Influences of aerosols and thin cirrus clouds on GOSAT XCO₂ and XCH₄ using Total Carbon Column Observing Network, sky radiometer, and lidar data

The column-averaged, dry air mole fractions of CO₂ and CH₄ (XCO₂ and XCH₄, respectively) were retrieved from short-wavelength infrared (SWIR) spectra observed by the Greenhouse gases Observing SATellite (GOSAT). Continuous measurements of SWIR spectra have been made via GOSAT since 2009, but there has been insufficient investigation of the effects of cirrus clouds and aerosols on the observations. In this work, we investigated the influences of aerosols and cirrus clouds on the differences between GOSAT observations and Total Carbon Column Observing Network (TCCON) data for XCO₂ and XCH₄ (ΔXCO₂ and ΔXCH₄) at three sites: Tsukuba and Saga in Japan, and Lauder in New Zealand. We used aerosol optical thickness (AOT), Angstrom exponents (AEs), and single scattering albedo (SSA), all obtained from sky radiometer observations, as well as vertical profiles of aerosols and thin cirrus clouds from lidar observations. Matchups were performed within ±0.1° latitude/longitude rectangular areas of each TCCON site, and within 30 min of the GOSAT overpass time. The results show a negative slope between ΔXCO₂ and AOT at 500 nm determined from sky radiometer data at Tsukuba and Saga. The GOSAT XCO₂ values tended to be lowered in the presence of cirrus clouds and dense boundary-layer aerosols. Moreover, a significant negative ΔXCO₂ was observed at times of large AOTs that resulted from dust-like events. At Lauder, ΔXCO₂ was negatively correlated with the AOT at 500 nm, although the AOT at this site was generally small. The mean ± standard deviation for ΔXCO₂ and ΔXCH₄ at Lauder are −0.80 ± 1.83 (ppm) and −5.27 ± 10.79 (ppb) with correlation coefficients <i>r</i> between GOSAT and TCCON of 0.94 and 0.83, respectively. Both ΔXCO₂ and ΔXCH₄ were significantly and negatively correlated with the AOT during Sep-Oct-Nov. In addition, stratospheric aerosols caused large negative biases of ΔXCO₂ and ΔXCH₄ at Lauder despite the small stratospheric aerosol optical depth at that site.