Earth's atmosphere is unique in the solar system, and there are a variety of geological and ecological factors that affect its composition.
First, in comparison with other planets in the solar system, Earth's atmosphere is uniquely rich in oxygen. Oxygen, of course, comes from plants as a by-product of photosynthesis. Let's compare Earth with our planetary neighbors, Venus and Mars. Both of these planet's atmospheres are rich in carbon dioxide. Venus has a very thick atmosphere while Mars has a thin atmosphere, but they are both mainly comprised of carbon dioxide.
So, in terms of ecology, the existence of photosynthetic organisms (both terrestrial and marine) have had a huge impact on the composition of Earth's atmosphere.
Geologically, volcanoes and other outlets do something we call "outgassing". Outgassing releases gases trapped in Earth's interior. These gases are primarily water vapor and smaller amounts of carbon dioxide, sulfur dioxide, and hydrogen.
If you consider humans part of "ecology", then, in the past 100 years, humans have had a considerable effect on Earth's atmosphere. The industrial revolution and the resulting technological advances (like cars, factories, and so on) have contributed to the slow but steady increase of carbon dioxide in the atmosphere. We think this is a problem because it may lead to global warming, which is an increase in the amount of energy in Earth's system. Therefore, global warming may lead to more frequent or more powerful weather events (like hurricanes) and more exagerrated weather patterns (like drier areas become drier and wetter areas become wetter, or vice versa). The problem with this is that humans are dependent on weather patterns for survival (and not just to help us decide what to wear on a given day!).
A related problem is that of the depletion of the ozone layer. This was caused by the development and widespread use of CFCs. Although the U.S. has banned the use of CFCs, it is taking the atmosphere a long time to recover.
The atmosphere is named in normal speech air. At 4.56 billion years ago, when earth globe appeared, hydrogen (H2) and helium (He) were already present.
Subsequently, due to the low density of these 2 gases, they will not be attracted by the planet, spreading progressive in cosmic space.
Thanks to the slow cooling of the Earth, volcanic activity, were brought to the surface various gases, which were the result of chemical reactions of the internal layers of the globe. These processes arose an atmosphere with a composition of about 80% water vapor (H2O), 10% carbon dioxide (CO2) and 5 to 7% hydrogen sulfide. This combination of gases can be seen today in the emanations and volcanic eruptions.
The climatology depends not only the shape of the atmosphere , but also its size. For an atmosphere 35 times thicker than today, would see the sun until the afternoon as a red spot and Heatstroke should be almost null. "Silver Clouds" that we see at night show heights of almost 80 km, and other facts allow us to say this atmosphere even beyond 250 km. In terms of utility, more interesting than the shape and dimensions of the atmosphere is it's temperature.
Immediately after the air temperature is relative air density and it's weight of each region. Aristotle knew that the air is heavy - even had made some attempts to weight it, but only in modern times found a reliable means of measuring the weight of the atmosphere, when it was discovered barometer (1644).