Explanation:

 The atmosphere contains many gases, most in small amounts, including some pollutants and greenhouse gases.  The most abundant gas in the atmosphere is nitrogen, with oxygen second.  Argon, an inert gas, is the third most abundant gas in the atmosphere.

Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%.  Gases like carbon dioxide, nitrous oxides, methane, and ozone are trace gases that account for about a tenth of one percent of the atmosphere.  Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is.