Estimating US fossil fuel CO2 emissions from measurements of 14C in atmospheric CO2

Abstract

We report national scale estimates of CO₂ emissions from fossil-fuel combustion and cement production in the United States based directly on atmospheric observations, using a dual-tracer inverse modeling framework and CO₂ and [Formula: see text] measurements obtained primarily from the North American portion of the National Oceanic and Atmospheric Administration's Global Greenhouse Gas Reference Network. The derived US national total for 2010 is 1,653 ± 30 TgC yr^-1 with an uncertainty ([Formula: see text]) that takes into account random errors associated with atmospheric transport, atmospheric measurements, and specified prior CO₂ and ¹⁴C fluxes. The atmosphere-derived estimate is significantly larger ([Formula: see text]) than US national emissions for 2010 from three global inventories widely used for CO₂ accounting, even after adjustments for emissions that might be sensed by the atmospheric network, but which are not included in inventory totals. It is also larger ([Formula: see text]) than a similarly adjusted total from the US Environmental Protection Agency (EPA), but overlaps EPA's reported upper 95% confidence limit. In contrast, the atmosphere-derived estimate is within [Formula: see text] of the adjusted 2010 annual total and nine of 12 adjusted monthly totals aggregated from the latest version of the high-resolution, US-specific "Vulcan" emission data product. Derived emissions appear to be robust to a range of assumed prior emissions and other parameters of the inversion framework. While we cannot rule out a possible bias from assumed prior Net Ecosystem Exchange over North America, we show that this can be overcome with additional [Formula: see text] measurements. These results indicate the strong potential for quantification of US emissions and their multiyear trends from atmospheric observations.

Keywords: atmospheric inverse modeling; fossil fuel CO2; radiocarbon.

Fig. 1.

Mean annual FF CO₂ from the Miller/CT prior of Fig. 2 over and around the United States, along with the location of observing sites with only CO₂ measurements (crosses) and with both CO₂ and ${\mathrm{\Delta }}^{14}{\text{CO}}_2$ measurements (circled crosses) within the map area in 2010. (Inset) The delineation of Western (orange), Central (green), and Eastern (blue) US regions mentioned in the text. The Western US region also contains Hawaii and Alaska.

Fig. 2.

Monthly prior (dashed lines/open symbols) and optimized (solid lines/filled symbols) US FF ${\text{CO}}_2$ estimates, along with the 2010 annual totals (gray region to the right). (A) The three pink lines represent the three FF ${\text{CO}}_2$ estimates starting from three different FF CO₂ priors (blue lines). (B) Results from three alternate aseasonal prior FF CO₂ estimates are shown. The pink shaded region in A denotes $\pm 1\sigma$ analytical uncertainty of optimized monthly FF CO₂, while the error bars in the gray regions denote the same for 2010 annual totals. Values are expressed as daily averages to account for months of varying lengths. Annual aggregate FF CO₂ for the inverse estimates and related priors are given in Table 1. Vulcan 3.0 monthly and annual aggregates are shown in A for comparison to the preferred inverse estimates. Adjustments made to Vulcan FF CO₂ for comparison to atmosphere-derived estimates are described in the text.

Fig. 3.

Modeled and observed (Obs) vertical gradients of ${\text{SF}}_6$ at eight continental US sites with respect to the mean free troposphere at each site, defined here as observed or simulated mole fractions between 5 and 8 km above sea level. The free tropospheric time series at each site was calculated by fitting a linear trend through the 5- to 8-km ${\text{SF}}_6$ mole fraction, which was then subtracted from all ${\text{SF}}_6$ mole fractions between 2001 and 2011 before averaging. The error bars represent twice the standard error in the mean difference between any level and the free troposphere. Site locations and date ranges of the observations are given in SI Appendix, Fig. S2. ${\text{SF}}_6$ emissions used here were from the EDGAR 4.2 inventory (3), after adjusting global totals to match the observed atmospheric growth rate (https://www.esrl.noaa.gov/gmd/hats/combined/SF6.html).

Fig. 4.

The annual total CO₂ flux (A) and the NEE (B) from two inversions, for the United States and the three large subdivisions of Fig. 1. The “CO₂+¹⁴CO₂” inversion is the dual tracer Miller/CT inversion of Fig. 2A, while the “CO₂ only” inversion is the same inversion without ${\mathrm{\Delta }}^{14}{\text{CO}}_2$ data and FF CO₂ optimization.