  We analyze dispersion measure (DM) variations of 37 millisecond pulsars in
the 9-year NANOGrav data release and constrain the sources of these variations.
Variations are significant for nearly all pulsars, with characteristic
timescales comparable to or even shorter than the average spacing between
observations. Five pulsars have periodic annual variations, 14 pulsars have
monotonically increasing or decreasing trends, and 13 pulsars show both
effects. Several pulsars show correlations between DM excesses and lines of
sight that pass close to the Sun. Mapping of the DM variations as a function of
the pulsar trajectory can identify localized ISM features and, in one case, an
upper limit to the size of the dispersing region of 13.2 AU. Finally, five
pulsars show very nearly quadratic structure functions, which could be
indicative of an underlying Kolmogorov medium. Four pulsars show roughly
Kolmogorov structure functions and another four show structure functions less
steep than Kolmogorov. One pulsar has too large an uncertainty to allow
comparisons. We discuss explanations for apparent departures from a
Kolmogorov-like spectrum, and show that the presence of other trends in the
data is the most likely cause.
