ABSTRACT

Human activities are changing the composition of the atmosphere not only directly through the emission of trace gases and aerosols, but also indirectly through perturbations in the physical, chemical, and ecological characteristics of the Earth System. These perturbations in turn influence the rates of production and loss of atmospheric constituents.

The impact of direct anthropogenic emissions on the atmosphere is often relatively easy to assess, especially if they are tied to major industrial activities, where accurate and detailed records are kept for economic reasons. Classical examples are the release of chlorofluorocarbons and the emission of CO2 from fossil fuel combustion. There are also cases, however, in which it is much more difficult to obtain accurate emission estimates. An example is biomass burning, for which no economic incentive for record keeping exists, and which takes on many forms, each with a different emission profile.

A more complex case exists in which human activities release a precursor compound, which is transformed in the atmosphere to a climatically active substance. This can be illustrated using the example of SO2, from which sulfate aerosol can be formed. The actual amount of radiatively active sulfate aerosol produced, however, is determined by a complex interplay of atmospheric transport processes, chemical processes in the gas phase, and interactions with other aerosol species.