Estimation of continuous anthropogenic CO 2 using CO 2, CO, δ 13 C(CO 2) and Δ 14 C(CO 2)

Abstract

We investigate different methods for estimating anthropogenic CO 2 using modelled 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 Δ 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 feasible at rural measurement sites due to the low signal to noise ratio of anthropogenic CO 2 estimates at such settings. At urban and polluted sites, potential future continuous Δ 14 C(CO 2) measurements with a precision of 5 ‰ or better are most promising for anthropogenic CO 2 determination (precision ca. 10–20%), but the insensitivity against CO 2 contributions from biofuel emissions may reduce its accuracy in the future. Other tracers, such as δ 13 C(CO 2) and CO could provide an accurate and already available alternative if all CO 2 sources in the catchment area are well characterized with respect to their isotopic signature and CO to anthropogenic CO 2 ratio. We suggest a strategy for calibrating these source characteristics on an annual basis using precise Δ 14 C(CO 2) measurements on grab samples. The precision of anthropogenic CO 2 determination using δ 13 C(CO 2) is largely determined by the measurement precision of δ 13 C(CO 2) and CO 2. The precision when using the CO-method is mainly limited by the variation of natural CO sources and CO 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. This allows significant improvement, validation and bias reduction of highly resolved emission inventories using atmospheric observation and regional modelling.