Estimating regional fossil fuel CO2 concentrations from 14CO2 observations: challenges and uncertainties

Atmospheric 14CH4, 14CO2 and 37Ar measurements around a Swiss pressurized water reactor during an annual revision period

Since the 1980s, radiocarbon (14C) has gained attention as a valuable tool to quantify the amount of fossil and non-fossil emissions of CO2 and CH4 in the atmosphere. Since the 1970s, however, important 14C emissions in the atmosphere also occur through the operation of nuclear power plants. The limited knowledge about these emissions challenges the use of 14C as a universal source apportionment tool. Depending on the reactor type, 14C is emitted in different forms; in particular, pressurized water reactors emit 14C as a mixture of 14CH4 and 14CO2. However, few atmospheric 14C measurements close to nuclear power plants are available, which mostly address 14CO2 emissions. Argon-37 (37Ar) can also be produced in nuclear reactors; however, its atmospheric measurement is challenging, resulting in limited available data. In this study, we sampled ambient air during 20-75 min into 18 individual bags around the pressurized water reactor in Gösgen, Switzerland, at the beginning of the annual revision period in 2019, when 14C and 37Ar emissions can be expected due to the depressurization of the reactor. These samples were analyzed for 14CH4, 14CO2 and partly for 37Ar. About 1 km downwind of the stack, we found background-corrected activities up to 1900, 370, and 93 mBq m-3 respectively. Considering corresponding background activities of 0.3, 48 and 2 mBq m-3 for 14CH4, 14CO2, and 37Ar, this represents an excess of about 6300, 7.4, and 47 times, respectively. Using an atmospheric dispersion model, we satisfactorily simulated the 14CH4 and 14CO2 activities in the surroundings of the reactor during this event. Our measurements emphasize the importance of nuclear power plants in the interpretation of atmospheric 14C measurements and show that pressurized water reactors represent a serious limitation in the use of 14C for source apportionment of CH4 sources. Our results also provide insights into the approximate magnitude of civilian 37Ar emissions from nuclear facilities specifically during maintenance operations.

Mid-infrared trace detection with parts-per-quadrillion quantitation accuracy: Expanding frontiers of radiocarbon sensing

Detection sensitivity is a critical characteristic to consider during selection of spectroscopic techniques. However, high sensitivity alone is insufficient for spectroscopic measurements in spectrally congested regions. Two-color cavity ringdown spectroscopy (2C-CRDS), based on intra-cavity pump-probe detection, simultaneously achieves high detection sensitivity and selectivity. This combination enables mid-infrared detection of radiocarbon dioxide ([Formula: see text]CO[Formula: see text]) molecules in room-temperature CO[Formula: see text] samples, with 1.4 parts-per-quadrillion (ppq, 10[Formula: see text]) sensitivity (average measurement precision) and 4.6-ppq quantitation accuracy (average calibrated measurement error for 21 samples from four separate trials) demonstrated on samples with [Formula: see text]C/C up to [Formula: see text]1.5[Formula: see text] natural abundance ([Formula: see text]1,800 ppq). These highly reproducible measurements, which are the most sensitive and quantitatively accurate in the mid-infrared, are accomplished despite the presence of orders-of-magnitude stronger, one-photon signals from other CO[Formula: see text] isotopologues. This is a major achievement in laser spectroscopy. A room-temperature-operated, compact, and low-cost 2C-CRDS sensor for [Formula: see text]CO[Formula: see text] benefits a wide range of scientific fields that utilize [Formula: see text]C for dating and isotope tracing, most notably atmospheric [Formula: see text]CO[Formula: see text] monitoring to track CO[Formula: see text] emissions from fossil fuels. The 2C-CRDS technique significantly enhances the general utility of high-resolution mid-infrared detection for analytical measurements and fundamental chemical dynamics studies.

A special issue preface: Radiocarbon in the Anthropocene

The Anthropocene is defined by marked acceleration in human-induced perturbations to the Earth system. Anthropogenic emissions of CO2 and other greenhouse gases to the atmosphere and attendant changes to the global carbon cycle are among the most profound and pervasive of these perturbations. Determining the magnitude, nature and pace of these carbon cycle changes is crucial for understanding the future climate that ecosystems and humanity will experience and need to respond to. This special issue illustrates the value of radiocarbon as a tool to shed important light on the nature, magnitude and pace of carbon cycle change. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.