In collaboration with Prof J.P. Reid, we have developed CRDS techniques for the study of ensembles of atmospheric aerosol particles, and for the study of optical properties of single particles less than 1 micron in diameter. The most recent work combines optical trapping and guiding of single particles within a Bessel beam with the spectroscopic methods for single particle study developed in our laboratory. This advance is allowing for the first time the study of the evolution of optical properties of a single accumulation mode aerosol particle, for example under conditions of changing ambient relative humidity to mimic nucleation of water droplets in the atmosphere.
Single aerosol particle measurements with CRDS.
The figure above shows how a single particle guided vertically into a laser beam trapped inside an optical cavity reduces the ring-down time of the cavity. The change in ring-down time depends on the extinction cross section of the particle (which in turn depends on the particle shape, size and composition) and its position in the laser beam. The particle is levitated by a Bessel beam optical trap and we use a feedback loop to maintain the particle at the centre of the cavity mode. The full apparatus is shown schematically in the figure below. A cell around the particle trapping region allows the relative humidity of the local environment to be controlled.
Apparatus used for single aerosol particle CRDS measurements. The instrument combines Bessel beam trapping and manipulation with CRDS measurement of optical extinction.
Illustrative outcomes are shown below: the measured extinction efficiency (Qext) of a NaCl droplet is compared to the results of a Mie scattering calculation. The droplet shrinks in radius over time as the relative humidity of the surrounding air reduces. Correct treatment of the change in refractive index of the aqueous NaCl solution in the calculations is necessary to give the good agreement shown with the experimental data.
Comparison of extinction efficiency of an NaCl droplet measured by CRDS (black line) with Mie scattering calculation (red line). The dashed lines show the envelope of CRDS measurements resulting from movement of the particle in the Bessel beam trap.
This work is supported by a grant from the Natural Environment Research Council (NERC).
Representative publications:
Deviations from plane-wave Mie scattering and precise retrieval of refractive index for a single spherical particle in an optical cavity, B.J. Mason, J.S. Walker, J.P. Reid and A.J. Orr-Ewing, J. Phys. Chem. A 118, 2083-2088 (2014).
Measurements of the evaporation and hygroscopic response of single fine-mode aerosol particles using a Bessel beam optical trap, M.I. Cotterell, B.J. Mason, A.E. Carruthers, J.S. Walker, A.J. Orr-Ewing and J.P. Reid, PCCP 16, 2118 – 2128 (2014).
Measurement of light extinction by single aerosol particles, J.S. Walker, A.E. Carruthers, A.J. Orr-Ewing and J.P.Reid, J. Phys. Chem. Lett. 4, 1748 – 1752 (2013).
Comparison of the accuracy of aerosol refractive index measurements from single particle and ensemble techniques, B.J. Mason, S.J. King, R.E.H. Miles, K.M. Manfred, A.M.J. Rickard, J. Kim, J.P. Reid and A.J. Orr-Ewing, J. Phys. Chem. A 116, 8547-8556 (2012).
CRDS measurements of single aerosol particle extinction part 1: The effect of position of a particle within the laser beam on extinction, T.J.A. Butler, J.L. Miller and A.J. Orr-Ewing, J. Chem. Phys. 126, 174302(1-7) (2007).
CRDS measurements of single aerosol particle extinction part 2: Extinction of light by an aerosol particle in an optical cavity excited by a cw laser, J.L. Miller and A.J. Orr-Ewing, J. Chem. Phys 126, 174303(1-7) (2007).
