A huge variety of chemical processes takes place on a continuous basis in the atmosphere; large molecules may be disintegrating to form smaller molecules, and small molecules may be coming together to form larger units, attaching themselves to small airborne particles. However, the important changes taking place within organic matter in the atmosphere can be understood without following every single one of the many thousands of substances present in the air.
Researchers were able to demonstrate that they only needed to investigate those few specific chemical properties that are particularly significant to the atmospheric behaviour of the substances. "For example, the ratio of oxygen to carbon in a substance affects its ability to absorb water -- and is therefore relevant to the ability of fine particulates to seed clouds," explains Andr้ Pr้v๔t, who leads the project at the Paul Scherrer Institute.
Development of fine particulates reconstructed in lab
The constant chemical changes taking place in the atmosphere also mean that the composition of fine particulates is similar in almost every corner of the world -- regardless of the precise source materials. However, researchers demonstrated that the properties of particular source materials can be recreated from the fine particulates. To do this, they initially used the smog chamber at the Paul Scherrer Institute to simulate the changes within individual materials in the atmosphere.
"We were able to use these results in conjunction with a complex statistical process to determine the type of source materials from which the fine particulates had originated. Additional procedures, such as the C14 method, can then be used to establish the exact sources -- whether, for example, substances come from woodland or from exhaust gases," explains Urs Baltensperger, Head of the Atmospheric Chemistry Laboratory at the Paul Scherrer Institute.
Fine particulates at different locations: a hazard to health or seeding for cloud formation
The detailed investigations into the make-up of the fine particulates were made possible by a novel type of device -- a special mass spectrometer -- which can be used to analyse the composition of the air with a time resolution of one minute. The researchers took measurements at 26 different sites in the northern hemisphere. PSI was responsible for two very different locations in Switzerland: Zurich's inner city and the Jungfraujoch -- in the Swiss Alps at 3450 meters. The Zurich measurements were important primarily from the point of view of the effect of gaseous emissions on health, while the measurements on the Jungfraujoch concentrated on issues involving cloud formation.
The work of the PSI researchers was supported by the Swiss National Science Foundation (SNSF).
- J.L. Jimenez, M.R. Canagaratna, N.M. Donahue, A.S.H. Prevot et al. Evolution of Organic Aerosols in the Atmosphere. Science, December 11, 2009 DOI: 10.1126/science.1180353
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