Do you recommend any changes to the best practices outlined in the Data Handling sections? If Yes, please explain below and indicate which section you are suggesting changes be made to.

Answer Explanations

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  Expert 9

  Section	Yes (please explain)	No
  Use reporting guidelines from relevant reviews	0	1
  Keep data at a granular level (e.g., per particle, per individual)	0	1
  Report raw counts/concentrations and contamination and recovery values	0	1
  Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes	1	0
  Share data using FAIR principles	0	1

  The manuscript states that "particle mass follows a reverse particle size distribution with less mass being held in the smallest size fraction. The distribution can be calculated from the particles measured in each study and then used to project particle concentrations into unobserved regions of the particle size distribution for each study so that a more comparable (e.g. 5 µm – 5 mm in the example) can be estimated for both studies. It should be noted that these conversions are more accurate when particle properties are measured at the per-particle level." While I see the logic behind this, I'm wary of recommending it without at the very least also recommending detailed explanation of how this was done being included in supporting information. Such an explanation is necessary for full transparency and permit readers to verify conversions have been made correctly.
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  Expert 3

  Section	Yes (please explain)	No
  Use reporting guidelines from relevant reviews	0	1
  Keep data at a granular level (e.g., per particle, per individual)	1	0
  Report raw counts/concentrations and contamination and recovery values	0	1
  Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes	1	0
  Share data using FAIR principles

  1) In this case, is more of a maybe, as this, as noted in the paper, would include "[e]ach row would fully describe a single particle", and, for larger studies, not sure this would be feasible...
  2) This is not always possible and may lead to erroneous estimates (as I have reviewed in some manuscripts submitted for different publications). These calculations involve some estimation, such as density, that, particularly for MPs in the environment and subject to weathering and biofilm/eco-corona formation, the actual particle densities may be very different from those estimated. This should perhaps be noted. 
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  Expert 4

  Section	Yes (please explain)	No
  Use reporting guidelines from relevant reviews	1	0
  Keep data at a granular level (e.g., per particle, per individual)	0	1
  Report raw counts/concentrations and contamination and recovery values	1	0
  Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes	1	0
  Share data using FAIR principles	0	1

  1 - Use reporting guidelines from relevant reviews
  In line 499 a restriction is made to reporting guidelines in the microplastics field. It is recommended not to make this restriction, but to consider reporting guidelines fin the microplastics field and beyond.
  2 - Report raw counts/concentrations and contamination and recovery values 
  Lines 536 -542. Here some general statements are made with regard to subtracting blank values. It is recommended to make these general statements more specific and provide guidance-like statements on how to subtract blancs: use averages or individual values, how to deal with uncertainties, how to correct for rcoveries, etc.
  3 - Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes
  There is something wrong with the flow of line 556.
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  Expert 1

  Section	Yes (please explain)	No
  Use reporting guidelines from relevant reviews	1	0
  Keep data at a granular level (e.g., per particle, per individual)	1	0
  Report raw counts/concentrations and contamination and recovery values	0	1
  Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes	0	1
  Share data using FAIR principles	0	1

  Line 500:  suggest inserting "and harmonization" after 'reproducibility'.
  
  On the subsection on 'keep data...level', I suggest inclusion of statements regarding QA/QC samples.  Along with sample data, QA/QC data should be reported for every dataset  (add this after line 522).  
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  Expert 5

  Section	Yes (please explain)	No
  Use reporting guidelines from relevant reviews	0	1
  Keep data at a granular level (e.g., per particle, per individual)	0	1
  Report raw counts/concentrations and contamination and recovery values	0	1
  Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes	0	1
  Share data using FAIR principles	0	1

  LINE 533 & 534 would be better worded as: "Low analyte amount is one source of non-detect; the other is analyte loss during sample extraction."
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  Expert 6

  Section	Yes (please explain)	No
  Use reporting guidelines from relevant reviews	0	1
  Keep data at a granular level (e.g., per particle, per individual)	0	1
  Report raw counts/concentrations and contamination and recovery values	0	1
  Transform microplastic values to comparable units when comparing across studies with different particle size ranges or classes	1	0
  Share data using FAIR principles	0	1

  When units are considered, the decision of concentration-based g/weight or g/volume count-based N/weight or N/volume or should be around the meaningfulness of the unit to application.