Dear Authors:
Thank you for your careful attention to the referee’s comments. After reading your responses and the revised manuscript carefully, I think that you have addressed many of the issues. However, I would like you to consider my one main suggestion and the minor suggestions below.

Main Suggestion: Pg 8 line 25-pg 9 line 3: I think this section would benefit from some more thought and analysis and I urge the authors to reconsider the reasoning in this section. In particular, I think that some of the correlations may be influenced by loss rates and lifetime issues. Please consider how formation, loss, and differing lifetimes may influence correlations and what this may tell us. Additionally, I would think that there would be a stronger correlation with isoprene nitrates if high NOx conditions enhanced formation. Finally how do the recent findings that MACR and MVK may be formed from reactions of isoprene hydroxy hydroperoxides on instrument surfaces (Rivera-Rios et al., 2014) influence this argument?
Minor Suggestions:
Pg 4 line 23: should be sulfate radical ion
Pg 8 line 20-21: please clarify if this is 3.7% of OM
Pg 8 line 21: Please correct typo: “y” (end of the line)
Pg 11 Line 14: “When increases the water content of the eluent…” Please revise
Pg 13 Line 2: Should this reference table S1?
Pg 14 line 27: “The precursors of bisulfate ion and sulfate radical insight…” I believe you are missing a word before “insight.”

~Ellie

References
Rivera-Rios, J. C., Nguyen, T. B., Crounse, J. D., Jud, W., St. Clair, J. M., Mikoviny, T., Gilman, J. B., Lerner, B. M., Kaiser, J. B., de Gouw, J., Wisthaler, A., Hansel, A., Wennberg, P. O., Seinfeld, J. H. and Keutsch, F. N.: Conversion of hydroperoxides to carbonyls in field and laboratory instrumentation: Observational bias in diagnosing pristine versus anthropogenically controlled atmospheric chemistry, Geophys. Res. Lett., 41(23), 2014GL061919, doi:10.1002/2014GL061919, 2014.