Anthropogenic pollution problems result from the enhancement of gas and aerosol-particle concentrations above background concentrations. In this chapter, the evolution of the background atmosphere is discussed. The discussion requires a description of the sun and its origins because sunlight has affected much of the evolution of the Earth's atmosphere. The description also requires a discussion of the Earth's composition and structure because the inner Earth affects atmospheric composition through outgassing, and the crust affects atmospheric composition through exchange processes, including soil-dust emission. Earth's earliest atmosphere contained mostly hydrogen and helium. Carbon dioxide replaced these gases during the onset of the Earth's second atmosphere. Today, nitrogen and oxygen are the prevalent gases. Processes controlling the changes in atmospheric composition over time include outgassing from the Earth's interior, microbial metabolism, and atmospheric chemistry. These processes still affect the natural composition of the air today.

THE SUN AND ITS ORIGIN

The sun provides the energy to power the Earth. Most of that energy originates from the sun's surface, not from its interior. The reason for this is discussed as follows.

About 15 billion years ago (b.y.a.), all mass in the known universe may have been compressed into a single point, estimated to have a density of 109 kg m−3 and a temperature of 1012 K (kelvin). With the “Big Bang,” this point of mass exploded, ejecting material in all directions. Aggregates of ejected material collapsed gravitationally to form the earliest stars.