trying to get details

Simple in principle. Mean abundance of a chemicals (in units of mass) divided by the emission rate (unit of mass/time) gives the lifetime. In practice, it might be difficult to establish the emission rate and calculate the abundance. Some chemicals have emission rates which varies very rapidly in space and time. For these, it becomes virtually impossible to assign a single lifetime. Then it becomes almost case by case.

In the Lagrangian frame of reference, the residence time of a given quantity of chemical species is determined by the location of the emission, the flow path, the lateral mixing, the vertical diffusion, the deposition (wet and dry), chemical reactions and the transfer between the gas phase and particles. The solution of the mass conservation equation for a given trajectory leads directly to the estimation of a residence time.

Considering the fact that all the parameters of the considered system exhibit a significant variability, we should look at the problem of the residence time more in the sense of the distribution of the residence times evaluated for the ensemble of the trajectories. Calculations can be accomplished using a Lagrangian model that includes all the relevant processes indicated in the first sentence

In the Eulerian frame of reference, we can use a method suggested by Professor Bolin and his coworkers in the article: "Residence time of atmospheric pollutants as dependent on source characteristics, atmospheric diffusion processes and sink mechanisms"

When considering the modifications of the method suggested in this paper, it is advisable to use a three-dimensional transport model with a realistic Planetary Boundary Layer scheme, good schemes for dry deposition coupled to a cloud model. In my opinion, we are still trying to master three-dimensional transport models with the properties required to make realistic estimates of residence time.

The remaining question is: which method is better?

In my opinion, the most accurate and realistic methods are based on the ensemble of trajectories because of their ability to estimate the distribution of the residence times.

you can see all the series

I agree with Dr Alain Robichand and Dr Janusz Pudykiewiez for their attractive explanations regarding residence time of the chemical species in atmosphere. On considering of all those factors, it is very difficult to highlight any fixed value of residence time and the values given in the literature are , it appears to me , approximate values.

Thank you Abhijit Mitra Phani Bhusan Ghosh Janusz Pudykiewicz for all your valuable answers.