  Dust evolution in protoplanetary disks from small dust grains to pebbles is
key to the planet formation process. The gas in protoplanetary disks should
influence the vertical distribution of small dust grains ($\sim$1 $\mu m$) in
the disk.Utilizing archival near-infrared polarized light and millimeter
observations, we can measure the scale height and the flare parameter $\beta$
of the small dust grain scattering surface and $^{12}$CO gas emission surface
for three protoplanetary disks IM Lup, HD 163296, and HD 97048 (CU Cha). For
two systems, IM Lup and HD 163296, the $^{12}$CO gas and small dust grains at
small radii from the star have similar heights but at larger radii ($>$100 au)
the dust grain scattering surface height is lower than the $^{12}$CO gas
emission surface height. In the case of HD 97048, the small dust grain
scattering surface has similar heights to the $^{12}$CO gas emission surface at
all radii. We ran a protoplanetary disk radiative transfer model of a generic
protoplanetary disk with TORUS and showed that there is no difference between
the observed scattering surface and $^{12}$CO emission surface. We also
performed analytical modeling of the system and found that gas-to-dust ratios
larger than 100 could explain the observed difference in IM Lup and HD 163296.
This is the first direct comparison of observations of gas and small dust grain
heights distribution in protoplanetary disks. Future observations of gas
emission and near-infrared scattered light instruments are needed to look for
similar trends in other protoplanetary disks.
