  Experimental observations suggest that there are differences between the
behavior of particles optically trapped in air and trapped in a liquid phase.
We present a modified version of Mie Debye Spherical Aberration theory to
numerically simulate such optical system in attempt to explain and predict
these effects. The model incorporates Mie scattering and focussing of the
trapping beam through media of stratified refractive index. Our results show a
geometrical optics approach cannot correctly describe our system and that
spherical aberration must be included. We successfully qualitatively explain
the observed phenomena and those of other authors, before discussing the limits
of our experimental techniques and methods to improve it. We draw the important
conclusion that when optically trapping aerosols the system does not behave as
a true `optical tweezers', varying between levitation and single beam gradient
force trapping depending on particle and beam parameters.
