  Dust properties appear to vary according to the environment in which the dust
evolves. Previous observational indications of these variations in the FIR and
submm spectral range are scarce and limited to specific regions of the sky. To
determine whether these results can be generalised to larger scales, we study
the evolution in dust emissivities from the FIR to mm wavelengths, in the
atomic and molecular ISM, along the Galactic plane towards the outer Galaxy. We
correlate the dust FIR to mm emission with the HI and CO emission. The study is
carried out using the DIRBE data from 100 to 240 mic, the Archeops data from
550 mic to 2.1 mm, and the WMAP data at 3.2 mm (W band), in regions with
Galactic latitude |b| < 30 deg, over the Galactic longitude range (75 deg < l <
198 deg) observed with Archeops. In all regions studied, the emissivity spectra
in both the atomic and molecular phases are steeper in the FIR (beta = 2.4)
than in the submm and mm (beta = 1.5). We find significant variations in the
spectral shape of the dust emissivity as a function of the dust temperature in
the molecular phase. Regions of similar dust temperature in the molecular and
atomic gas exhibit similar emissivity spectra. Regions where the dust is
significantly colder in the molecular phase show a significant increase in
emissivity for the range 100 - 550 mic. This result supports the hypothesis of
grain coagulation in these regions, confirming results obtained over small
fractions of the sky in previous studies and allowing us to expand these
results to the cold molecular environments in general of the outer MW. We note
that it is the first time that these effects have been demonstrated by direct
measurement of the emissivity, while previous studies were based only on
thermal arguments.
