Department of Earth System Science

Fluctuations in the concentrations of stratospheric trace gases are often correlated over a large range of space and time scales, an observation frequently used to infer the existence of various chemical processes. Three-dimensional models provide a tool to examine the causes and variations of trace gas relationships, because they can realistically simulate the interplay between stratospheric photochemistry and meteorology. Thus such models can aid the interpretation of observed trace gas relationships. We use the general circulation model of the Goddard Institute for Space Studies to simulate the evolution and distribution of N₂O, CO₂, and O₃ over a year. In the modeled lower stratosphere the constituents N₂O and CO₂ have well-correlated spatial variations, but the slope of the regression line depends on both the season and the direction of sampling. This departure from a universal form is due both to the annual cycle in tropospheric CO₂ and to transport of air from the upper stratosphere photochemically depleted in N₂O. Due to the short photochemical lifetime of tropical O₃, its relationship with N₂O is still more varied. In particular, the slope of the O₃−N₂O regression line changes significantly from middle to high latitudes, behavior relevant to the use of N₂O for estimating the rate of polar winter O₃ depletion. In general, a tight correlation between two trace gases such as N₂O and O₃ is often observed, but this datum cannot be used to infer a similar universal relationship because a different direction of sampling may change the slope and the scatter about it.