40 years of theoretical advances in mass-independent oxygen isotope effects and applications in atmospheric chemistry: A critical review and perspectives

The discovery of chemically induced mass-independent isotope fractionation in ozone formation 40 years ago revealed a new type of isotope effect that was not considered within the conventional framework of single-isotope ratio measurements. Since this landmark discovery, advances to our theoretical understanding of isotopic physical chemistry effects have helped establish numerous new applications in geochemistry and cosmochemistry. Given that many mass-independent isotope effects are linked to gaseous or multiphase (gas-liquid-solid) reactions, such as the ozone effect arising from the basic symmetry-dependent bond-formation reaction and the photochemical effect associated with wavelength-dependent bond-dissociation reactions, they have wide applications in atmospheric sciences and also contribute to enhancing the fundamental knowledge of quantum chemical effects. Within this context, the present review critically elucidates the historical development of research on mass-independent isotope effects over the past four decades, with a particular emphasis on investigation in atmospheric chemistry. We underscore the paramount importance of a deeper mechanistic understanding of mass-independent isotope effects to effectively harness cutting-edge insights from isotope geochemistry in atmospheric chemistry research. With a focus on triple oxygen isotope compositions of oxygen-bearing molecules in the modern Earth's atmosphere, we critically identify and appraise key unresolved questions pertaining to mass-independent isotope effects. Addressing these questions in the future may allow new and exciting interdisciplinary discoveries, not solely confined to atmospheric chemistry but also encompassing the realms of earth and planetary sciences, given the existence of analogous intricacies in the highly complex isotope effects related to sulfur and other elements, both in the present and the deep past.