Photoinitiated Chemistry in Single Levitated Aerosol Droplets With Cavity Ring Down Spectroscopy

XU ZHANG, Michael Cotterell, Andrew J. Orr-Ewing, University of Bristol

     Abstract Number: 164
     Working Group: Aerosol Physics

Abstract
The photochemistry occurring in atmospheric aerosol droplets has a substantial impact on the Earth's climate. However, the interaction between light and aerosol droplets is complex and can be greatly enhanced in localised regions by the excitation of standing-wave modes. This work investigates the dependence of photobleaching kinetics on particle size, initial droplet composition, light intensity, and illumination wavelength. We use a linear electrodynamic quadrupole to levitate a single droplet, which has an adjustable initial diameter ranging from 1-4µm, to the centre of a cavity ring-down spectroscopy (CRDS) probe beam. Simultaneously, we measure the distribution of angle-resolved elastic light scattering when the droplet is irradiated with another laser beam. These measurements enable accurate and precise retrieval of the changing refractive indices of absorbing or non-absorbing droplets as they undergo physicochemical changes. The refractive indices are directly related to the chemical composition. By tracking the changes in these refractive indices over time, the rates of the chemical processes can be quantified. A series of experiments were carried out on 1,2,6-hexanetriol (HT) to test the accuracy of the experimental method in obtaining refractive index. The results show that the measured extinction cross-sections are in excellent agreement with predictions from Lorenz-Mie theory over of the measured range of particle size. We then studied binary droplets composed of HT doped with a dye (VIS441). VIS441 is a photo-reactive dye that absorbs light strongly at wavelengths close to 441 nm and undergoes bleaching to form non-absorbing products. We introduced another 405 nm laser beam to initiate the photochemical reaction and observed binary aerosol droplets undergoing photobleaching. Similarly, we studied the photobleaching of a chromophore present in atmospheric aerosols: imidazole-2-carboxaldehyde (IC). We have observed the onset of photobleaching in the binary aerosol system of IC/HT. The degree of photobleaching is correlated with the exposure time to a photobleaching light source. Our ongoing work is quantifying the rate of the photo-reaction and the influence of factors such as laser power and droplet size.