High spectral resolution (0.0024 cm-1), ground-based infrared (IR) solar spectra were recorded at Rikubetsu, Japan (43.5 ¿N, 143.8 ¿E), by a Fourier transform infrared (FTIR) spectrometer covering the 4.8-μm fundamental absorption band of carbon monoxide (CO). Total vertical CO columns and tropospheric concentrations, from May 1995 to May 1996, were retrieved from these spectra using an iterative inversion algorithm. Microwindows at 2111.00--2112.00 cm-1 and 2157.40--2159.20 cm-1 were used for the retrievals. Tropospheric CO concentrations above Rikubetsu retrieved from the IR solar spectra are in good agreement with aircraft measurements performed in the nearby area in recent years. The data exhibit seasonal variations in tropospheric CO concentrations. The averaged mixing ratios between 0 and 3 km are 219¿26 parts per billion by volume (ppbv) in April and 115¿7 ppbv in September. The data also show CO concentrations and seasonal variations decreasing with altitude. The vertical change in CO concentration is the greatest in spring and the smallest in fall. A maximum CO column of 3.31¿0.34¿1018 molecules cm-2 in April and a minimum of 2.04¿0.10¿1018 molecules cm-2 in September were observed. The CO column above Rikubetsu is consistent with observations at Kitt Peak, the International Scientific Station of the Jungfraujoch (ISSJ), and Zvenigorod, considering differences in altitude and latitude between the observatories.¿ 1997 American Geophysical Union