Posts Tagged ‘Rudolf Criegee’

Criegee intermediates further unsettle climate science

April 26, 2013

Far from being a “settled” science, global warming in particular and “climate science” in general are looking decidedly shaky these days!

What is not in doubt is that clouds and their formation are of critical importance for our climate. But clouds can both “warm” and “cool”. They can attenuate the sun’s radiation that reaches the earth during the day and they can prevent the radiation of heat  from the earth into space during the night. They can absorb some of the sun’s radiation and transfer that heat into the atmosphere and radiate some of it back into space as well. The net effect of clouds is uncertain. Solar effects themselves can affect the formation of clouds (Svensmark’s theory) as has been confirmed recently by experiments at CERN. Current climate models speculate that carbon dioxide can affect the moisture levels and therefore increase clouds in the atmosphere. But no mechanisms are known and these assumptions are more fanciful than based on any evidence. Moreover the assumed enhanced warming due to the increased moisture (positive forcing) is even more fanciful since it is also not known as to whether any such extra moisture results and whether any exists as clouds. Computer models – in the absence of any known mechanisms for such forcing – merely assume some “net, resultant” level of the forcing which (of course) causes warming and can be attributed to carbon dioxide. These assumptions about the forcing due to carbon dioxide effectively presuppose the forcing and are little more than “fudge factors”.

But even the chemistry of and the chemical reactions in the upper atmosphere are uncertain. A new paper  provides new evidence of how Criegee intermediate molecules in the atmosphere could help in aerosol and cloud formation and contribute to cooling in the atmosphere.

A  Criegee intermediate is a carbonyl oxide with two free radical centres which act independently of each other. These molecules could help to break down sulfur dioxide and nitrogen dioxide in the atmosphere and their existence formation of Criegee biradicals was first postulated in the 1950s by Rudolf Criegee. 

Rudolf Criegee (1902-1975) was a German chemist. He studied in Tübingen, Greifswald, and Würzburg and received his doctorate at Würzburg in 1925. He proposed a reaction mechanism for ozonolysis in 1953. The Criegee intermediate and the Criegee rearrangement are named after him. In this context, his research on cyclic reactions and cyclic rearrangement-mechanisms led him, independently of the Nobel-Prize winning work of R.B.Woodward and R.Hoffmann (Woodward-Hoffmann rules), to the same conclusions as theirs, but he failed to publish his findings in time.

File:Carbonyl oxide (Criegee zwitterion).svg

Carbonyl oxide (Criegee zwitterion): wikipedia

The new paper is published in Science:

Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOOCraig A Taatjes et al, Science 12 April 2013: Vol. 340 no. 6129 pp. 177-180 DOI: 10.1126/science.1234689

Abstract: Although carbonyl oxides, “Criegee intermediates,” have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10−14 ± 0.4 × 10−14 centimeter3 second−1.

From the Manchester University Press Release:

Scientists have discovered further evidence for the existence of new molecules in the atmosphere that have the potential to off-set global warming by reacting with airborne pollutants.

Researchers from The University of Manchester, Bristol University, Southampton University and Sandia National Laboratories in California have detected the second simplest Criegee intermediate molecule – acetaldehyde oxide – and measured its reactivity.

Intermediates are molecules that are formed during a chemical reaction and react further to produce the final chemicals of the reaction. Criegee intermediates – carbonyl oxides – were first identifies by the team in January last year and shown to be powerful oxidisers, reacting with pollutants such as nitrogen dioxide and sulphur dioxide.

The authors, whose latest study is again published in the journal Science, believe Criegee intermediates have the potential to cool the planet by converting these pollutants into sulphate and nitrate compounds that will lead to aerosol and cloud formation.

Professor Carl Percival, who led the Manchester team in the University’s School of Earth, Atmospheric and Environmental Sciences, said: “We have carried out the first ever studies on the second simplest Criegee intermediate and were able to show that it also reacts extremely quickly with sulphur dioxide to produce sulphates under experimental conditions.

“We can therefore say that the reaction of these intermediates with sulphur dioxide will have a significant impact on sulphuric acid production in the atmosphere if they follow the pattern established by these two studies.

He continued: “One of the main questions from our first study was if this increased reactivity would be observed for other Criegee intermediates, so with these findings we now have additional evidence that Criegee intermediates are indeed powerful oxidisers of pollutants such as nitrogen dioxide and sulphur dioxide.

“What this study suggests is that the biosphere could have a significant impact on aerosol production and thus potentially climate cooling via the formation of Criegee intermediates. The next steps will be to carry out modelling studies to quantify the impact of Criegee intermediates on climate and to quantify the level of alkene present in various environments.”

The formation of Criegee intermediates or biradicals was first postulated by the German chemist Rudolf Criegee in the 1950s but, despite their importance, it had not been possible to study the chemicals in the laboratory. The detection of the molecules was made possible through a unique apparatus that uses light from a third-generation synchrotron facility at the Lawrence Berkeley National Laboratory.

The latest study has also revealed which of the two isomers of acetaldehyde oxide is the most reactive. Isomers are molecules that contain the same atoms but arranged in different combinations, while conformational isomerism refers to the way the atoms of a molecule are rotated around a single chemical bond.

“In this new paper we have been able to show that the reactivity depends on the conformer of acetaldehyde oxide in a dramatic way,” said Professor Percival. “The ‘anti’ conformer is much more reactive than the ‘syn’ conformer, which we believe more likely to be formed in the atmosphere. This enabled us to measure the rate coefficient for reaction with water for the first time; the removal, via reaction with water, is of vital importance if we want to understand the role of Criegee intermediates in the atmosphere.”

Sandia combustion chemist Craig Taatjes, the lead author on the paper, added: “Observing conformer-dependent reactivity represents the first direct experimental test of theoretical predictions. The work will be of tremendous importance in validating the theoretical methods that are needed to accurately predict the kinetics for reactions of Criegee intermediates that still cannot be measured directly.”

Related: 

Criegee Intermediates Found to Have Big Impact On Troposphere

Offsetting Global Warming: Molecule in Earth’s Atmosphere Could ‘Cool the Planet’

 


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