  We report on the effect of exposure to atmospheric conditions on the THz
conductivity of thin films of the topological insulator Bi$_2$Se$_3$. We find:
1) two contributions of mobile charge carriers to the THz conductivity
immediately after growth, and 2) the spectral weight of the smaller of these
decays significantly over a period of several days as the film is exposed to
ambient conditions, while the other remains relatively constant. We associate
the former with a bulk response, and the latter with the surface. The surface
response exhibits the expected robustness of the carriers from 2D topological
surface states. We find no evidence for a third spectral feature derived from
topologically trivial surface states.
