Observations of radiocarbon (14C) in Earth’s atmosphere and other carbon reservoirs are important to quantify exchanges of CO2 between reservoirs. The amount of 14C is commonly reported in the so-called Delta notation, i.e., Δ14C, the decay- and fractionation-corrected departure of the ratio of 14C to total C from that ratio in an absolute international standard; this Delta notation permits direct comparison of 14C/C ratios in the several reservoirs. However, as Δ14C of atmospheric CO2, Δ14CO2 is based on the ratio of 14CO2 to total atmospheric CO2, its value can and does change not just because of change in the amount of atmospheric14CO2 but also because of change in the amount of total atmospheric CO2, complicating ascription of change in Δ14CO2 to change in one or the other quantity. Here we suggest that presentation of atmospheric 14CO2 amount as mole fraction relative to dry air (moles of 14CO2 per moles of dry air in Earth’s atmosphere), or as moles or molecules of 14CO2 in Earth’s atmosphere, all readily calculated from Δ14CO2 and the amount of atmospheric CO2 (with slight dependence on δ13CO2), complements presentation only as Δ14CO2, and can provide valuable insight into the evolving budget and distribution of atmospheric 14CO2.

14CO2 activity in air samples collected at Kakrapar Gujarat Site, India, was measured, and site-specific dilution factor for 14CO2 has been evaluated. 14CO2 activity in air samples was monitored for 72 different sampling events at onsite stack of Nuclear Power Plant (NPP) and at ESL meteorology laboratory (at 1.6 km from NPP stack). 14CO2 activity in air at stack of NPP and at ESL meteorology laboratory was observed to 0.10–0.18 TBq (GWe.year)–1, with mean value 0.12 TBq (GWe.year)–1 and ≤0.04–0.13 Bq m–3, with mean value 0.08 Bq m–3 respectively. The results were correlated with meteorological parameters. Site specific dilution factor for 14CO2 in air was evaluated at 1.6 km and was found to be in the range of 4.6E-05 to 21E-05 s m–3. Inter angle (degree) between plume direction and fixed sampling location and rainfall (mm) are found to be the important influencing parameters for dilution factor of 14CO2 in air.

The extent of 14C-imazaquin and 14C-imazethapyr abiotic vs. biotic degradation in soil was investigated. Degradation was measured in an in vitro system which allowed 90% recovery of applied herbicide. Triallate biodegradation is well documented and therefore used as a standard. Herbicide degradation was compared in two soils, a Cisne silt loam and a Drummer silty clay loam. Herbicide degradation in gamma-irradiated soil was compared to fresh soil. Biomass quantities were measured for the duration of the experiments. 14CO2 evolution, extractable parent, metabolites, and unextractable residue were measured. After 12 weeks of incubation, 95% of the radioactivity could be extracted as parent from sterilized soil. In unsterilized soil, imazaquin and imazethapyr degraded at a similar rate which was dependent upon soil type. All herbicides degraded slower in the Drummer soil and triallate degraded two to three times faster than the imidazolinones in either soil. 14C-imazaquin degradation products included 14CO2 and unextractable residues. The major product from 14C-imazethapyr degradation was 14CO2. Evolution of 14CO2 from an imazethapyr-treated Cisne soil, containing a serial dilution of activated charcoal, demonstrated that adsorption of herbicide was negatively correlated with degradation. Therefore imidazolinone microbial degradation is regulated by the amount of herbicide in soil solution as determined by soil characteristics.