The authors have done a thorough job of responding to the concerns raised by the reviewers and the manuscript is now ready for publication. I tend to agree with the reviewer that it would be better to consistently refer to the mixing ratios in units of ppb, but I leave it to the discretion of the authors. (The authors indicate that the precision of the measurements is ~3 ppb, and the enhancements under discussion are often in the range of tens or hundreds of ppb. The authors' argument about using ppm to avoid 'unnecessary' significant figures does not make that much sense to me, but I think it is a matter of preference. Once the manuscript has been edited (or not) to make this change, it is ready for publication.