The effect of coarse biogenic aerosol on the downwelling infrared flux at the surface was estimated from the enhancement of spectral IR zenith radiances measured by the ground-based Fourier transform spectrometer Emission-Infrared-Spectrometer for Atmospheric Research (EISAR) at Potsdam, Germany. The enhancement in IR downwelling flux was found to 10.40 W m-2 for a pine pollen concentration of ~2000 pollens per m3 per day, which is ~8 times the monthly mean at the site in May where pine pollens are present in the air. The enhancement is ~2 times larger than caused by a 10% change in precipitable water vapor (PWV) at midlatitudes in summer and also roughly 2 times the amount that would be caused by conventional aerosol models at 5 km visibility. The enhancement at average pine pollen abundance is estimated to ~1--2 W m-2 and is not negligible if the effect found is not pine pollen specific. The effect of pollens on the downwelling radiance is a spectrally smooth curve with a maximum of 11 mW (m2 sr cm-1)-1 at 1000 cm-1. A broad dip in the radiance enhancement was found between 850 and 1000 cm-1 that could not be assigned to known atmospheric absorbers. It is thus most likely pollen specific and is probably caused by pollen exines. The spectral enhancement in IR zenith radiance caused by the pollens was compared with changes induced by PWV variations and the abundance of aerosol types. A distinct radiance enhancement by pine pollens of up to 80% in maximum and 20% on average was found around the 4.67 μm CO fundamental band used for remote sounding of CO from ground and satellites. ¿ 2000 American Geophysical Union