Dear authors,

I have received one review of your revised manuscript; the reviewer was not satisfied with the lack of a propagation of uncertainties, necessary for a validation/closure study such as this, and felt that comments from the previous round of review were not addressed adequately. I agree that more should be done in this regard before the manuscript can be accepted for publication. As a result I request further revisions; please see the reviewer's detailed comments on this and other subjects below:

"This revised manuscript describes a comparison between the recently developed CAPS PMssa monitor (Aerodyne Research, Inc.) with more established techniques. In response to an earlier review, the authors have clarified that the intent of the manuscript is a comparison, rather than a fundamental evaluation of the performance and accuracy of the instrument. This confusion was a result of the clearly stated first sentence of the Abstract which (still) says, "An evaluation of the performance and accuracy of a Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor . . . was conducted. . . .", and, on line 62, "the present optical closure study intends to quantify uncertainties in the measurement of the primary optical properties and the resulting SSA by the CAPS PMssa. . . ". Instead, on line 212, the revised manuscript now states, "This paper does not address in any explicit way, nor was it designed to address, the question of the absolute uncertainties of the different measurement techniques. It was designed to address the question of how well they correlate." This last statement is clearly at odds with the earlier statements. Furthermore, a "closure" study--which is mentioned at several points in the manuscript--is not an evaluation of "correlation", but of how well the instruments compare when various measurements are combined. Correlation is not particularly useful--what if the measurements were highly correlated by disagreed in scale by a factor of 10?

Essential to any closure study is a propagation of uncertainties from the raw extinction and scattering measurements through to calculated SSA so that the closure can be quantitatively evaluated. One hopes that, at the end of such a study, a final statement such as, "the measurements agreed with each other within expected uncertainties" can be made. Unfortunately, because errors have not been propagated in the comparisons between the techniques, the usefulness of this multi-instrument comparison is questionable. The instruments certainly appear to agree very well, but is this level of agreement within the expected uncertainties stated in Table 6? Please note that the uncertainties may be lower than the values cited in this table because of the averaging done for this analysis; this needs to be properly accounted for.

In addition to this issue, the authors have not responded to concerns expressed in the first review regarding the slopes of the fitted lines. They pointed out that the lines shown on the graphs were 1:1 lines, not fitted slopes (and have so indicated in the figure captions), but did not respond to the concern that the values in the tables did not match regressions performed by the reviewer on the raw data in the supplementary tables. Please confirm that the regressions are correct. The regressions are forced through zero, although there is no justification stated for this choice, and the regressions are one-sided and are not weighted for uncertainties, which are likely heteroscedastic, in the data. This should be rectified, as it will affect the uncertainties in the fit parameters.

The equations need to be reordered and the variables fully defined. Equation (5) should precede Eqs (3) and (4), as it is described first in the text. "C'(n)" is not defined. In Eqs (1) and (2), the various sigmas are not defined. They are defined instead following Eq (6), when they should already have been defined. Angstrom exponent is defined again after Eq. (6).

The description of the SSA measurement as a "direct" measurement or a "derived" measurement, and that of scattering, is mixed in the manuscript. For example, line 39 says that the measurement of SSA requires the simultaneous but independent observation of two parameters--extinction and scattering. At line 172, the manuscript says that "the monitor should be thought of as providing separate extinction and SSA values with the scattering channel a derived measurement", presumably because scattering is calibrated with the extinction measurement, making it not a fully independent measurement. Also line 380 says that the CAPS PMssa provides a "direct measurement of the scattering and extinction coefficient. . . as well as indirect measurement of the absorption coefficient and single scatter albedo" These statements should be reconciled.

There are a number of typos in the manuscript; a partial list follows:

Line 32: Change "aerosols" to "aerosols'" (possessive)
Line 34 and throughout manuscript; formatting of references.
Line 47 and elsewhere: capitalize only trade names, not instrument types (e.g., integrating nephelometer, cavity ringdown, ammonium sulphate, black carbon).
Line 70: The atomizer is a "Collison-type", not "collision-type"
Line 80: "measured SSA measured"
Line 103: "of the scattering"
Line 117: "and to"
Line 119: "given in Tab and n"
Line 131: "corrections"
Table 5 has a different font
Lines 205, 206: "data were". "data" is a plural noun.
Table 6: "sep", "ssp", "sap" undefined, typos "polydispers" and "sigam".
Line 230: "from the generation system"
Line 235: "aerosolsused"
Line 319: "offsets" should be "intercepts"
Line 326: "3.64" needs units
Line 343: Fig. 14 contains points from AS, which contains no light-absorbing fraction
Line 371: What is meant by "the accuracy was 91%"? The slope of the regression line? Accuracy has not been evaluated."

Best wishes,

Andrew

Dear authors,

Thank you for your revised manuscript; I have received one review of it from a referee of the previous version. The referee and I appreciate the additional work put in here, particularly in the estimation of uncertainties. The referee has a number of minor comments on this version (mostly language issues), which I have reproduced below with some notes of my own.

I would be grateful if you could further revise the manuscript to address the below comments. I expect that, after this, I will accept the manuscript for publication in AMT without going out for another round of review. I know that this paper has been in review/revision for somewhat longer than normal and appreciate your patience. These comments are as follows:

I'd like to thank the authors for including a thorough analysis of the uncertainty propagation of the various combinations of instruments, and for making other improvements in the manuscript. I strongly recommend that they incorporate the uncertainties into their discussion and conclusions, focusing on whether agreements between the different instrument combinations agree within these carefully calculated uncertainties. This would be facilitated by plotting these uncertainties, rather than standard deviations, on the x-y plots. With these changes, the manuscript should be acceptable for publication following a substantial number of technical and minor edits. [Editor’s note: a few additional sentences, and the plot modifications, would be good here. If the goal is to assess the agreement then in general I think uncertainty is more helpful to show than standard deviation.]

Minor changes and technical edits:

1) Line 23: change "lowest" to "best". "Lowest" is ambiguous; is low accuracy good or bad?

2) Line 29: change "measurement" to "determination". SSA is not directly measured.

3) Line 40: define sigma_ep, sigma_sp, and sigma_ap here; they are not defined in Eq. 1.

4) Lines 45-65, and elsewhere in the manuscript: consistently use the mathematical notation for sigma_ep etc. rather than writing out the variable name each time.

5) Lines 53 and 59: Change "measure" to "determine"

6) Lines 71 and 72: Change "aerosol" to "material"--it's not an aerosol until it's nebulized

7) Lines 113 and 114: Move this sentence to just before Eq. 5.

8) Line 116: Move Eqs. 8 and 9 to just after Eq. 4. You are using Angstrom exponent here; you need to go ahead and define it here.

9) Table 4: The column labels "Blue", "Green" and "Red" make no sense. The row label "Angstrom Exponent" makes no sense. The row label should be "Wavelength Pairs", and the column headings a(B/G) etc. should be replaced with "450 nm/550 nm", "450 nm/700 nm", and "550 nm/700 nm". Get rid of the first row with the unneeded column labels "Blue" etc.

10) Eq. 6: Define sigma_PSAP here, not after Eq. 7.

11) Eq. 7: Why use omega_naught for SSA when you've been using SSA throughout the text?

12) Table 5: Explain what the values in parentheses on the last row are.

13) Line 151: Remove "change in". When particles are introduced into the cavity, there is a phase shift, not a change in the phase shift. (I think?)

14) Fig. 2: Make larger. Define LO_out. Use sigma_ep instead of sigma_e, for consistency with the text.

15) Line 172: Negligible differences between what and what?

16) Line 181: Change to, "The noise levels were < 1Mm^-1. . . ."

17) Line 182: Replace "lower than" with "<". Also, won't the truncation uncertainty vary with the size of the particles generated? For ambient conditions the uncertainty may be <4%, but maybe not for these lab tests.

18) Eqs. 8 and 9: There is no need to introduce the variable "w" in Eq. 9; just use "x" and then the two equations make sense with each other (but move both to just after Eq. 4/5).

19) Line 206: Again, this statement is non-sensical. Correlation is not useful and is not a goal; two measurements could be highly correlated yet disagree quantitatively. This study examines if the instrument combinations agree within uncertainties for a range of laboratory aerosols.

20) Table 6: Undefined parameters SEP, SSP, and SAP are introduced. Please use consistent nomenclature.

21) Line 215: I suggest making the Appendix a Supplemental Materials section instead. [Editor’s note: I leave this up to you. Practically, the main difference is that an Appendix will be part of the final journal pdf file while a Supplement will be a separate file also available for download from the final article webpage.]

22) Line 226: "is principle"

23) Fig. 4: In the caption explicitly state the time resolution of the data.

24) Line 254: Replace "bellow" with "<".

25) Line 256: Here is a place to clearly state, "These differences are within the combined uncertainties in measurements." Great!

26) Table 7: Are the values "Std m" and "std b" really standard deviations, or uncertainties in the slope and intercept, respectively?

27) Line 279: Replace "exclusive" with "purely".

28) Line 316: Here you refer to Table A3 as a "supplemental table". I agree the appendix should be a "Supplemental Materials".

29) Line 326: Replace "below" with "<".

30) Lines 330-337: You may wish to say that the PM_ssa technique measures absorption with uncertainties comparable to that of the PSAP for absorption values greater than some minimum value.

31) Section 3.4: In Fig. 12, the SSA value measured by PM_ssa for the purely scattering aerosol is far outside of the uncertainty value of 3% shown in Table 6. However, the expected uncertainty for a low scattering value is probably worse than 3% (Fig. 13 in the Appendix). In fact, it's hard to reconcile the broad-ranging uncertainties in the SSA values shown in Figs. 13 and 14 with the values in Table 6. Would it not be better to give a range of uncertainties and refer to the figures?

32) Generally, I suggest you focus on the level of agreement in Fig. 12 and state if they are within expected uncertainty in the discussion and conclusions. Throughout the manuscript, it would be much better to plot the uncertainty bars (derived from the analysis in the Appendix/Supplemental Materials) in Figs. 5, 9, and 11, rather than the standard deviation of each measurement, to show the level of agreement between the various methods.

33) There are numerous typographical and formatting errors in the references. For example, some references capitalize the title (e.g., Corbin et al.); sometimes the journal name is fully spelled out (e.g. Haywood et al.) and sometimes it's abbreviated (e.g., Heintzenberg et al.). There is an extra carriage return on line 409. These types of errors are characteristic of EndNote-type software; they format irregularly and must always be hand-corrected. [Editor’s note: the journal typesetters will give specific queries related to these and apply consistent formatting at the page proofs stage, but it may be worth checking through yourselves as well first.]

Please let me know if you have any questions.

Best wishes,

Andrew

Dear authors,

Thank you for your further revision of this manuscript. I have read it through and am happy to accept this version for publication in AMT.

Best wishes,

Andrew