English for Today by Ganiu Abisoye Bamgbose (Dr GAB)
Are you an authority in/on your field?
He is an authority ~in~ English grammar (nonstandard).
He is an authority *on* English grammar (standard).
Note that “authority” collocates with “on” to talk about expertise, but “expert” and “specialist” collocate with “in” for the same usage:
He is a specialist ~on~ English grammar (nonstandard).
He is a specialist *in* English grammar (standard).
He is an expert ~on~ English grammar (nonstandard).
He is an expert *in* English grammar (standard).
GAB
No comment