Field calibration and correction of air water concentration measurements

Infrared gas analyzers (IRGAs) are commonly used in Eddy Covariance (EC) system and are used for, in particular, the ecosystem water cycle. However, they suffer from a measurement drift of absolute concentrations with time, leading to the increasing bias of readings. It is recommended in the manuals to do a factory calibration once every 1-2 years (e.g., LI-6262) or user calibration when considerable drift occurs (e.g., LI-7000). However, our experience shows that a significant drift can occur within a few days already. At our semi-arid EC site of Yatir Forest (31˚20'N, 35˚03'E, Israel), we are measuring a vertical air humidity profile (absolute humidity, C_w in mmol×mol^-1, and relative, RH, %),  to study the VPD regime within the canopy and to analyze dew formation events, which requires highly accurate RH measurements, however accurate RH measurements are difficult to achieve.

Air humidity in Yatir is measured by three different instruments: (1) LI-7000 close-pass IRGA above the canopy for EC flux calculations; (2) LI-6262 close-pass IRGA with inlets in 4 different heights from above the ground up to the sonic height, used for humidity profile measurements; (3) Rotronic HC2S3 air humidity (RH) and temperature (T) sensor above the canopy. Both IRGAs are placed within a temperature-controlled box, and calibrated for zero and span with N2, dew point generator and laboratory standard gases every 1-2 weeks. The Rotronic sensor has very low drift and does not require calibration, but is assumed to be less accurate, especially under high and low RH.

To achieve highly accurate measurements on daily time scale we propose a correction routine that rely on the stability of the RH probe, and the accuracy of the IRGAs after calibration. Every time the IRGA is calibrated, a correction-1 to the RH probe is produced. Between calibrations, the trends in the drifting IRGAs data are corrected (correction-2) to the interpolated stable RH probe data.

For the flux measurements, the mean absolute Cw error before correction was 1.0 mmol×mol^-1, which translates under average temperature of 25^°C and RH of 50% to errors of RH, VPD and dew point of 3.0%, 93.5 Pa and 0.9^°C, respectively. Following our correction procedure, reduced the error to 0.5 mmol×mol^-1, which decreased the errors in RH, VPD and dew point under the same conditions to 1.5%, 47 Pa and 0.4^°C, respectively. For the humidity profile, Cw error after correction decreased from 1.9 mmol×mol^-1 to 0.5 mmol×mol^-1, which decreased the errors in RH, VPD and dew point under the same conditions by 4.1%, 131 Pa and 1.2^°C, respectively.

We will describe the method in more detail and demonstrate its application to our field measurements.