Observations of XCO₂ and XCH₄ with ground-based high-resolution FTS at Saga, Japan, and comparisons with GOSAT products

Abstract. Solar absorption spectra in the near-infrared region have been continuously acquired with a ground-based (g-b) high-resolution Fourier transform spectrometer (FTS) at Saga, Japan, since July 2011. Column-averaged dry-air mole fractions of greenhouse gases were retrieved from the measured spectra for the period from July 2011 to December 2014. Aircraft measurements of CO₂ and CH₄ for calibrating the g-b FTS data were performed in January 2012 and 2013, and it is found that the g-b FTS and aircraft data agree to within ± 0.2 %. The column-averaged dry-air mole fractions of CO₂ and CH₄ (XCO₂ and XCH₄) show increasing trends, with average growth rates of 2.3 and 9.5 ppb yr⁻¹, respectively, during the ∼ 3.5 yr of observation. We compared the g-b FTS XCO₂ and XCH₄ data with those derived from backscattered solar spectra in the short-wavelength infrared (SWIR) region measured with Thermal And Near-infrared Sensor for carbon Observation–Fourier Transform Spectrometer (TANSO-FTS) onboard the Greenhouse gases Observing SATellite (GOSAT): NIES SWIR Level 2 products (versions 02.xx). Average differences between TANSO-FTS and g-b FTS data (TANSO-FTS minus g-b FTS) are 0.40 ± 2.51 and −7.6 ± 13.7 ppb for XCO₂ and XCH₄, respectively. Using aerosol information measured with a sky radiometer at Saga, we found that the differences between the TANSO-FTS and g-b FTS XCO₂ data are moderately negatively correlated with aerosol optical thickness and do not depend explicitly on aerosol size. In addition, from several aerosol profiles measured with lidar located right by the g-b FTS, we were able to show that the presence of cirrus clouds tends to cause an overestimation in the TANSO-FTS XCO₂ retrieval, while high aerosol loading in the lower troposphere tends to cause an underestimation.