When do we use “arrive at” versus “arrive in”?
Is the usage at or in correct in following sentences?
a. Flight has arrived in Tokyo.
b. Flight has arrived at Tokyo airport.
Both of your sentences are correct. American English speakers, and Brits as far a I know, would say "in" a city and "at" an airport. The word "in" is a little less precise than "at", and so fits better with the generalized city vs the more specific location of the airport.
We normally use "arrive in" for cities, towns, and countries, and "arrive at" for other places. So both your sentences are correct and:
arrive at the airport
arrive at Tokyo Airport
arrive in Tokyo
But if you're referring to a point on a journey, this also becomes possible:
P.S. "Flight" is a Countable Noun so you need either an Article or a Possessive Pronoun to go with it
You need an article before "flight". You need an indefinite article before "Tokyo airport" because there is Narita airport and Haneda airport in Tokyo. While "... arrived at a Tokyo airport" does not sound natural to me, the grammar is correct in both.
Both of your suggested phrases sound good to me. I don't think that the opposites work ("at Tokyo" and "in Tokyo airport").