Simulation and Understanding of the Atmospheric Radical Budget for Regions with Large Emissions from Plants

Project Information

SARLEP

Grant agreement ID: 681529

DOI

10.3030/681529

Project closed

EC signature date 22 February 2016

Start date 1 March 2016

End date 28 February 2021

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 850 000,00

EU contribution

€ 1 850 000,00

Coordinated by

FORSCHUNGSZENTRUM JULICH GMBH
Germany

Periodic Reporting for period 4 - SARLEP (Simulation and Understanding of the Atmospheric Radical Budget for Regions with Large Emissions from Plants)

Reporting period: 2020-09-01 to 2021-02-28

Summary of the context and overall objectives of the project

Approximately one giga-ton carbon per year of volatile organic compounds (VOCs) is released into our atmosphere from man-made and natural sources, of which approximately 90% is from biogenic sources. Most of the organic compounds are processed within the lowest part of the atmosphere (troposphere) by oxidation reactions, which lead to oxidized organic compounds, particles and ozone, all of which are harmful to humans and the environment. Actions to improve air quality are guided by the predictions of numerical models, which include chemical reaction schemes. A detailed understanding of the underlying oxidation processes of organic compounds is necessary for accurate predictions.

The most important oxidant agent in the troposphere is the hydroxyl radical (OH) mainly produced by photochemical reactions driven by sunlight. Its outstanding importance for the oxidation capacity of the atmosphere relies on its quasi-catalytic cycling: After VOCs have been attacked, peroxy radicals are produced, which can regenerate OH, so that one OH radical originating from photolysis can oxidize a large number of VOCs before being lost in radical termination reactions. Understanding the recycling mechanisms is essential to predict the removal rate of pollutants out of the atmosphere and the formation of secondary pollutants such as ozone and particles. Recommendations for actions to improve air quality are based on the knowledge of atmospheric oxidation processes driven by radical chemistry.

In order to improve our understanding of atmospheric oxidation processes, simulation experiments in the unique large outdoor chamber SAPHIR at Forschungszentrum Jülich were performed. In order to ensure high accuracy of data, quality-assurance is essential. Therefore, one objective of the project was to develop instrumentation for the detection of short-lived atmospheric radicals and organic compounds.

By combining highly precise and accurate measurements in simulation experiments and quantum-chemical calculations, it could be shown that new reaction pathways of peroxy radicals impact the radical regeneration rate specifically in forested environments. The project significantly contributed to a better understanding of chemical atmospheric processes in these regions that lead to the formation of secondary pollutants in the oxidation chain of biogenic organic compounds. Results have been published and can now be implemented in chemical models used for air quality and climate warming predictions.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Highly sensitive and accurate detection of radical and trace gas species were required. In the experiments in this project, hydroxyl radical concentrations were measured by established instruments that make use of either long-pass absorption or laser-induced fluorescence. In order to quantify, if artefacts interfered with measurements in this project, a chemical zeroing scheme was successfully implemented and characterized. No significant unaccounted interference was observed in the experiments in this project. An instrument for the detection of hydroperoxy radicals (HO2) using mass spectrometry was developed and characterized. This new instrument performed measurements in the experiments in this project concurrently with a second established instrument. Agreement of measurements by both instruments gives confidence that measurements are not affected by interferences.

New mass spectrometry instruments applying different ion chemistry were also used for the detection of organic species. Characterization and calibration for organic species that are produced in the experiments needed to be done and partly developed. Due to the different ion chemistry the classes of organic species and/or their sensitivity for specific compounds differ, so that the range of oxidized organic compounds that are detected in the experiments could be greatly enlarged also for the future.

In addition, measurements of OH reactivity, the inverse lifetime of the OH radicals gave a quantification of unidentified organic species. A large fraction of OH reactivity instruments (in total 9) operated worldwide in field experiments participated in an international campaign at the chamber. Results demonstrated the high accuracy of OH reactivity measurements performed in the chamber and could contribute to the improvement of the quality of OH reactivity measurements not only for the experiments in SAPHIR but also for the community worldwide.

The photo-oxidation of the most important biogenic emissions of organic species was investigated in simulation experiments in the chamber. Separate series of experiments with the important oxidation products were performed, in order to distinguish between effects from the initial attack of oxidants to isoprene and the subsequent chemistry of organic oxidation products. In these experiments, a large set of instruments from cooperation partners participated, in order to ensure a complete characterisation of oxidation products.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The detection of atmospheric constituents specifically of radical species in the atmospheric chamber SAPHIR were of highest quality. This is, for example, ensured by the unique feature of the simultaneous detection of radical species by independent techniques. OH radicals were detected by fluorescence and absorption instruments and HO2 radicals by chemical conversion to OH that is detected by fluorescence. The new mass spectrometer instrument was the first one worldwide for the detection of HO2 that was tested against established HO2 detection methods. Concerning the detection of organic compounds, the SAPHIR chamber is one of the first facilities worldwide that is now equipped with a VOCUS-PTR instrument. It allows measuring a much wider range of different oxygenated organic compounds than previous PTR instruments.

The photo-oxidation and night-time oxidation were performed under atmospherically relevant conditions unlike laboratory experiments which are typically done at high concentrations of reactants. The experiments with isoprene showed that the widely used oxidation mechanisms do not describe the observations in the chamber regarding the regeneration of radicals. The analysis of experiments revealed that the importance of the subsequent chemistry of oxidation products is much higher than previously thought. Based on experimental observations in chamber experiments and quantum mechanical calculations, the mechanistic understanding of the isoprene oxidation was extended over a wide range of chemical conditions.

The analysis of the experiments of a wide range of monoterpene species emitted by plants and their oxidation products demonstrates that the reaction of monoterpenes with the OH radical favours different pathways than currently assumed in models. These results help to explain previous field observations where similar results were found in environments that were dominated by monoterpene emitting plants.

Results from the investigation of single compounds were extended by the investigation of complex mixtures of emitted species and oxidation products. This was achieved by either using emissions from real plants that are housed in a Teflon bag or by investigating ambient air in the simulation chamber. The forest nearby the SAPHIR chamber provided a mixture of natural forest emissions. This allowed to analyse, if results achieved in the experiments with single compounds can be transferred to a realistic mixture of reactive compounds.

SARLEP investigated the atmospheric fate of organic compounds emitted by plants.