The authors have substantively responded to all of the reviewer's comments, which were minor in nature. The following are worth noting, however.

Reviewer #1, Comment #5: The authors respond “ … variation in composition across a particle population would produce a distribution of gamma values, though it is not expected that the gamma value that represents the average particle composition is greatly different to the average of the gamma values across the particle population.” This is an important point, and one that would have been worthy of including in the text.

Reviewer #2, Comment #2: The response does not directly answer the question, which asks about correlations between NO2 concentrations and aerosol surface area rather than random variability in aerosol surface area. The point is likely minor and does not require change in the manuscript, but the authors may wish to make note of it.