Dear Authors

thankyou for your revision of your manuscript "Laboratory evaluation of particle size-selectivity of optical low-cost particulate matter sensors". It is clear that the reviewers consider that your work is a useful contribution, and could be published. However I don't think it is quite ready to publish yet. Accordingly I am asking that you make revisions to clarify parts of the manuscript. Some specific comments are provided below.

regards
Murray Hamilton
(Assoc. Ed. AMT)

**************************************************************
In the comments below the line numbers refer to the "authors comments" version of the manuscript.

I don't think that the response to the reviewers is adequate, in regard to the question of how 10 steps in the program of the GP50 might lead to 30 size bins. To be sure, you have said that the 10 steps are not related to the fact that there are 30 size bins (lines 164 to 169). However the source of this confusion remains. At line 141 you state that the aerosols are monodisperse, and the implication of this sentence is that this is controlled by the GP50. Thus in the light of your response, one wonders how the different size bins are chosen and/or created. Next at line 158 there is a sentence that still says that the GP50 program (of 10 steps) produced particles sizes in a logarithmically distributed range (0.45 to 9.78 microns). This is still the main clue to the reader as to how you arrived at 30 size bins.
Further, though this may not have been intentional, it could contradict the earlier assertion that your aerosols are monodisperse, since your use of "logarithmically distributed" is ambiguous - it could refer to a continuous statistical distribution. It would be better to say "logarithmically spaced", which is what I think you mean.

Thus I cannot work out where your 30 size bins come from (actually I count 28 bins in figure 5), until much later in the paper at line 204. There I see that the bins are actually determined by the APS measurements. But if you are to get logarithmic spacing something else must predetermine what the mean diameter (CMD) should be. So I'm still puzzled.

Another response that I consider inadequate is in regard to the comment where reviewer 1 raises the possibility of there being detections of multiple particles. It is a off-topic to give expected concentrations in China - what really matters here are the concentrations in your experimental setup, at the detectors. I cannot find any estimate of this number in your revised manuscript. Also the assertion that Mie theory gives the total intensity of the scattered light scaling as the sixth power of diameter is too simple here. The scaling is much more complicated because of the rapidly growing forward scattering lobe (with increasing diameter), so the scaling at a particular angle needs to be considered.

Additional comments:
Referring to an "optical aerosol spectrometer" could confuse. The spectrum determined is a size spectrum, not an optical spectrum. So saying "optical aerosol size spectrometer" would get around this.

At line 154 "The GP50 was operated in a method-mode, meaning that an automated program was used to dispense the liquids." is a bit confusing because "method-mode" is meaningless to me. Better to say "The GP50 used an automated program for dispensing the liquids."

In Figure 3, there is nothing that is white, only grey, in spite of the caption referring to something white.

The explanation of figure 4 needs improvement - in particular, in panel c) there is a legend that presumably refers to size bins as determined by the device under test. Things like this need to be be made more explicit.

At line 286 - South Coast of which country?
**************************************************************

Dear Authors,
I am satisfied with your response and can now recommend your revised manuscript for publication in AMT. There is one small typo that I noticed which is that the text in the supplementary material refers to a red circle in Fig. S3. It is in fact in Fig S4.
regards
Murray Hamilton
(Assoc. Ed. AMT)