  We present dust continuum observations of the protoplanetary disk surrounding
the pre-main sequence star AS 209, spanning more than an order of magnitude in
wavelength from 0.88 to 9.8 mm. The disk was observed with sub-arcsecond
angular resolution (0.2"-0.5") to investigate radial variations in its dust
properties. At longer wavelengths, the disk emission structure is notably more
compact, providing model-independent evidence for changes in the grain
properties across the disk. We find that physical models which reproduce the
disk emission require a radial dependence of the dust opacity \kappa_{\nu}.
Assuming that the observed wavelength-dependent structure can be attributed to
radial variations in the dust opacity spectral index (\beta), we find that
\beta(R) increases from \beta<0.5 at \sim20 AU to \beta>1.5 for R>80 AU,
inconsistent with a constant value of \beta\ across the disk (at the 10\sigma\
level). Furthermore, if radial variations of \kappa_{\nu} are caused by
particle growth, we find that the maximum size of the particle-size
distribution (a_{max}) increases from sub-millimeter-sized grains in the outer
disk (R>70 AU) to millimeter and centimeter-sized grains in the inner disk
regions (R< 70 AU). We compare our observational constraint on a_{max}(R) with
predictions from physical models of dust evolution in proto-planetary disks.
For the dust composition and particle-size distribution investigated here, our
observational constraints on a_{max}(R) are consistent with models where the
maximum grain size is limited by radial drift.
