This paper presents a compilation of atmospheric radiocarbon for the period 1950–2019, derived from atmospheric CO₂ sampling and tree rings from clean-air sites. Following the approach taken by Hua et al. (2013), our revised and extended compilation consists of zonal, hemispheric and global radiocarbon (¹⁴C) data sets, with monthly data sets for 5 zones (Northern Hemisphere zones 1, 2, and 3, and Southern Hemisphere zones 3 and 1–2). Our new compilation includes smooth curves for zonal data sets that are more suitable for dating applications than the previous approach based on simple averaging. Our new radiocarbon dataset is intended to help facilitate the use of atmospheric bomb ¹⁴C in carbon cycle studies and to accommodate increasing demand for accurate dating of recent (post-1950) terrestrial samples.

A series of ¹⁴ C measurements in Ocean Drilling Program cores from the tropical Cariaco Basin, which have been correlated to the annual-layer counted chronology for the Greenland Ice Sheet Project 2 (GISP2) ice core, provides a high-resolution calibration of the radiocarbon time scale back to 50,000 years before the present. Independent radiometric dating of events correlated to GISP2 suggests that the calibration is accurate. Reconstructed ¹⁴ C activities varied substantially during the last glacial period, including sharp peaks synchronous with the Laschamp and Mono Lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments. Simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in ¹⁴ C production rate together with plausible changes in deep-ocean ventilation and the global carbon cycle during glaciation.

Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of greenhouse gases (GHGs), we developed the first comprehensive monitoring systems of CO₂ emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012–2013 dormant season showed a statistically significant increase of about 20% (from 4.5 to 5.7 MtC ± 0.23 MtC) compared to the Hestia CO₂ emission estimate, a state‐of‐the‐art building‐level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to affect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO₂ boundary inflow and to the different prior emissions (i.e., ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the uncertainties associated with undetermined parameters using an ensemble of inversions. The total CO₂ emissions based on the ensemble mean and quartiles (5.26–5.91 MtC) were statistically different compared to the prior total emissions (4.1 to 4.5 MtC). Considering the relatively small sensitivity to the different parameters, we conclude that atmospheric inversions are potentially able to constrain the carbon budget of the city, assuming sufficient data to measure the inflow of GHG over the city, but additional information on prior emission error structures are required to determine the spatial structures of urban emissions at high resolution.

The Indianapolis Flux Experiment (INFLUX) aims to develop and assess methods for quantifying urban greenhouse gas emissions. Here we use CO₂, ¹⁴CO₂, and CO measurements from tall towers around Indianapolis, USA, to determine urban total CO₂, the fossil fuel derived CO₂ component (CO₂ff), and CO enhancements relative to background measurements. When a local background directly upwind of the urban area is used, the wintertime total CO₂ enhancement over Indianapolis can be entirely explained by urban CO₂ff emissions. Conversely, when a continental background is used, CO₂ff enhancements are larger and account for only half the total CO₂ enhancement, effectively representing the combined CO₂ff enhancement from Indianapolis and the wider region. In summer, we find that diurnal variability in both background CO₂ mole fraction and covarying vertical mixing makes it difficult to use a simple upwind‐downwind difference for a reliable determination of total CO₂ urban enhancement. We use characteristic CO₂ff source sector CO:CO₂ff emission ratios to examine the contribution of the CO₂ff source sectors to total CO₂ff emissions. This method is strongly sensitive to the mobile sector, which produces most CO. We show that the inventory‐based emission product (“bottom up”) and atmospheric observations (“top down”) can be directly compared throughout the diurnal cycle using this ratio method. For Indianapolis, the top‐down observations are consistent with the bottom‐up Hestia data product emission sector patterns for most of the diurnal cycle but disagree during the nighttime hours. Further examination of both the top‐down and bottom‐up assumptions is needed to assess the exact cause of the discrepancy.

We use the nearly ideal tracer ¹⁴CO₂ to estimate the fossil fuel CO₂ enhancement in boundary layer air at two sites in New England and Colorado. Improved Δ¹⁴C measurement precision of 1.6–2.6‰ provides fossil fuel CO₂ detection capability of 0.8–1.5 ppm. Using the indirect tracers CO and SF₆, we obtain two additional independent estimates of the fossil fuel CO₂ component, and we assess the biases in these methods with respect to the ¹⁴CO₂‐based estimates. The SF₆‐based estimates vary considerably from the ¹⁴CO₂‐based estimates, and are at times implausibly large. The CO‐based estimates are less variable, but show seasonally coherent biases with respect to the ¹⁴CO₂ method.