|The revised manuscript by Costabile at al. uses Po Valley field data to show that AAE depends more on the OA-to-BC ratio than it does on OA levels alone. High AAE, indicative of brown carbon, are found mostly in the droplet mode, which they correctly identify as evidence of secondary brown carbon formation by aqueous processing. AAE is also correlated with high nitrate concentrations.|
The main aspect that is missing is the authors do not show how AAE depends on ammonia, other than a high correlation in Table 1. Since ammonia has been identified as a key reactant in brown carbon formation (1-8), this relationship is of special interest to atmospheric chemists, and the paper’s impact would be greatly increased by including this data analysis, e.g. in Figure 2.
Line 66: Can the authors clarify the relationship between Gilardoni et al. (2016) and the current study? For example, Figure 4b in Gilardoni et al. (2016) is very similar to Figure 5c, even if it is taken from a different dataset. The manuscript does not yet do enough to explain the relationship between the two papers.
Line 80: How/why is dust associated with high fOA? Or is this only an attribute of HULIS?
Line 453: Somewhere in the manuscript the authors should explain why these index of refraction values are “peculiar”
Figure 5: Two of these data sets (a and c) have been shown in previous figures on the same axes, but with different variables used for color coding. Given that the color coding looks about the same, this suggests that a graph of albedo vs droplet mode aerosol score might be useful. Just a suggestion.
Line 29: This statement, while correct, should be accompanied by several literature references.
Line 43: This statement is inaccurate unless qualified. Are the authors referring only to field measurement studies?
Line 94: The abbreviation PSAP is used here before it is defined 3 paragraphs later.
Line 166: Please include the 50% cut-off diameter for the impactor at the SMPS flow rates used.
Line 384: The reference to Figure 1 in the supplementary material is confusing. The figure caption makes no mention of showing case study data, and the figure does not identify the case study data in any way.
Line 399: “mycrophisical” should be “microphysical”
Figure 1: the PC2 symbol in the legend above the graph is not the same as the one on the graph
Figure 4: Is the local time at the Po Valley sampling site the same at UTC? There should be a statement to this effect.
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