Macquarie Island hourly radon data set 2013-2021

Full description

Atmospheric radon observations constitute a convenient and unambiguous indicator of recent terrestrial influence on air masses. Since most anthropogenic gaseous and aerosol pollutants are also of terrestrial origin, high-quality radon observations serve as a proxy for the ‘pollution potential’ of air masses in remote regions (e.g., Chambers et al. 2018; Williams and Chambers 2016).

Radon-222 (radon) is a gaseous decay product of Uranium-238. Its immediate parent, Radium-226, is ubiquitous in soils and rocks. Radon is a noble gas, poorly soluble, and radioactive (t0.5=3.82 d), so it does not accumulate in the atmosphere on greater than synoptic timescales. Its average source function from unfrozen terrestrial surfaces is relatively well constrained, and 2-3 orders of magnitude greater than that from the open ocean. Furthermore, on regional scales radon’s terrestrial source function is not significantly affected by human activity. This combination of physical characteristics enables air masses that have been in contact with terrestrial surfaces to be tracked over the ocean, or within the troposphere, for 2-3 weeks.

The radon concentration of air masses that have been in long-term equilibrium with the Southern Ocean is typically 30-50 mBq m-3 (e.g., Zahorowski et al. 2013). Consequently, key requirements of radon detectors deployed in such remote locations are: a detection limit of ≤50 mBq m-3, stable absolute calibrations, and low maintenance. Macquarie Island is small (34 x 5 km) and situated roughly midway between Australia and Antarctica. Radon and meteorological observations are made at the “Clean Air Laboratory” on an isthmus at the northern end of the island (~54.5 degrees S). At Macquarie Island, radon is measured with a 1500L, single head, dual flow loop, two-filter detector (Whittlestone and Zahorowski 1998). Sample air is drawn at ~45 L m-1 from an inlet ~5 m above ground level (a.g.l.) on a 10 m mast. The detector is automatically calibrated monthly using a 19.58±4% kBq 226Ra source injecting for 6 hours at a flow rate of ~170 cc min-1. Instrumental background checks are performed quarterly by automatically stopping the internal and external flow loop blowers for 24 hours.

An updated dataset was provided in September, 2022.

This corrected dataset supersedes all previous datasets of radon measurements at MQA under AAS 4167, and extends into the first year of MQA radon measurements under the new project AAS 4621.

This dataset has been improved and enhanced relative to previous versions in the following ways:
(1) Based on tests conducted during the last two field trips to MQA, the data have been corrected for the problems that we encountered with the calibration system over the last few years. These problems have been largely resolved, although we have had to reject many of the calibrations in the period Apr 2017 – Apr 2019 and interpolate between them. Luckily, radon calibration coefficients can be assumed to change slowly and smoothly in time between head refurbishments, especially at a “clean” site like MQA, and we are now confident in the accuracy of the corrected dataset.
(2) The radon data has had a simple response-time correction applied, in the form of a 30 min lag applied to the half hourly data prior to conversion to hourlies. Note that a more sophisticated response time correction can be applied if requested, based on the method outlined in Griffiths et al. 2016 (Atmos. Meas. Tech. 9, 2689–2707, https://doi.org/10.5194/amt-9-2689-2016).
(3) Although no STP corrections have been applied to the radon data, additional columns have been included in the file which will allow such a correction to be applied if desired (air temperature, relative humidity and air pressure inside the detector volume).
(4) A new column has been added providing an estimate of the counting error associated with the calibrated hourly radon concentrations (square root of the total hourly count, expressed as a concentration).

Additional information is included in the “Notes” tab of the dataset .xlsx file.

Lineage

Progress Code: completed

Statement: 2013-2021 data are considered final. QA/QC checks and measures have been applied to the downloaded data by remote analysis at ANSTO in Sydney. Note that processed radon data may always be subject to future revisions / corrections.

Notes

Purpose
Support interpretation of greenhouse gas and other atmospheric pollution data gathered at Macquarie Island as part of AAS 4167 (radon is a tracer of terrestrial influences on sampled atmospheric data).