Lawrence Berkeley National Laboratory Recent Work Title Assessing fossil fuel CO 2 emissions in California using atmospheric observations and models

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. Fossil fuel combustion is the primary cause of increasing atmospheric CO2 concentration and associated radiative forcing [1]. Over 1990–2014, CO2 emissions from fossil fuel combustion and cement production (ffCO2) are estimated to have increased by ∼60% globally [2] but by only ∼9% in the United States [2]. The 2015 Paris Agreement of the UN Framework Convention on Climate Change adopted greenhouse gas mitigation pledges from individual countries [3], and many other mitigation policies are being implemented on the subnational scale. California’s Global Warming Solutions Act of 2006 and subsequent policies set out to progressively reduce greenhouse gas emissions to meet targets for 2020 and 2030, with extensions planned for 2050. ‘Bottom-up’ estimates of CO2 emissions from fossil fuel combustion and cement production are based on calculations using data on fuel production and usage, carbon content of fuel, combustion efficiency of sources, and information on individual emitting activities [2, 4]. California’s in-state fossil fuel CO2 emissions in 2014–15 were 91 million tonnes of carbon per year (MtC yr−1), with another 26 MtC yr−1 emitted by out-of-state electricity production, the military, and interstate and international shipping and aviation [4]. Fossil fuel CO2 © 2018 The Author(s). Published by IOP Publishing Ltd Environ. Res. Lett. 13 (2018) 065007 Figure 1. Observations of CO2 and ffCO2 from the California network. Observations for each site shown by quartile using boxplots for (A) total CO2 concentration and (B) ffCO2 concentration. The three campaigns conducted in 2014–15 are shown in different colors. Sites are ordered vertically according to location. The full range for total CO2 at CIT in January–February is 409–476 ppm. (C) Annual mean ffCO2 emissions from Vulcan v2.2(19) within the US for 2002 and from EDGAR v4.2 FT2010(29) outside the US for 2008. The sites in the observation network are shown as circles on the map: Trinidad Head (THD), Sutter Buttes (STB), Walnut Grove (WGC), Sutro (STR), Sandia-Livermore (LVR), Victorville (VTR), San Bernardino (SBC), Caltech (CIT) and Scripps Inst. Oceanography (SIO). Lines show the boundaries of the 16 subregions of California used in the inversion, with four major regions labeled. Individual observations conducted approximately every 3 d are shown in (D)–(G) for the October–November campaign. Total CO2 concentration is shown in (D) and (E) and ffCO2 concentration is shown in (F) and (G) for Northern California sites and Southern California sites separately. Uncertainty in ffCO2 is approx. ±1.5 ppm, indicated by the vertical dashed lines in (B). emissions from California are about 1% of the global total. Bottom-up calculations of fossil-derived CO2 (fossil fuel CO2 or ffCO2) emissions have historically been regarded as having relatively small uncertainties, and natural carbon fluxes often produce stronger spatial and short-term variations in atmospheric CO2. Therefore, ‘top-down’ studies of atmospheric CO2 incorporating atmospheric measurements and modeling have historically focused on natural carbon fluxes including photosynthesis and respiration [5, 6]. However, 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]. These discrepancies suggest that top-down studies incorporating the measurement of a tracer that distinguishes fossil-derived CO2 (ffCO2) could be useful for evaluating ffCO2 emissions on regional scales [11]. Top-down studies for ffCO2 emissions are still in the early stages of development [12–15], in comparison to relatively well-developed applications of top-down emissions estimates for other greenhouse gases such as methane [16, 17] and hydrofluorocarbons [18] that have revealed biases in corresponding bottomup estimates, which tend to have large uncertainties. Top-down studies estimate the distribution and magnitude of emissions that minimizes differences with observations, typically also minimizing the deviation from a prior estimate of emissions following Bayesian statistics. Estimates of the spatial distribution of 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 (figure 1). 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.