In this manuscript, the authors derived water activity and surface tension from hygroscopic growth of aerosol particles smaller than 100 nm composed of aqueous ammonium sulfate, D-glucose, and their mixtures. The hygroscopic growth of particles was measured over a wide range of RH from 2.0% to 99.6% using a high humidity tandem differential mobility analyzer (HHTDMA). The derived water activity and surface tension were compared with those from electrodynamic balance and bulk measurements and thermodynamic model predictions (E-AIM and UNIFAC). Overall, the manuscript is well written, and the topic fits the scope of Atmospheric Chemistry and Physics very nicely. I recommend the manuscript for publication after the authors address the following comments.

Major comment:

The authors derived water activity and surface tension using differential Kohler analysis, i.e., fitting parameters for the 𝑠_𝑤(𝐷_𝑠) dependence with the same 𝑔_𝑏 (i.e., Eq. 3). This approach assumes that water activity and surface tension depend on 𝑔_𝑏 only (i.e., independent of 𝐷_𝑠). However, due to surface-bulk partitioning, both water activity and surface tension also depend on 𝐷_𝑠 (i.e., surface area to volume ratio). I would suggest that the authors include relevant discussions on how such dependence affects the accuracy of derived water activity and surface tension and conclusion of this study. 

Minor comments:

Line 188-189: The growth factor is a monotonic function of RH (first derivative is consistently above zero).

Line 205-210: Please provide more details on how the uncertainties are estimated.