Determination of mid-latitude radon-222 flux from the Southern Ocean using atmospheric radon-222 concentration measurements at an island ground station
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Date
2004-10-24
Journal Title
Journal ISSN
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Publisher
International Atomic Energy Agency
Abstract
Terrestrial radon (radon-222) fluxes typically exceed oceanic fluxes by 2-3 orders of
magnitude [1]. This sometimes leads to the oceanic flux being neglected in atmospheric
models [2, 3]. However, direct comparisons between observed and simulated atmospheric
radon concentration at remote sites with extended oceanic fetch would benefit from the
inclusion of a realistic oceanic radon flux within models [4].
Existing estimates of oceanic radon fluxes are not well constrained, with values ranging over
two orders of magnitude (0.0011 - 0.15 atoms cm-2s-1) [1, 5]. At present, the primary factors
contributing to the poor characterisation of oceanic radon fluxes are the limited number of
observations, and the representativeness of the published results. Previous studies have
employed either the accumulation [1] or gradient methods [5]. Both methods are based on
spot measurements and as such, are subject to local conditions. More importantly, it would be
difficult, if not impossible, to relate such results to a wider range of environmental parameters
such as wind speed and sea state, which have a significant effect on ocean-atmosphere
exchange.
We present a method for the determination of regional oceanic radon fluxes. The method is
applied to a subset of high sensitivity hourly atmospheric radon concentration observations
from 1999 to 2003 made at Cape Grim, Tasmania (40°41’S, 144°41’E), a World
Meteorological Organisation Global Atmosphere Watch (WMO GAW) station. A simple
expression for an average oceanic radon flux is derived and applied using a subset of the
observations considered to be representative of air parcels with an extended oceanic fetch.
We discuss the dataset using the notion of an ‘oceanic event’, which is defined here as any set
of consecutive hourly observations coming from the oceanic sector. Typically, the duration of
a single oceanic event will vary from a few hours to a few days. The intermittent nature of
boundary layer wind fields can result in short-term transitions across the oceanic sector
boundaries when the air mass has mixed origins. Since the terrestrial radon source is much
stronger than the oceanic source, only oceanic events that persist on synoptic time-scales are
likely to include hourly observations that are representative of a minimally perturbed oceanic
fetch. The evolution of radon concentration in the composite oceanic event was examined. It
was found that radon concentrations in the first 20-30 hours after change to the oceanic sector
are strongly perturbed from oceanic values. After the initial 20-30 hours, mean radon
concentrations in the composite oceanic event are within the 95% confidence interval. This
suggests that radon concentrations from this portion of the composite oceanic event are
minimally perturbed from typical oceanic values. The lowest value in the range of estimated mean radon flux from the region of the Southern
Ocean within the Cape Grim radon measurement fetch is about 0.0026 atoms cm-2s-1. This
value is thought to constitute a lower limit estimate since it was obtained assuming negligible
loss of radon from the marine boundary layer to the free troposphere. Taking into account the
entrainment of radon from the marine boundary layer to the lowest layer within the free
troposphere, derived from airborne measurements of mixing of dimethyl sulphide and aerosol
particles over the Southern Ocean, leads to an upper limit estimate of about 0.006 atoms cm-
2s-1. Based on the 10 and 90 percentile radon concentration and wind speed observations, and
assuming a mechanically driven mixing height, the regional oceanic radon flux may vary
from 0.0014 to 0.008 atoms cm-2s-1 with changes in the sea state induced by wind and other
environmental parameters.
Our findings support the common assumption that oceanic radon fluxes are 2-3 orders of
magnitude lower than terrestrial radon fluxes, which are typically within the range 0.5-2
atoms cm-2s-1. Our values are lower than some experimental spot estimates of oceanic radon
flux rates made in the seventies [1, 5]. However, they are in close agreement with more recent
estimates of the flux derived from model evaluation studies and also with radon flux values
assumed in an intercomparison of the convective and synoptic processes of 20 global
atmospheric transport models sponsored by the World Climate Research Program [4].
The new method of oceanic radon flux derivation can be applied to other sites around the
world where ‘clean’ ocean air can be clearly identified. The method offers an alternative to
experimental local/spot estimates of oceanic radon flux such as the accumulation method and
gradient methods. More importantly, it is representative of a large region and allows results to
be related to a wider range of environmental parameters that influence sea state (e.g. wind
speed), which have a significant effect on ocean-atmosphere exchange.
Description
Keywords
Antarctic Ocean, Radon 222, Tasmania, Concentration, Environment, WMO, Monitoring, Wind, Seas, Atmospheric circulation
Citation
Zahorowski, W., Chambers, S., & Henderson-Sellers, A. (2004). Determination of mid-latitude radon-222 flux from the Southern Ocean using atmospheric radon-222 concentration measurements at an island ground station. Paper presented to International Conference on Isotopes in Environmental Studies – Aquatic Forum 2004 Monte-Carlo, Monaco 25–29 October 2004. Book of extended synopses. Retrieved from https://inis.iaea.org/collection/NCLCollectionStore/_Public/36/003/36003223.pdf?r=1#page=5&zoom=auto,-15,800