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

Abstract

We retrieved CO 2 volume mixing ratios (VMRs) from solar absorption spectra in the 1.6‐ μ m CO 2 (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 CO 2 VMRs ( X CO2 ) from December 2001 to December 2007. The average difference between the X CO2 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 X CO2 retrieval by determining information about the ILS simultaneously with X CO2 . 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 X CO2 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 X CO2 derived from the FTS measurements demonstrated that X CO2 could be retrieved with a precision of ∼0.2%. The retrieved X CO2 values were compared with CO 2 VMRs obtained from aircraft sampling measurements up to 7 km altitude over Sagami Bay, Japan. The seasonal amplitude of the retrieved X CO2 agreed within 1 ppm with those of the CO 2 VMRs obtained from the aircraft sampling measurements above 3 km. The average seasonal amplitude of X CO2 over Tsukuba was ∼8 ppm. In addition, X CO2 showed an increasing trend, with a growth rate of ∼2 ppm/a.