  There is growing observational evidence of dust coagulation in the dense
filaments within molecular clouds. Infrared observations show that the dust
grains size distribution gets shallower and the relative fraction of small to
large dust grains decreases as the local density increases. Ultraviolet (UV)
observations show that the strength of the 2175 {\AA} feature, the so-called UV
bump, also decreases with cloud density. In this work, we apply the technique
developed for the Taurus study to the Orion molecular cloud and confirm that
the UV bump decreases over the densest cores of the cloud as well as in the
heavily UV irradiated {\lambda} Orionis shell. The study has been extended to
the Rosette cloud with uncertain results given the distance (1.3 kpc).
