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| Titel |
Determination of 222-Rn exhalation rates in an urban area - comparison of top-down and bottom-up approach |
| VerfasserIn |
Miroslaw Zimnoch, Paulina Wach, Kazimierz Różański, Jolanta Godlowska, Jadwiga Mazur, Krzysztof Kozak |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052097
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| Zusammenfassung |
| Radon-222 is a radioactive noble gas, widely used as a tracer of atmospheric transport. The
radon precursor, 226Ra, belongs to 238U series and is ubiquitous in the Earth’s crust and in the
soils. Due to its relatively long half-life (T1-2 = 3.8 days), 222Rn mostly diffuses out of the
soil into the atmosphere, where it decays to 218Po. The exhalation rate of 222Rn into the
atmosphere is controlled by the source term i.e. 226Ra content in the soil and its vertical
distribution, by physical properties of the upper soil layer such as mineral structure,
porosity and water content and to some extent by short-term variations of physical
parameters characterizing the soil-atmosphere interface (atmospheric temperature and
pressure).
Radon-222 has been repeatedly used in the past decade as a tool to assess surface
emissions of greenhouse gases such as CO2 and CH4 into the atmosphere, originating from
distributed sources. The method requires a priori knowledge of 222Rn exhalation rate in the
given area and its temporal variability.
In this study we present a comparison of two independent methods of deriving exhalation
rates of 222Rn in an urban environment: (i) direct measurements utilizing the static chamber
method, and (ii) determination of 222Rn exhalation rates through measurements of
mixing layer height in the lower atmosphere, combined with measurements of 222Rn
specific activity in near-ground atmosphere. The comparison was performed in
Krakow, southern Poland, with parallel measurements of 222Rn exhalation rates
running from September 2005 to September 2006. Only night-time 222Rn fluxes were
compared.
Direct measurements of 222Rn flux (static chamber method) were performed using the
AlphaGUARD PQ2000 PRO® monitor together with an accumulation chamber working in
an automatic system which made it possible to perform 4 measurements during 24 hours.
The system was developed at the Institute of Nuclear Physics, Polish Academy of
Sciences. The mixing layer height was measured with vertical doppler sodar system
developed at the Department of Monitoring and Modelling Air Pollution, Institute of
Meteorology and Water Management. Atmospheric concentrations of 222Rn were
measured using radon monitor based on alpha spectrometry of 222Rn daughters
captured from the air stream on a glass filter placed directly over the surface barrier
detector measuring alpha particles emitted by 222Rn daughter products. The radon
monitor used for atmospheric 222Rn measurements was developed at the Institute of
Environmental Physics, University of Heidelberg, Germany, and made available for this
study.
The night-time monthly mean 222Rn exhalation rates were calculated from the data
obtained using the methods briefly described above. Generally, high values of 222Rn
exhalation rates were observed during summer months (July, August, September) and
reduced values during winter months (February, March). The maximum monthly mean value
of 222Rn exhalation rate (ca. 105 Bq m-2 h-1) measured with the aid of chamber method
was observed in September 2005, to be compared with ca. 65 Bq m-2 h-1 obtained from the
indirect method. The minimum 222Rn exhalation rate was recorded in February 2006 (ca. 20
Bq m-2 h-1and 10 Bq m-2 h-1, for the sodar-assisted and for the chamber measurements,
respectively). Although the amplitude of seasonal changes of 222Rn exhalation rate
derived from chamber measurements was significantly higher than that obtained
using the indirect method based on sodar measurements of mixing layer height and
atmospheric measurements of 222Rn concentration, a reasonably good agreement
between both methods was obtained (r2 = 0.67). The principal reason for the observed
differences in the measured 222Rn fluxes can be attributed to different footprint of
both methods. While the chamber method yields 222Rh exhalation rates averaged
over an area of ca. 0.04 m2 covered by the chamber, the footprint of the indirect
method is in the order of several square kilometers, comparable to the size of the
city.
The research was supported by the Ministry of Science and Higher Education (project No.
4132/B/T02/2008/34), and the statutory funds of the AGH University of Science and
Technology (project No.11.11.220.01) |
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