Tall Tower Trace Gas Observatory

The Tall Tower Trace Gas Observatory is currently supported by the National Science Foundation, Lower Atmosphere Chemistry Section (ATM-0546476) and the Office of Science (BER) United States Department of Energy (DE-FG02-06ER64316). We express our sincere thanks to Minnesota Public Radio (KCMP 89.3 "The Current") and Tom Nelson for their logistical support.






Over the past year we have gained valuable experience combining tunable diode laser (TDL) spectroscopy with the eddy covariance (EC) approach and for the first time have made direct measurement of biosphere-atmosphere isotopic CO2 exchange. This new technique has recently been deployed at the Tall Tower Observatory (240m/798ft) site located within a heavily managed landscape. This system is being used to quantify CO2 mixing ratios and isotopic fluxes at 100 m and 200 m above the ground surface. It is calibrated every 6 minutes against CMDL working standards and should, therefore, provide high-precision concentration data useful for inverse carbon budget studies. The photographs above show the sonic anemometer and air sample inlet mounted at the 100 m level and the tunable diode laser and sampling system located at the base of the tower. The primary goal of the projects is to improve the scientific understanding of the biophysical processes and isotope discrimination mechanisms controlling the exchange of carbon dioxide between the land and the atmosphere within a heavily managed landscape.




Below we show four examples of new data obtained from this system that will prove invaluable in constraining landscape- and regional-scale carbon budgets. First, net ecosystem CO2 exchange is being measured using an EC-TDL and EC-IRGA system--providing an integrated flux footprint ranging from 3 to 10 km depending on atmospheric stability. This footprint represents a number of land use types and agricultural management practices and should provide a more representative view of agricultural impact on land- atmosphere CO2 exchange. The concentration footprint can be approximately 20-fold greater than the flux footprint and will provide a regional CO2 signature.

The figure below illustrates that flux measurements at a height of 100 m for the EC-IRGA and EC-TDL systems are in excellent agreement (upper left-hand panel) and that the net uptake of carbon is relatively large at this time of year (i.e. approximately 6 weeks since planting).




In the lower left-hand panel the isotopic composition of the ambient air is shown--illustrating that daytime photosynthesis causes 13CO2 and C18O16O enrichment of the ambient air. Compared to our previous measurements taken a few meters above corn and soybean canopies, the enrichment is much less pronounced and can be attributed to the influence of boundary layer dynamics and the larger scales of eddy motion. The signal, therefore,represents a very large source area. Continuous measurement of the isotopic composition of the ambient air should prove very important in improving our understanding of the rectifier effect. Such measurements are unprecedented. The right-hand panels show the corresponding flux ratio and Keeling plots and indicate that the isotopic composition of net CO2 exchange is approximately -18.4 per mil over the 3-day period. Based on the above data, and a simple isotopic mass balance, we estimate that approximately 60% of the total CO2 exchange can be attributed to C4 (corn) production. This direct measurement provides an excellent opportunity to better understand how land use change may impact the carbon balance and the composition of the atmosphere. It will also provide a rigorous constraint on model validation at the landscape and regional scales. Further, with the addition of water vapor flux measurements and water use efficiency of C3/C4 vegetation we should be able to estimate the relative contributions of managed ecosystems to changes in atmospheric water vapor.


Eddy Covariance TDL Isotope Experiment


Ecosystem/Regional Scale Carbon Cycle Study