Franz et al.:

I suggest you make some changes to the Abstract. To say "the availability of only little concrete evidence for chemical structures" in reference to SOA is not accurate. With high-resolution MS, a number of groups have identified hundreds of compounds in SOA. Maybe better to say that identification of SOA compounds is demanding and not necessarily complete. Using the word "putative" in isoprene derived SOA particle constituents is not warranted. These compounds have been unequivocally detected and confirmed in both the laboratory and ambient samples. To say "epoxides and tetrols have been proposed to be present" implies that their presence has not been confirmed, which is false. Epoxides and tetrols have been confirmed beyond question. (We now routinely synthesize all the epoxide isomers in our laboratory.) Overall, this is a novel study, but referee #1 who is an expert in this area states, correctly, that "a compound's existence in an SOA sample can neither be confirmed, discounted, nor quantified by SFG spectroscopy". Please give a final read through to make sure you are not overselling the method.