Broadband Light Scattering and Cavity Ring-Down Spectroscopy Measurements on Single Light-Absorbing Aerosol Particles

AIDAN RAFFERTY, Andrew J. Orr-Ewing, Jonathan P. Reid, Michael Cotterell, University of Oxford

     Abstract Number: 47
     Working Group: Instrumentation and Methods

Abstract
Aerosol in the atmosphere has a profound effect on the Earth’s energy budget and climate. Current climate models have a large uncertainty associated with the aerosol contribution. In part, this is due to a poor understanding of the physical and chemical properties of these aerosol particles. Of particular importance is the complex refractive index of aerosol, m = n + ik. The relative values of n and k determine the fraction of light scattered and absorbed by an aerosol particle. This in turn determines whether aerosol has a local cooling or heating effect. Values of k for absorbing aerosol are currently poorly characterised. Thus, novel techniques for measuring k must be developed. A good way to measure aerosol properties is to make measurements on single aerosol particles to overcome ensemble averaging effects. One method that enables this approach is optical trapping. Optical trapping uses laser light to levitate an aerosol particle and enable it to be investigated over long periods of time. Most commonplace optical trapping approaches do not work for absorbing aerosol, but the use of counterpropagating hollow beams overcomes this. When focused to different points, the hollow beams create an “optical bottle” in which a single absorbing aerosol particle can be trapped. Once trapped, we investigate the particle’s optical properties using a combination of two techniques: broadband light scattering (BLS) and cavity ring-down spectroscopy (CRDS). CRDS allows for highly sensitive determination of the extinction cross-section of a levitated particle. However, it cannot disentangle the various factors contributing to this on its own. By combining with BLS, we can disentangle these factors and provide wavelength-dependent information on n and k. We will provide an overview of our experimental setup, the data analysis framework, and some preliminary results.

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