Atmosphere boundary thickness

Hi all

I am trying to quantify gas emissions into the atmosphere. But the gas diffusion is highly dependant on atmospheric thickness, isn't it? Whats the most reasonable value for this? Stefan, I saw in one of your document that you took the D1=0.005m (the atmospheric center to the center of connection with soil). Is it a good value for all the conditions?

 Stefan, I read your paper in 2004 (Ghezzehei et al., 2004). This thickness depends on wind speed, so when I have different wind speeds, I should change the connection distance? How can I apply the impact of wind speed in diffusion? Increasing the diffusion coefficient with square root of wind speed?

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  • Madjid,

    The wind speed and surface roughness determine the thickness of the laminar boundary layer at the land surface. I do not know how the two are related, though. At any rate, I would adjust the nodal distance (rather than the diffusion coefficient) to account for wind effects. See also a previous post.


  • Dear Stefan

    I learnt alot from that post. Thank you!

    You said there that" The most mechanistic approach is to define an air-mass fraction in the atmosphere that corresponds to the relative humidity value".

    1-1 It means that if RH is 40% in reality, saturation (water content) is 60% and air content is 40% in my atmospheric boundary layer of my model? I think it keeps RH constant, is it reasonable?

    2-2 Is it doable to calculate the evaporation by TMVOC? Or it has to be done by TOUGH2 and its EOSs?

    But I have more questions now!

    2-1 In case of a routine gas emissions into the atmosphere, what is your suggestion for ICP and IRP for both porous media and atmospheric layer? 

    2-2 Is there any available reference for atmospheric boundary layer thickness that I plug in my values into that equation and obtain the appropriate nodal distance for my atmospheric layer (this layer will be the thickness for diffusion). Without that, I can assign E-8, E-6, E-4, ... which affects the mass rate significantly. 

  • Madjid,

    1-1 It is essential that you fully understand the difference between RH (which is related to saturated vapor pressure and water mass fraction), and saturation (or water content), which are related to phase volume ratios. What you propose will always produce 100%RH!

    1-2 Yes, TMVOC can do evaporation

    2-1 See my many posts related to specifying atmospheric boundary conditions (e.g. this) - but you need to resolve 1-1 first!

    2-2 The values you propose seem way too low, but I don't really know. Please look at cfd and low-Mach turbulence studies and let us all know what you find!


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