolo Laj