| Summary: | Nine years (2004-2013) of hourly Radon-222, carbon dioxide and ozone concentration observations at Cape Grim are used to assess the residual terrestrial influence on air masses with radon concentrations below the 100 mBq m-3 threshold traditionally used for ‘baseline’ delineation (Figure 1a). Subsequently, a two step radon-only approach for estimating ‘baseline’ constituent concentrations on monthly timescales is proposed. Based on a stringent 40mBq m-3 radon threshold followed by a simple 10th/90th percentile constitute outlier removal, the technique is completely independent of meteorological or aerosol observations. An initial evaluation of the techniqueusing hourly carbon dioxide and ozone records yielded monthly ‘baseline’ concentration estimates more consistent with expectations of minimally perturbed Southern Ocean air masses than existing baseline selection techniques (Figure 1c). This work builds upon prior studies that have identified radon as a valuable baseline criteria [e.g Gras and Whittlestone, 1992; Molly and Galbally, 2014]. CSIRO Oceans and Atmosphere Flagship Aspendale, Victoria, Australia.
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