Context :

Aims : Dust properties appear to vary according to the environment in which the dust evolves .
Previous observational indications of these variations in the far-infrared ( FIR ) and submillimeter ( 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 millimeter ( mm ) wavelengths , in the atomic and molecular interstellar medium ( ISM ) , along the Galactic plane towards the outer Galaxy .

Methods : We correlate the dust FIR to mm emission with the HI and CO emission , which are taken to trace the atomic and molecular phases , respectively .
The study is carried out using the DIRBE data from 100 to 240 { \mu m } , the Archeops data from 550 { \mu m } to 2.1 mm , and the WMAP data at 3.2 mm ( W band ) , in regions with Galactic latitude |b| \leq 30 \degr , over the Galactic longitude range ( 75 \degr < l < 198 \degr ) observed with Archeops .

Results : 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 { \mu m } .
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 .

Conclusions :