Geoscience Reference
In-Depth Information
Table 2 Estimated minimum detectable fluxes based on instrument precision (for flux gradient,
diffusion coefficient = 0.1 m
2
s
1
m
1
,
d
z = 1.5 m, spectra time 3 min; for
fl
ux chamber, spectra
time 2 min)
FTIR-precision
(1
r
, 10 min)
Estimated FTIR-precision
(1
r
) for 3 and 2 min
spectra
Minimum detectable
flux
'
flux gradient
method
'
Minimum detectable
flux
'
flux chamber
method
'
CO
2
0
:
02 lmol mol
1
0
:
04 and 0
:
05 lmol mol
1
0
:
2
l
mol m
2
s
1
0
:
003
l
mol m
2
s
1
9
:
7 lgm
2
s
1
0
:
12 lmol m
2
s
1
CH
4
0
:
2 nmol mol
1
0
:
36 and 0
:
45 nmol mol
1
2
:
0 nmol m
2
s
1
0
:
09 nmol m
2
s
1
35
:
6ngm
2
s
1
1
:
3ngm
2
s
1
0
:
2 nmol mol
1
0
:
36 and 0
:
45 nmol mol
1
2
:
2 nmol m
2
s
1
0
:
3 nmol m
2
s
1
CO
62
:
2ngm
2
s
1
7
:
6ngm
2
s
1
N
2
O
0
:
06 nmol mol
1
0
:
11 and 0
:
13 nmol mol
1
0
:
67 nmol m
2
s
1
0
:
02 nmol m
2
s
1
4ngm
2
s
1
02 ng m
2
s
1
29
:
0
:
mol mol
1
)attwo
where in
d
C is the difference in concentration of gas x (
l
xed
cient (m
2
s
1
), and F the
different inlet-heights (
d
zm
ðÞ
) and K is the diffusion coef
mol m
2
s
1
). The diffusion coef
fl
ux (
l
cient can be parameterized using the data
from a sonic anemometer.
Considering the FTIRs concentration detection limits (Table
2
), an increased
inlet-height difference can give higher concentration differences and decreases the
method
s detection limits. However, a too high distance between the inlets may lead
to a different footprint per inlet (e.g. measuring distinctive types of soil, ecosystems
or environmental conditions). Also, the lower inlet position should be higher than
nearby vegetation or other disturbances. The sonic anemometer should be placed
between the 2 inlets (Baldocchi
2003
).
Air sampling was done continuously. For 30 min per hour, the air
'
ows were
directed to air sampling bags, after that the bag inlet was closed until analysis.
Before analysis, the measurement-cell was evacuated. Afterwards, the cell and
sample lines were
fl
flushed with sample air to avoid contamination. For each air
sample, a 3 min-spectrum was taken.
fl
2.2 The Flux Chamber Technique
Six soil collars (50 cm
50 cm) were inserted up to 10 cm depth a week before the
start of the experiment. Positions for soil collars were selected to be undisturbed and
representative. Two transparent
50 cm) were
switched between collars every few days to avoid major environmental disturbance.
The
fl
ux chambers (50 cm
50 cm
fl
flux chambers (open dynamic chambers) were tightened by use of clamps and
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