  Near-infrared spectroscopy from APOGEE and wide-field optical photometry from
Pan-STARRS1 have recently made possible precise measurements of the shape of
the extinction curve for tens of thousands of stars, parameterized by R(V).
These measurements revealed structures in R(V) with large angular scales, which
are challenging to explain in existing dust paradigms. In this work, we combine
three-dimensional maps of dust column density with R(V) measurements to
constrain the three-dimensional distribution of R(V) in the Milky Way. We find
that variations in R(V) are correlated on kiloparsec scales. In particular,
most of the dust within one kiloparsec in the outer Galaxy, including many
local molecular clouds (Orion, Taurus, Perseus, California, Cepheus), has a
significantly lower R(V) than more distant dust in the Milky Way. These results
provide new input to models of dust evolution and processing, and complicate
application of locally derived extinction curves to more distant regions of the
Milky Way and to other galaxies.
