We could just say 'air', but once we modify it to 'air around us' it sounds better to use a determiner - especially as it sits among a group of similar items. We are referring, in the structure of the sentence, to a particular part of 'air', although of course we know in reality it's all air.
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We could just say 'air', but once we modify it to 'air around us' it sounds better to use a determiner - especially as it sits among a group of similar items. We are referring, in the structure of the sentence, to a particular part of 'air', although of course we know in reality it's all air.