  Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm
HI column densities, along with estimates of the local dust abundances, we
measure the volume densities of a sample of actively star-forming giant
molecular clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution
scale of 170 pc. Our approach is based on an equilibrium model for the cycle of
molecular hydrogen formation on dust grains and photodissociation under the
influence of the FUV radiation on the cloud surfaces of GMCs. We find a range
of total volume densities on the surface of GMCs in M83, namely 0.1 - 400 cm-3
inside R25, 0.5 - 50 cm-3 outside R25 . Our data include a number of GMCs in
the HI ring surrounding this galaxy. Finally, we discuss the effects of
observational selection, which may bias our results.
