Motivation for the proposed study:
Studies of the global carbon cycle are motivated by the need to predict future trends in atmospheric CO2, to define responses of the terrestrial biosphere and oceans to changing CO2 and climate, and to determine permissible emissions for selected future levels of atmospheric CO2. The confidence in predictions of future CO2 is limited by uncertainties in the magnitude of terrestrial and oceanic sinks and associated biophysical mechanisms. Currently the best constraints on marine and terrestrial fluxes are derived by inverse analysis of concentration data for CO2, CO2/ CO2, and O2 from the global surface observing network, using global biophysical/transport models. One of the largest sources of uncertainty for inverse studies is the “rectification” of seasonal and diurnal oscillations of CO2 fluxes, atmospheric concentration gradients caused by correlation between biophysical fluxes and rates for transport that are difficult for models to simulate, and for which validation data are currently unavailable. The proposed study will measure the CO2 gradients due to rectification phenomena over North America and will separate the influence of biogenic and combustion fluxes on CO2 distributions at continental scale, using measurements of CO2, CO, O2, CO2 SF6, and other tracers. The goal is to provide unique, valuable new constraints on the global carbon cycle.

Objectives of the proposed measurements:

• Obtain comprehensive extensive measurements of the vertical and horizontal distributions of CO , CO, O , 13CO 222

SF6, and other tracers in and above the planetary boundary layer (PBL) across North America and adjacent ocean regions in two seasons (summer, winter) using airborne in situ instrumentation and grab samples.

• Obtain intensive diurnal observations in and above the planetary boundary layer in selected regions to complement the extensive measurements, to quantify the effect of the diurnal rectifier on distributions of these tracers over land, and to obtain regional flux estimates.

Deliverables:

• Extensive and intensive tracer distributions and covariances will be analyzed to determine the influence on observed concentrations over North America of each of the three primary processes in the global carbon cycle: fossil fuel burning, terrestrial biotic exchange, and oceanic exchange.

• The data from intensive regional studies will be analyzed to define regional emission rates.

• Analysis of data from the proposed experiments will define the magnitude of gradients over the continent of North America associated with both the seasonal and diurnal rectifiers, and will determine the influence of rectification on concentrations measured at marine boundary layer stations by comparison of our airborne data to concentrations measured at remote stations at the same time.

• The measurements will be compared to model predictions to test parameterizations of mixing and rectification, to help define continental source and sink distributions/magnitudes, and to assess the impact of rectification phenomena on inverse analyses of the global carbon cycle. The work will provide primary data to aid in assessing the design of a carbon cycle observing network, such as Carbon America, that expands upon current surface measurements. The data will provide the first direct measurement of photosynthetic and respiratory quotients over large regions, essential parameters for understanding the relationship between global CO2 and O2 cycles currently available only from laboratory measurements.

• We plan to examine the accuracy of continental fluxes derived by scaling the observed ratios of CO2 concentration changes associated with terrestrial fluxes vs. fossil fuel emissions by accounting for the influence of rectification on observed concentration gradients, using the known magnitude of the fossil fuel combustion.