Dear Authors,

As you can see in the referee's comments, some significant concerns and questions remain with your manuscript. While you have revised your original manuscript, this did not properly address many of the initial major comments from the referees in the first round of reviews. In some cases your revisions seem to have added more confusion and convolution to the manuscript. I agree with the referees that the revised manuscript is still quite difficult to follow and that many of the claims and conclusions are not supported in a clear and logical or consistent manner by the data presented.

Please carefully take the referees' comments, from both rounds of reviews, into account as you revise your paper to address these concerns. Your paper will require major revisions to reach the quality necessary for publication in ACP.

Best regards,
Ryan Sullivan

Dear Authors,

Thank you for submitting your revised manuscript. Both Referees found that the revised manuscript was largely an improvement over the previous version. There are still some specific issues that require further proper revision before the paper can be accepted to ACP. Please carefully address the remaining issues raised by the Referees.

In particular, note the comment from Referee 2 regarding the optical property measurements and relation to the BC-to-OA ratio: "First, the finding that there is a significant dependence of BrC on the BC-to-OA ratio (e.g. line 360-362) does not necessarily follow from the data. The dependence of AAE on BC-to-OA ratio shown in Figure 3 can be due to the mere fact that at low BC-to-OA ratios OA has more weight in dictating AAE."
"Second, the concern that I raised in the last two reviews about AAE plateauing at ~1.8 for large BC-to-OA ratios still holds."

Also, as Referee 3 stated, please clearly explain in more detail how this manuscript relates to the closely related paper from your group by Gilardoni et al. that was published in PNAS recently. The two papers clearly relate strongly to each other. This overlap and similarity is not necessarily a negative, provided that the two papers add to each other and do not repeat an excessive amount of information and results, and provided that the relationship between the two papers is presented clearly.

Also note that both Referees rated the presentation quality of this manuscript as only "Fair". This reflects the often clunky and unclear language used. Please carefully review your writing and rephrase to increase the clarity of the writing as needed.

Ryan Sullivan

Dear Authors,

Thank you for your revised submission. You have now addressed the major questions and concerns from the Referees, and your manuscript is now suitable for publication in ACP. Please work carefully with the copy-editor to ensure your manuscript's language sand writing meets the journal's standards such that your science can be clearly communicated with the readership.

I did notice an error regarding the stated cut size of the impactor used for the SMPS. This was in response to Referee 3's question:

"Line 166: Please include the 50% cut-off diameter for the impactor at the SMPS flow rates used.
AA. We indicated the 50% cutoff diameter = 0.457 μm at the aerosol flowrate = 0.6 lpm"

I think you are using the nozzle diameter, 0.0457 cm, incorrectly as the cut size. The cut size is not usually this small at such a low flow rate. It also wouldn't make sense to make SMPS measurements up to d_m = 750 nm if the impactor is removing particles with d_a > 457 nm. Please recalculate the cut size for the impactor flow rate and nozzle diameter you used. The SMPS manual, or Hinds' text provides the equations to calculate this.

Best regards,
Ryan Sullivan
ACP Co-Editor