Classification

The principle underlying diagnostic use of modelling of tracer transport is that the space–time distributions of trace-gas concentrations in the atmosphere reflect the space–time distributions of their sources and sinks. Observations of these distributions can in turn provide information about the processes involved. Tracer-inversion calculations use the relation between fluxes and concentrations in order to make specific estimates of fluxes. The possibility of using atmospheric-concentration data to make more direct inferences about processes is addressed in Chapter 12.

In this chapter, we review the general characteristics of sources that will influence the effectiveness of such inversions. In practice, the question of which source characteristics are important depends on the objectives of the inversion.

In considering the sources, there are two questions that we need to ask.

• What do we wish to learn about the sources and sinks?

• What prior information is available for Bayesian estimation?

The first question is addressed in very broad terms in the following section. To put the discussion in context, we consider some of the generic ways of classifying sources and sinks of greenhouse gases.

Location. This is the primary information required by forward modelling of atmospheric transport of trace constituents. Conversely, information about source strengths at specific locations is the most direct output from inverse modelling.