Abundance of Cellular Material and Proteins in the Atmosphere

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Science  01 Apr 2005:
Vol. 308, Issue 5718, pp. 73
DOI: 10.1126/science.1106335


Suspended atmospheric particles play a crucial role in any global climate scenario: They can both enforce and suppress radiative forcing. In developing climate modeling further, a deeper understanding of atmospheric aerosol is needed. Because of extreme local and temporal variations, proper incorporation of aerosols into models requires modeling of the aerosol itself. It turns out that cellular material and proteins compose up to 25% of the atmospheric aerosol. Consequently, the source strength of the biogenic aerosol in general must be corrected and should be estimated on the order of other major aerosol sources.

Atmospheric aerosols play a crucial role in regulating the global climate and can either enforce or suppress anthropogenic forcing. Their influence on climate forcing (natural as well as anthropogenic) has been estimated (1), but a better understanding of the composition and sources of atmospheric aerosols is needed to improve climate models. Here we report evidence that particles injected directly from the biosphere constitute a major portion of atmospheric aerosols.

Cellular (and protein) particles injected directly into the atmosphere include fur fibers, dandruff, skin fragments, plant fragments, pollen, spores, bacteria, algae, fungi, viruses, protein “crystals,” and more, ranging in size from tens of nanometers to millimeters. Knowledge about “dead” or fragmented biological particles in the atmosphere is greatly limited. Tropical forests have been proposed as sources (2), and filter samples (3) in western Siberia at ground and aloft show ∼3 μg/m3 of protein, but cellulose and protein make up only a fraction of primary biological aerosol particles (PBAPs).

The meteorological relevance of cellular particles could be high. Pollen grains attract water at relative humidity well below 100% and thus might act locally as cloud condensation nuclei, influencing the formation of clouds. Other biological particles, including decaying vegetation (and associated bacteria) and marine plankton, are excellent ice nuclei. Ice nuclei trigger precipitation and thereby remove water from the atmosphere. One can easily imagine the influence on global cloud cover, climate forcing and feedbacks, and precipitation distribution if the source and distribution of cellular atmospheric particles varies on a regional to global scale.

Atmospheric chemists and modelers have previously considered the biosphere a minor source of primary particles (4), and bioaerosols were thought to occur only in minute concentrations, with insignificant global emissions (1) for the year 2000 [56 Tg/year of biogenic carbonaceous aerosols (>1 μm in size) compared with 3300 Tg/year for sea salt and 2000 Tg/year for mineral dust]. Recently, however, a greater contribution to the atmosphere of particles from biological activities of the oceans has been reported (5). Still, some surveys (6) report as much as 20 to 40% of the aerosol measured as compositionally unidentified.

We observed PBAPs at several geographical locations and aloft, covering all seasons and many characteristic environments (Fig. 1). These data reveal the complete absence of a pronounced annual cycle, despite the common expectation that concentrations in spring or summer should be higher than in winter. A detailed analysis shows that the fractions of different biological compounds do vary, though: In spring, pollen is more abundant, whereas in winter, decaying cellular matter prevails. We have also observed that resuspension from exposed surfaces acts as a source in winter and in dry periods. This might be important for cellular particle production even from the cryosphere. Measurements in Ireland (1998, at a marine location) (7) also indicate a rather high portion for the PBAP fraction, on the order of 25%. Not surprisingly, recent measurements (2001) in a tropical forest reveal that particles smaller than 1 μm compose up to 40%, and particles larger than 1 μm up to 80%, of the total aerosol number concentration of PBAPs (8).

Fig. 1.

Observation of PBAPs greater than 0.2 μm in radius. Particles were collected and stained with a protein dye; protein-containing particles then developed a bluish tint. The shape of the particles, the presence of characteristic elements, and their stability in an electron beam (microscope) were also used for identification. Cellular (and all) particles were individually sized and counted. (A) In Mainz, Germany (1990 to 1998, at a semirural location), a rather stable concentration plateau was observed. The PBAP fraction varied between 5% and almost 50%. The common assumption that, in winter, the biological fraction and its concentration are low was not confirmed. (B) Data from Lake Baikal, Russia (1996 to 1997, at a remote continental location) (10) support the year-round stable presence of PBAPs.

The biosphere is thus a major source (9) for primary aerosol particles, and cellular (protein-containing) particles are a major fraction of the atmospheric aerosol.

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