As a result of photochemistry, some relationship between the stratospheric age of air or mean age and the amount of tracer contained within an air sample is expected. The existence of such a relationship allows inferences about transport history to be made from observations of chemical tracers. This paper lays down the conceptual foundations for the relationship between age and tracer amount for long-lived tracers, developed within a Lagrangian framework. Although the photochemical loss depends not only on the age of the parcel but also on its path, we show that under the average path approximation that the path variations are less important than parcel age. The average path approximation then allows us to develop a formal relationship between the age spectrum and the tracer distribution. Using this relationship, tracer-tracer correlations can be interpreted as the result of mixing which connects parts of the single-path photochemistry curve, a universal path-independent curve that describes the photochemical loss in terms of the total photon exposure. This geometric interpretation of mixing gives rise to constraints on trace gas correlation curves as can be seen in the atmospheric trace molecule spectroscopy observations. ¿ 2000 American Geophysical Union