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Tropospheric ozone (O3) is an important greenhouse gas and controls the oxidation capacity of the atmosphere. Over the past decades, it has also acted as an important pollutant and O3 concentrations regularly exceed health threshold values due to anthropogenic emissions of O3 precursors. Since O3 and other oxidants are unstable in any paleoclimate archive, variation in the oxidation capacity of the atmosphere in the past can only be studied indirectly. The abundance of atmospheric O2 with two heavy isotopes (e.g., 18O18O, expressed by ∆36) is a potential tracer for ozone photochemistry [1]. Yeung et al. [2] showed that ∆36 in ice core air O2 carries information about past tropospheric O3 levels. Here we present measurements of ∆36 in air O2 extracted from a Greenland ice core [3] covering the later part of the last glacial period and the Holocene, and present-day air to investigate variations of oxidant levels in the pre-industrial atmosphere. In the glacial period ∆36 was significantly higher than in the pre-industrial period due to lower glacial tropospheric temperatures and O3 burden. During the Holocene, we observed millennial-scale variability in ∆36 values with a significantly lower ∆36 values around the Mid Holocene. This implies considerable variability of oxidant levels over the Holocene, a period assumed to have experienced relatively stable climate. The temporal evolution of ∆36 matches qualitatively with the evolution of methane (CH4), a potent greenhouse gas, suggesting that the oxidative removal of CH4 is a key factor controlling the millennial-scale variability of CH4 over the Holocene. Simulations of ∆36 in the 3D atmospheric chemistry model EMAC [4] have been used to investigate the contribution of changing temperatures, oxidant precursors and atmospheric transport on oxidant levels in the different climate states.
[1] Laskar, A.H., Peethambaran, R., Adnew, G.A., & Roeckmann, T. (2019), Rapid Commun. Mass Spectrom. 33, 981-994.
[2] Yeung, L.Y., Murray, L.T., Martinerie, P. et al. (2019), Nature 570, 224–227.
[3] Dahl-Jensen, D., Albert, M. R., Aldahan, A. et al. (2013), Nature 493, 489-494.
[4] Gromov, S., Röckmann, T., Laskar, A.H. & Peethambaran, R. (2019). Geophysical Research Abstracts, 21, EGU2019-16938-1.
