|Garimella et al. have significantly revised and improved upon their original manuscript. In particular they have added details regarding their experimental methods and data analysis procedures that have clarified their results. The majority of my initial questions and concerns have been addressed, and I feel the revised paper is now suitable for publication in ACP.|
I feel I must comment on the extensive unsolicited “review” submitted by Kumar. While ACPD is designed to encourage public comments from the readership, I strongly feel that the nature of Kumar’s comments and demands of the authors to significantly modify their work are inappropriate, and not in the spirit of the “short comments” permitted by ACPD. Kumar seems determined to force these and all others studying the hygroscopicity of mineral dust particles to analyze their data within the adsorption isotherm FHH paradigm that he supports. Previously Kumar published a paper where he re-analyzed hygroscopicity data of minerals reported by Koehler et al. (2009) and Sullivan et al. (2009) to fit the FHH adsorption activation model (Kumar et al., 2009). While Kumar is certainly entitled to his opinion, and to make /suggestions/ to other authors regarding how to analyze their data, it is not appropriate to try and force other researchers to conform to his paradigm, as he has done here with the extensive “short comment” he submitted.
Garimella et al. have nicely demonstrated that by not correcting for the multiple-charge and particle shape issues, especially for mineral particle sizes smaller than 300 nm, the retrieved hygroscopicity data can be quite inaccurate. This may explain why Kumar’s previous data appears to better fit the FHH model rather than Kappa-Kohler theory, as he made measurements of quite small particles and did not account for these important method artifacts.
The authors might note that Sullivan et al. (2010a, 2010b) also reported similarly low kappa values for 200 and 300 nm dry-generated ATD particles of kappa = 0.004 and 0.002, respectively, adding further support for the reliability of using Kappa-Kohler model to treat the CCN activation of mineral particles presented here. These should be added to Figure 11.
I am still troubled by the few data points collected in the various CCN activation curves (Fig. 3). Often only 3 useable data points are available to fit the activating regions of the curve to. Given the great lengths the authors have expended to produce the proper monodisperse aerosol and perform various corrections, it is surprisingly that the activation curves themselves were not more carefully measured. Fitting a curve to only 3 points in the active region of the sigmoidal fit introduces significant uncertainty into the retrieved kappa values. Particularly when the activation curves are somewhat shallow and not steep, as is commonly observed for mineral particles. I urge the authors to measure higher resolution curves in future work.
The authors should also note that the importance of selecting a monodisperse aerosol from the correct region of a polydisperse aerosol for accurate hygroscopicity measurements from CCN activation curves was previously discussed in detail by Petters et al. (2007).
Koehler, K., Kreidenweis, S. M., DeMott, P. J., Petters, M. D., Prenni, A. J. and Carrico, C. M.: Hygroscopicity and cloud droplet activation of mineral dust aerosol, Geophys. Res. Lett., 36, L08805, doi:doi:10.1029/2009GL037348, 2009.
Kumar, P., Nenes, A. and Sokolik, I. N.: Importance of adsorption for CCN activity and hygroscopic properties of mineral dust aerosol, Geophys. Res. Lett., 36, L24804, doi:doi: 10.1029/2009GL040827, 2009.
Petters, M. D., Prenni, A. J., Kreidenweis, S. M. and DeMott, P. J.: On measuring the critical diameter of cloud condensation nuclei using mobility selected aerosol, Aerosol Sci. Technol., 41(10), 907–913, 2007.
Sullivan, R. C., Miñambres, L., DeMott, P. J., Prenni, A. J., Carrico, C. M., Levin, E. J. T. and Kreidenweis, S. M.: Chemical processing does not always impair heterogeneous ice nucleation of mineral dust particles, Geophys. Res. Lett., 37(24), L24805, doi:10.1029/2010GL045540, 2010a.
Sullivan, R. C., Moore, M. J. K., Petters, M. D., Kreidenweis, S. M., Roberts, G. C. and Prather, K. A.: Effect of chemical mixing state on the hygroscopicity and cloud nucleation properties of calcium mineral dust particles, Atmos. Chem. Phys., 9, 3303–3316, 2009.
Sullivan, R. C., Petters, M. D., DeMott, P. J., Kreidenweis, S. M., Wex, H., Niedermeier, D., Hartmann, S., Clauss, T., Stratmann, F., Reitz, P., Schneider, J. and Sierau, B.: Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation, Atmos. Chem. Phys., 10, 11471–11487, 2010b.