  Sulfur is an abundant element which remains undepleted in diffuse
interstellar gas (Av<1) but it is traditionally assumed to deplete on dust
grains at higher densities and larger Av. Photodissociation regions (PDRs) are
an interesting intermediate medium between translucent and dark clouds where
the chemistry and energetics are dominated by the illuminating FUV radiation
field. Thus they can provide new insights about the sulfur depletion problem.
However, physical and chemical gradients in PDRs take place at small angular
scales (~1 to 10''). Aperture synthesis observations are therefore required to
resolve such gradients. Besides, a complete understanding of molecular
excitation is needed to correctly determine molecular abundances but also the
preavailing physical conditions. Hence, multi-J observations at increasing
frequencies are also required. Such high angular resolution and broad frequency
coverage observations will be provided by ALMA in the near future. In this work
we present IRAM-PdBI observations of the CS J=2-1 line toward the Horsehead PDR
complemented with IRAM-30m observations of several rotational lines of
different sulfur bearing molecules (CS, HCS+, SO, H2S, etc.). Photochemical and
nonlocal, non-LTE radiative transfer models adapted to the Horsehead geometry
have been developed. The gas phase sulfur abundance has been inferred in the
PDR.
