Variations between Dust and Gas in the Diffuse Interstellar Medium. 3.
  Changes in Dust Properties
  We study infrared emission of 17 isolated, diffuse clouds with masses of
order solar masses, to test the hypothesis that grain property variations cause
the apparently low gas-to-dust ratios that have been measured in those clouds.
Maps of the clouds were constructed from WISE data and directly compared to the
maps of dust optical depth from Planck. The mid-infrared emission per unit dust
optical depth has a significant trend toward lower values at higher optical
depths. The trend can be quantitatively explained by extinction of starlight
within the clouds. The relative amounts of PAH and very small grains traced by
WISE, compared to large grains tracked by Planck, are consistent with being
constant. The temperature of the large grains significantly decreases for
clouds with larger dust optical depth; this trend is partially due to dust
property variations but is primarily due to extinction of starlight. We updated
the prediction for molecular hydrogen column density, taking into account
variations in dust properties, and find it can explain the observed dust
optical depth per unit gas column density. Thus the low gas-to-dust ratios in
the clouds are most likely due to `dark gas' that is molecular hydrogen.
