[sarasi]

Why is the first kanji in "空気" there? Because the Japanese word for "air" has always been written like that. It may help you if you simply think of the word "空気" as "air" and don't try to dissect it. Learn kanji compounds together as words, not as separate characters, and don't spend too much time worrying about why those particular kanji together have that meaning, otherwise you will get really bogged down.