system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does the air seem "fresher" in the morning?
gpt
Are you referring to outside air or your perception upon waking in general?  Speaking to outside air, generally speaking (noting that each climate is different) humidity raises in the morning as the air is being warmed by the sun.  As humidity increases, our perception of smell increases which would be one contributing factor for a fresher smell.

Also as the ground and air warms, more insects that interact with the plants (bees, flies, etc..) are moving around, stirring up sent particles which also can contribute.  Wind invariably still moves regardless of the time of day however insects move inside of folds and cavities of plants where a more dense consultation of sent particles are contained.

tl;dr Humidity increases in the morning, heightening our sense of smell.  Same reason your farts smell worse in the shower.