This is a jukugo word, which usually means on'yomi readings from the kanji. Since 大 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Imagine an atmosphere consisting entirely of ties and keys (たいき), floating around. The ties are getting tangled together, and the keys keep bashing you on the head. Bring back the normal atmosphere, this one's horrible.
I check the atmospheric pollution map everyday.
The sky is blue because blue and green lights are scattered in all directions by the air molecules in Earth's atmosphere.
Air conditions are unstable nationwide again today.