Estimation of continuous anthropogenic CO<sub>2</sub>: model-based evaluation of CO<sub>2</sub>, CO, δ<sup>13</sup>C(CO<sub>2</sub>) and Δ<sup>14</sup>C(CO<sub>2</sub>) tracer methods

Abstract

Abstract. We investigate different methods for estimating anthropogenic CO 2 using modeled continuous atmospheric concentrations of CO 2 alone, as well as CO 2 in combination with the surrogate tracers CO, δ 13 C(CO 2 ) and 14 C(CO 2 ). These methods are applied at three hypothetical stations representing rural, urban and polluted conditions. We find that, independent of the tracer used, an observation-based estimate of continuous anthropogenic CO 2 is not yet feasible at rural measurement sites due to the low signal-to-noise ratio of anthropogenic CO 2 estimates at such settings. The tracers δ 13 C(CO 2 ) and CO provide an accurate possibility to determine anthropogenic CO 2 continuously, only if all CO 2 sources in the catchment area are well characterized or calibrated with respect to their isotopic signature and CO to anthropogenic CO 2 ratio. We test different calibration strategies for the mean isotopic signature and CO to CO 2 ratio using precise 14 C(CO 2 ) measurements on monthly integrated as well as on grab samples. For δ 13 C(CO 2 ), a calibration with annually averaged 14 C(CO 2 ) grab samples is most promising, since integrated sampling introduces large biases into anthropogenic CO 2 estimates. For CO, these biases are smaller. The precision of continuous anthropogenic CO 2 determination using δ 13 C(CO 2 ) depends on measurement precision of δ 13 C(CO 2 ) and CO 2 , while the CO method is mainly limited by the variation in natural CO sources and sinks. At present, continuous anthropogenic CO 2 could be determined using the tracers δ 13 C(CO 2 ) and/or CO with a precision of about 30 %, a mean bias of about 10 % and without significant diurnal discrepancies. Hypothetical future measurements of continuous 14 C(CO 2 ) with a precision of 5 ‰ are promising for anthropogenic CO 2 determination (precision ca. 10-20 %) but are not yet available. The investigated tracer-based approaches open the door to improving, validating and reducing biases of highly resolved emission inventories using atmospheric observation and regional modeling.

A CO-based method to determine the regional biospheric signal in atmospheric CO₂

“…Given both increasing and increasingly uncertain anthropogenic emissions (Ballantyne et al, 2015), this method might also provide an approach complementing the 14 CO 2 -based method of investigating CO 2, A (Gamnitzer et al, 2006;Vardag et al, 2015). As also pointed out in our study, the approach has some caveats such as the uncertainty associated with the secondary production of CO from VOC oxidation, which should be investigated in more detail.…”

Evaluation of 4 years of continuous <i>δ</i><sup>13</sup>C(CO<sub>2</sub>) data using a moving Keeling plot method

“…However, the disadvantage of using such a bottom-up approach is that usually only information from a few specific sites are available, which need then to be upscaled correctly such that they are representative of the entire catchment area. For a determination of δ F during the entire year, one can possibly utilize 14 C(CO 2 ) and CO/CO 2 measurements (following Vardag et al (2015b)) or O 2 /N 2 measurements (e.g., Sturm et al 2006;Steinbach et al, 2011), all of which exhibit their own deficiencies, which are discussed elsewhere (e.g., Ciais et al, 2015;Vardag et al, 2015b).…”

“…An uncertainty of ±2.5 ‰ for δ F is rather large if we want to use this observation-based top-down result for further quantitative source apportionment. Vardag et al (2015b) showed that a misassignment of 2.5 ‰ in δ F leads to a bias in the continuous fuel CO 2 estimate of about 15 % for an urban measurement site like Heidelberg. The observation-based biospheric end member δ bio has an uncertainty of only about 1.0 ‰ in Figure 5.…”

“…For example, Vardag et al (2015b) used the isotopic signatures of δ bio (assumed to be known within a fixed uncertainty) and δ F (obtained by calibration with 14 C(CO 2 )) to calculate the fossil fuel CO 2 contribution from the (continuously) measured CO 2 and δ 13 C(CO 2 ) signal. However, knowing the isotopic signatures δ bio and δ F over the entire course of the year requires an extensive number of measurements at the relevant sources throughout the year and further assumptions of how to scale up these point measurements to a mean source signature of all relevant sources.…”

Bayesian Optimization of the Community Land Model Simulated Biosphere–Atmosphere Exchange using CO2 Observations from a Dense Tower Network and Aircraft Campaigns over Oregon

“…The fossil fuel offsets were removed using the CO:CO 2 ratios from road traffic sources after removing a baseline from the observations. The use of CO as a combustion tracer and for inverse model studies was successfully demonstrated in previous studies (e.g., Gerbig et al 2003;Vardag et al 2015). In contrast to the four other observation towers that are located in rural and remote regions, regional and local anthropogenic emissions of CO 2 significantly affect the measured mixing ratios at the OSI tower that represents the large Willamette valley ecoregion.…”