Constraining annual and seasonal radon-222 flux density from the Southern Ocean using radon-222 concentrations in the boundary layer at Cape Grim

Abstract

Radon concentrations measured between 2001 and 2008 in marine air at Cape Grim, a baseline site in north-western Tasmania, are used to constrain the radon flux density from the Southern Ocean. A method is described for selecting hourly radon concentrations that are least perturbed by land emissions and dilution by the free troposphere. The distribution of subsequent radon flux density estimates is representative of a large area of the Southern Ocean, an important fetch region for Southern Hemisphere climate and air pollution studies. The annual mean flux density (0.27 mBq m−2 s−1) compares well with the mean of the limited number of spot measurements previously conducted in the Southern Ocean (0.24 mBq m−2 s−1), and to some spot measurements made in other oceanic regions. However, a number of spot measurements in other oceanic regions, as well as most oceanic radon flux density values assumed for modelling studies and intercomparisons, are considerably lower than the mean reported here. The reported radon flux varies with seasons and, in summer, with latitude. It also shows a quadratic dependence on wind speed and significant wave height, as postulated and measured by others, which seems to support our assumption that the selected least perturbed radon concentrations were in equilibrium with the oceanic radon source. By comparing the least perturbed radon observations in 2002–2003 with corresponding ‘TransCom’ model intercomparison results, the best agreement is found when assuming a normally distributed radon flux density with σ=0.075 mBq m−2 s−1

Characterising terrestrial influences on Antarctic air masses using Radon-222 measurements at King George Island

“…Median monthly radon concentrations decreased from 72 mBq m −3 in February to 44 mBq m −3 in November, with corresponding 10th percentiles (representing the least terrestrially influenced air), reducing from 49 mBq m −3 to 29 mBq m −3 . The latter range is similaralthough opposite in phase -to the seasonal variability in Southern Ocean baseline air masses as observed at Cape Grim in Tasmania (27 to 44 mBq m −3 ; Zahorowski et al, 2013), where "baseline" here represents the least terrestrially perturbed air. Monthly 90th percentile concentrations were highly variable due to the station's proximity to South America (∼ 900 km), from which passing weather systems occasionally bring terrestrially influenced air to KSG year round (Fig.…”

“…However, the high January concentrations are consistent with summertime observations at the nearby Syowa Station (Ui et al, 1998) in the range 150-270 mBq m −3 . In late winter (JulyAugust), median values are 25-39 mBq m −3 , similar to the oceanic baseline values observed at Cape Grim (Zahorowski et al, 2013), and corresponding 10th percentile values are 1-3 mBq m −3 .…”

“…As shown in previous sections, despite the remoteness of most Antarctic coastal regions from significant terrestrial radon sources, mean concentrations well above oceanic baseline levels (27-44 mBq m −3 ; Zahorowski et al, 2013) are frequently observed, particularly in summer. In addition to the influences of local radon sources and the direct transport of continental air masses to Antarctica within the boundary layer by passing synoptic weather systems (radonic storms), it has been hypothesised that indirect transport to polar regions through the mid-to upper-troposphere may play an important role in the seasonal radon variability (Heimann et al, 1990;Balkanski and Jacob, 1990;Polian et al, 1986;Hogan et al, 1982).…”

“…A noble gas, and poorly soluble, its primary atmospheric sink is radioactive decay (half-life, t 0.5 = 3.82 days). Since radon's oceanic sources are 2 orders of magnitude smaller than its terrestrial sources (Zahorowski et al, 2013), and its atmospheric lifetime is comparable to that of short-lived anthropogenic pollutants (e.g. NO x or SO 2 ), the residence times of water and aerosols, and the timescale of many important aspects of atmospheric dynamics, radon is an ideal tracer for transport studies focusing on distant terrestrial pollution.…”

Improving the Representation of Cross-Boundary Transport of Anthropogenic Pollution in East Asia Using Radon-222

“…6 is the build-up of radon before dawn in all seasons. Assuming a negligible oceanic radon flux (Zahorowski et al, 2013), this demonstrates a local-scale contribution to Gosan observations, since mean nocturnal winds are too small to bring radon from neighbouring source regions (China, Korea or Japan) over the course of a single night. To investigate the possibility of similar local influences on pollution observations we analysed diurnal cycles of CO and SO 2 (Fig.…”