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Marbles on the horizon
Scientists make new discoveries about the nature of natural fine aerosol particles
An international team of scientists from Finland, the United States, and Germany – with important contributions from the Bielefeld chemist Professor Dr. Thomas Koop – has made some remarkable new discoveries about natural fine aerosol particles. In their article published in Nature, the authors describe experiments showing that, contrary to previous assumptions, the aerosol particles produced by forests are not liquid but solid. This makes it necessary to rethink and reevaluate how they impact on atmospheric processes and Earth's climate.
Aerosols – fine particulate matter in air – are not just a human health concern. They are also strongly involved in atmospheric processes such as the formation of clouds or the chemical breakdown of environmentally hazardous substances. Natural forests are one of the main sources of fine particulate matter: they release gaseous compounds such as terpenes that react with other chemicals to form so-called secondary organic aerosols (SOA). Up to now, it had been assumed that such biogenic SOA particles were little liquid droplets (each about ten-thousandths of a millimetre in diameter). However, the new studies show that shortly after their formation, these particles have already become amorphous solids: they act very much like tiny little glass marbles. This transforms our basic understanding of how SOA particles form, how they interact with water and reactive trace gases, and thereby also how natural fine particulate matter influences Earth’s atmosphere and climate.
"It was about two years ago. Based on theoretical considerations and laboratory measurements we suggested that glassy aerosol particles should exist in the atmosphere," says Prof. Dr. Thomas Koop, one of the two corresponding authors of the study. "When my Finnish colleagues presented their data to me at a conference last year, we quickly realized that we had to be dealing with such particles."
However, what is so special about these particles? They are not liquid, but solid. Or, to be more precise they are glassy. This means that they are made up of extremely viscous liquids that solidify when the air is somewhat drier – rather like superglue as it dries. This behaviour also implies that the contents of these particles are protected from chemical breakdown processes, so they survive longer in the atmosphere. At the same time, the uptake of water and other trace gases is strongly retarded in glassy particles. This prevents them from participating actively in cloud formation processes, which is why they are less likely to be precipitated in rainfall. The solid state of the particles might well explain the frequent discrepancies that had been found in earlier experiments on the properties of SOA. "SOA particles consist of a myriad of different chemical compounds, and we do not know enough about their properties. Our studies are just a start here, and only future research will show how strongly the actual influence of SOA particles on the atmosphere deviates from our earlier concepts," says Koop.
Original publication:
A. Virtanen, J. Joutsensaari, T. Koop, J. Kannosto, P. Yli-Pirilä, J. Leskinen, J.M. Mäkelä, J.K. Holopainen, U. Pöschl, M. Kulmala, D.R. Worsnop, A. Laaksonen; An amorphous solid state of biogenic secondary organic aerosol particles, Nature (14.Oktober 2010),
http://dx.doi.org/10.1038/nature09455
Contact:
Professor Dr. Thomas Koop, Bielefeld University
Faculty of Chemistry
Telephone: +49. (0)521. 106-6135
Email: thomas.koop@uni-bielefeld.de