The goal of the project is to to better understand and improve the robustness of the North American CO2 budget computed by NOAA�??s CarbonTracker by developing a Lagrangian optimization module for CarbonTracker and to perform a set of CO2 and multi-species optimizations. To this end, the project will upgrade the NOAA Air Resources Laboratory�??s HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model to compute accurate sampling footprints (influence functions that describe the sensitivity of an air sample to upwind sources and sinks) corresponding to all North American measurements used in CarbonTracker for the period 2007 to 2010. This work will improve the accuracy of CO2 flux estimates for North America at regional and local scales and will represent a significant step toward addressing the scientific and societal need for independent verification of fossil fuel CO2 emissions estimates from inventories. All products and software to be developed by this project (including the footprint library, optimized fluxes, and the upgraded HYSPLIT model) will be made publicly available.