Revised methodology for CO2 and CH4 measurements at remote sites using a working standard-gas-saving system

Abstract

We have revised a calculation method of mole fractions and uncertainties for in situ CO2 and CH4 measurements with a working standard-gas-saving system. It uses on-site compressed air to track the baseline drift of sensors. The Japan–Russia Siberian Tall Tower Inland Observation Network (JR-STATION) is made up of this system, which was installed across nine different sites in Siberia. The system acquires semi-continuous data by alternating between sampling air from multiple altitudes through switched flow paths and recording several minutes of averaged data for each altitude. We estimated the sensor repeatability (ur) based on the measurement of on-site compressed air. The ur for CO2 and CH4 was mostly around 0.05 ppm and below 5 ppb, respectively. The combined standard uncertainties (uc(x)) of time-averaged ambient air measurements were sometimes higher than the ur for each period because the data included atmospheric variability during the measurement period of several minutes. Data users should consider the difference between the ur and uc(x) to select optimal data, depending on their focusing spatial scale. The CO2 and CH4 data measured with a non-dispersive infrared (NDIR) analyzer and a tin dioxide sensor (TOS) exhibited good agreement with those measured by cavity ring-down spectroscopy (CRDS).