Abstract
The most abundant gases in dry air, N2 ( ≈ 78.1% by volume) and O2 ( ≈ 20.95% by volume), represent the equilibrium state of global biogeochemical processes that have operated on time scales of many millions of years. Among the remaining gases, the noble gas argon ( ≈ 0.93% by volume) is by far most abundant. Because of their great abundance and long lifetimes, human activities have not affected the concentrations of these major atmospheric constituents to any significant degree. Any changes due to human activities can, therefore, only reflect themselves in the concentrations of less abundant and shorter-lived gases. During the most recent centuries, Mankind has been changing atmospheric chemistry by the emissions of typical fossil and biomass fuel derived air pollutants, such as CO, SO2, NO, NO2, hydrocarbons, soot and sulphate particles, as well as a number of long-lived gases such as CH4, N2O, and the chlorofluorocarbons CFCl3 and CF2Cl2, which are not directly harmful to life, but exert a major influence on global atmospheric chemistry and climate.
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