|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. They say, "[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 statement is clearly at odds with the first sentence of the Abstract, which 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. . . ". In the "Summary and Outlook" section , the authors state that their analysis has demonstrated an "accuracy of 96% and 99% for the extinction coefficient and scattering coefficient channels." This is not correct; they have instead compared suites of instruments whose accuracies they have neither stated nor evaluated.
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 "absolute 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. And are these uncertainties actually propagated from measurement uncertainties, or do they just represent the standard deviations of the measurements? This is not clear.
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. However, these lines do not pass through the axis origins, so they cannot be 1:1 lines. In addition, the authors 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.
Line 25 says that the measurement of SSA requires the simultaneous but independent observation of two parameters--extinction and scattering. At line 137, the manuscript says that "the monitor should be thought of as providing separate extinction and SSA values with the scattering channel a derived measurement". This latter statement is incorrect and should be amended.
There are a number of typos and minor wording changes needed in the manuscript; a partial list follows:
Line 32: Change "aerosols optical depth" to "aerosol optical depth".
Line 45: Replace "aerosol optical parameters" with "coefficients". SSA is an "optical parameter" also.
Line 47 and elsewhere: capitalize only trade names, not instrument types (e.g., integrating nephelometer, cavity ringdown, ammonium sulphate, black carbon should not be capitalized).
Line 70: The atomizer is a "Collison-type", not "collision-type”.
Lines 94-95: The text describing the roles of the MFCs still does not match Fig. 1.
Line 111 and many places elsewhere in the text: "data were". "data" is a plural noun.
Lines 123 and 124: Don't abbreviate "approximately"
Line 125: Define "LOD".
Line 181: "from the generation system"
Table 3: I don't understand what the columns "M", "Std m", "B" and "Std b" are. From the caption, there is a "standard deviation of the mean, intercept, standard intercept and R2". What do these mean? What is a “standard intercept”?
Fig. 5: The error bars are very small, suggesting that the instruments don't agree within experimental uncertainty. This is because the errors have not been appropriately calculated and propagated. The error bars should be much larger.
Line 217: Change "neither" to "either" and "nor" to "or" (because you have "no" in front of "systematic", this is currently a double negative).
Tables 3 and 4: Change the column label of "SSA" to "Nominal SSA" to be consistent with the text.
Fig. 10 caption: What is "the absorption channel”? There are two lines here. One is from the CAPS PMssa, which does not have an "absorption channel".
Line 277: What is meant by, "the deviations being randomly distributed around zero"? Shouldn't they be randomly distributed about the mean of the measured value?
Line 284: Place "relative" in front of "uncertainty".
Table 6: The Table caption says "Absolute uncertainty of the SSA measurement". Is this in fact the absolute uncertainty, or is it the standard deviation of the mean value (which is not the same as the uncertainty)?