Continuous low-maintenance CO₂/CH₄/H₂O measurements at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia

Abstract. To monitor the continental carbon cycle, a fully automated low maintenance measurement system is installed at the Zotino Tall Tower Observatory in Central Siberia (ZOTTO, 60°48' N, 89°21' E) since April 2009. A cavity ring-down spectroscopy (CRDS) analyzer continuously measures carbon dioxide (CO₂) and methane (CH₄) from six heights up to 301 m a.g.l. Buffer volumes in each air line remove short term CO₂ and CH₄ mixing ratio fluctuations associated with turbulence, and allow continuous, near-concurrent measurements from all tower levels. Instead of drying the air sample, the simultaneously measured water vapor is used to correct the dilution and pressure-broadening effects for the accurate determination of dry air CO₂ and CH₄ mixing ratios. The stability of the water vapor correction was demonstrated by repeated laboratory and field tests. The effect of molecular adsorption in the wet air lines was shown to be negligible. The low consumption of four calibration tanks that need recalibration only on decadal timescale further reduces maintenance. The measurement precision (accuracy) of 0.04 ppm (0.09 ppm) for CO₂ and 0.3 ppb (1.5 ppb) for CH₄ is compliant with the WMO recommendations. The data collected so far (until April 2010) reveals a seasonal cycle amplitude for CO₂ of 30.4 ppm at the 301 m level.