Dear authors,

I have read your responses to reviewers comments and the revised manuscript. One of the reviewers has further comments (please see below). The manuscript will be published once these comments are addressed.

Best,
Sally

Reviewer further comments:
1. Ultra low NO. Although not the focus of the paper, NO levels less than 100 ppt are common where isoprene is oxidized (even happens in the US). I suggest that the authors explain the sources of HCHO (and MVK/MACR) in this regime as the yield shown in this study is not explained by the branching ratios produced in OH chemistry illustrated in the manuscript.

2. Discussion of the ISOPOO + HO2 rate. Suggest that the authors not cite Paulot et al. for the slow rate of this reaction found in some versions of RACM. Paulot adopted this rate FROM RACM and later revised the mechanism in both GEOS-CHEM and in the EPA isoprene mechanism described in Xie et al. to use the much faster (and more likely correct) rate.