Measurements of CO 2 Molar Mixing Ratio by Infrared Absorption Spectroscopy C, mole fraction (μmole mole -1 = ppm) LI-COR analyzers measure absorbance at 4.26 μm V = kA (voltage proportional to absorbance) ρ c = P c /RT = CP/RT (molar density of absorber) ρ c L = CPL/RT (absorber amount in absorption cell)
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Measurements of CO 2 Molar Mixing Ratio by Infrared Absorption Spectroscopy C, mole fraction (μmole mole -1 = ppm) LI-COR analyzers measure absorbance.
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Measurements of CO2 Molar Mixing Ratio
by Infrared Absorption Spectroscopy
C, mole fraction (μmole mole-1 = ppm)
LI-COR analyzers measure absorbance at 4.26 μm
V = kA (voltage proportional to absorbance)
ρc = Pc/RT = CP/RT (molar density of absorber)
ρcL = CPL/RT (absorber amount in absorption cell)
LiCor internal schematic
100 cc/min
Virial Equation of State (REAL not IDEAL):
PV = nRT(1 + nB(T)/V + nC(T)/V2 + …)
neglect higher order terms
Solve for n:
n = -(V/2B){1 – (1 + 4PB/RT)1/2}
B (10-6 m3 mole-1) (in air at T = 300 K)
air -7.7
CO2 -42.3
Using this, we find the equivalence of (at 300 K and 1 Bar):
volume mixing ratio 370.0 x 10-6 m3 CO2 / m3 air
molar mixing ratio 370.5 x 10-6 mole CO2 / mole air
• LiCor Analyzer Response Curve:
C = [a0 + a1 (V P0/P) + a2 (V P0/P)2] T/T0
C is CO2 mole fraction
P0, T0 are pressure and temperature during calibration
Pressure Broadening
effective pressure: Pe = PN2 + Σbi Pi
Gas Coef (bi) % of air
N2 1.00 78.084
O2 0.81 20.946
Ar 0.78 0.934
H2O ≈1.57 ≈1
Pressure Broadening
Example calibration curve in air (PO2 = 20 kPA, PN2 = 80 kPA):
Pe = 80 kPa x 1.00 + 20 kPa x 0.81 = 96.2 kPa
C = 326.61 + 0.1738 V + 4.5095 x 10-5 V2
A LiCor response of 300 mV implies CO2 = 382.82 ppm
What if you calibrated using pure N2? (PN2 = Pe = 100 kPa)
C = 326.61 + 0.1807 V + 4.8723 x 10-5 V2
Now a response of 300 mV gives CO2 = 385.21 ppm !
What error is made using synthetic (no Argon) vs. real air?
Water Vapor
(1) Pressure broadening of CO2 line
bH2O ≈ 1.57
(2) Dilution of air – adding 1% H2O displaces 1% of CO2 (≈ 3.7 ppm)