Mid-Infrared Extinction and its Variation with Galactic Longitude
  Based on the data obtained from the Spitzer/GLIPMSE Legacy Program and the
2MASS project, we derive the extinction in the four IRAC bands, [3.6], [4.5],
[5.8] and [8.0] micron, relative to the 2MASS Ks band (at 2.16 micron) for 131
GLIPMSE fields along the Galactic plane within |l|<65 deg, using red giants and
red clump giants as tracers. As a whole, the mean extinction in the IRAC bands
(normalized to the 2MASS Ks band), A_[3.6]/A_Ks=0.63, A_[4.5]/A_Ks=0.57,
A_[5.8]/A_Ks=0.49, A_[8.0]/A_Ks=0.55, exhibits little variation with wavelength
(i.e. the extinction is somewhat flat or gray). This is consistent with
previous studies and agrees with that predicted from the standard interstellar
grain model for R_V=5.5 by Weingartner & Draine (2001). As far as individual
sightline is concerned, however, the wavelength dependence of the mid-infrared
interstellar extinction A_{lambda}/A_Ks varies from one sightline to another,
suggesting that there may not exist a "universal" IR extinction law. We, for
the first time, demonstrate the existence of systematic variations of
extinction with Galactic longitude which appears to correlate with the
locations of spiral arms as well as with the variation of the far infrared
luminosity of interstellar dust.
