Experimental evaluation of water vapour cross-sensitivity for accurate eddy covariance measurement of CO₂ flux using open-path CO₂/H₂O gas analysers

Figures & data

Fig. 1 Schematic setup for testing water vapour cross-sensitivity. Compressed sample gas is supplied to the LI-7200 IRGA after adjustment of the water vapour mole fraction with or without a dew point generator and then passing or not passing through a two-step dehumidification unit consisting of an electric cooler and a chemical trap. The ‘zero gas’ refers to sample gas with no CO₂. See text for more details.

Fig. 2 Time-series example of the CO₂ absorptances (upper) with H₂O mole fractions (lower) in humidified (solid lines) and dehumidified (dotted lines) CO₂ zero sample gas detected by the LI-7200 IRGA. The CO₂ absorptance was corrected using the value of X _wc supplied by the manufacturer. All data are plotted as 1-minute averaged values from 10 Hz raw data.

Fig. 3 Example of CO₂ mixing ratio outputs in CO₂ zero gas as a function of H₂O mole fraction: dehumidified gas (□), humidified gas without the correction coefficient X _wc (♦), humidified gas with X _wc value supplied by the manufacturer (▪) and humidified gas with the X _wc value adjusted in this study (×). This study adjusted the value of X _wc such that the corrected CO₂ outputs with X _wc were almost the same as the CO₂ outputs with dehumidified gas. Mixing ratio outputs were calculated as average values over a period of 3 minutes after the raw signals from the LI-7200 were stabilised.

Fig. 4 Examples of effects of cross-sensitivity due to direct absorption interference as a function of H₂O mole fraction, using single-path, dual-wavelength IRGAs listed in Table 1. The CO₂ mixing ratios of all IRGAs calculated without any correction show several biases, with the assumption that the constant CO₂ mixing ratio (400 ppm) is calculated for the changes of H₂O mole fractions when all IRGAs are corrected by their individual values of X _wc .

Table 1. Correction coefficients for cross-sensitivity due to direct absorption of CO₂ due to H₂O (X _wc ) and zero factors and span adjustment terms for CO₂ (Z _c and S _c ) and H₂O (Z _w and S _w ) for the listed IRGAs in Fig. 4

Serial number	72H-0361		75H-0140	75H-0298		75H-0370	75H-0731
Factory calibration day (month year)	Jun 2012		Oct 2000	Mar 2002	Apr 2006	May 2002	Apr 2004
Xwc	−0.0007 (supplied)	0.0010 (adjusted)	0.0153	0.0072	0.0022	0.0100	0.0015
Zc	0.908		0.880	1.035	0.957	0.886	0.947
Sc	1.002		1.003	1.000	0.999	0.994	1.001
Zw	1.029		1.033	1.032	0.652	0.562	0.714
Sw	1.009		1.005	0.998	0.995	0.996	1.005

The first IRGA (serial number 72H-0361) is a model LI-7200, and the others are LI-7500 models.

Fig. 5 Example of CO₂ mixing ratio outputs in ambient (403.7 ppm) CO₂ gas as a function of H₂O mole fraction: dehumidified gas (□), humidified gas with the manufacturer's values of a _w (1.15) and X _wc (−0.0007) (▪), humidified gas with the manufacturer's a _w and adjusted X _wc (0.001) values (♦), and humidified gas with adjusted values of X _wc and a _w from this study (×). The adjusted value of a _w (1.78) was determined so that the CO₂ output in humidified gas was almost the same as the CO₂ output with dehumidified gas. The adjusted value of X _wc was determined in Section 3.1.

Fig. 6 Two examples of CO₂ mixing ratios in dehumidified ambient CO₂ gas (□) and in humidified ambient CO₂ gas with adjusted values of X _wc (0.001) and a _w (1.78) (•), as functions of H₂O. Different conditions of sea salt attachment on optical windows were applied, although the AGC values were equivalent (68.75) and remained constant during both tests.

Fig. 7 Sensitivity tests of CO₂ mixing ratio outputs in dry [□, r_c(dry)] and humidified [•, r_c(humid)] standard gases with known CO₂ mixing ratios (359.7, 401.0 and 421.3 ppm CO₂) under the condition of optical window contamination shown in Fig. 6b. The outputs in humidified gases were corrected with the adjusted values of X _wc and a _w , and were humidified at the highest level (~3.0%) during the test.