We present a new technique for measuring the extinction cross section, σext, for single aerosol particles, using a Bessel laser beam (BB) optical trap to finely control and optimise the particle position to the centre of a cavity ring-down spectrometer. By measuring the evolution of σext with time and changing environmental conditions and fitting to cavity standing wave Mie theory, the particle refractive index can be both accurately and precisely determined. Development of this technique will be important in reducing the uncertainties associated with atmospheric aerosol radiative forcing, among the largest uncertainties in climate modelling.