Tracing the Physical Conditions in Active Galactic Nuclei with
  Time-Dependent Chemistry
  We present an extension of the code ProDiMo that allows for a modeling of
processes pertinent to active galactic nuclei and to an ambient chemistry that
is time dependent. We present a proof-of-concept and focus on a few
astrophysically relevant species, e.g., H+, H2+ and H3+; C+ and N+; C and O; CO
and H2O; OH+, H2O+ and H3O+; HCN and HCO+. We find that the freeze-out of water
is strongly suppressed and that this affects the bulk of the oxygen and carbon
chemistry occurring in AGN. The commonly used AGN tracer HCN/HCO+ is strongly
time-dependent, with ratios that vary over orders of magnitude for times longer
than 10^4 years. Through ALMA observations this ratio can be used to probe how
the narrow-line region evolves under large fluctuations in the SMBH accretion
rate. Strong evolutionary trends, on time scales of 10^4-10^8 years, are also
found in species such as H3O+, CO, and H2O. These reflect, respectively, time
dependent effects in the ionization balance, the transient nature of the
production of molecular gas, and the freeze-out/sublimation of water.
