Nadir looking infrared spectra of a forest fire smoke plume off the south shore of Long Island, New York, were obtained from a NASA ER-2 aircraft during two spatially coincident over-flights on the morning of August 25, 1995. These spectra exhibit enhanced CO column densities at the same geographic locations over the smoke plume on both over-flights with a peak CO column density ~2.6¿1018 cm-2, ~6&sgr; above the clear air background. Meteorological conditions suggest the smoke plume was confined to the planetary boundary layer (PBL), pressures ≥850 mb, and perhaps to a thin region near the top of the PBL. Constraining the excess CO to the PBL yields a CO mixing ratio ~1,400 ppbv. Further constraining the CO to the model layer nearest the top of the PBL, 852--878 mb, yields ~4,300 ppbv. From the spatial overlap of the spectra, the estimated width of the CO rich portion of the plume is ≤2.8 km vs. a plume width of ~5 km in GOES-8 satellite visible images. ¿ American Geophysical Union 1996