When modeling climate, scientists must account for a large number of variables. One of the most challenging is the effect of small particulates suspended in the atmosphere, which can either reflect or absorb incoming radiation from the sun and thus alter its influence. The most common types of aerosols are soot, ash and other man-made particles as well as naturally derived dust and salt. Until now, plants and animals have been considered a small source of particulate pollution. But a new study suggests that up to 25 percent of aerosols worldwide could be coming from biological sources, including fur, skin, pollen and bacteria.

Ruprecht Jaenicke of the Institute of Atmospheric Physics in Mainz, Germany, gathered air samples from around the globe at different times of the year and analyzed their content. He found evidence of a variety of cellular and protein particles--from dandruff to algae, bacteria to viruses--injected directly into the air. There was no identifiable annual cycle in the total amount of these bioaerosols, contrary to expectations that concentrations in the spring or summer should be higher than those present during the winter months. The proportions of different biological compounds did demonstrate variability, however. Pollen is most abundant in the spring, for example, whereas the amount of decaying cellular matter peaks in winter.

Jaenicke's report, published today in the journal Science, concludes that the biosphere is a major source of primary aerosol particles ranging in size from tens of nanometers to millimeters in size. Because they can influence cloud formation and trigger precipitation, understanding their global distribution is important, he notes, because "the meteorological relevance of cellular particles could be high."