Dalton's Law of Partial Pressures
For a fixed temperature and volume (container),
the total pressure of a mixture of gases is the sum
of the partial pressures that each of the individual
component gases would have if placed alone in
the same volume (container).

Earth's atmosphere is a mixture of gases:
78.08% Nitrogen
20.95% Oxygen
0.93% Argon
0.00182% Neon
0.00052% Helium
0.00005% Hydrogen
0.0000114% Krypton
0.000009% Xenon
0.1% to 4% Water Vapor (variable)...maximum in tropics, minimum at South Pole
0.035% Carbon Dioxide (variable)...increasing about 0.4% per year
0.00017% Methane (variable)
0.00003% Nitrous Oxide (variable)
0.000004% Ozone (variable)
0.00000001% Chlorofluorocarbons [CFCs (variable)]