  We report a study of high quality pnictide superconductor
BaFe$_{1.84}$Co$_{0.16}$As$_2$ thin films using time-domain THz spectroscopy.
Near T$_c$ we find evidence for a coherence peak and qualitative agreement with
the weak-coupling Mattis-Bardeen form of the conductivity. At low temperature,
we find that the real part of the THz conductivity is not fully suppressed and
$\sigma_2$ is significantly smaller than the Matthis-Bardeen expectation. The
temperature dependence of the penetration depth $\lambda$ follows a power law
with an unusually high exponent of 3.1. We interpret these results as
consistent with impurity scattering induced pair-breaking. Taken together our
results are strong evidence for an extended s$\pm$ symmetry order parameter.
