Dear Authors,
The two reviewer judged the revised manuscript. At least one reviewer still showed great concern about the scientific quality. Particularly the reviewer requests some more concrete evidence that lactic acid is produced by microorganisms, and to argue the possibility of other sources (e.g., photochemistry) more carefully. I find that revision is on the right track but would need to consider if the authors could address to these points.

My additional concern is on Figure 9. How to interpret the negative correlation with RH during nighttime? Better not to show regression lines when the correlation is not statistically significant? (also for other figures)


Co-Editor: Yugo Kanaya

After the revision, I need to say that overall integrity was reduced.
I will reconsider after major revisions.

1. Two major flaws: First, monoacids will not react with O3. Second, the new sentence in Abstract that "concentrations of LMW monoacids did not correlate with nss-SO4^2- and NO3^-, indicating that LMW monoacids are derived from the local sources" has a logical flaw. The fact indicates that they are not likely from urban sources, and never indicate about the local sources.

2. Uta et al. (2012) was cited many times in text as important source of information for this manuscript, but it did not appear in the reference list.

3. It was good to include soil analysis, but the sampling was made at a different site, for which no justification was provided. Detection of high concentrations of lactic acid, C1 and C2 monoacids in soil supported the idea that they originate from soil or bacterial processes, but the fact that gaseous lactic acid did not show positive correlation with C1-C6 monoacids (page 5, lines 11-12) did not support the idea. Further, the authors claim first that branched chain monoacids including iC4 (and iC5, isopentanoic acid) are known as common metabolites of bacteria (page 5, line 3) and regarded them as a tracer from that emission sector. But they did not show up in high concentrations from soil analysis, denying their assumption. Overall my impression is that the soil analysis introduced more puzzle. Analysis of soil from the same site as atmospheric measurements were made is important.

4. C3-C6 monoacids will also survive at least for a couple of days (against OH oxidation) and their mention "short-lived" (in section 4) is not supported. For which conclusion do the authors need these sentences (here and also those in page 4, lines 32-35)?

5. Figure 7. Caption is wrong. No data on formic acid are shown.

I would suggest that the authors need to "clean up" their logic. After revision on individual point raised by the reviewers, clarity of discussion is lacking.

Dear Authors,

Previous concerns were mostly adequately addressed. But one important point is not fully resolved, as to if the soil analysis supports the conclusion, which the two reviewers thought critical. I still recommend performing ON-SITE soil sampling in the similar season. Otherwise, the authors should provide strong justification of similarity in the environment of the two places.

Best wishes,
Yugo Kanaya, Co-Editor

Dear Authors,

I now find that the manuscript reached quality standard of ACP and recommend publication.

Best wishes,
Yugo Kanaya, ACP Co-Editor