Molecular Study of the Effects of Chemical Processing on Heterogeneous Ice Nucleation: Role of Active Sites and Product Formation

Abstract:

Mineral dust aerosol is the largest global source of ice nuclei, but the identity of the active sites for nucleation is unknown. During atmospheric transport, mineral dust aerosol can encounter and react with sulfuric acid, which affects the ice nucleation activity either due to changes to reactive surface sites or product formation. In this study, we reacted two types of clays found in mineral dust, kaolinite and montmorillonite, with sulfuric acid. Variation in the mineral due to acid treatment was separated from product formation through rinsing techniques. The samples were subsequently reacted with a probe molecule, (3,3,3-trifluoropropyl)dimethylchlorosilane, that selectively binds to edge hydroxyl groups that are bonded to a silicon atom with three bridging oxygens. Hydroxyl groups are considered potential active sites, because they can hydrogen bond with water and facilitate ice nucleation. Attachment to these sites was quantified by ¹⁹F magic angle spinning nuclear magnetic resonance (MAS NMR) of the ¹⁹F atoms on the probe molecule, which provided a direct correlation of the number of hydroxyl groups. Our results indicate that the number of edge-site hydroxyl groups increases with exposure to acid. Ice nucleation measurements indicate that the sulfuric acid-treated mineral is less ice active than the untreated mineral. Surprisingly, no difference between the nucleation activity of the untreated mineral and acid-treated, rinsed mineral is observed. As a result, we hypothesize that once a critical density of active sites is reached for ice nucleation, there is no further change in nucleation activity despite a continued increase in active sites. We additionally propose that the reduced activity of the acid-treated mineral is due to product formation that blocks active sites on the mineral, rather than changes to active sites.