A technique is presented which enables single aerosol particles to be optically trapped within a Bessel laser beam (BB), while the evolving optical properties are examined with cavity ring-down spectroscopy (CRDS) and elastic light scattering (or phase functions, PF) measurements. An aerosol particle can be confined for extended timescales while the relative humidity (RH) is varied. The refractive index (RI) of the confined particle can be retrieved through fitting PFs to Mie theory or the extinction cross section (obtained in CRDS measurements) to cavity standing wave Mie theory. The motivation of these measurements is to reduce the uncertainty associated with the interaction of atmospheric aerosols with solar and terrestrial radiation through improved representations of the optical constants aerosols.