Estimation of continuous anthropogenic CO2: model-based evaluation of CO2, CO, δ13C(CO2) and Δ14C(CO2) tracer methods

S. N. Vardag, C. Gerbig, G. Janssens-Maenhout, I. Levin
2015 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> We investigate different methods for estimating anthropogenic CO<sub>2</sub> using modeled continuous atmospheric concentrations of CO<sub>2</sub> alone, as well as CO<sub>2</sub> in combination with the surrogate tracers CO, &amp;delta;<sup>13</sup>C(CO<sub>2</sub>) and Δ<sup>14</sup>C(CO<sub>2</sub>). These methods are applied at three hypothetical stations representing rural, urban and polluted conditions. We find that, independent of the tracer used, an
more » ... ion-based estimate of continuous anthropogenic CO<sub>2</sub> is not yet feasible at rural measurement sites due to the low signal-to-noise ratio of anthropogenic CO<sub>2</sub> estimates at such settings. The tracers &amp;delta;<sup>13</sup>C(CO<sub>2</sub>) and CO provide an accurate possibility to determine anthropogenic CO<sub>2</sub> continuously, only if all CO<sub>2</sub> sources in the catchment area are well characterized or calibrated with respect to their isotopic signature and CO to anthropogenic CO<sub>2</sub> ratio. We test different calibration strategies for the mean isotopic signature and CO to CO<sub>2</sub> ratio using precise Δ<sup>14</sup>C(CO<sub>2</sub>) measurements on monthly integrated as well as on grab samples. For &amp;delta;<sup>13</sup>C(CO<sub>2</sub>), a calibration with annually averaged <sup>14</sup>C(CO<sub>2</sub>) grab samples is most promising, since integrated sampling introduces large biases into anthropogenic CO<sub>2</sub> estimates. For CO, these biases are smaller. The precision of continuous anthropogenic CO<sub>2</sub> determination using δ<sup>13</sup>C(CO<sub>2</sub>) depends on measurement precision of δ<sup>13</sup>C(CO<sub>2</sub>) and CO<sub>2</sub>, while the CO method is mainly limited by the variation in natural CO sources and sinks. At present, continuous anthropogenic CO<sub>2</sub> could be determined using the tracers δ<sup>13</sup>C(CO<sub>2</sub>) 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 Δ<sup>14</sup>C(CO<sub>2</sub>) with a precision of 5 &amp;permil; are promising for anthropogenic CO<sub>2</sub> 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.</p>
doi:10.5194/acp-15-12705-2015 fatcat:cdzatkk4vjbmfgk77wxb4bokji