Cloud drop activation of insoluble particles: impact of surface properties

A number of studies have reported experimental CCN activation properties of water insoluble particles, mainly various minerals and soots, during the past decade or so (e.g. Kumar et al., 2011; Lathem et al., 2011, Dalirian et al., 2018). A popular theoretical framework for interpreting the results is the FHH adsorption activation theory (Sorjamaa and Laaksonen, 2007), which is a combination of the two-parameter FHH adsorption isotherm model and the Kelvin equation. However, it has become clear that the FHH activation theory tends to overpredict critical supersaturations quite substantially when the FHH parameters are determined from experimental water adsorption isotherms (Laaksonen et al., 2016; Hatch et al., 2019). A possible reason for the discrepancy is surface roughness of the particles, not accounted for in the FHH adsorption activation theory (Laaksonen et al., 2016). One way to quantify the extent of the surface roughness is through the surface fractal dimension, which can be determined e.g. with the help of nitrogen adsorption measurements. We showed earlier (Laaksonen et al., 2016) that employing the surface fractal dimension within the FHH framework does seem to improve the theoretical predictions. However, our data for water and nitrogen adsorption measurements were obtained from literature sources, and therefore the surface properties of a given mineral species employed in the adsorption measurements and in the CCN experiments were not necessarily similar. Therefore, the uncertainty limits of the surface fractal dimension -corrected predictions were rather high. Here, we compare theoretical and experimental critical supersaturations for several metal oxide and mineral aerosols. The materials used in the water and nitrogen adsorption measurements are the same as those used in the CCN experiments, allowing us to improve the reliability of our conclusions regarding the quality of the theoretical predictions.    Dalirian, M, Ylisirniö, A., Buchholz, A., Schlesinger, D., Ström, J., Virtanen, A. and Riipinen, I. (2018). Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility.  Atmos. Chem. Phys. 18, 12477-12489. Hatch, C.D., Tumminello, P.R., Cassingham, M.A., Greenaway, A.L., Meredith, R. and Christie, M.J. (2019). Technical note: Frenkel, Halsey and Hill analysis of water on clay minerals: toward closure between cloud condensation nuclei activity and water adsorption. Atmos. Chem. Phys. 19, 13581-13589. Kumar, P, Sokolik, I.N. and A. Nenes, A (2011). Measurements of cloud condensation nuclei activity and droplet activation kinetics of fresh unprocessed regional dust samples and minerals. Atmos. Chem. Phys. 11, 3527–3541. Laaksonen, A., Malila, J. and Nenes A (2016). Surface fractal dimension, water adsorption efficiency, and cloud nucleation activity of insoluble aerosol. Sci. Rep. 6, 25504. Lathem, T., Kumar, P., Nenes, A., Dufek, J., Sokolik, I.N., Trail, M. and Russell, A. (2011). Hygroscopic properties of volcanic ash. Geophys. Res. Lett. 38, L11802. Sorjamaa, R. and Laaksonen A. (2007). The effect of H2O adsorption on cloud drop activation of insoluble particles: a theoretical framework. Atmos. Chem. Phys. 7, 6175–6180.