Observations of XCO 2 and XCH 4 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.