There is strong evidence that North America terrestrial ecosystems are currently a substantial sink of carbon dioxide. The magnitude of the sink has a large range of uncertainty, we have a limited understanding of how it has varied over time, and the processes responsible for this sink are not entirely clear. Our limited understanding is linked to methodological limits. Quantifying spatial patterns and temporal variability of carbon dioxide sources and sinks at continental to regional scales remains a challenging problem.

In response to this challenge a rapid expansion of the North American carbon cycle observational network is underway. This expansion includes a network (AmeriFlux) of continuous, eddy-covariance based CO2 flux measurements and a continental CO2 mixing ratio observing network of comparable precision and accuracy to the marine flask network. To date, inverse studies of the North American carbon budget have not utilized these emerging data sources directly (i.e. tower fluxes and continental mixing ratio observations).

We propose a program that will turn the emerging wealth of data in North America to our advantage. This can be accomplished by merging research groups at the forefronts of terrestrial boundary layer CO2 flux and mixing ratio observations, and high resolution, land-atmosphere carbon cycle modeling. The study will explore the potential for fusion of CO2 flux and mixing ratio observations in a coupled land-atmosphere data assimilation framework. We will accomplish this via inverse modeling incorporating the emerging North American CO2 mixing ratio observational network, forwards modeling built upon the North American flux network, and cross-evaluation of these two model-data fusion approaches.

Expected products include: 1) development and evaluation of a comprehensive analysis system for estimation of monthly CO2 exchange across North America at high spatial resolution; 2) dramatic reduction in the uncertainty in the annual net North American CO2 flux and its interannual variations, as compared to currently published results; 3) attribution of CO2 sources between fossil fuel combustion and ecosystem exchange using CO and other trace gases; 4) application of AmeriFlux observations to evaluate the mechanisms responsible for seasonal to interannual responses of ecosystem carbon exchange to climate; 5) evaluation of the flux and mixing ratio predictions of the forwards and inverse models; 6) evaluation of the atmospheric and ecosystem models, and the flux and mixing ratio observational networks used in these studies. The methods explored here will be portable to other parts of the globe.