  Superconducting radio-frequency (SRF) niobium cavities are the modern means
of particle acceleration and an enabling technology for record coherence
superconducting quantum systems and ultra-sensitive searches for new physics.
Here we report a systematic effect observed on a large set of bulk SRF cavities
- an anomalous decrease of the resonant frequency at temperatures just below
the superconducting transition temperature - which opens up a new means of
understanding the physics behind nitrogen doping and other modern cavity
surface treatments relevant for future quality factor and coherence
improvements. The magnitude of the frequency change correlates systematically
with the near-surface impurity distribution in studied cavities and with the
observed $T_c$ variation. We also present the first demonstration of the
coherence peak in the real part of the AC complex conductivity in Nb SRF
cavities and show that its magnitude varies with impurity distribution.
