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On the importance of high-frequency air-temperature fluctuations for spectroscopic
corrections of open-path carbon dioxide flux measurements
OverviewThe IRGASON flux system consists of an open-path
infrared H2O and CO2 gas analyzer and a colocated 3D
sonic anemometer. The IRGASON measures CO2 mixing
ratio in situ by using the fast-response humidity corrected
sonic temperature measurement to account for the
temperature related density fluctuations. CO2 flux can be
computed directly using instantaneous CO2 mixing ratio,
eliminating the need for density (WPL) corrections in post
processing.
Instrumentation1. IRGASON – Integrated CO2 & H2O open-path flux system
with colocated sonic and gas sensing volumes2. EC155 – CO2 & H2O closed-path analyzer 3. CSAT3A – Stand alone sonic anemometer/thermometer4. Temperature probe in a five plates radiation shield5. Quad-Disk Pressure Probe (Nishiyama et al., 1989)Site DetailsLocation: +41° 46' 3.89" N, –111°51' 21.19" EMeasurement height: 1.65 m – IRGASON, 2.00 m – CSAT3Sampling rate: 20 Hz
LiteratureGrelle, A., Burba, G. (2007). Fine-wire thermometer to correct CO2 fluxes by open-path analyzers for artificial density fluctuations. Ag. For. Meteorol.,
147:48-57.Webb E., G. Pearman, and R. Leuning (1980). Correction of flux measurements for density effects due to heat and water vapour transfer. Quart. J. Roy.
Meteorol. Soc., 106: 85-100.Jamieson, John A., R. McFee, G. Plass, R. Grube, R. Richards. (1963). Infrared Physics and Engineering. McGraw Hill Book Company, Inc.Griffith, D. (1982). Calculations of carrier gas effects in non-dispersive infrared analyzers I. Theory. Tellus, 34, 376-384.
Acknowledgements: The first author thanks Campbell Scientific for the support of this study and Steve Sargent for the discussions and the review of the poster.
MotivationA growing number of studies report systematic differences
in CO2 flux estimates obtained with the two main types of
gas analyzers, open and closed path. Compared to eddy-
covariance systems based on closed-path (CP) gas
analyzers, systems with open-path (OP) gas analyzers
consistently overestimate CO2 uptake during daytime
periods with high positive sensible heat fluxes.
Ivan Bogoev(1)*, Manuel Helbig(2), Oliver Sonnentag(2)
(1) Research and Development, Campbell Scientific, Inc., Logan, Utah, USA; *ivan@campbellsci.com (2) Département de géographie, Université de Montréal, Montréal, Québec, Canada
Field experimentWe conducted a field inter-comparison with IRGASON
(open-path) and EC155 (closed-path) systems operating
side-by-side and using a common sonic anemometer.
Hypothesis: Inadequate correction of fast temperature related spectroscopic effectsThe strength and the half-width of the CO2 absorption lines are affected by temperature. Single-path, dual-wavelength,
non-dispersive infrared gas analyzers use air-temperature measurements to correct for these spectroscopic effects. The
probes used for the compensation have limited frequency response due to their thermal mass and are inadequate to
compensate CO2 absorption measurements for fast temperature variations.
1
4
3
2
5
Findings1. During periods of high sensible heat flux, the IRGASON measures more negative CO2 flux. Under negative heat flux
regimes, the IRGASON measures more positive CO2 flux (Figure 1).
2. Hourly mean CO2 concentrations measured with the EC155 and the IRGASON agree to within 3% (Figure 2).
3. During daytime under high sensible heat flux conditions, CO2 spectra measured by the two analyzers agree well in the 0
to 0.005 Hz range, but the IRGASON measures increased CO2 variations above 0.01 Hz (Figure 3).
4. Nighttime CO2 spectra agree well in the frequency range 0 to 0.1 Hz. The EC155 measured increased CO2 variations
above 0.1 Hz (Figure 4).
5. Compared to the EC155, under high sensible heat flux conditions, the co-spectral response of the IRGASON is
increased (Figure 5).
6. During nighttime the agreement of the co-spectral responses extends to higher frequencies up to 0.1 Hz (Figure 6).
-0.6 -0.4 -0.2 0 0.2 0.4
-0.6
-0.4
-0.2
0
0.2
0.4
Hourly CO2 Flux
EC155 Fc [mg m-2
s-1
]
IRG
AS
ON
Fc [
mg m
-2 s
-1]
1:1
Hs[W]
-50
0
50
100
150
200
Figure 1
400 420 440 460 480 500 520
400
420
440
460
480
500
Hourly Mean CO2 Concentration
EC155 CO2 [mol mol
-1]
IRG
AS
ON
CO
2 [m
ol m
ol-1
] slope: 1.0297 offset: -11.6571
R2 = 0.99404
1:1
-10
0
10
20
30
T [C]
Figure 2
10-2
100
10-2
100
102
Power Spectrum of CO2
Frequency [Hz]
S C
O2
[p
pm
2]
EC155
IRGASON
-5/3
Daytime
Figure 310
-210
010
-2
100
102
104
Power Spectrum of CO2
Frequency [Hz]
S C
O2 [
ppm
2]
EC155
IRGASON
-5/3
Nighttime
Figure 4
10-2
100
10-2
100
Cospectrum of CO2 and w
Frequency [Hz]
Co C
O2 [
ppm
m s
-1]
IRGASON
EC155
-7/3
Daytime
Figure 510
-210
010
-4
10-2
100
Cospectrum of CO2 and w
Frequency [Hz]
Co C
O2 [
ppm
m s
-1]
IRGASON
EC155
-7/3
Nighttime
Figure 6
𝐴 =𝑁
Δν 𝜈1
𝜈2
1 − exp −𝑆𝑖α𝑖cL
𝜋 𝜈 − 𝜈02 + 𝛼𝑖
2𝑑𝜈
A = Absorbed light energy in the spectral interval
N = Number of absorption lines in the spectral interval
ν = Wave number of the individual spectral line
c = Density of the absorbing gas
L = Path length
Si = Strength of the individual line
αi = Half-width of the individual line
Si = f(T,P) αi = f(T,P)
T = Air temperature, P = barometric pressure
-50 0 50 100 150 200 250
-5
0
5
FcOP
- FcCP
Hs [W m-2
]
Fc
OP -
Fc
CP [ m
ol m
-2s
-1]
=-0.014257*Hs+0.066828
=-0.0035314*Hs+0.06371
Slow Temp
fit
Fast Temp
fit
-0.6 -0.4 -0.2 0 0.2 0.4-0.6
-0.4
-0.2
0
0.2
0.4
Hourly CO2 Flux
EC155 Fc [mg m-2
s-1
]
IRG
AS
ON
Fc [
mg m
-2 s
-1]
slope = 1.086
R2 = 0.9371
slope = 1.026
R2 = 0.9771
Slow Temp
Fast Temp
ConclusionsUsing fast air-temperature measurements to correct for the absorption line broadening effects of open-path CO2 flux,
reduces the differences with fluxes measured by a closed-path analyzer.
Slow-response air temperature measurement
Fast-response air temperature measurement
Results of using fast-response air temperature in the conversion of absorption into CO2 densityThe difference between OP and CP flux measurements is reduced and less dependent on sensible heat flux.
2280 2300 2320 2340 2360 2380 24000
0.2
0.4
0.6
0.8
1
CO2 Absorption
Wave Number
Absorp
tion
T: 240 K
T: 330 K
Figure 7
Figure 8 Figure 9
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