I am lightest on the ground but I am heaviest in the air. What am I?
It could be a
Hot air balloon
It's lightest on the ground because
the air outside the balloon is heavier than inside. This is due to the higher temperature inside the balloon, thus lower pressure than outside the balloon.
It's heaviest in the air because
the atmospheric pressure is lower higher up (i.e. the air is lighter), making the balloon relatively heavier.
Otherwise it would just go up up up.
My guess is
Lightest on the ground
Lots of CO2 is trapped in our soil through various means. It is among the lightest compounds found in soil.
Heaviest in the air:
CO2 is the heaviest compound (that I know of) in the Earth's atmosphere. I believe Argon is the second heaviest, with an atomic weight of 18, but I could be mistaken.