Water adsorption constrained Frenkel–Halsey–Hill adsorption activation theory: Montmorillonite and illite: Fid-Bau Portal

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

Hatch, Courtney D. / Greenaway, Ann L. / Christie, Matthew J. / Baltrusaitis, Jonas

Abstract Fresh mineral aerosol has recently been found to be effective cloud condensation nuclei (CCN) and contribute to the number of cloud droplets in the atmosphere due to the effect of water adsorption on CCN activation. The work described here uses experimental water adsorption measurements on Na-montmorillonite and illite clay to determine empirical adsorption parameters that can be used in a recently derived theoretical 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 FHH and B FHH, to be 98 ± 22 and 1.79 ± 0.11 for montmorillonite and 75 ± 17 and 1.77 ± 0.11 for illite, respectively. The A FHH and B FHH values obtained from water adsorption measurements differ from values reported previously determined 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 determined from water adsorption measurements were then used to calculate the critical super-saturation (s c) for CCN activation using FHH-AT. The relationship between s c and the dry particle diameter (D dry) gave CCN activation curve exponents (x FHH) of −0.61 and −0.64 for montmorillonite and illite, respectively. The x FHH 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 dry and the hygroscopicity parameter exhibits a broader variability with D dry compared to more soluble aerosols. Despite the differences in A FHH, B FHH and x FHH, the FHH-AT derived CCN activities of montmorillonite and illite are quite similar to each other and in excellent agreement with experimental 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 atmospheric models to study the effect of mineral dust aerosol on cloud formation and climate.

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Highlights • CCN activation of illite and montmorillonite determined from water adsorption. • Theoretical CCN activation was in excellent agreement with previous measurements. • FHH adsorption parameters differed from previous values due to different methods. • Inferred hygroscopicity parameters were found to vary with dry particle diameter. • FHH adsorption parameters reported will reduce uncertainties in climate models.