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Well-known oxidation mechanism also operative in the atmosphere – with far-reaching effects


It is already known since 1875 that hydrocarbons can be oxidized by oxygen. Up to now, this so-called “autoxidation process” has been investigated solely in the condensed phase. An international team of scientists with participation of the Leibniz-Instituts für Troposphärenforschung (TROPOS) provided evidence for the importance of this process in the atmosphere as well.

Laboratory experiments have been conducted with latest analytical techniques allowing to discover the reaction mechanism of autoxidation as far as possible. This mechanism describes how hydrocarbons from vegetation are converted to products with up to 12 O-atoms at a time scale of seconds.

Field measurements at the TROPOS station in Melpitz at Leipzig helped to identify the intermediates on the way to the formation of ELVOCs.

Foto: Tilo Arnhold/TROPOS

Laboratory set-up with CI-APi-TOF (chemical ionisation - atmospheric pressure interface – time-of-flight) mass spectrometers for the detection of highly oxidized RO2 radicals end closed-shell product

Foto: Dr. Torsten Berndt/ TROPOS

The resulting, highly oxidized products represent extremely low-volatility compounds effecting particle growth substantially, and subsequently the formation of clouds and the climate system as stated by the scientist from Germany, Finland and from US in the journal Angewandte Chemie. This new insight is useful for a better understanding for the impact of natural emissions for air quality and climate.

Forests emit large amounts of volatile organic compounds (VOC). Alpha-pinene and limonene, the typical flavour of conifer forest, belong to them. Their reaction products form the so-called secondary organic aerosol. In this case, gases are converted to particle constituents. The atmospheric particles are able to reflect solar radiation or act as nuclei for cloud droplets.

These processes have a major influence on climate and are therefore of special scientific interest. However, the corresponding model calculations are very inaccurate at the moment because there are still large gaps in our knowledge concerning the role of natural emissions for the process of particle formation and growth. As long as these processes are poorly understood, it is difficult to give accurate predictions. This uncertainty also affects all climate models.

Large uncertainties do exist especially for the growth of newly formed particles towards cloud nuclei on which water condenses, and thus, initiating the formation of clouds. The particle growth starting from a diameter of about two through to 100 nanometers requires probably low-volatile organic vapors, as has been speculated. These "extremely low-volatility organic compounds (ELVOCs)” have been detected recently and this finding was published in 2014 in the journal NATURE. Latest improvements of measurement techniques made their detection possible.

Potential formation pathways for ELVOCs were very speculative. In the present study scientists from TROPOS and from the University of Helsinki in collaboration with US colleagues solve this mystery as far as possible. Intermediates forming finally ELVOCs were identified as a result of laboratory experiments conducted in the flow tube in TROPOS as well as from field experiments in Melpitz near Leipzig and in Hyytiäla (south Finland).

“Our investigations showed that the most abundant monoterpenes, like alpha-pinene and limonene, produce highly oxidized RO2 radicals with up to 12 O-atoms within a time scale of seconds, and subsequently also the corresponding closed-shell products. The overall process in the atmosphere proceeds in a similar way as the well-known autoxidation in the condensed phase” states Dr. Torsten Berndt from TROPOS. Autoxidation is a chemical process converting slowly substances with the help of oxygen from air under room-temperature conditions. The autoxidation is responsible for a couple of processes, for instance for embrittling of plastics and the spoilage of food.

„Field measurements above the meadows in Melpitz and in the boreal forest in Hyytiäla confirm the finding from laboratory and point to the importance of the autoxidation for the oxidation of hydrocarbons in the atmosphere. Very probably, the highly oxidized RO2 radicals and the corresponding close-shell products have a very low volatility and are crucial for the growth of atmospheric aerosols. Thus, they influence the interactions between aerosols, clouds and the climate”, emphasises Tujia Jokinen from the University of Helsinki. The new findings will help to better assess the effects of vegetation and different land use for climate. As a result, the climate models can be improved, which up to now did not describe particle growth in a proper way.
Tilo Arnhold

Jokinen, T., Sipilä, M., Richters, S., Kerminen, V.-M., Paasonen, P., Stratmann, F., Worsnop, D., Kulmala, M., Ehn, M., Herrmann, H. and Berndt, T. (2014): Rapid autoxidation forms highly oxidiced RO2 radicals in the atmosphere. Angewandte Chemie (International Edition). E-pub ahead of print. doi: 10.1002/ange.201408566 10.1002/ange.201408566 
The research was funded by the European Commission, the Academy of Finland and the European Research Council ERC (ATMNUCLE).

Further information:
Dr. Torsten Berndt, Prof. Hartmut Herrmann
Leibniz Institute for Tropospheric Research (TROPOS)
phone +49-341-2717-7032, -7024
Tuija Jokinen, Dr. Mikael Kristian Ehn
Universität Helsinki
phone +358-294-151698, -151076
Tilo Arnhold, TROPOS Public Relations
phone +49-341-2717-7060

Laboratory investigations on particle formation and early growth at TROPOS:
TROPOS Research site Melpitz
Centre of Excellence in Atmospheric Science – From Molecular and Biological processes to The Global Climate
SMEAR II station of the University of Helsinki in Hyytiäla

New gas-phase compounds form organic particle ingredients (Press release, 26-Feb-2014)
Plants moderate climate warming (Press release, 28-Apr-2013)
in German:
A new atmospherically relevant oxidant of sulphur dioxide - Nature (press release, 08 August 2012):
in German:

The Leibniz Institute of Tropospheric Research (TROPOS) is member of the Leibniz Society which consists of 89 independent research institutes. Research at these institutes ranges from natural-, engineering- and environmental research to economy, regional and social research to the humanities. Leibniz institutes work on questions relevant to society, economy and ecology. The institutes focus on knowledge- and application-oriented basic research. They operate scientific infrastructure and offer research-based service. The Leibniz society puts focus on knowledge transfer toward policy, science, economy, and the public sector. Leibniz institutes are in intensive cooperation with universities – including the ScienceCampi -, with industry and other partners in Germany and abroad. The institutes undergo high-quality, independent and transparent evaluations. Because of their importance for Germany, the Federal Government and the federal states fund the Leibniz-Society together. The Leibniz-Institutes employ around 17.200 staff, among them are 8.200 scientists. The total annual budget of the institutes is 1.5 Billion Euro. 

Tilo Arnhold | TROPOS

Further reports about: Atmosphere TROPOS autoxidation climate models clouds mechanism organic compounds

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