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Age as a diagnostic of stratospheric transportEstimates of stratospheric age from observations of long-lived trace gases with increasing tropospheric concentrations invoke the implicit assumption that an air parcel has been transported intact from the tropopical tropopause. However, because of rapid and irreversible mixing in the stratosphere, a particular air parcel cannot be identified with one that left the troposphere at some prior time. The parcel contains a mix of air with a range of transit times, and the mean value over this range is the most appropriate definition of age. The measured tracer concentration is also a mean over the parcel, but its value depends both on the transit time distribution and the past history of the tracer in the troposphere. In principle, only if the tropospheric concentration is increasing linearly can the age be directly inferred. We illustrate these points by employing both a one-dimensional diffusive analog of stratospheric transport, and the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS). Within the limits of the GCM, we estimate the time over which tropospheric tracer concentrations must be approximately linear in order to determine stratospheric age unambiguously; the concentration of an exponentially increasing tracer is a function only of age if the growth time constant is greater than about 7 years, which is true for all the chlorofluorocarbons. More rapid source variations (for example, the annual cycle in CO2) have no such direct relationship with age.
Document ID
19950031230
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Hall, Timothy M.
(NASA Goddard Institute for Space Studies, New York, NY United States)
Plumb, R. Alan
(Massachusetts Inst. of Technology, Cambridge, MA United States)
Date Acquired
August 16, 2013
Publication Date
January 20, 1994
Publication Information
Publication: Journal of Geophysical Research
Volume: 99
Issue: D1
ISSN: 0148-0227
Subject Category
Environment Pollution
Accession Number
95A62829
Distribution Limits
Public
Copyright
Other

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