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Degradation of ozone-depleting methyl halides in the troposphere characterized by isotopes

Periodic Reporting for period 1 - DEMETROPIS (Degradation of ozone-depleting methyl halides in the troposphere characterized by isotopes)

Reporting period: 2016-10-01 to 2018-09-30

Methyl chloride and methyl bromide are each the largest natural sources of chlorine and bromine in the stratosphere. Consequently, these compounds contribute to the destruction of the stratospheric ozone layer which protects flora and fauna from ultraviolet radiation that causes skin cancer and genetic damage. Sources and sinks of these compounds are not sufficiently well quantified with known sinks outweighing known sources by about 20% for both compounds. These unknown emissions, often referred to as “missing sources”, may complicate the prediction of the future state of the stratospheric ozone layer. It is currently assumed that stratospheric ozone levels are recovering due to the decrease of atmospheric concentrations of human-made chemicals, such as chlorofluorocarbons. Emissions from natural sources of methyl halides are, however, strongly dependent on temperature and thus on climate change. Higher temperatures, for instance, may cause increased emissions from oceans, plants and peatlands and counteract the decrease of atmospheric chlorofluorocarbon levels. Hence it is of great importance to better understand emission and degradation processes for these compounds in order to predict the future state of the ozone layer which protects us from the adverse effects of sunlight.
The overall objective of this project was to test and apply an isotopic approach for the characterisation of the main degradation pathway (sink) of these compounds in the atmosphere: the reaction with hydroxyl radicals. To achieve this goal, it was necessary to adapt and, if not available, to develop appropriate methods for the isotopic measurement of the various isotopes for these compounds. In a second step reference laboratory experiments were proposed to study hydroxyl radical reactions with methyl halides under controlled laboratory conditions and to determine the isotope effects caused by these reactions. These parameters are indispensable prerequisites for future studies that plan to use stable isotopes for source apportionment of methyl halides.

The applicant returned to the EU and was given the opportunity to reintegrate in a scientific environment in Germany where he amplified his research skills. Apart from maintaining previous collaborations, new contacts were established to deepen the scientific exchange. Additionally, the applicant benefitted from training in all aspects relevant for running international research projects such as managing the project, co-supervision of students and participating in teaching activities within and outside the host institution.
The proposed research project addressed the gap of missing isotopic information of sink processes of methyl halides. The aims of the first objective were fully achieved. Existing isotopic methods (hydrogen, carbon, bromine) were tested and adapted for methyl halides and are now fully applicable to address the different research questions associated with the unbalanced budget of methyl halides. In addition, a completely new, simpler, and previously unavailable technique for the measurement of chlorine isotopes was developed. This method is not only applicable for methyl chloride but also universally applicable to a wide array of chlorinated compounds. Reference laboratory experiments for OH-radical reactions with methyl halides were carried out during a secondment at Leibnitz Institute for Tropospheric Research. Results from these experiments delivered inconclusive results, probably due to additional processes besides OH-radical reactions in the reaction chamber. Hence contingency actions were undertaken. Firstly, another mechanism for the production of OH radicals was conceived, and the experimental equipment adapted accordingly. Secondly, an alternative but not less important abiotic degradation process was investigated in the meantime: hydrolysis and halide exchange in ocean water. In addition, the potential isotope effects of air-water partitioning of halogenated compounds were studied which constitute crucial knowledge in this context.

The results of the project were presented at international conferences (Goldschmidt 2017, ISOTOPES 2017). The analytical method for the chlorine isotopic measurement was published (open access) in Analytical Chemistry. Manuscripts of the experimental results of the abiotic degradation experiments and the water-air partitioning of halogenated organics have been prepared and are freely available as preprints (on ChemRxiv preprint server). Final submission and publication is envisioned for the coming months.
The results obtained in this project present important scientific achievements that go beyond the state of the art in this field of research. The analytical technique for chlorine isotope measurements presents the first universal method and is applicable for a wide array of organics. This method was not only used for methyl chloride within this project but also for other important contaminants such as trichloroethene, organophosphates and pesticides and has already resulted in several additional publications of other colleagues at the host institution who benefitted from this method. Also, for the near future, this method will considerably increase the number of studies using multidimensional isotope analysis of a wide range of organic compounds.

For methyl halides, sink processes were studied throughout this project. The isotopic data for halogens and hydrogen are the first of their kind and as such provided completely new mechanistic insights and delivered first isotopic enrichment factors. These enrichment factors are a crucial requirement for any future isotope-based study that aims at resolving the unbalanced budgets of methyl halides. Thus, the results of this project contribute to a better understanding of the sink processes of these ozone-depleting substances. Together with isotopic data of other sources and sinks the results of this project may be used in future isotope-based models and lead to a more precise prediction of the future state of the ozone layer in the stratosphere.