Calibration

The LI-COR LI-6262 is calibrated against two standard gases with known concentrations (320 ppm and 420 ppm) once a day, short before midnight. This method is more common in the field of long term ecosystem carbon dioxide exchange studies than the one described in section . The advantage of using calibration gases is the avoidance of the complex calibration procedure described in section . The disadvantage is the interruption of the data collection during the calibration period. The analyzer runs in absolute mode.

Calibration starts at 23h 46min UTC+1, and finishes at midnight. The programmable timer switches between the standard gases in every 2 minutes so that each first minute is used to flush the sampling cell, while the signal measured during the second minute is integrated to determine the response of the instrument, to track the zero and span drift (LI-COR, 1996).

Factory calibration (fifth order polynomial for CO and third order polynomial for HO) is applied to convert the raw voltage signal into mole fraction (LI-COR, 1996). Dry air mixing ratio is calculated in the same way as it is described in section to avoid using the Webb-correction (Webb et al., 1980). Span and zero drifts are taken into account using a linear stretch of the time series based on the measured mixing ratios of the calibration gases. The validity of the linear approximation for the instrument in the range of interest is described in section .