  The aromatic features in M101 were studied spectroscopically and
photometrically using observations from all three instruments on the Spitzer
Space Telescope. The global SED of M101 shows strong aromatic feature (commonly
called PAH feature) emission. The spatially resolved spectral and photometric
measurements of the aromatic feature emission show strong variations with
significantly weaker emission at larger radii. We compare these variations with
changes in the ionization index (as measured by [NeIII]/[NeII] and [SIV/SIII],
which we probe over the ranges 0.03-20 and 0.044-15 respectively) and
metallicity (expressed as log(O/H)+12, which ranges from 8.1 to 8.8). Over
these ranges, the spectroscopic equivalent widths of the aromatic features from
7 HII regions and the nucleus were found to correlate better with ionization
index than with metallicity. This implies that the weakening of the aromatic
emission in massive star forming regions is due primarily to processing of the
dust grains in these environments, not to differences in how they form
(although formation could still be important on a secondary basis). The
behavior of the correlation between the aromatic feature equivalent widths and
ionization index can be described as a constant equivalent width until a
threshold in ionization index is reached ([NeIII]/[NeII] ~ 1), above which the
equivalent widths decrease with a power law dependence. This behavior for M101
HII regions is also seen for the sample of starburst galaxies presented in the
companion study of Engelbracht et al. (2008) which expands the range of
[NeIII]/[NeII] ratios to 0.03-25 and log(O/H)+12 values to 7.1-8.8. The form of
the correlation explains seemingly contradictory results present in the
literature. [abridged]
