Atmospheric Composition and Structure

Composition of the Atmosphere

The atmosphere is composed of a mixture of gases and aerosols that, when considered in their elemental states, result in a surprisingly small number of major gases, trace gases, and a number of variable components that can be present depending on the location and conditions. The most important of these atmospheric constituents are the major gases, as their percent by volume of the atmosphere rarely changes from day to day or location to location. Given a random sample of air at sea level (therefore, representative of a 100% gaseous mixture), we can express the relative composition of individual gases and components in the atmosphere by their percentages. The three most prominent gases in the atmosphere, based on a relatively recent composition analysis are (1) nitrogen in its diatomic state (N2), comprising approximately 78.1% of our air sample; (2) oxygen in its diatomic state (O2; perhaps the best-known component of the atmosphere due to its respiratory function within the Kingdom Animalia), which comprises approximately 21% of the atmosphere; and (3) the noble gas argon (Ar) in its elemental state, which comprises less than 1% of the atmosphere. If one were to add up a more scientifically precise percent by volume for our sample of dry air by not rounding for simplicity's sake, these three components would comprise approximately 99.964% of all the detectible gases in the air mixture. Therefore, for each breath inhaled by human beings, the vast majority of that air consists of inert diatomic nitrogen and the relatively inert noble gas argon, with less than 21% of the air that we breathe consisting of oxygen. What gases and aerosols then comprise the remaining 0.036%?

The remaining 0.036% is changing in percent by volume at all times due to the dramatic increase [Page 137]in the fourth most prevalent atmospheric gas, carbon dioxide (CO2). At the time when careful measurements of the atmosphere were being conducted to determine atmospheric composition, CO2 comprised nearly the entire 0.036% remaining. However, in recent years, the level of CO2 in the atmosphere has been increasing both in percent by volume and in the number of molecules of a selected gas per million molecules sampled, known as parts per million or ppm. Expressed in this way, a few years ago, the concentration of CO2 in the atmosphere was nearly 360 ppm (or almost 0.036%). Today, the concentration of CO2 in the atmosphere is slightly more than 380 ppm (or a little more than 0.038%), resulting in slightly less of a percent by volume of the three major gases but only by a small fraction of a percent. However, this small fraction of a percent is leading many scientists today to worry about “climate change,” as there are inherent properties associated with the concentration of CO2 in the atmosphere that affect the temperature of the atmosphere as well.

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