Tropospheric carbon monoxide (CO) column densities were retrieved from 491 nadir infrared spectra obtained by the University of Wisconsin's High-Resolution Interferometer Sounder (HIS) during a cross-country flight of a NASA ER-2 on September 12--13, 1993 (UT). A classical, Fourier-based signal-processing technique was utilized to retrieve the CO column density from the 2135--2200 cm-1 portion of the CO 1-0 vibration-rotation band. Error analysis indicates column CO retrieval uncertainties of approximately ¿10%. Retrieved CO column densities over the western United States from this flight, ~1.2¿1018 cm-2, agree well with previous ground-based CO column measurements over the western United States at the same season. Derived CO sensitivity functions for these measurements possess a broad peak centered in the 500--600 mbar region, thus implying the retrieved tropospheric CO mixing ratios are indicative of the mean free troposphere (mft). Across the United States, three spatial groupings of CO were observed: west, mft CO~76 parts per billion by volume (ppbv); midwest, mft CO~56 ppbv; and east, mft CO~71 ppbv. Weather map and back trajectory analyses indicate different source regions for the air parcels in each of these regions. The low CO amounts over the midwest most likely result from the mixing of rapidly subsiding air naturally low in CO with a low-altitude inflow of similarly clean subtropical air off the Gulf of Mexico. These results demonstrate the utility of satellite infrared spectrometer observations to globally map tropospheric CO abundances and further reveal the impact of synoptic scale atmospheric circulations on CO abundances.¿ 1997 American Geophysical Union