Aerosol-cloud interactions are the largest uncertainty in our understanding of the climate system. Of particular interest are cirrus (ice) clouds, which are ubiquitous in the upper troposphere, cause a net warming effect on the climate, and affect the transport of water vapor into the stratosphere. Ice particles in these clouds tend to nucleate through heterogeneous pathways, in which an aerosol acts as a seed for the nucleation of ice. Heterogeneous nucleation is thought to depend on the number of surface active sites on the particle, with each type of surface active site having its own energetics for ice nucleation. While these surface active sites are presumably molecular in origin, they remain poorly described. One of the main goals of the PI's lab is to characterize materials of interest for ice nucleation on the molecular scale to characterize the number and types of active sites available on a given atmospherically relevant surface. True molecular-level information on the ice nucleation process is seldom found in the literature.
Mineral dust aerosol is the largest global source of ice nuclei. In particular, aluminosilicate clay minerals compose the majority of dust from Asia and Africa, the largest sources of dust aerosol. As mineral dust is transported in the atmosphere, it reacts with pollution plumes and atmospheric trace gases. Laboratory studies have shown that the activity of particles toward certain modes of heterogeneous ice nucleation decreases when the particles are treated with acids, suggesting that the cloud formation properties of mineral dust aerosol particles are altered due to these processes. The PI's group has shown that changes to lattice spacings due to product formation and structural changes in the aluminosilicate clay minerals can explain the reduction in ice nucleation activity. To further investigate these changes, we have used solid state NMR to quantify changes to the number of reactive hydroxyl groups on the edges of the clay minerals. We have found that the number of reactive hydroxyl groups increases as a result of acid-treatment of the minerals. Both of these studies characterize why a reduction in ice nucleation activity is observed through the investigation of molecular scale properties of these minerals.
Our proposal expands our solid state NMR studies to further investigate changes to hydroxyl groups under a wider variety of chemical conditions. In addition, we propose to use the high-field magnets at EMSL to determine changes to the speciation of aluminum and silicon as a result of acid treatment. To obtain complementary information about the surface of the minerals, we plan to use high resolution transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). Using these techniques, we can characterize changes to the most common global ice nuclei on the molecular scale, and determine what constitutes ice active sites on these minerals.