aosc 634 air sampling and analysis vertical flux eddy correlation (eddy covariance) and vertical...
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AOSC 634Air Sampling and Analysis
Vertical FluxEddy Correlation (Eddy Covariance)
And Vertical Gradient
Copyright Brock et al. 1984; Dickerson 2013
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Copyright © 2013 R.R. Dickerson 2
Destruction by Dry Deposition
O3
Hei
ght
This is a typical ozone profile in a rural or remote area.
Deposition Velocity – the apparent velocity (cm/s) at which an atmospheric species moves towards the surface of the earth and is destroyed or absorbed.
Vd = H/Ĉ dC/dt
Where H = mixing height (cm)
Ĉ = mean concentration (cm-3)
C = concentration (cm-3)
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Copyright © 2013 R.R. Dickerson 3
Destruction by Dry Deposition
O3H
eigh
t
From the deposition velocity, Vd, and mixing height, H, we can calculate a first order rate constant k’.
k’ = Vd /H
For example if the deposition velocity is 0.5 cm/s and mixing height at noon is 1000 m the first order loss rate is lifetime is 0.5/105 s-1 = 5x10-6 s-1 and the lifetime is 2x105 s or 56 hr (~2.3 d). At night the mixed layer may be only 100 m deep and the lifetime becomes 5.6 hr.
Deposition velocities depend on the turbulence, as well as the chemical properties of the reactant and the surface; for example of plant stomata are open or closed. The maximum possible Vd for stable conditions and a level surface is ~2.0 cm/s.
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Copyright © 2013 R.R. Dickerson 4
Tech Note
X
Hei
ght
For species emitted into the atmosphere, the gradient is reversed (black line) and the effective deposition velocity, Vd, is negative. From the height for an e-folding in concentration, we can calculate the eddy diffusion coefficient (units m2/s)
1/k’ = t = H/ Vd = H2/Kz
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Gradient Method
Copyright © 2013 R.R. Dickerson 5
Deposition velocity: Vd = H/Ĉ dC/dt
Where H = mixing height (cm)
Ĉ = mean concentration (cm-3)
C = concentration (cm-3)
k’ = Vd /H = 1/t
Kz = Eddy Diffusion Coefficient (m2/s)
Characteristic diffusion time: t = H2/Kz
Global mean Kz ~ 10 m2s-1, so the average time to tropopause
~ (104m)2/10(m2s-1) = 107 s = 3 months
Compare this to updraft velocities in Cb.
In convectively active PBL Kz ~ 100 m2 s-1
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Useful technique for calculating fluxes or lifetimes.
•When the atmosphere shows horizontal uniformity, production and loss reduce to a 1 D problem.•This holds when vertical gradients are much greater than horizontal gradients and when the species X is in steady state.•Let z be altitude (m), F flux (g m-2s-1), [X] concentration (g/m3), k’ the pseudo first order rate constant (s-1) for loss of X, t is lifetime of X.
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Example for fertilized soil NO emissions:
• We want to know the emission rate.• We have the NO profile at night; this only works at night. • NO goes from 20 mg/m3 at the surface to essentially zero at 100 m with a scale height of 10 m.• The column content is therefore
10m*20x10-6g m-3 = 2x10-4 g m-2
• We know ozone is roughly constant at 50 ppb, therefore at RTP the lifetime is ~100 s. More generally, you can integrate with [O3](z) and k(z).• If t is a constant then k’ is a constant:
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Example for crop soil NO emissions, continued:
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Guangzhou Tower
O3
NO2
NO
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0 5 10 15 20 250
100000000000002000000000000030000000000000400000000000005000000000000060000000000000700000000000008000000000000090000000000000
100000000000000
F(NO)_molec/(cm^2*s)
0 5 10 15 20 250
5000000000000100000000000001500000000000020000000000000250000000000003000000000000035000000000000400000000000004500000000000050000000000000
F(NO)_121
0 5 10 15 20 250
20000000000000
40000000000000
60000000000000
80000000000000
100000000000000
120000000000000
140000000000000
F(NO)_454
Using the average O3 and NO for the 121-454 layer
assuming that the O3 and NO concentrations at 121 m represent those in the 0-121 layer
assuming that the O3 and NO concentrations at 454 m represent those in the 0-454 layer
Average diurnal variation of ground temperature was calculated and applied in the k(T) calculations.
k = 1.08e+12*exp(-1370 K/T) cm**3/(mol * s)
Atkinson et al. (1997)
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Example: What is the lifetime of SO2 over the eastern US?
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The flux is monitored.
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Figure IIa
SO2 Emissions (tons/day)
0-20
20-75
75-150
150-300
300-500
Locations of flights made with aircraft (shown with black airplanes). Location of power plants emitting SO2 shown in pink circles (size of circle represents size of emissions for July 13, 2002).
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Lifetime of SO2 over the eastern US. See Lee et al., (2011).
0
5
10
15
20
25
30
35
0 10 20 30 40 50 60
SO2 lifetime (hours)
Fre
qu
en
cy
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Guenther, A., et al. (1996), Isoprene fluxes measured by enclosure, relaxed eddy accumulation, surface layer gradient, mixed layer gradient, and mixed layer mass balance techniques, Journal of Geophysical Research-Atmospheres, 101(D13), 18555-18567.
Lee, C., et al. (2011), SO2 emissions and lifetimes: Estimates from inverse modeling using in situ and global, space-based (SCIAMACHY and OMI) observations, Journal of Geophysical Research-Atmospheres, 116.
Wesely, M. L., and B. B. Hicks (2000), A review of the current status of knowledge on dry deposition, Atmospheric Environment, 34(12-14), 2261-2282.