Conference Agenda

The clumped isotopic composition (Δ¹³CH₃D and Δ¹²CH₂D₂) of methane (CH₄) has recently been developed as additional tracers to constrain atmospheric CH₄ sources and sinks. Atmospheric CH₄ levels are controlled by a complex interplay of natural and anthropogenic sources, atmospheric OH levels, and other sink reactions. Understanding the contribution of each of these factors is crucial for a comprehensive understanding of the global CH₄ cycle.

Technical advancements in low-concentration sample extraction now facilitate direct measurements of the Δ¹³CH₃D and Δ¹²CH₂D₂ of CH₄ from ambient air. The first such measurements reveal distinct signatures of approximately 1 ± 0.3 ‰ for Δ¹³CH₃D and 44 ± 3 ‰ for Δ¹²CH₂D₂, with Δ¹²CH₂D₂ being higher than in all major source categories.

However, these measurements do not align with existing model predictions, highlighting significant knowledge gaps in the known clumped isotopic composition of CH₄ sources and the kinetic isotopic fractionation associated with sinks. Although the number of source-specific measurements remains limited, these discrepancies are unlikely to be attributed only to under-sampled sources. Instead, uncertainties in sink fractionation are more likely to explain the observed differences. However, precisely quantifying the fractionation effects in CH₄ sinks remains a challenge.

To further investigate this discrepancy, we reconstructed the first historical record of atmospheric Δ¹³CH₃D and Δ¹²CH₂D₂ using firn air samples collected at East-GRIP. Our measurements show an unexpected increase of 10 ‰ in Δ¹²CH₂D₂ between 1993 and 2018. This dataset constrains a two-box atmospheric model of the five most abundant CH₄ isotopologues. Additionally, the optimized model assesses the influence of various sources and sinks on the clumped isotopic composition of CH₄ over time while also providing future projections.

Currently, we are working on expanding the database of the clumped isotopic composition of major CH₄ sources and measuring ambient air under different conditions to further constrain the clumped isotope budget.

The triple isotope composition (Δ¹⁷O) of atmospheric oxygen (O₂) has been proposed as a tracer of the global (land + ocean) biosphere productivity. Yet, the Δ¹⁷O of tropospheric O₂ is determined by both the productivity of the biosphere (i.e., photosynthesis) and stratospheric photochemistry (Luz et al., 1999). Previous studies reconstructed the global biosphere productivity from measurements of Δ¹⁷O in O₂ from ice cores. They reported a reduced global biosphere productivity during Last Glacial Maximum (LGM) compared to the pre-industrial period (Blunier et al., 2002, 2012; Landais et al., 2007; Yang et al., 2022). However, our present understanding of isotope fractionation effects in the past stratosphere is limited. For example, climate-chemistry model experiments suggest that the LGM stratosphere was warmer, had reduced ozone levels, and experienced variable stratosphere-troposphere exchange (Wang et al., 2020; Fu et al., 2020). These changes could have altered oxygen isotope fractionation, which could, in turn, influence estimates of global productivity.

To address this issue, we conducted numerical experiments using a box model describing the Δ¹⁷O budget of atmospheric O₂ and CO₂ under LGM boundary conditions. We explore how Δ¹⁷O of O₂ is sensitive to factors such as stratospheric temperatures, concentrations of CO₂ and O₃, and the stratosphere-troposphere exchange flux. Preliminary results indicate that an LGM-like stratosphere reduces the magnitude of Δ¹⁷O depletion in stratospheric O₂, which implies a global biosphere productivity lower than today. This illustrates that, if our current estimates of the past global biosphere productivity are to be refined, remaining uncertainties associated with past stratospheric photochemistry and isotope fractionation by biological/hydrological processes should be addressed and reduced.

Keywords: Triple isotope composition, biosphere productivity, Last Glacial Maximum, Stratosphere

Cities are major contributors to global anthropogenic carbon dioxide (CO₂) emissions. Accurately quantifying urban emissions at the local scale is therefore important for climate change mitigation. Traditional methods, such as emission inventories, often face significant uncertainties at the city-level, emphasizing the potential need for independent, observation-based approaches. Measurements of atmospheric CO₂ concentrations and the respective stable isotopic composition (δ¹³C) can characterize and quantify emissions from different sources. Within the Vienna Urban Carbon Laboratory project, we installed a cavity ring-down laser isotope spectrometer (Picarro G2131-i) on the Arsenal Radio Tower in Vienna. Since May 2022, this setup delivers continuous, high-resolution measurements of CO₂ mixing ratios and δ¹³C-CO₂ 144 meters above ground level. Diel and seasonal variations in these parameters reveal distinct source contributions. In summer, increase in δ¹³C coupled with daytime CO₂ reductions indicates strong photosynthetic activity. In autumn, elevated CO₂ levels and , decrease in δ¹³C suggest increased contributions from ecosystem respiration. Bayesian inverse analyses of CO₂ sources shows a consistent contribution from gasoline combustion throughout the year. Emissions from natural gas combustion is particularly high during winter when winds originate from the east and southeast, which can be linked to heating systems in buildings and power plants. Our contribution will also present results from footprint analyses using the Stochastic Time-Inverted Lagrangian Transport (STILT) model, to infer the spatial distribution of emission sources.

The recent geological epoch, the Holocene, is assumed to have been climatically stable, though this is challenged by discrepancies between climate proxies for temperature and model results. Furthermore, trends of the greenhouse gas methane are not understood. Atmospheric oxidants control the atmospheric methane abundance, but variations of oxidants in the past are not known since they are not preserved in paleo-climate archives. We present measurements of the clumped isotopes of atmospheric O₂ (abundance of ¹⁸O¹⁸O denoted by ∆₃₆) extracted from a Greenland ice core, covering the Holocene and the late glacial period, to provide new insights into past variations of temperature and oxidant levels. In the glacial period ∆₃₆ was 0.07 ‰ higher than in the Late Holocene, attributed to low temperatures and a low tropospheric O₃ burden. Remarkably, ∆₃₆ shows pronounced millennial-scale variations over the Holocene, with mid-Holocene ∆₃₆ values being 0.06 ‰ lower than in the Late Holocene, and 0.03 ‰ below present-day conditions. Our analyses with an atmospheric chemistry-climate model and a box model suggest that the low ∆₃₆ values in the mid-Holocene can be explained by a combination of high oxidant levels and high upper tropospheric temperatures, potentially augmented by changes in stratosphere-troposphere transport. The millennial scale variability of ∆₃₆ matches the temporal evolution of CH₄, which suggests that the mid-Holocene minimum in CH₄ is largely driven by tropospheric oxidants. Our ∆₃₆ data suggest that key atmospheric features, notably oxidant levels and temperature, have varied significantly during the Holocene.