Water adsorption constrained Frenkel-Halsey-Hill adsorption activation theory: Montmorillonite and illite.

Fresh mineral aerosol has recently been found to be effective cloud condensation nuclei (CCN) and contribute to the no. of cloud droplets in the atm. due to the effect of water adsorption on CCN activation. The work described here uses exptl. water adsorption measurements on Na-montmorillonite and illite clay to det. empirical adsorption parameters that can be used in a recently derived theor. framework (Frenkel-Halsey-Hill Activation Theory, FHH-AT) that accounts for the effect of water adsorption on CCN activation. Upon fitting the Frenkel-Halsey-Hill (FHH) adsorption model to water adsorption measurements, we find FHH adsorption parameters, A and B, to be 98 ± 22 and 1.79 ± 0.11 for montmorillonite and 75 ± 17 and 1.77 ± 0.11 for illite, resp. The A and B values obtained from water adsorption measurements differ from values reported previously detd. by applying FHH-AT to CCN activation measurements. Differences in FHH adsorption parameters were attributed to different methods used to obtain them and the hydratable nature of the clays. FHH adsorption parameters detd. from water adsorption measurements were then used to calc. the crit. super-satn. (s) for CCN activation using FHH-AT. The relationship between s and the dry particle diam. (D) gave CCN activation curve exponents (x) of -0.61 and -0.64 for montmorillonite and illite, resp. The x values were slightly lower than reported previously for mineral aerosol. The lower exponent suggests that the CCN activity of hydratable clays is less sensitive to changes in D and the hygroscopicity parameter exhibits a broader variability with D compared to more sol. aerosols. Despite the differences in A, B and x, the FHH-AT derived CCN activities of montmorillonite and illite are quite similar to each other and in excellent agreement with exptl. CCN measurements resulting from wet-generated clay aerosol. This study illustrates that FHH-AT using adsorption parameters constrained by water adsorption is a simple, valid method for predicting CCN activation of fresh clay minerals and provides parameters that can be used in atm. models to study the effect of mineral dust aerosol on cloud formation and climate. [on SciFinder(R)]