"A solution is a homogenous mixture of two or more substances and in such mixture, the solute is a substance dissolved in another substance known as a solvent."
The concentration of solute in a solution is always less than that of the solvent.
Solute and solvent in air
In the air, considering the concentration of the gases, gas with a higher concentration would be solvent, and gas with lower concentration would be the solute.
Since the concentration of nitrogen (N2) is maximum in air. Hence, the solvent is Nitrogen gas and other gases like Oxygen and other gases act as solute.