Infrared in situ radiance measurements in the 9.5-through 11.5-μm spectral region resulted in a direct determination of the IR volume extinction coefficient. Aerosols were collected for an analysis of size distributions and chemical composition. Mie calculations based on these data show that at 10-μm wavelength more then 95% of the IR extinction is caused by absorption. A comparison of the calculated with the measured aerosol absorptivity resulted in values for the imaginary part of the IR refractive index of 0.47 and 0.19 for the urban and rural aerosols, respectively.