Spectroscopic, ab initio, and Smog-Chamber Studies of Molecular Aggregates

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The research proposal describes a program to study molecular aggregates, which are of relevance to atmospheric chemistry, spanning size regimes from the microscopic, molecular world to the nano-scale. The proposed studies on smaller complexes and clusters are enabled by revolutionary advances in high-resolution rotational spectroscopy in terms of both the development of powerful spectroscopic techniques and developments of effective software to aid in the complex spectroscopic assignment processes. At the heart of theses studies is the characterization of 'critical nuclei', which play a crucial role in the formation and growth of atmospheric aerosol particles. These critical nuclei are thought to consist of several molecules, including organic acids, sulfuric acid, and water, but 'microscopic', spectroscopic analyses have thus far not been provided. ***Helium nanodroplets can consist of about a thousand to millions of helium atoms, attain a temperature of ~0.4 K, and are superfluid. These properties make them an 'ideal' matrix to study not only unstable molecules, but also low-temperature reactions. Many atmospheric reactions involve radicals and are barrier-less or have low barriers. In addition to radical reactions, for example, between tolyl radical and molecular oxygen, we will also investigate photo-induced reactions. An example is the photo-induced decomposition of oxalic acid, the most abundant dicarboxylic acid in the atmosphere, which could be a major atmospheric sink for this type of compounds. ***Even larger systems, namely secondary organic aerosol particles and soot particles, will be studied using a photo-reaction or 'smog' chamber. Soot particles are produced in incomplete fossil fuel combustion, for example, and are emitted in great quantities into the atmosphere. Of particular interest is the ageing process of soot particles. Atmospheric soot can be coated with secondary organic aerosol, perhaps produced from organic material also released during fossil fuel burning. This coating changes not only the surface properties of the particle but the increased surface tension causes a collapse of the soot core. Both coating and collapse lead to changes in optical properties and affect the contributions of soot particles to radiative forcing.***The proposed research will lead to greater insights into atmospheric processes, including aerosol particle growth and ageing, and the results will have impact on atmospheric modelling and will likely reduce the uncertainties associated with the direct and indirect effects of aerosol on the global climate. **