Regional CO2 Fluxes:Three Approaches

1) Bakwin et al., Carbon dioxide budget in the continental atmosphere, submitted to Tellus.

2) Helliker et al., Regional-scale measurements of CO2 flux and stable isotope discrimination across the planetary boundary layer, in preparation.

3) Styles et al., Constraining the terrestrial carbon budget at regional to continental scales with surface concentration measurements, work in progress.

Gloor, M., et al., What is the concentration footprint of a tall tower?J. Geophys. Res., 106, 17831-17840, 2000.

∂CO2/∂t = Fc/Zmix – Ui (∂CO2/∂Xi)

Approach #1: Bakwin et al.Atmospheric budget on a monthly time scale.

Approach #2: Helliker et al.Boundary layer cuvette

Where does this come from?The Lagrangian budget equation is:

but … this is a Lagrangian equation advection has been ignored!

m

i

i

mm

im

c wdtz

zcc

tc

zF

qqcc

FFm

m

qc

• = regional flux estimate, o = tower NEE

You can’t argue with success???

Approach #3: Styles et al.Uncalibrated flux sites

•T is a period of time which starts in the morning before CBL growth begins, and ends in the afternoon when the CBL is fully developed

•cB is CO2 within the CBL

•c+ is CO2 above the CBL

•h = zi (CBL height)

B Bc

c c hF

T

Approach #3: Styles et al.Uncalibrated flux sites

Wait a minute, is this right???

•What about the initial state of the CBL? (h ≠ 0 at t = 0 !)

•This assumes that CO2 above the CBL in the a.m. = c+

Nevertheless, we forge ahead …

B Bc

c c hF

T

You could get (cB – c+) from accurate measurements at a tower (cB) and take c+ as equal to the MBL value.

BUT, flux tower CO2 mixing ratios are usually not well calibrated!

Hence, Styles et al. substitute:

(cmin – cavg) for (cB – c+)

where cmin and cavg are daily minimum and daily average CO2 measured at the flux tower.

Whaaat??? That can’t work!

rain

Time series of daily average CO2 at each height

cB – c+

cmin – cavg

cmin – cavg = 0.915 (cB – c+) – 5.80, r2 = 0.89

cB from 396 m tower data (daily min)c+ from MBL data

cmin and cavg from30 m tower data

SiteLatitude Longitude Elevation Vegetation type

Harvard forest 42.5 -72.2 300 Deciduous

Park Falls 45.8 -90.1 470 Deciduous/coniferous

Willow Creek 45.8 -90.1 520 Deciduous

Boreas NSA 55.9 -98.5 260 Coniferous

Great Mountain 42.0 -73.2 430 Deciduous/coniferous

Univ. MichiganBiol. Station

45.6 -84.7 230 Deciduous/coniferous

Blodgett 38.9 -120.6 1320 Coniferous

Walker Branch 36.0 -84.3 360 Deciduous

Santarem logged -3.0 -55.0 200 Rainforest

WeidenBrunnen 50.2 11.9 780 Coniferous

Hesse 48.7 7.8 300 Deciduous

FLUXNET sites used in Styles et al. analysis

All sites

y = 1.0621x - 4.6905R2 = 0.8158

-30

-25

-20

-15

-10

-5

0

-20 -15 -10 -5 0 5

Fc*daylength/h/rho

avg

dai

ly m

in C

O2

- av

g C

O2

Conclusions

• Three very different approaches, each with very dubious simplifying assumptions, all give some reasonable-looking results.

• Are we just lucky? Stupid? Both?