Leipzig. Highly sensitive detection methods allow ever deeper insights into complex chemical processes in the atmosphere: Researchers at the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig found a series of new product channels in a detailed product study on the oxidative degradation of isoprene in the gas phase, which allows a better mechanistic understanding of this important process for atmospheric chemistry. The results were published in the journal Nature Communications .
Isoprene (C 5 H 8 ), mainly produced by deciduous forests, is one of the most important non-methane compounds with an annual emission rate of about 600 million metric tonnes of carbon released into the atmosphere. The degradation of isoprene takes place almost exclusively in the gas phase via reaction with the OH radical, which is regarded as an oxidative washing agent of the atmosphere . This initially produces structurally different HO-C 5 H 8 O 2 peroxy radicals, which are in equilibrium with each other. Despite their immense importance, their further reaction pathways are not yet fully understood. Isoprene reactions dominate the gas-phase processes, especially in the tropics, alongside those of methane (CH 4 ) and carbon monoxide (CO).
The experiments at TROPOS were carried out in a large flow system at 1 bar air and room temperature. The reaction control was designed in such a way that product formation could be monitored for atmospheric concentrations of the reactive intermediates. Highly sensitive mass spectrometry was used to selectively detect the formation of peroxy radicals and stable products. The good agreement of the results using different ionisation schemes confirmed the reliability of the findings.
The results of the investigations in the laboratory in Leipzig were largely in agreement with previous knowledge on the degradation of isoprene. However, some C 4 - and C 5 -products and their formation pathways were described for the first time. Of particular interest was the tracing of the formation of highly oxidised peroxy radicals of the composition C 5 H 9 O 8 and C 5 H 9 O 9 , which are formed via so-called autoxidation processes on a short time scale of seconds. "The molar yield of the highly oxidised species as a function of competing processes is only 0.3 % at most, which seems negligible at first," says Dr Torsten Berndt from TROPOS, who planned and carried out the experiments. "However, the huge emission rate of isoprene makes the formation of C 5 H 9 O 8 and C 5 H 9 O 9 potentially very interesting on an absolute scale," Berndt explains further.
In order to further assess the significance of these reaction pathways, they were then implemented in a global chemistry-climate model. The model simulations carried out showed that around 4 million metric tonnes of highly oxidised isoprene peroxy radicals (C 5 H 9 O 8 and C 5 H 9 O 9 ) are formed globally every year. "Interestingly, the modelled production rates are of the same order of magnitude as the formation of the analogous products (C 10 H 17 O 7 and C 10 H 15 O 8,10 ) from the oxidation of α-pinene. This formation pathway was previously regarded as the most important process for the formation of highly oxidised gas-phase products in the atmosphere," add the scientists responsible for the global modelling, Dr Erik Hoffmann and Dr Andreas Tilgner.
In a further reaction step, the C 5 H 9 O 8 and C 5 H 9 O 9 radicals react in the atmosphere mainly with nitrogen monoxide (NO) and hydroperoxy radicals (HO 2) . The reaction with NO leads mainly to the formation of the corresponding highly oxidised organic nitrates if the chemical C 5 -structure is retained. The product formation in the case of the reaction with HO 2 is still speculative and requires further investigation.
In conclusion, the scientists emphasise that the new research findings raise a number of questions. The highly oxidised products of isoprene could influence a variety of processes such as the growth of atmospheric aerosol particles and secondary organic matter formation. These processes are closely linked to important aerosol-cloud interactions and their global climate and radiative effects. Therefore, considerably more studies are needed to adequately clarify the significance of the new oxidation pathways and the resulting products for the atmosphere.
Nature Communications
Experimental study
Not applicable
Highly oxidized products from the atmospheric reaction of hydroxyl radicals with isoprene
28-Feb-2025
The authors declare that they have no competing interests.