Volume 16, No. 3, March 2016, Pages 885-899 PDF(604 KB)
Towards a Universal “Baseline” Characterisation of Air Masses for High- and Low-Altitude Observing Stations Using Radon-222
Scott D. Chambers1, Alastair G. Williams1, Franz Conen2, Alan D. Griffiths1, Stefan Reimann3, Martin Steinbacher3, Paul B. Krummel4, L. Paul Steele4, Marcel V. van der Schoot4, Ian E. Galbally4, Suzie B. Molloy4, John E. Barnes5
1 Australian Nuclear Science & Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW, 2232, Australia
2 Environmental Geosciences, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland
3 Laboratory for Air Pollution & Environmental Technology (Empa), Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600, Dübendorf, Switzerland
4 CSIRO Oceans and Atmosphere Flagship, Aspendale, Victoria, 3195, Australia
5 NOAA/Mauna Loa Observatory, Hilo, HI, USA
- Simple technique for reliable identification of “baseline” conditions using radon.
- Objective identification of baseline conditions for high- and low-altitude sites.
- Characterisation of air masses in long-term equilibrium with the remote ocean.
- Characterisation of “background” atmospheric composition at continental sites.
- Relationship between degree of terrestrial influence and air mass composition.
We demonstrate the ability of atmospheric radon concentrations to reliably and unambiguously identify local and remote terrestrial influences on an air mass, and thereby the potential for alteration of trace gas composition by anthropogenic and biogenic processes. Based on high accuracy (lower limit of detection 10–40 mBq m–3), high temporal resolution (hourly) measurements of atmospheric radon concentration we describe, apply and evaluate a simple two-step method for identifying and characterising constituent mole fractions in baseline air. The technique involves selecting a radon-based threshold concentration to identify the “cleanest” (least terrestrially influenced) air masses, and then performing an outlier removal step based on the distribution of constituent mole fractions in the identified clean air masses. The efficacy of this baseline selection technique is tested at three contrasting WMO GAW stations: Cape Grim (a coastal low-altitude site), Mauna Loa (a remote high-altitude island site), and Jungfraujoch (a continental high-altitude site). At Cape Grim and Mauna Loa the two-step method is at least as effective as more complicated methods employed to characterise baseline conditions, some involving up to nine steps. While it is demonstrated that Jungfraujoch air masses rarely meet the baseline criteria of the more remote sites, a selection method based on a variable monthly radon threshold is shown to produce credible “near baseline” characteristics. The seasonal peak-to-peak amplitude of recent monthly baseline CO2 mole fraction deviations from the long-term trend at Cape Grim, Mauna Loa and Jungfraujoch are estimated to be 1.1, 6.0 and 8.1 ppm, respectively.
222Rn; Clean air; Mountain site; Terrestrial influence; Greenhouse gases.