ABSTRACT.A sample with a radiocarbon concentration estimated to be greater than 10 5 times Modern was inadvertently graphitized and measured in the Xi'an AMS system last year. Both the sample preparation lines and the ion source system were seriously contaminated and a series of cleaning procedures were carried out to remove the contamination from them. After repeated and careful cleaning as well as continuous flushing with dead CO 2 gas, both systems have recovered from the contamination event. The machine background is back to 2.0 × 10 -16 and the chemical blank is beyond 50 kyr. CONTAMINATION LEVELSamples that contain high concentrations of radiocarbon ("hot" samples) are a catastrophe for a lowbackground accelerator mass spectrometer (AMS) laboratory. The memory effect induced by contamination in the sample preparation lines and/or the ion source is very difficult to eliminate. As a national AMS platform in China, the Xi'an AMS Center receives various samples from many differing groups. Though we do our best to inform our colleagues about the dangers of "hot" sample contamination to our multi-element AMS laboratory, a water sample (XA6007) was received last summer that proved to have extremely high 14 C content. The count rate of this "hot" sample was so high that the data acquisition system was grossly overloaded (the dead time was 100%). To make things worse, the data acquisition system unaccountably printed out a falsely low count rate of 968 cps (ratio of 14 C/ 12 C = 7.5 × 10 -14 ) for this sample, while an archaeological sample (XA6008), which immediately followed this "hot" sample, had a count rate of 10 times Modern. We were finally able to determine which sample was hot by lowering the 12 C beam current from microamperes to nanoamperes. Unfortunately, the hot sample was sputtered when in the ion source for 30 min total (this was the time required to complete its analysis). The average beam current in that analysis was ~30 A. The data from this test yielded an estimated 14 C concentration for the hot sample of greater than 10 5 times Modern! It is well known that about 20 years ago, both the LLNL (Vogel et al. 1990) and Arizona AMS laboratories (Jull et al. 1990) were exposed to 14 C contamination of 30,000 and 5000 Modern, respectively. Fortunately, these hot samples were discovered quickly and were only analyzed for 10 seconds and <2 min, respectively; thus, only their sample preparation lines and not their ion source system were affected by the "hot" samples. In our case, the ion source system was seriously contaminated, as were the sample preparation lines. In order to check the level of contamination in the ion source system, we remeasured 8 samples (Table 1), 5 of which were standard samples (sugar), 2 were blank samples (charcoal), and the last was a bone sample. All these samples were prepared, prior to the arrival of the "hot" sample, in uncontaminated sample preparation lines and pressed into holders using the same drill stem. Comparing the pre-and post-contamination results for these samp...
Carbon dioxide (CO 2 ) is the dominant greenhouse gas (GHG) species not only regarding the total anthropogenic emissions but also for its climatic impact, since it contributes roughly 80% of the total radiative forcing (Folberth et al., 2015). A significant part of a country's emissions likely originates from certain key areas, and results from a few representative cities can provide robust verification of reported emissions reductions over time at national or regional scale (Mckain et al., 2012). According to the most recent census bureau data (National Bureau of Statistics, NBS, 2021), China currently has 15 megacities with more than 10 million inhabitants and is among the top CO 2 emitters globally. Hence, China has pledged to reach its peak CO 2 emissions around 2030 and make best efforts to achieve this goal earlier. Given the different energy structures and levels of economic development across the country, China has delegated emission-reduction targets to the lower administrative units (Liu Abstract Identifying the sources of atmospheric Carbon dioxide (CO 2 ) is an important prerequisite for developing effective mitigation strategies. Here we conducted regular observations of the atmospheric CO 2 mixing ratio and its carbon isotope compositions (i.e., Δ 14 C and δ 13 C) in Xi'an and Beijing during winter, to estimate source contributions of CO 2 emissions in Chinese megacities. The results showed that CO 2 emissions in both Xi'an and Beijing originated mainly from fossil-fuel sources, which contributed 65 ± 3% and 82 ± 2% of the total CO 2 enhancement, respectively, during the sampling period; the results also revealed a substantial biogenic CO 2 contribution during winter. We further separated the fossil-fuel sources into contributions from coal, oil and natural gas combustions. We found that coal combustion was the dominant anthropogenic source in Xi'an, accounting for 54 ± 4% of the total fossil-fuel emissions, and oil and natural gas contribute almost equally to the emissions. In contrast, emission from natural-gas combustion was the main fossil-fuel source in Beijing, accounting for more than half of the total fossil-fuel emissions, whereas, coal combustion contributed only 17 ± 10%. These top-down results are generally consistent with emission inventory when seasonal variations of emissions are considered; some differences between the two methods indicated that the inventory for Xi'an might be underestimating the emissions from oil consumption. This pilot study confirms the potential of direct verification between top-down and bottom-up methods from the perspective of source attribution.Plain Language Summary Quantifying Carbon dioxide (CO 2 ) sources from cities is crucial for formulating policies and evaluating targets because cities have become the key region and basic unit for regulating emissions. Here, we take Xi'an and Beijing, two Chinese megacities, as the study case to conduct source attribution of atmospheric CO 2 based on the powerful tracer ability of dual carbon isotopes ( 14 C and 13 ...
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