North American biogenic carbon dioxide (CO ) fluxes and total methane (CH.i) emissions remain poorly diagnosed at regional scales. Regional scales (areas smaller than the entire continent – the MidWest com belt, or MidAtlantic forests, for example) are critically important because they are the scales (biomes, geopolitical units) over which management activities take place, and over which climate and ecological processes drive terrestrial fluxes. A rapid expansion in North American, tower-based greenhouse gas (GHG) measurements over the last decade, and substantial advances in atmospheric inversion methodology provide an excellent opportunity for improving our understanding of regional CO2 and CH-1 fluxes. This project will take advantage of an extensive continental GHG measurement network (over 80 continuous, tower-based measurements, and over 20 flask measurement and aircraft profiling sites) and a newly operational continental-scale flux inversion system, CarbonTracker – Lagrange, to diagnose North American GHG fluxes at regional spatial and sub-seasonal temporal resolution from 2007 2018. These inverse flux estimates will be cross-evaluated with comparisons of posterior mole fractions to independent atmospheric GHG mole fraction observations from the Atmospheric Carbon and Transport (ACT) – America Earth Venture Suborbital (EVS) flight campaigns, and comparisons of posterior fluxes to regional clusters of flux towers. Sensitivity studies to inversion system inputs will explore the robustness of the results. The resulting inverse flux estimates will be compared to climate variations and trends in an effort to diagnose causal relationships between fluxes and climate conditions. The inverse fluxes will also be compared to terrestrial biogeochcmical models and inventory flux estimates when available, to demonstrate the utility of the atmospheric data for cross-evaluation of model and inventory methods of esti matin g regional GHG fluxes. Second, a range of data remo va l experiments will illustrate the value of currently available tower and aircraft data in resolving both spatia l structure and temporal patterns in GHG fluxes. This portion of the study will demonstrate the value of continued operation of an expanded in situ observational network and inform the design of a future integrated carbon observing system. This project responds to the AC4 request for studies of long-term trends in observations of atmospheric composition. In particular, the investigators will ..explain various trends, patterns and extremes detectable in the existing long-term observational records.” We will illuminate “long-term trends patterns, anomalies and extremes in long-term trends, intra- and interannual variability and change, changes in amplitude of seasonal cycle, local or regional changes in the long-term trends” in continental GHG fluxes and the associated records of atmospheric composition. We will also provide new insight into regional CH4 emissions, thus responding to the request from the National Academy of Sciences for proposals that focus on anthropogenic methane emissions. Finally, by using complementary data from non-NOAA sources, the project will, “demonstrate future expansion capabilities and/or to test how detection limits for trends can be lowered.” This project advances the Climate Program Office’s objective to advance scientific understanding, monitoring, and prediction of climate, and will guide future investment in observational and analytic capabilities.