What is that sensor on the Airbus A220?
That's one of the two ice detectors manufactured by Collins. Below is not precisely the model used by A220, but serves to illustrate the general layout of the unit:



Image ref: Collins

From this Collins fact sheet, ice detectors 


  use a magnetostrictive technology to drive the sensing probe to resonate at its natural frequency. As ice accretes on the probe, a shift in resonance frequency occurs. When the resonance frequency reaches the set point, an ice signal is activated...