Air / Water Gas Exchange The distribution of a chemical across the air-water interface between the atmospheric gas phase and the water dissolved phase •Equilibrium transfer of organic chemical between Air and Water K H = P a / w C w Appropriate for: Exchange between air and falling raindrop (over ~10 m fall) Low MW organic gases exchanging between peat water and bubbles (in wetlands and marshes) Confined headspace over a solution Sheltered systems with more or less constant water and atmospheric conditions Inappropriate for : Large Lakes Flowing rivers Spills in both rivers and lakes Oceans ( sometimes ! ) In these you must consider Mass Transport (absolute and net fluzes)
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Air / Water Gas Exchange The distribution of a chemical across the air-water interface between the atmospheric gas phase and the water dissolved phase.
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Air / Water Gas ExchangeThe distribution of a chemical across the air-water interface between the atmospheric gas phase and the water dissolved phase
•Equilibrium transfer of organic chemical between Air and WaterKH = Pa / w Cw
Appropriate for:Exchange between air and falling raindrop (over ~10 m fall)Low MW organic gases exchanging between peat water and bubbles
(in wetlands and marshes)Confined headspace over a solutionSheltered systems with more or less constant water and atmospheric
conditionsInappropriate for :
Large LakesFlowing riversSpills in both rivers and lakesOceans ( sometimes ! )In these you must consider Mass Transport (absolute and net fluzes)
Processes of Air / Water Exchange
Depiction of the physical processes responsible for the movement of chemicals through four zones spanning an intact “air-water” interface (i.e. no bubbles or aerosols).
Figure from Schwarzenbach, Gschwend and Imboden, 1993
Processes of Air / Water Exchange
“Little” Mixing: Stagnant, 2-film model“More” Mixing: surface renewal modelWave Breaking: intense gas transfer ( breaking bubbles)
Figure from Schwarzenbach, Gschwend and Imboden, 1993
Stagnant Boundary Layer Model of Air / Water Exchange –Whitman Two Film Model
Figure from Schwarzenbach, Gschwend and Imboden, 1993
Two Film Model
Figure from Schwarzenbach, Gschwend and Imboden, 1993
Net Flux = Kol * (Cw – Ca/H*)
resistance to transport * Concentration gradient relative to equilibrium
H* is “dimesnionless” Henry’s Law Constant at ambient temperature
1/ Kol = ( 1/ Kw + 1/ (Ka H*) )
= (1 / Dw / zw) + (1/ Da/ za H*)
where Dw = diffusivity in water Da = diffusivity in air zw = water film thickness za = air film thickness
un-measurable parameters: zw, za
Two Film Model- Continued
Fw = - Dw ( Cw/a – Cw ) / zw
So, at steady state:
Fw = - Dw ( Cw/a – Cw ) / zw = -Da (Ca – Ca/w) / za = Fa