Kelly J. Baustian

Internally mixed sulfate and organic particles as potential ice nuclei in the tropical tropopause region

Cirrus clouds are ubiquitous in the tropical tropopause region and play a major role in the Earth’s climate. Any changes to cirrus abundance due to natural or anthropogenic influences must be considered to evaluate future climate change. The detailed impact of cirrus clouds on climate depends on ice particle number, size, morphology, and composition. These properties depend in turn on the nucleation mechanism of the ice particles. Although it is often assumed that ice nucleates via a homogeneous mechanism, recent work points to the possibility that heterogeneous ice nucleation is important in the tropical tropopause region. However, there are very few studies of depositional ice nucleation on the complex types of particles likely to be found in this region of the atmosphere. Here, we use a unique method to probe depositional ice nucleation on internally mixed ammonium sulfate/palmitic acid particles, namely optical microscopy coupled with Raman microscopy. The deliquescence and efflorescence phase transitions of the mixed particles were first studied to gain insight into whether the particles are likely to be liquid or solid in the tropical tropopause region. The ice nucleating ability of the particles was then measured under typical upper tropospheric conditions. It was found that coating the particles with insoluble palmitic acid had little effect on the deliquescence, efflorescence, or ice nucleating ability of ammonium sulfate. Additional experiments involving Raman mapping provide new insights into how the composition and morphology of mixed particles impact their ability to nucleate ice.

Characterizing the Morphology of Organic Aerosols at Ambient Temperature and Pressure

The aerosol direct effect, which characterizes the interaction of radiation with aerosol particles, remains poorly understood. By determining aerosol composition, shape, and internal structure, we can predict aerosol optical properties. In this study, we performed a feasibility study to determine if tapping-mode atomic force microscopy (TM-AFM) and Raman microscopy can be effectively used to obtain information on aerosol composition, shape, and structure. These techniques are advantageous because they operate under ambient pressure and temperature. We worked with model aerosol particles composed of organic components of varying solubility mixed with ammonium sulfate. In particular, we explored whether aerosols could be differentiated on the basis of the solubility of the organic component. We also characterized the aerosol internal structure and investigated how this structure changes as the solubility of the organic compound is varied. To obtain indirect chemical information from AFM, we imaged particles supported on both polar, SiO(x)/Si(100), and nonpolar, highly ordered pyrolytic graphite, surfaces. We have found that AFM can be used to differentiate the solubility of the organic component. In some cases, AFM can also be used to identify internal structure. With Raman microscopy, we can differentiate between core-shell structures and homogeneous structures. Surprisingly, we find that even for the most soluble compounds, core-shell structures are observed. To discuss consequences of our results for climate studies, we calculate the difference in radiative forcing caused by having a core-shell aerosol rather than a homogeneous particle. Overall, these techniques are promising for characterizing composition, shape, and internal structure of atmospheric particles.

Metal oxide formation by serine and cysteine proteases

Silicatein-α, a member of the cathepsin-L family of protease enzymes, has previously been identified as a mediator for the formation of metal oxides such as silica (K. Shimizu, J. Cha, G. D. Stucky and D. E. Morse, Proc. Natl. Acad. Sci. U. S. A., 1998, 95, 6234). The active site of silicatein-α, a serine-histidine-asparagine triad, prompted us to investigate metal oxide formation in protease enzymes with similar active sites. Three enzymes were explored: the cysteine proteases papain and TEV (tobacco etch virus protease), and the serine protease trypsin. It was observed that papain and trypsin were able to mediate the formation of titania at room temperature and neutral pH. Only papain was able to mediate the formation of silica, and TEV was unable to mediate the formation of either titania or silica. The metal oxide formation activities of papain and trypsin were inhibited by heat denaturation, suggesting a role of protein tertiary structure on condensation activity. These activities are discussed in terms of sequence homology to silicatein-α and active site accessibility.