introduction of composition and structure
Answers
This chapter discusses the structure and composition of the Earth's atmosphere. The Earth's atmosphere is a reservoir of several gases. The gas molecules in this reservoir are continuously exchanged because of the interaction of different sources and sinks. The average time spent in the atmosphere by the molecules of a given gas is called average residence time, or simply residence time. The concept of the residence time gives a good basis for classifying the atmospheric gases. It is to be noted that the relative and absolute quantities of the present quasi-permanent gases were variable during geological times. This means that their concentration can only be considered loosely as permanent if one studies the atmospheric phenomena on a time scale short compared to the age of the Earth. The residence time of atmospheric gases has a very important characteristic: the longer the residence time of a given constituent the smaller the variability of its concentration in time and space.