Observations of XCO.sub.2 and XCH.sub.4 with ground-based high-resolution FTS at Saga, Japan, and comparisons with GOSAT products

• Published in: Atmospheric measurement techniques Vol. 8; no. 12; pp. 5263 - 10525
• Main Authors: Ohyama, H, Kawakami, S, Tanaka, T, Morino, I, Uchino, O, Inoue, M, Sakai, T, Nagai, T, Yamazaki, A, Uchiyama, A, Fukamachi, T, Sakashita, M, Kawasaki, T, Akaho, T, Arai, K, Okumura, H
• Format: Journal Article
• Language: English
• Published: Copernicus GmbH 17.12.2015
• Subjects: Air pollution; Greenhouse gases; Remote sensing
• ISSN: 1867-1381; 1867-8548

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 CO.sub.2 and CH.sub.4 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 CO.sub.2 and CH.sub.4 (XCO.sub.2 and XCH.sub.4) show increasing trends, with average growth rates of 2.3 and 9.5 ppb yr.sup.-1, respectively, during the ∼ 3.5 yr of observation. We compared the g-b FTS XCO.sub.2 and XCH.sub.4 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 XCO.sub.2 and XCH.sub.4, respectively. Using aerosol information measured with a sky radiometer at Saga, we found that the differences between the TANSO-FTS and g-b FTS XCO.sub.2 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 XCO.sub.2 retrieval, while high aerosol loading in the lower troposphere tends to cause an underestimation.