What does the atmosphere contain?
There are many things in the atmosphere, but basically it is made
up of two different elements which are gases. Measured by volume
(not weight) completely dry air contains approximately 78% nitrogen
and 21% oxygen with about 1% of everything else. The extra 1% or so
of total air content consists of many different things. It is
mostly argon plus small amounts of carbon dioxide and a number of
other gases. However, air is not normally dry so you have to factor
in an average of about 1% or more of water vapor. In addition to
those things, air also contains dust, pollen, spores, ash, and
various chemical pollutants. If you consider the air by mass or
weight instead of volume then some of those gases like water vapor
and carbon dioxide which are heavier would consist of a larger
percentage of the air. The same would be true of particles and
other pollutants in the air.