acerbate vs irk

acerbate

verb
  • To exasperate; to irritate. 

  • To make bitter or sour. 

adj
  • Embittered; having a sour disposition or nature. 

irk

verb
  • to irritate; annoy; bother 

How often have the words acerbate and irk occurred in a corpus of books? (source: Google Ngram Viewer )