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AOSC 634Air Sampling and Analysis

Vertical FluxEddy Correlation (Eddy Covariance)

And Vertical Gradient

Copyright Brock et al. 1984; Dickerson 2013

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)

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.

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

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

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?

The flux is monitored.

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).

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

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.

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