A Lagrangian view of stratospheric trace gas distributions

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 sam ple is expected. The existence of such a relationship allows inferences abo ut transport history to be made from observations of chemical tracers. This paper lays down the conceptual foundations for the relationship between ag e and tracer amount for long-lived tracers, developed within a Lagrangian f ramework. Although the photochemical loss depends not only on the age of th e parcel but also on its path, we show that under the "average path approxi mation" that the path variations are less important than parcel age. The av erage path approximation then allows us to develop a formal relationship be tween the age spectrum and the tracer distribution. Using this relationship , tracer-tracer correlations can be interpreted as the result of mixing whi ch connects parts of the "single-path photochemistry curve," a universal pa th-independent curve that describes the photochemical loss in terms of the total photon exposure. This geometric interpretation of mixing gives rise t o constraints on trace gas correlation curves as can be seen in the atmosph eric trace molecule spectroscopy observations.