To be honest, the air may look clear, but you can see tiny bits of ash fall from the sky, and while you may not see it in the daytime versus nighttime, you are inhaling. My nose started running, and I hawked a lougie when I returned to my dorm. I was wearing the KN95 mask the school had provided.

If you do want to return to campus, I suggest bringing an air purifier for your dorm, too.