Seasonal evolutions of N 2 O, O 3 , and CO 2 : Three‐dimensional simulations of stratospheric correlations

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 N 2 O, CO 2 , and O 3 over a year. In the modeled lower stratosphere the constituents N 2 O and CO 2 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 CO 2 and to transport of air from the upper stratosphere photochemically depleted in N 2 O. Due to the short photochemical lifetime of tropical O 3 , its relationship with N 2 O is still more varied. In particular, the slope of the O 3 −N 2 O regression line changes significantly from middle to high latitudes, behavior relevant to the use of N 2 O for estimating the rate of polar winter O 3 depletion. In general, a tight correlation between two trace gases such as N 2 O and O 3 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.