We demonstrate a widely tunable laser heterodyne radiometer operating in the thermal IR during an atmospheric observation campaign in the solar occultation viewing mode. An external cavity quantum cascade laser tunable within a range of 1120 to $1238{\mathrm{cm}}^{-1}$ is used as the local oscillator (LO) of the instrument. Ultra-high-resolution ( $60\mathrm{MHz}$ or $0.002{\mathrm{cm}}^{-1}$ ) transmission spectroscopy of several atmospheric species (water vapor, ozone, nitrous oxide, methane, and dichlorodifluoromethane) has been demonstrated within four precisely selected molecule-specific narrow spectral windows ( $\sim 1{\mathrm{cm}}^{-1}$ ). Atmospheric transmission lines within each selected window were fully resolved through mode-hop-free continuous tuning of the LO frequency. Comparison measurements were made simultaneously with a high-resolution Fourier transform spectrometer to demonstrate the advantages of the laser heterodyne system for atmospheric sounding at high spectral and spatial resolutions.