Dear Authors,

I received two referee reports for the revised version of your manuscript. I concur with the referees that some minor comments should be addressed before the publication of this manuscript can go forward. Please see comments below.

With kindest regards,

Daniel Knopf

Minor revisions:
Regarding the small spectral shifts presented as evidence for changes in DOPC conformation (e.g. P10L33, P11L7), the authors state in their response that "Spectra were averaged over 2000 scans, and IRRAS measurements were repeated at least three times to ensure reproducibility." What was the variability in the reported IRRAS peaks over the three trials? Was it smaller than the reported spectral shifts (i.e. < 2 cm-1)? I think that it would be very useful to clarify this in the text—I know that dark controls were performed, but the variability of the IRRAS bands for both the dark controls and the illuminated samples would be helpful to report.

P1L1 "Photosensitizing compounds like brown carbon can absorb UV light"
BrC isn't a "compound"-this makes it sound like it is an individual chemical species, whereas it is actually a complex mixture of light-absorbing species.

P1L1 "can absorb UV light and produce low volatile organic compounds (O:C ratio of 0.25 to 1)"
Where is this O:C range coming from? I wonder if a more general/"big picture" opening statement would be helpful here? Where are the low-volatility species coming from?

P2L4
Literature citations for the fates of phospholipids would be helpful.

P3L5-20 "These water-soluble organic materials like IC are termed humic-like substances (HULIS) due to their similar properties to macromolecular humic substances"
I wouldn't classify IC itself as HULIS, since IC is a small-molecular-weight species, whereas HULIS are much higher in molecular weight.

P5L7 "All size fractions of SOA we collected were involved in photosensitizing reaction."
Does this mean that all of the aluminum foil substrates were combined for extraction? This should be clarified in the text.

P6L12-P7L6 "The collapse pressure of DOPC monolayer on pure artificial seawater decreased from 46 to 28 mN/m with the addition of IC. The collapse pressure for the DOPC monolayer on the artificial seawater containing HA was even lower."
It would be helpful here to explicitly state the concentrations, or at least mention that the collapse pressure would vary with concentration of the two additives—otherwise, the inference for the reader (I think) is that HA reduces the collapse pressure for DOPC more than IC.

"In contrast to IC and HA, the collapse pressures of DOPC monolayer were higher in the PM2.5 and SOA samples. The inorganic ions from the PM2.5 and SOA samples may contribute to the assembly of organic monolayer."
A reference for this statement would be helpful—how would the inorganic ions affect the assembly of the monolayer (what does assembly mean in this context)?

Dear Authors,

I am happy to accept your manuscript for publication in Atmospheric Chemistry and Physics.

With best regards,

Daniel Knopf