The changes in atmospheric composition over the Anthropocene are quantified mainly from measurements of air inclusions in polar ice and firn. The natural and anthropogenic emissions inferred from these records inform model future predictions and policy decisions. However, the past rate of removal from the atmosphere for many gases is poorly constrained, limiting the understanding of past emissions and the ability to predict future climate change or stratospheric ozone levels under given emissions scenarios.

This ambitious project further explores the treasure of atmospheric trace gas information from Law Dome by quantifying changes in the oxidising capacity of the atmosphere over the industrial period. We measured the carbon-14 isotope of carbon monoxide (¹⁴CO), an integrating proxy of the concentrations of the hydroxyl radical (OH), the main tropospheric oxidant. To do this in ice, significant scientific and technical challenges were overcome using novel ice drilling, air extraction and analytical techniques developed over recent decades.

In summer 2018-19, a joint US and Australian team sampled and measured air from firn and ice at Law Dome DE08-OH. Trapped air was immediately extracted from the ice using an on-site large-volume ice melting system. Trace gases measured in the samples confirm ice core air ages spanning from the 1870s at 240 m depth to the early 2000s. Firn-air samples capture air from the early 2000s to present. Analyses of [CO] and halocarbons show a relatively low and stable procedural CO blank and demonstrate that the samples are unaffected by ambient air inclusion. ¹⁴CO analyses in these firn and ice core air samples are in progress at the time of this submission. The ¹⁴CO history will be interpreted with the aid of the GEOS-Chem chemistry-transport model to place the first observational constraints on the variability of Southern Hemisphere [OH] since ≈1870 AD.