14C-AMS measurements in modern tree rings to trace local fossil fuel-derived CO2 in the greater Xi'an area, China

Tracing fossil fuel CO2 by 14C in maize leaves in Guanzhong Basin of China

Quantifying fossil fuel CO₂ (CO_2ff) in the atmosphere provides a benchmark method to monitor anthropogenic carbon emissions. Radiocarbon (¹⁴C) in atmospheric CO_2ff has been widely studied using the ¹⁴C in plants to document regional CO_2ff patterns. However, annual CO_2ff variations, reflecting spatial distributions based on plant samples, are still scarce. In this paper, the spatial distribution and temporal CO_2ff changes in the Guanzhong Basin is established using Δ¹⁴C measurements from maize leaves collected in 2011 and 2012. With regard to spatial distribution, samples collected around Xi'an City showed lower Δ¹⁴C values (more CO_2ff), while sites located near the perimeter of the basin showed higher Δ¹⁴C values (less CO_2ff). This is likely due to the concentration of anthropogenic activities in the center of the Guanzhong Basin. The observed CO_2ff mole fractions generally matched with PKU CO₂ inventory and the ODIAC CO₂ inventory data in the spatial distribution trend. However, it seems that thermal power plants were not well captured by the PKU CO₂ inventory. Our results provide useful information for the improvement of the inventory and verification of regional carbon cycle models.

Radiocarbon in the atmospheric gases and PM10 aerosol around the Paks Nuclear Power Plant, Hungary

Our study shows a one-year-long, monthly integrated continuous monitoring campaign of gaseous radiocarbon emission and ambient air compared with 4 event-like, weekly (168 h) atmospheric aerosol radiocarbon data in every season of 2019, at 4 locations (n = 16 aerosol sample) around the Paks Nuclear Power Plant, Hungary. The study shows the first aerosol radiocarbon results around a nuclear power plant measured by accelerator mass spectrometry in Hungary. There was no dominant contribution detected in the atmospheric CO₂ gas fraction, but we could detect excess radiocarbon in the total gaseous carbon fraction at almost every sampling point around the Paks Nuclear Power Plant. The highest Δ¹⁴C value in the total gaseous carbon form was 157.9 ± 4.6‰ in November and the highest Δ ¹⁴C value in the CO₂ fraction was 86.1 ± 4.0‰ in December during 2019. Observed ¹⁴C activity excess is not higher than previously published values around the Paks Nuclear Power plant at the same sampling points (Molnár et al., 2007; Varga et al., 2020). Our aerosol radiocarbon measurements show that there is no significant contribution from the nuclear power plant to the atmospheric PM₁₀ fraction. We could not detect a Δ ¹⁴C value higher than 0‰ in any season. The results show that the simple aerosol sampling, without pre-treatment of the filters, is appropriate for the measurement of excess radiocarbon at the vicinity of nuclear power plants. The applied preparation and measurement method can be applicable for detection of hot (¹⁴C) particles and early identification of radiocarbon emission from nuclear power plants in the PM₁₀ fraction.

Fabricating intramolecular donor-acceptor system via covalent bonding of carbazole to carbon nitride for excellent photocatalytic performance towards CO2 conversion

Photocatalytic conversion of CO₂ into hydrocarbon fuels is an ideal technology of mitigating greenhouse effect caused by excessive emission of CO₂. However, the high recombination rate of electron-hole pairs and limited charge carriers transport speed constrained the catalytic performance of many semiconductor catalysts. In this contribution, a series of carbon nitride (g-CN) samples with intramolecular donor-acceptor (D-A) system were successfully prepared by introducing organic donor into their structures. Characterization results confirmed that carbazole was successful connected to the structure of g-CN via chemical bond. The formation of intramolecular D-A system greatly enlarged the light response region of g-CN-xDbc. In addition, a new charge transfer transition mode was formed in g-CN-0.01Dbc due to the incorporation carbazole, which enable it to use light with energy lower than the intrinsic absorption of g-CN. Meanwhile, the D-A structure led to the spatial separation of electrons and holes in g-CN-xDbc and significantly decreased the recombination rate of electron-hole pairs. The g-CN-0.01Dbc presented the best catalytic performance and the CO evolution rate was 9.6 times higher than that of g-CN. Moreover, the reaction was performed in water without any additive, which made it green and sustainable. DFT simulation confirmed the D-A structure and charge carrier migration direction in the prepared samples.

Radiocarbon bomb-peak signal in tree-rings from the tropical Andes register low latitude atmospheric dynamics in the Southern Hemisphere

South American tropical climate is strongly related to the tropical low-pressure belt associated with the South American monsoon system. Despite its central societal role as a modulating agent of rainfall in tropical South America, its long-term dynamical variability is still poorly understood. Here we combine a new (and world's highest) tree-ring ¹⁴C record from the Altiplano plateau in the central Andes with other ¹⁴C records from the Southern Hemisphere during the second half of the 20th century in order to elucidate the latitudinal gradients associated with the dissemination of the bomb ¹⁴C signal. Our tree-ring ¹⁴C record faithfully captured the bomb signal of the 1960's with an excellent match to atmospheric ¹⁴C measured in New Zealand but with significant differences with a recent record from Southeast Brazil located at almost equal latitude. These results imply that the spreading of the bomb signal throughout the Southern Hemisphere was a complex process that depended on atmospheric dynamics and surface topography generating reversals on the expected north-south gradient in certain years. We applied air-parcel modeling based on climate data to disentangle their different geographical provenances and their preformed (reservoir affected) radiocarbon content. We found that air parcel trajectories arriving at the Altiplano during the bomb period were sourced i) from the boundary layer in contact with the Pacific Ocean (41%), ii) from the upper troposphere (air above the boundary layer, with no contact with oceanic or continental carbon reservoirs) (38%) and iii) from the Amazon basin (21%). Based on these results we estimated the ∆¹⁴C endmember values for the different carbon reservoirs affecting our record which suggest that the Amazon basin biospheric ¹⁴C isoflux could have been reversed from negative to positive as early as the beginning of the 1970's. This would imply a much faster carbon turnover rate in the Amazon than previously modelled.

Accuracy Improvement of the 14C Method Applied in Biomass and Coal Co-Firing Power Stations

The 14C method is an approach used to determine the proportion of carbon derived from biomass and fossil fuel in the co-fired flue gas. Its accuracy is mainly limited by the deviations between the applied biomass fuels’ 14C activity reference value and virtual value. To enrich the theoretical basis of the 14C method when applied to a Chinese biomass and coal co-firing power station, this study performed field sampling experiments and established a new evaluation method based on domestic literature. Unlike previous studies, this study revealed that the 14C activity of biomass far away from fossil carbon sources was 0.7–1.3 pMC lower than the local atmosphere. The 14C activity laws between tree rings and barks, specifically between eucalyptus bark and poplar bark were different, due to different growth models and different bark regeneration cycles, respectively. According to the test results and renewal conclusions, this study proposed a reasonable idea for constructing the prediction equation of referential biomass fuels’ 14C activity. Following this equation, the biomass fuels’ 14C activities of biomass direct-fired power stations at different Chinese cities were obtained.