AoA
AoA (АоА) may mean "Age of stratospheric Air" [1][2] or "Age of Air" (referring the same stratospheric air) [3].
This quantity has sense of time spent by an air parcel in the stratosphere since its entry the stratosphere across the tropopause (boundary in the Earth's atmosphere between the troposphere and the stratosphere; roughly, at 17 km above equatorial regions, and about 9 km over the polar regions).
Such a "definition" does not yet indicate the unique (and exhausting) way to measure this parameter; so, its value is expected to depend on the methods of evaluation.
However it is still supposed, that the estimates in various methods give similar values; to, this term may be used without to specify, which chemical (or isotope) is used for the evaluation, where is the placed margin between troposphere and stratosphere, and which model of degradation of the chemical content is used.
The ambiguity is aggravated by the diffusional drift down of the most heavy components, for exaple,
History
The idea of modification of content of the air is mentioned in 1999 by Shuhua Li, Darryn W.Waugh. They considered the transfer of chlorofluorocarbons (CFCs), nitrous oxide (N2O), methane (CH4), ozone (O3) and discuss the role of the stratosphere and the solar radiation. [4].
Since that time, the numerous attempts to provide the formal definition of term AoA are observed.
In the first approximation, the chemical and isotopic content of the air in any macroscopic domain is qualified with a single parameter, that has dimension of time.
This approaches assumes, that the irradiation with solar light, relaxation of metastable states of molecules and nuclei, loss of water and heavy components with precipitation and other natural processes affect the content of air in similar way; and this content allows to estimate the time the air remains in the upper layers of the atmosphere.
References
- ↑ https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2000RG000101 Darryn Waugh and Timothy Hall. AGE OF STRATOSPHERIC AIR: THEORY, OBSERVATIONS, AND MODELS. First published: 31 December 2002 https://doi.org/10.1029/2000RG000101 We review the relationship between tracer distributions and transport timescales in the stratosphere and discuss the use of timescales to evaluate and constrain theories and numerical models of the stratosphere. The “age spectrum,” the distribution of transit times since stratospheric air last made tropospheric contact, provides a way to understand the transport information of tracers, how sensitive different tracers are to various transport processes, and how to use tracers in combination to constrain transport rates. Trace gas observations can be used to infer aspects of the age spectrum, most commonly the “mean age,” but also the shape of the spectrum. Observational inferences of transport timescales provide stringent tests of numerical models independent of photochemistry, and comparisons of these observations with chemical transport models have highlighted certain problems with transport in the models. Age simulations and comparisons with data can now be considered standard tests of stratospheric models.
- ↑ https://www.nature.com/articles/ngeo397 Darryn Waugh. The age of stratospheric air. Nature Geoscience, volume 2, pages 14–16 (2009). Climate models predict that increasing greenhouse gas levels will invigorate the circulation in the upper atmosphere. But a close look at observations of the age of stratospheric air over 30 years reveals no acceleration in the circulation.
- ↑ 2020.05.15. https://www.atmos-chem-phys.net/20/5837/2020/acp-20-5837-2020.pdf Rostislav Kouznetsov, Mikhail Sofiev, Julius Vira, and Gabriele Stiller. Simulating age of air and the distribution of SF6 in the stratosphere with the SILAM model. Atmos. Chem. Phys., 20, 5837–5859, 2020 https://doi.org/10.5194/acp-20-5837-2020 // Abstract. The paper presents a comparative study of age of air (AoA) derived from several approaches: a widely used passive-tracer accumulation method, the SF6 accumulation, and a direct calculation of an ideal-age tracer. The simulations were performed with the Eulerian chemistry transport model SILAM driven with the ERA-Interim reanalysis for 1980–2018.// The Eulerian environment allowed for simultaneous application of several approaches within the same simulation and interpretation of the obtained differences. A series of sensitivity simulations revealed the role of the vertical profile of turbulent diffusion in the stratosphere, destruction of SF6 in the mesosphere, and the effect of gravitational separation of gases with strongly different molar masses.// The simulations reproduced well the main features of the SF6 distribution in the atmosphere observed by the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite instrument. It was shown that the apparent very old air in the upper stratosphere derived from the SF6 profile observations is a result of destruction and gravitational separation of this gas in the upper stratosphere and the mesosphere. These processes make the apparent SF6 AoA in the stratosphere several years older than the ideal-age AoA, which, according to our calculations, does not exceed 6–6.5 years. The destruction of SF6 and the varying rate of emission make SF6 unsuitable for reliably deriving AoA or its trends. However, observations of SF6 provide a very useful dataset for validation of the stratospheric circulation in a model with the properly implemented SF6 loss.
- ↑ https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/1999JD900913 Shuhua Li, Darryn W.Waugh. Sensitivity of mean age and long‐lived tracers to transport parameters in a two‐dimensional model. Journal of Geophysical Research: Atmospheres, Volume104, IssueD23. 20 December 1999. Pages 30559-30569. Recent comparisons of modeled and observed stratospheric mean age show large differences between model predictions and data as well as between individual models, indicating large variations (and deficiencies) in model transport. We explore here the sensitivity of the mean age to different components of the transport by examining the effect of varying the transport parameters within a two‐dimensional model. The mean age is shown to be sensitive to changes in advective circulation and diffusion coefficients, with the sensitivity being largest for changes in the circulation strength. In most cases the magnitudes, but not the orientation, of the mean age isopleths change. However, if the horizontal mixing is made very small within middle latitudes or large within low latitudes, large changes occur in the orientation of mean age isopleths. The effects of these transport changes on chemically active long‐lived tracers are also examined. It is found that the lower stratospheric concentrations are relatively insensitive to the transport changes if these changes do not alter the general shape of mean age isopleths. However, significant changes occur when the transport parameters are modified so as to change the orientation of the mean age (and long‐lived tracer) isopleths.