Radon Measurements to Help Scientists Estimate Carbon Dioxide Exchange

 
Published: 15 January 2007

Researchers installed a continuous 222Rn monitor at the base of the 60-meter tower at the SGP Central Facility. A sampling tube connected to the tower supplies air to the container, where the radon is measured.

In November, ARM scientists and researchers from the National Oceanic and Atmospheric Administration’s Climate Monitoring and Diagnostics Laboratory began a collaborative field campaign at the ARM Southern Great Plains (SGP) site. The science objective of the Radon Measurements of Atmospheric Mixing (RAMIX) field campaign is to quantify the mixing rate of air between the surface and the atmospheric boundary layer—the layer of air from Earth’s surface up to about 1 km. This information is valuable for studies of the terrestrial carbon cycle and cloud formation. In particular, uncertainties in estimates of regional carbon dioxide (CO2) exchange (that rely on models of atmospheric transport) are dominated by model error in the vertical mixing rate. The RAMIX campaign will provide an independent measure of mixing rate that scientists will combine with measured variations in atmospheric CO2 mixing ratio to estimate regional CO2 exchange.

Carbon dioxide is a greenhouse gas and has been identified as a major contributor to global warming. Because terrestrial vegetation absorbs carbon dioxide from the atmosphere and stores the carbon in soils, scientists believe that carefully chosen land use and management (e.g., conservation tillage) can be effective in reducing global warming. Measuring the regional carbon exchange is one way of measuring the net rate at which warming and cooling is occurring.

Radon (222Rn) provides a valuable tracer of atmospheric mixing because it is emitted relatively ubiquitously from the land surface and has a short enough half-life (3.8 days) to allow characterization of mixing processes based on time variations and/or vertical profile measurements. To provide accurate estimates of atmospheric mixing rate, the RAMIX team will use both 222Rn concentration measurements and a spatially explicit model of surface 222Rn flux that will be tested with in situ measurements. Beyond the tower measurements of 222Rn, airborne 222Rn concentration measurements will increase the accuracy of the mixing rate estimates. Data from RAMIX are expected to contribute to a separate but related measurement and modeling effort, the Aircraft Carbon field campaign, which will also assess the nature of the carbon cycle by looking at carbon profiles in the atmosphere.