ABSTRACT
The most abundant species in the atmosphere are nitrogen (N2), oxygen (O2), water (H2O), argon (Ar) and carbon dioxide (CO2). However, in terms of variations in atmospheric chemistry, there is a series of less abundant gases (trace gases) which have a greater impact than their concentrations might suggest. The most important of these are ozone (O3), methane (CH4), nitric oxide (NO), nitrogen dioxide (NO2), and hydrocarbons. The chemical reactions of these trace gases are influenced by many factors, including the concentration of particle surfaces, the amount of water available, and intensity of solar radiation. All these factors vary widely, not only within the lower atmosphere, but also between the major layers of the atmosphere (see 43.4). The stratosphere is dry and exposed to very high energy photons from the Sun. It is also relatively free of hydrocarbons and dust. In contrast, the troposphere has more water and dust, and it contains a cocktail of hydrocarbons released by biological processes. The troposphere is also protected from high-energy photons because these are absorbed by matter in the stratosphere above.
