Allocation of Terrestrial Carbon Sources Using ¹⁴CO₂: Methods, Measurement, and Modeling

Abstract

The radiocarbon content of whole air provides a theoretically ideal and now observationally proven tracer for recently added fossil-fuel-derived CO2 in the atmosphere (Cff). Over large industrialized land areas, determination of Cff also constrains the change in CO2 due to uptake and release by the terrestrial biosphere. Here, we review the development of a Δ14CO2 measurement program and its implementation within the US portion of the NOAA Global Monitoring Division's air sampling network. The Δ14CO2 measurement repeatability is evaluated based on surveillance cylinders of whole air and equates to a Cff detection limit of <0.9 ppm from measurement uncertainties alone. We also attempt to quantify additional sources of uncertainty arising from non-fossil terms in the atmospheric 14CO2 budget and from uncertainties in the composition of “background” air against which Cff enhancements occur. As an example of how we apply the measurements, we present estimates of the boundary layer enhancements of Cff and Cbio using observations obtained from vertical airborne sampling profiles off of the northeastern US. We also present an updated time series of measurements from NOAA GMD's Niwot Ridge site at 3475 m asl in Colorado in order to characterize recent Δ14CO2 variability in the well-mixed free troposphere.

Observations of 14CO2 in ecosystem respiration from a temperate deciduous forest in Northern Wisconsin

“…For comparison, the mean atmospheric Δ 14 CO 2 at LEF from June to August 2012 was 31.8‰, which by subtraction gives Δ 14 C disequilibria with respect to atmosphere of 90‰ and 10‰ for the June and August nights, respectively. This results in a mean age of R eco for June of 16–19 years, and for August of 1–4 years, based on measurements of tropospheric 14 CO 2 for the appropriate Northern Hemisphere zone [ Hua , ] and from Niwot Ridge [ Lehman et al ., ]. (See methods for age determination in the supporting information.)…”

“…Whole air process control samples with CO 2 concentrations and Δ 14 C near ambient levels were collected from high‐pressure cylinders in individual extraction aliquots as described by Lehman et al . [] or, in the case of extractions performed at CAMS, from PFPs filled from cylinders. The control gases extracted at CAMS, which are identified as LARS1 and LARS2, had a standard deviation of 3.4‰ Δ 14 C ( N = 23, Table S1 in the supporting information).…”

Constraints on emissions of carbon monoxide, methane, and a suite of hydrocarbons in the Colorado Front Range using observations of <sup>14</sup>CO<sub>2</sub>

“…The background is estimated by applying a smoothing algorithm (Thoning et al, 1989) to the NWR data (a curve-fit of 3 polynomials, 4 harmonics, and added low-pass filtered residuals), after filtering out samples influenced by upslope flows carrying locally influenced air, characterized by high CO/CO 2 ratios, as in Turnbull et al (2007). Smoothed NWR results used here are from Lehman et al (2013). The standard deviation of the residuals from the smoothing fit are calculated to be 1.7‰ .…”

“…While 14 CO 2 is produced naturally in the upper atmosphere from cosmogenic radiation, the abundance of 14 CO 2 in the modern atmosphere was strongly impacted by above-ground nuclear testing that occurred in the middle part of the 20th century. Since the atmospheric nuclear weapons test ban was put in place, the decrease in 14 CO 2 , which has been observed at a number of global background monitoring sites (Levin and Kromer, 2004;Turnbull et al, 2007;Currie et al, 2011;Graven et al, 2012a, b;Lehman et al, 2013), has been influenced primarily by the exchange of atmospheric 14 CO 2 with the oceanic and terrestrial carbon reservoirs. In recent years, however, the atmospheric decline has been increasingly influenced by isotopic dilution due to the Seuss effect, as fossil fuel combustion increases and as the atmosphere, ocean, and terrestrial carbon reservoirs approach equilibrium with the "bomb spike".…”

Strong regional atmospheric 14C signature of respired CO2 observed from a tall tower over the midwestern United States

“…Over industrialized continental regions, depletions in the ratio of radiocarbon to total carbon in atmospheric CO 2 ( 14 CO 2 , expressed as Δ 14 CO 2 ) are driven largely by geographic gradients in fossil fuel CO 2 emissions [ Djuricin et al ., ; Gamnitzer et al ., ; Hsueh et al ., ; Lehman et al ., ; Levin et al , , , ; Miller et al ., ; Randerson et al ., ; Riley et al ., ; Turnbull et al ., , ; Vay et al ., ; Vogel et al ., , ]. As a consequence, a growing number of studies have used measurements of Δ 14 CO 2 in the troposphere to derive mole fractions of recently added fossil CO 2 (C ff ) while recognizing the need to correct for what are usually modest contributions to the 14 CO 2 budget from nonfossil sources.…”