Northern Hemisphere 23 days and southern hemisphere 28 days.
Source: PgNo228, Atmopsheric Chemistry and Physics, Second Edition, Authors:John H. Seinfeld and Spyros N Pandis.
Dear Rohit,
First of all, the length of the residence time (RT) of a chemical component in a reservoir is typically calculated as the ratio of the amount of this particular chemical already contained in this reservoir (say in kg or moles), divided by either the influx of that chemical into (or the outflow out of) the reservoir (in kg/s or moles/s).
If the rates of influx and outflow of that chemical component differ, the content of the reservoir keeps changing (depending on the relative strengths of the sources and sinks), and the RT is ambiguous as it can take multiple values. Hence, RT is typically estimated for steady-state systems, i.e., in situations where the amount of the chemical component in the system is stable. Note that a common mistake is to define the RT as a ratio of the amount contained by the net flux. This is of course meaningless, since the net flux of the component in a steady state system is null, so the residence time becomes undefined or infinitely large.
If you compare two such steady-state reservoirs, and they exhibit different RT values, it implies either that the contents, or that the influx or outflow rates are different. Specifically, a smaller RT means either a lesser reservoir content, or a larger flux into or out of the reservoir.
In the particular case of tropospheric ozone, if the residence time is shorter in the northern hemisphere (a fact I cannot confirm or infirm, and again assuming this is a steady-state situation), it must be because its total ozone content is smaller, or because the processes that generate and destroy ozone there are more prevalent or efficient.
Remember that pollution sources are mostly located over land surfaces and where human population density is the largest: both of these factors are predominantly located in the northern hemisphere, and they both affect the production and the destruction of ozone.
Regarding the difference between the hemispheres, it is also important to note (1) that inter-hemispheric concentration differences have been observed in many other chemical compounds, and (2) that though the general circulation is a global phenomenon, the atmosphere tends to 'behave' as a set of loosely connected but somewhat independent reservoirs. Indeed, one can treat the troposphere and the stratosphere separately in first approximation: there are exchanges of mass, energy and momentum at the tropopause, but these are limited in space and time. Similarly, atmospheric circulation in each hemisphere dynamically evolves primarily as a response of phenomena and processes taking place within it: inter-hemispheric mixing and transfers mostly take place around the Intertropical Convergence Zone (ITCZ) or when tropical cyclones force such exchanges along near-equatorial trajectories, but weather forecasting can be done on a hemispherical basis with good approximation, at least for short periods of time.
Hence the claim that the lifetime of ozone is different in both hemispheres is the result of strengths of the sources and sinks of ozone in each hemisphere, combined with the relative independence between the atmospheric circulations within these hemispheres, which prevents to some extent a complete homogenization on a global scale.
Regards, Michel.
n our measurements of ozone in a 19 m deep well showed that within a meter or two the levels of O3 went from 90 ug/m3 at the top to about 12 ug/m3 (the latter is abot the sensitivity of our detector).
The gist of it is that when there is no sunshine, (like several meters deep in the well where there is no direct sunshine), O3 is not generated and quickly (about a minute lifetimes) disappears. The formation and the decay (reaction) rates are in the order of minutes.
Dr Holub, thanks for the response but I was wondering why there has been a difference in the residence among hemispheres. And how actually it is defined for each pollutant.
I pointed out it's a question how long the air and the concentration of its pollutants/nutrients (nutrients, like CO2), depends on the presence of sunshine (the energy source) and also how close it is to the CO2 source (the earth). So, perhaps, there is less sunshine, or less CO2 sources, in the southern hemisphere. compared to northern hemisphere. To prove it scientifically would require a huge grant. Not likely to be solved in the near future.