Radiocarbon analysis of stratospheric CO₂ retrieved from AirCore sampling

Abstract. Radiocarbon (¹⁴C) is an important atmospheric tracer and one of the many used in the understanding of the global carbon budget, which includes the greenhouse gases CO₂ and CH₄. Measurement of radiocarbon in atmospheric CO₂ generally requires the collection of large air samples (a few liters) from which CO₂ is extracted and then the concentration of radiocarbon is determined using accelerator mass spectrometry (AMS). However, the regular collection of air samples from the stratosphere, for example using aircraft and balloons, is prohibitively expensive.

Here we describe radiocarbon measurements in stratospheric CO₂ collected by the AirCore sampling method. AirCore is an innovative atmospheric sampling system, which comprises a long tube descending from a high altitude with one end open and the other closed, and it has been demonstrated to be a reliable, cost-effective sampling system for high-altitude profile (up to  ≈  30 km) measurements of CH₄ and CO₂. In Europe, AirCore measurements have been being performed on a regular basis near Sodankylä (northern Finland) since September 2013. Here we describe the analysis of samples from two such AirCore flights made there in July 2014, for determining the radiocarbon concentration in stratospheric CO₂. The two AirCore profiles were collected on consecutive days. The stratospheric part of the AirCore was divided into six sections, each containing  ≈  35 µg CO₂ ( ≈  9.6 µgC), and stored in a stratospheric air subsampler constructed from 1∕4 in. coiled stainless steel tubing ( ≈  3 m). A small-volume extraction system was constructed that enabled  >  99.5 % CO₂ extraction from the stratospheric air samples. Additionally, a new small-volume high-efficiency graphitization system was constructed for graphitization of these extracted CO₂ samples, which were measured at the Groningen AMS facility. Since the stratospheric samples were very similar in mass, reference samples were also prepared in the same mass range for calibration and contamination correction purposes. The results show that the Δ¹⁴CO₂ values from tropopause up to about 19(±1) km for the sample collected on 15 July was 18 ± 6 ‰ (samples 1–4), very similar to the current tropospheric value. On the other hand, Δ¹⁴CO₂ values from tropopause up to about 18(±1) km for the sample collected on 16 July (samples 1–4) showed a large gradient from −62 to 21 ‰. The next sample in the profile, corresponding to about 18(±1)–22(±2) km (one sample from each profile), shows slight enrichment of 80 ± 20 ‰. The last section from both profiles, containing air from the upper stratosphere, was contaminated with pre-fill air.