Abstract. In this decade understanding the impact of human activities on climate has been one of the key issues of discussion globally. In that respect, the continuous rise of the concentration of greenhouse gases, e.g., CO2, CH4, etc. in the atmosphere, predominantly due to human activities requires continuous monitoring to understand the dynamics. 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 collection of large air samples (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 of a long tube descending from a high altitude with one end open and the other closed, and 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 are 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). Each section was separately stored in a ¼ inch coiled stainless steel tubing (≈ 3m) for radiocarbon measurements. A small-volume extraction system was constructed which enabled ≈ 100% CO2 extraction from the stratospheric air samples. Also, a new small-volume high-efficiency graphitization system was constructed for graphitization of these extracted CO2 samples, which were later 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 to correct for contaminations. The results show that the Δ 14CO2 values for lower stratosphere up to about 18(± 1) km (first four samples from each profile) are very similar (10 ± 8‰) and represent the current tropospheric value. The next sample in each profile, corresponding to about 18(± 1)-22(± 2) km showed slight enrichment of 80 ± 20‰. The last section from one profile, corresponding to altitudes above 22(± 2), also showed enhanced Δ 14CO2 value of 79.1 ± 30‰. The last section from the other profile was spoiled during preparation.
Radiocarbon analysis of stratospheric CO<sub>2</sub> retrieved from AirCore sampling.
