Air pollutants trapped beneath frequent temperature inversions in the Los Angeles basin bring about surface radiance enhancements of up to fifty precent in the middle-infrared window (8--12 microns). This constitutes an anthropogenic modification to the downwelling longwave flux which can be as large as 9 W/m2. A Fourier Transform Infrared (FTIR) spectroradiometer has been used to measure middle-infrared atmospheric emission spectra under Los Angeles smog, and these 1 cm-1 resolution spectra demonstrate that both anthropogenic aerosols and increased tropospheric ozone abundance contribute to enhancements in surface longwave radiation. ¿ American Geophysical Union 1994