Column‐averaged volume mixing ratio of CO2 measured with ground‐based Fourier transform spectrometer at Tsukuba

Abstract

We retrieved CO2 volume mixing ratios (VMRs) from solar absorption spectra in the 1.6‐μm CO2 (30012‐00001) band measured with a ground‐based high‐resolution Fourier transform spectrometer (FTS) at Tsukuba, Japan, using profile retrieval and scaling retrieval algorithms. We derived the time series of the column‐averaged CO2 VMRs (XCO2) from December 2001 to December 2007. The average difference between the XCO2 values obtained with the two algorithms was approximately 0.8 ppm. This difference was attributed to differences between the column averaging kernels of the algorithms. We corrected the distortion effect of an instrumental line shape (ILS) on XCO2 retrieval by determining information about the ILS simultaneously with XCO2. Aircraft in situ measurements were made simultaneously with the FTS measurements over the FTS site on 10 August 2004 and 30 March 2005. The differences between the XCO2 values derived from the FTS measurements and from the aircraft in situ measurements complemented by model data were less than 1%. The diurnal variations of XCO2 derived from the FTS measurements demonstrated that XCO2 could be retrieved with a precision of ∼0.2%. The retrieved XCO2 values were compared with CO2 VMRs obtained from aircraft sampling measurements up to 7 km altitude over Sagami Bay, Japan. The seasonal amplitude of the retrieved XCO2 agreed within 1 ppm with those of the CO2 VMRs obtained from the aircraft sampling measurements above 3 km. The average seasonal amplitude of XCO2 over Tsukuba was ∼8 ppm. In addition, XCO2 showed an increasing trend, with a growth rate of ∼2 ppm/a.