Abstract
Analysis systems incorporating atmospheric observations could provide a powerful tool for validating fossil fuel CO2 (ffCO2) emissions reported for individual regions, provided that fossil fuel sources can be separated from other CO2 sources or sinks and atmospheric transport can be accurately accounted for. We quantified ffCO2 by measuring radiocarbon (14C) in CO2, an accurate fossil-carbon tracer, at nine observation sites in California for three months in 2014–15. There is strong agreement between the measurements and ffCO2 simulated using a high-resolution atmospheric model and a spatiotemporally-resolved fossil fuel flux estimate. Inverse estimates of total in-state ffCO2 emissions are consistent with the California Air Resources Board’s reported ffCO2 emissions, providing tentative validation of California’s reported ffCO2 emissions in 2014–15. Continuing this prototype analysis system could provide critical independent evaluation of reported ffCO2 emissions and emissions reductions in California, and the system could be expanded to other, more data-poor regions.
Highlights
Recent work has shown there can be large differences in national fossil fuel CO2 emissions estimated by different groups [7, 8], and uncertainties in emissions are much larger at sub-national scales [9, 10]
Estimates of the spatial distribution of fossil fuel CO2 (ffCO2) emissions have been produced using data from large point sources such as power plants and allocating other emissions using proxy data such as population, road networks or light observed at night by satellites [10, 19, 20], which can be used for prior emissions estimates in top-down studies
Incorporating the observed and simulated ffCO2 into Bayesian inverse estimates of ffCO2 emissions following Fischer et al [27], we find that California in-state total ffCO2 emissions are 83.8 MtC yr−1 for May, 85.9 MtC yr−1 for October–November and 87.7 MtC yr−1 for January–February with 95% confidence intervals of ±13 to ±15 MtC yr−1 (table 1, figure 3(A))
Summary
We conducted a field study to observe ffCO2 with high spatial and temporal resolution over the statewide California region using radiocarbon (14C) as a fossil fuel tracer, and to use the observations in a top-down calculation of California’s ffCO2 emissions. Measurements of CO2 concentration and Δ14C in CO2 were conducted at nine existing observation sites across California and used to calculate ffCO2 with uncertainty of ±1–1.9 ppm (1-σ) (detailed methods, figure 1(A), table S1 available at stacks.iop.org/ERL/
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