Elucidating atmospheric reaction mechanisms by means of carbon stable isotopes

Author: Nana Khundadze
Keywords: Isoprene, OH radicals, stable isotopes, KIE, mass balance calculation, SAPHIR chamber.
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Isoprene dominates (~50%) the flux of non-methane volatile organic compounds (VOC) emitted by the biosphere. Once released, it is chemically transformed, until being removed from the atmosphere. The dominant chemical degradation pathway here is the photo-oxidation, leading to secondary pollutant formation (e.g., of the greenhouse gas ozone). In mixtures of atmospheric vapors isoprene oxidation products can suppress both particle number and mass of secondary organic aerosol (SOA). Isoprene emission rate already exceeds that of all man-made VOC by approximately a factor of five. In addition, since efforts are made to reduce anthropogenic emissions, it is likely that isoprene will steadily gain in importance in terms of reactivity in the atmosphere, especially during periods of increased temperature and high photo-chemical activity. Despite being the focus of numerous studies over the past years, the current understanding of many important aspects of isoprene chemistry is still incomplete. This can be attributed to the complexity of parameters governing the photochemistry of isoprene and uncertainties in measuring all oxidation products. Measurements of carbon stable isotope ratios are considered a powerful tool to get insight into sources and chemical reactions of air pollutants and to study their life cycle in the atmosphere. The source strength of a precursor VOC can be derived from measurement of its ambient mole fraction and isotopic composition (13C), given that the isotopic signature at the point of emission and its fractionation due to chemical removal are known. Change in isotope ratios can thus be used to quantify the extent of chemical reaction for a given compound. On that account, stable isotope studies can serve as the additional information needed in elucidating some aspects of the intricate reaction network within the investigated chemical mechanism of the isoprene photo-oxidation. The goal of this project is to get insight into the atmospheric isoprene photo-oxidation mechanism by using δ^13C measurements together with mass balance calculations for the precursor and secondary organic compounds formed in the first and subsequent reaction generations. The use of stable isotopes for process understanding within the atmospheric research is a highly innovative field.