Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%. These gases precentages stay fairly constant day to day. The other atmospheric gases appear in trace ammounts and can vary day by day.
So that got me to wondereing,....why are the gasses on Earth so different from the rest of the Solar system, (or Universe in general), which is composed mainly of Hydrogen - 75%, and Helium - nearly 25%? I couldn't find an answer, and it all gets further complicated because we don't know what "dark matter' is composed of. However, I did find out something truly amazing regarding how important the percentages of Nitrogen and Oxygen are to life on earth, and what would happen if those amounts changed only a few percent: "Consider this: If the abundance of oxygen was any greater, all life would be at risk. The probability of forest fire increases by 70 percent for each 1 percent rise in oxygen concentration above the present level. If it formed 25% of the air instead of 21% now, oxygen would leave all land vegetation in raging fires. Everything from the driest Arctic tundra to the wettest tropical rain forest would burst into flame. And if the level of nitrogen, which is largely responsible for air pressure, were to fall to 75% from its present level of 78%, our planet would enter ice-age – possibly permanent."