Radiocarbon analysis of stratospheric CO2 retrieved from AirCore sampling

Radiocarbon (C-14) 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 approximate to 30 km) measurements of CH4 and CO2. In Europe, AirCore measurements have been being performed on a regular basis near Sodankyla (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 approximate to 35 mu g CO2 (approximate to 9.6 mu gC), and stored in a stratospheric air subsampler constructed from 1/4 in. coiled stainless steel tubing (approximate to 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 Delta(CO2)-C-14 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, Delta(CO2)-C-14 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