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)

LiCor internal schematic

100 cc/min

Virial Equation of State (REAL not IDEAL) : PV = nRT(1 + nB(T)/V + nC(T)/V 2 + …) neglect higher order terms Solve for n: n = -(V/2B){1 – (1 + 4PB/RT) 1/2 } B (10 -6 m 3 mole -1 ) (in air at T = 300 K) air -7.7 CO Using this, we find the equivalence of (at 300 K and 1 Bar): volume mixing ratio370.0 x m 3 CO 2 / m 3 air molar mixing ratio370.5 x mole CO 2 / mole air

LiCor Analyzer Response Curve: C = [a 0 + a 1 (V P 0 /P) + a 2 (V P 0 /P) 2 ] T/T 0 C is CO 2 mole fraction P 0, T 0 are pressure and temperature during calibration

Pressure Broadening effective pressure:P e = P N 2 + Σb i P i GasCoef (b i )% of air N O Ar H 2 O≈1.57 ≈1

Pressure Broadening Example calibration curve in air (P O2 = 20 kPA, P N2 = 80 kPA): P e = 80 kPa x kPa x 0.81 = 96.2 kPa C = V x V 2 A LiCor response of 300 mV implies CO 2 = ppm What if you calibrated using pure N 2 ? (P N2 = P e = 100 kPa) C = V x V 2 Now a response of 300 mV gives CO 2 = ppm ! What error is made using synthetic (no Argon) vs. real air?

Water Vapor (1)Pressure broadening of CO 2 line b H2O ≈ 1.57 (2)Dilution of air – adding 1% H 2 O displaces 1% of CO 2 (≈ 3.7 ppm) Solution: DRY THE AIR! How dry is dry enough? 0.1 ppm / 370 ppm = 0.027% → H 2 O ≤ m 3 /m 3 air Error in CO 2 Dewpoint 0.1 ppm o C 0.2 ppm o C

What if we had 3 standard gases instead of 4 ???

The results differ for different LiCor CO 2 analyzers under the same conditions!!!