The Optical -- Mid-infrared Extinction Law of the l=165 Sightline in the
  Galactic Plane: Diversity of Extinction Law in the Diffuse Interstellar
  Medium
  Understanding the effects of dust extinction is important to properly
interpret observations. The optical total-to-selective extinction ratio, Rv =
Av/E(B-V), is widely used to describe extinction variations in ultraviolet and
optical bands. Since the Rv=3.1 extinction curve adequately represents the
average extinction law of diffuse regions in the Milky Way, it is commonly used
to correct observational measurements along sightlines toward diffuse regions
in the interstellar medium. However, the Rv value may vary even along different
diffuse interstellar medium sightlines. In this paper, we investigate the
optical--mid-infrared (mid-IR) extinction law toward a very diffuse region at l
= 165 in the Galactic plane, which was selected based on a CO emission map.
Adopting red clump stars as extinction tracers, we determine the
optical-to-mid-IR extinction law for our diffuse region in the two APASS bands
(B, V), the three XSTPS-GAC bands (g, r, i), the three 2MASS bands (J, H, Ks,
and the two WISE bands (W1, W2). Specifically, 18 red clump stars were selected
from the APOGEE--RC catalog based on spectroscopic data in order to explore the
diversity of the extinction law. We find that the optical extinction curves
exhibit appreciable diversity. The corresponding Rv ranges from 1.7 to 3.8,
while the mean Rv value of 2.8 is consistent with the widely adopted average
value of 3.1 for Galactic diffuse clouds. There is no apparent correlation
between Rv value and color excess E(B-V) in the range of interest, from 0.2 to
0.6 mag, or with specific visual extinction per kiloparsec, AV/d.
