AOSC 634 Air Sampling and Analysis Vertical Flux Eddy Correlation (Eddy Covariance) And Vertical Gradient Copyright Brock et al. 1984; Dickerson

Copyright © 2013 R.R. Dickerson2 Destruction by Dry Deposition O3O3 Height 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. V d = H/Ĉ dC/dt Where H = mixing height (cm) Ĉ = mean concentration (cm -3 ) C = concentration (cm -3 )

Copyright © 2013 R.R. Dickerson3 Destruction by Dry Deposition O3O3 Height From the deposition velocity, V d, and mixing height, H, we can calculate a first order rate constant k’. k’ = V d /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 0.5/10 5 s -1 = 5x10 -6 s -1 and the lifetime is 2x10 5 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 V d for stable conditions and a level surface is ~2.0 cm/s.

Copyright © 2013 R.R. Dickerson4 Tech Note X Height For species emitted into the atmosphere, the gradient is reversed (black line) and the effective deposition velocity, V d, is negative. From the height for an e-folding in concentration, we can calculate the eddy diffusion coefficient (units m 2 /s) 1/k’ =  = H/ V d = H 2 /K z

Gradient Method Copyright © 2013 R.R. Dickerson5 Deposition velocity: V d = H/Ĉ dC/dt Where H = mixing height (cm) Ĉ = mean concentration (cm -3 ) C = concentration (cm -3 ) k’ = V d /H = 1/  Kz = Eddy Diffusion Coefficient (m 2 /s) Characteristic diffusion time: t = H 2 /K z Global mean K z ~ 10 m 2 s -1, so the average time to tropopause ~ (10 4 m) 2 /10(m 2 s -1 ) = 10 7 s = 3 months Compare this to updraft velocities in Cb. In convectively active PBL K z ~ 100 m 2 s -1

6 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 -2 s -1 ), [X] concentration (g/m 3 ), k’ the pseudo first order rate constant (s -1 ) for loss of X,  is lifetime of X.

7 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  g/m 3 at the surface to essentially zero at 100 m with a scale height of 10 m. The column content is therefore 10m*20x10 -6 g 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 [O 3 ](z) and k(z). If  is a constant then k’ is a constant:

8 Example for crop soil NO emissions, continued:

9 Guangzhou Tower O3O3 NO 2 NO

Using the average O3 and NO for the layer assuming that the O3 and NO concentrations at 121 m represent those in the layer assuming that the O3 and NO concentrations at 454 m represent those in the 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)

11 Example: What is the lifetime of SO 2 over the eastern US?

The flux is monitored.

Figure IIa SO 2 Emissions (tons/day) Locations of flights made with aircraft (shown with black airplanes). Location of power plants emitting SO 2 shown in pink circles (size of circle represents size of emissions for July 13, 2002).

Lifetime of SO 2 over the eastern US. See Lee et al., (2011).

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), Lee, C., et al. (2011), SO 2 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),