Air quality monitoring devices for a320
Far as I can tell, there are no passive or active air quality monitoring devices on the A320. The cabin air systems on modern airliners are designed against an air quality standard, and their performance is tested with equipment that is installed on the test aircraft for that specific purpose. 

EASA published the results of an air quality campaign test program in 2017. 


  In total, 69 measurement flights were performed between July 2015 and
  June 2016 on 8 types of aeroplane/engine configurations. This included
  61 flights on aeroplanes equipped with engine bleed air systems, and 8
  flights on Boeing 787 which is equipped with electrical compressors
  (‘bleed free’ system). For all flights, measurement equipment was
  installed in the cockpit and in the cabin.
  
  The results show that the cabin/cockpit air quality is similar or
  better than what is observed in normal indoor environments (offices,
  schools, kinder gardens or dwellings). No occupational exposure limits
  and guidelines were exceeded. Special attention was paid to
  organophosphates, in particular TCP (Tri Cresyl Phosphates) with the
  use of high sensitivity analysis techniques. Only traces of TCP meta-
  and para-isomers were sometime detected in all aeroplane types, but no
  ortho-isomers were detected.