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Towards a Universal “Baseline” Characterisation of Air Masses for High- and Low-Altitude Observing Stations Using Radon-222

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Chambers Scott D., Williams Alastair G., Conen Franz, Griffiths Alan D., Reimann Stefan, Steinbacher Martin, Krummel Paul B., Steele L. Paul, van der Schoot Marcel V., Galbally Ian E., Molloy Suzie B., Barnes John E.,
Project ICOS-CH: Integrated Carbon Observation System in Switzerland
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Original article (peer-reviewed)

Journal Aerosol and Air Quality Research
Volume (Issue) 16(3)
Page(s) 885 - 899
Title of proceedings Aerosol and Air Quality Research
DOI 10.4209/aaqr.2015.06.0391

Open Access

URL http://www.aaqr.org/files/article/322/33_AAQR-15-06-SIMtS-0391_885-899.pdf
Type of Open Access Publisher (Gold Open Access)

Abstract

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.
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