Plein air are the French words we like to toss around explaining painting outdoors all at one go—plein air always suggesting just plain outside air. (French meaning of plein air is in the open air.)
Not plain; in fact, it's within buildings where the plain air usually is, plain, restricted, usually only partially or disappointingly lit. Out in the wild air there is not only the cutting bite of heat or cold, of winds tumultuous or calm—there are the smells of trees, cars, clouds, soil. So plain air spelled with an a would be—enclosed air, inside most of our buildings.