system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does the air at night smell nicer/different?
gpt
During the day, the sun heats the air and it causes air to get mixed all over the place due to convection.  This kicks up a lot of stuff and generally makes it smell worse.  In the colder night, the air is stiller and thus cleaner.  It also decreases the airs ability to hold water, so the moisture condenses in our noses and increases our sense of smell (same reason dogs have wet noses).