1. Characterisation of mixing processes in the lower atmosphere using Rn-222 and climate-sensitive gases P. Schelander, A. Griffiths, A.G. Williams, S. Chambers and W. Zahorowski ANSTO Institute for Environmental Research, PMB 1, Menai, NSW, Australia 2234. Background A need has been identified within the international scientific community for improved knowledge and predictions of diurnal, seasonal and inter-annual cycles of surface emissions, mixing and movement of anthropogenic pollutants in the lower atmosphere. One aim for this project is to utilise measurements of the naturally occurring radioactive gas radon ( 222 Rn) to progress understanding of vertical mixing processes in the lower atmosphere. Ultimately, it is hoped that this information will lead to the development of improved parameterisations of vertical transport processes in regional and global models. Radon-222 Radon-222 is produced through the α-decay of Radium- 226 and is the only gaseous decay product of the Uranium-238 series. Radium-226 is ubiquitous to most soil and rock types and results in a fairly consistent flux of radon from all terrestrial surfaces of between 0.8 – 1.2 atoms cm -2 s -1. Radon is a noble gas, so its only significant atmospheric sink is via radioactive decay. Since the half-life of radon (3.8 days) is comparable to the lifetimes of some common atmospheric pollutants and many atmospheric processes, it is an ideal atmospheric tracer on these timescales. Within the well-mixed daytime CBL only a small reduction of radon with height is expected since the mixing timescale (<1 hour) is much shorter than the radon half-life. The gradient of radon observed above the CBL can be related to the rate of exchange of CBL air with the troposphere as well as the rate of vertical transport within the lower troposphere. Budget of total radon in the ABL vertical column: Time change Surface flux Decay Encroachment Entrainment Aircraft radon sampling A sampling system for measurements of radon concentrations from airborne platforms has been developed at ANSTO, and has been successfully deployed using a light aircraft. The radon sampler consists of 6 charcoal traps and samples are collected over 5 minute intervals. Radon sampler in the pod of Airborne Research Australia’s motor-glider Atmospheric surface layer observations A 50 meter tower at Lucas Heights is used for monitoring radon, CO 2, H 2 0 and meteorological variables in the lower atmosphere. Radon intakes are mounted at 2 and 50 meters, and gas samples are continuously collected from six different levels. The tower is equipped with two 3-D ultrasonic anemometers at 10 and 50 meters for flux computations. In addition to standard meteorological instrumentation, ground instrumentation allows a full surface energy balance to be computed. The ANSTO 50m meteorological tower at Lucas Heights with two radon detectors sampling air from intakes at 2 and 50 meters. Expected outcomes Radon is an ideal passive tracer in the atmosphere. The data set being collected at the ANSTO tower, with continuous measurements of radon, tracers and meteorological parameters, will result in a long-term record with an uncommon mix of data. Similarly, the dataset resulting from aircraft campaigns will be unlike others. The combination of these two datasets offers the potential for unique insights into the atmospheric vertical mixing process. Gas analysis Closed-path broadband Infra-Red Gas Analysers (IRGAs) sample air from six heights on the tower, measuring CO 2 /H 2 O concentration and, using eddy correlation, flux at 10 m. A Fourier Transform Infra-Red Spectrometer (FTIR) from University of Wollongong samples air from the same inlets. The FTIR also measures C0 2 and H 2 O concentration—cross-referenced with the IRGA measurement—in addition to N 2 O, CO and CH 4 concentration and HDO/H 2 O ratio. This system is beginning continuous measurements and will run for at least a year at the ANSTO site. Acknowledgements Carol Tadros, Ot Sisoutham, Sylvester Werczynski, Adrian Element and Airborne Research Australia Scintec flat-array sodar deployed at Goulburn Airport during a field campaign. Putting it all together Radon concentration in isolation tells little about atmospheric mixing. The other meteorological and trace gas data can be viewed as helping to interpret the radon data. The boundary layer depth (h in the radon budget equation, leftmost column) is a critical meteorological parameter. This quantity is estimated from sodar data in ground-based measurements. Comparison of radon concentration with trace gas concentration allows the mixing of climate-sensitive gases to be studied as part of this effort. Radon profiles aloft under a variety of cloud conditions