According to current concepts, the composition of the atmosphere is largely determined by dynamic processes. The atmosphere represents a geochemical reservoir through which the elements are cycled by an exchange of matter with the oceans and with terrestrial surface waters and soils. Much of the exchange is mediated by biological processes, so that the biosphere is an important source of atmospheric constituents. A typical atmospheric cycle of an element such as sulfur or nitrogen begins with its release into the air as a volatile compound, usually in chemically reduced form. Thereafter the compound undergoes transport and chemical transformation within the atmosphere, but eventually the element is returned to the earth's surface by precipitation scavenging and dry deposition. The natural cycles of atmospheric sulfur and nitrogen are now massively perturbed by an additional emission of huge quantities of SO2 and NOx associated with the combustion of fossil fuels for energy production. It is thus necessary to study the unperturbed atmosphere in more detail in order to assess the impact of anthropogenic emissions upon remote atmospheric regions.