Effect of coarse biogenic aerosol on downwelling infrared flux at the surface

The effect of coarse biogenic aerosol on the downwelling infrared flux at t he surface was estimated from the enhancement of spectral IR zenith radianc es measured by the ground-based Fourier transform spectrometer Emission-Inf rared-Spectrometer for Atmospheric Research (EISAR) at Potsdam, Germany. Th e enhancement in IR downwelling flux was found to 10.40 W m(-2) for a pine pollen concentration of similar to 2000 pollens per m(3) per day, which is similar to 8 times the monthly mean at the site in May where pine pollens a re present in the air. The enhancement is similar to 2 times larger than ca used by a 10% change in precipitable water vapor (PWV) at midlatitudes in s ummer and also roughly 2 times the amount that would be caused by conventio nal aerosol models at 5 km visibility. The enhancement at average pine poll en abundance is estimated to similar to 1-2 W m(-2) and is not negligible i f the effect found is not pine pollen specific. The effect of pollens on th e downwelling radiance is a spectrally smooth curve with a maximum of 11 mW (m(2) sr cm(-1))(-1) at 1000 cm(-1). A broad dip in the radiance enhanceme nt was found between 850 and 1000 cm(-1) that could nor be assigned to know n atmospheric absorbers. It is thus most Likely pollen specific and is prob ably caused by pollen exines. The spectral enhancement in IR zenith radianc e caused by the pollens was compared with changes induced by PWV variations and the abundance of aerosol types. A distinct radiance enhancement by pin e pollens of up to 80% in maximum and 20% on average was found around the 4 .67 mu m CO fundamental band used for remote sounding of CO from ground and satellites.