With prior support from NOAA�??s Climate Program Office and Global Monitoring Division, we have greatly expanded the use of high-precision ??14CO2 measurements as a tracer of recently-emitted CO2 from combustion of fossil fuels and cement manufacture (FFCO2) within the NOAA Global Greenhouse Gas Reference Network (GGGRN) and developed the computational machinery needed in order to use the new observations in a dual-tracer inversion framework capable of separating fossil and biospheric CO2 fluxes. In recent work, we obtain inverse estimates of US FFCO2 emissions based on network observations from 2010 indicating emissions 5-7 % larger than from the most widely-used international emissions inventories. These differences are statistically significant with respect to rigorously-derived posterior FFCO2 flux uncertainties and apparently robust to differing specifications of the magnitude and seasonality of prior emissions. Furthermore, derived monthly FFCO2 emissions estimates are similar to monthly, national 2010 totals from the (forthcoming) US Vulcan 3.0 inventory. Here we propose to leverage the now long-term ??14CO2 NOAA measurement effort led by the PIs in order to undertake a similar analysis of US monthly emissions for the decade of GGGRN CO2 and ??14CO2 observations spanning 2010 through 2019. Our overall goal will be to determine whether differences between widely-used international emissions inventories and our estimates of US FFCO2 emissions from observations are real and persistent over time, based on a proposed analysis of uncertainties and possible biases in our dual-tracer inversion framework arising from assigned a priori fluxes and atmospheric transport. Our effort will include extending high resolution US Vulcan inventories through 2019 (Northern Arizona Univ.) and comparison to annual US EPA FFCO2 inventories reported to the UNFCCC. This proposal is directly responsive to CPO ESSM�??s AC4 competition goals, aimed at the use of long-term observational records to constrain trends and processes influencing atmospheric composition and climate. The proposed work bears directly on a key CPO mandate, namely to advance scientific monitoring in support of effective decision making. The proposed work can be expected to provide improved understanding of US FFCO2 emissions and their uncertainties, informing the US carbon budget, US emissions reporting, and estimates of climate forcing. Detection of persistent biases in the US country portion of international emissions inventories would suggest that such biases may be widespread and of even greater significance.