One-filter α- or β-activity
monitors
One-filter detectors measure the decay rates of aerosol-bound 222Rn
progeny directly accumulated by air filtration. Their α and/or
β activity is then measured in situ with dedicated detector systems.
Since they normally consist of only a filter head, counting electronics, and
a pumping device, they are much more compact than two-filter radon monitors.
A disadvantage of the one-filter method, however, is that atmospheric
222Rn activity concentrations can only be determined by making
assumptions about the radioactive disequilibrium between 222Rn and its
measured progeny in the atmosphere. This disequilibrium changes with height
above ground and the atmospheric mixing state (Jacobi and André, 1963).
Furthermore, aerosol removal processes, such as dry or wet deposition, may
bias the measurement. Depending on the location of the station and the
meteorological conditions (atmospheric humidity and precipitation events),
these latter effects may be as large as 30 % (e.g. Xia et al., 2010).
Also, one-filter detectors sample not only 222Rn progeny but also the
aerosol-bound decay products of 220Rn. Although the activity
concentrations of 220Rn itself are 1 to 2 orders of magnitude
smaller than those of 222Rn in the continental atmosphere (Jacobi and
André, 1963; Volpp, 1984), its long-lived progeny 212Pb (T1/2=10.6 h) may accumulate on static filters. The α activity of its
progeny 212Po thus needs to be carefully separated, e.g. by
spectroscopy, in such systems (see e.g. Levin et al., 2002, and Sect. 2.2.1.).
Heidelberg one-filter α monitor (HRM)
The original HRM
was designed in the 1990s and is described in detail by Levin et al. (2002).
Briefly, the system consists of a homemade filter holder carrying a Whatman
quartz filter (QMA Ø 47 mm), which continuously collects all aerosols from
an ambient airflow of ca. 1 m3 h-1, monitored with a mass flow
meter (Bronkhorst, model F-112AC-AAD-22-V). The face velocity is
approximately 0.15 m s-1 and the pressure drop over the filter about 5 kPa.
Except for situations of very high ambient aerosol concentration, which
could then block the filter, the filter is changed once per month. A surface
barrier detector (Canberra CAM 900 mm2 active surface) with
pre-amplifier is mounted in the filter holder about 5 mm from the loaded
filter's surface to measure the α particles from the decaying
222Rn and 220Rn progeny. Half-hourly integrated α-spectra
are stored and allow separation of the 222Rn-derived 214polonium
(214Po) from the high energy 220Rn-derived 212polonium
(212Po) counts. The methodology of separating 218Po and
212bismuth (212Bi) counts from the spectra, and calculating the
α activity of 214Po on the filter, is explained in detail by
Levin et al. (2002). From the flow rate through the filter the atmospheric
214Po activity concentration can be calculated, taking into account the
filter efficiency and the solid angle of the detector (which depends on the
distance of the detector from the filter).
In 2010 the original HRM design was modernized by implementing
state-of-the-art electronics, data acquisition, and evaluation hardware and
software (Rosenfeld, 2010). The filter holder was also slightly modified to
allow more direct air flow from the intake onto the filter (avoiding
potential loss of aerosols at the surfaces of the filter holder). Other
aspects, however, including the solid angle of the detector and all other
parameters, were kept the same. Long-term comparisons between a modernized
HRM and our reference monitor that has been running at Heidelberg station
since 1999 with regular checks of its measurement efficiency using a
241americium (241Am) α source showed no significant
difference between the first- and the second-generation monitors (see also
Sect. 2.3).
The HRM is not calibrated as such. Except for a calculation of the solid
angle of the detector (solid angle = 0.265; Cuntz, 1997), we assume that
the detector efficiency for α particles is 100 %. The filter
efficiency has been determined to be 100 %, except for the first few hours
after filter change, when the aerosol loading is still very low. The mass
flow meter has been calibrated by the company to within ±2 %.
Atmospheric 222Rn activity concentrations can then be derived from
atmospheric 214Po activity concentration, if the disequilibrium between
222Rn and its progeny at the measurement site is known (see below).
Finnish Meteorological Institute (FMI) one-filter β-activity monitors
The FMI standard one-filter β-activity monitor in Helsinki is based
on a pair of filter-holder/GM-tube assemblies, together with supporting
electronics. Glass fiber filters (Whatman GF/A, 130 × 120 mm2) are placed
around cylindrical filter holders with Geiger–Müller (GM) tubes inside
(Paatero et al., 1994). Air is drawn through the filters alternately in
4 h periods at ca. 23 m3 h-1, and counts from both
GM counters are read and saved in 1 min intervals. Filters are changed
every 1 to 2 weeks. The particle removal efficiency of the glass fiber
filter was measured to be better than 99 % with a face velocity of
0.10 m s-1 and a pressure drop of 6 kPa (Mattsson et al., 1965). The
filter-holder/GM-tube assemblies are surrounded by lead shielding to reduce
the background count rate. 222Rn activity concentration is calculated
assuming (i) equilibrium between 222Rn and its short-lived progeny
nuclides and (ii) there is no significant amount of long-lived beta
activity (artificial or from 212Pb from the 220Rn series) present.
The β-counting efficiencies are taken to be 0.96 % for 214Pb
and 4.3 % for 214Bi, determined with an analyser utilizing an
alpha–beta pseudo-coincidence technique (Mattsson et al., 1996). These
counting efficiencies are used for both FMI systems at Pallas (FMI-1) and in
Helsinki (FMI-2), as the counting geometries and GM-tube models are
identical. This type of monitor was originally designed and employed in the
early 1960s for radiation monitoring purposes; it was not specifically
designed for 222Rn measurements.
LSCE active deposit moving filter progeny monitor
The LSCE monitor (Polian, 1986; Biraud, 2000) determines 222Rn
activity from measurements of its short-lived progeny 218Po and
214Po and uses the so-called active deposit method with a moving filter
tape. The measurement is a two-stage process with a sampling period, where
attached radon progeny are collected on the cellulose filter
(Pöllman–Schneider), followed by a counting period, which begins after
the exposed portion of filter tape (13.8 cm2) has been advanced under
the detector. Ambient air is pumped through the filter (deposition velocity
ca. 1 m s-1) for 2 h at a flow rate of about 12–14 m3 h-1.
Following this sampling period the filter tape advances under an
α spectrometer (scintillator from Harshaw Company and
photomultiplier from EMI, Electronics Ltd) to measure the radioactive decay
for 2 h. During this counting period, the radioactive decay of
218Po, 214Po and 212Po (to determine the 220Rn activity)
on the filter is logged every 10 min. Knowing the temporal evolution of the
α decays on the filter during the 2 h counting, atmospheric
222Rn (resp. 222Rn progeny) activity when the sample was being
collected can be calculated (Biraud, 2000).
Bundesamt für Strahlenschutz α/β monitor (P3)
The Federal Office for Radiation Protection (BfS) developed the
α/β monitor (so-called P3) in the late 1950s to continuously
monitor the natural (220Rn / 212Po and 222Rn) and
artificial β-activity concentrations in ambient air. The technique
applied is based on a static one-filter detection system (see Stockburger,
1960, and Stockburger and Sittkus, 1966, for details). The electronics for
counting and data recording as well as the pumping system was modernized
several times since 1966, but the detector system is still unchanged. Ambient
air is drawn continuously with an airflow of ca. 50 m3 h-1 through
a cellulose nitrate membrane filter (pore size 1.2 µm, Sartorius
Stedim Biotech). On this filter, aerosols, including the progeny of
222Rn and 220Rn, are quantitatively collected and the activities
are measured with a (custom-made) sandwich counter, consisting of three
independent proportional gas flow counters (counting gas: 100 % methane
2.5). The exposed effective filter size is 300 cm2
(0.23 × 0.13 m2), the face velocity 0.46 m s-1, and the
pressure drop ca. 22 kPa. The high voltages of the counters as well as the
thickness of the foils between them are adjusted in such a way that the lower
energy α particles are measured by the first counter above the filter,
the high-energy α particles by the middle counter, and only the
β particles are measured by the third counter. The α activity
of the short-lived 222Rn progeny 218Po (αE=6.0 MeV, T1/2=3.05 min) and 214Po (αE=7.69 MeV, T1/2=164 µs) collected on the filter is measured
in situ, mainly by the counter positioned directly above the filter. Only the
high-energy α particles (8.78 MeV) from the decay of 212Po from
the 220Rn decay chain could be measured in the middle proportional
counters. Based on this count rate, corrections are made for activity
contributions coming from the progeny of 220Rn to the ones of 222Rn
measured by the lower counter. From this corrected count rate, the
atmospheric 222Rn activity concentration is derived, assuming an
equilibrium of 222Rn with the measured progeny. Finally, the artificial
β activity is calculated.
Tracerlab Working Level Monitor (WLM) one-filter system
The Tracerlab WLM is a one-filter instrument (using a quantitatively
collecting cellulose nitrate membrane filter, pore size 0.8 µm,
effective diameter 25 mm) that measures the potential α-energy
concentration (typically given in units of J m-3, however, here as
“radon equivalent” in Bq m-3). The monitor uses α spectroscopy, so discrimination between 218Po, 214Po, and also
214Po and 212Po is possible. The atmospheric 222Rn activity
concentration is estimated using the ratio of three 222Rn progeny
(218Po, 214Po, 214Pb) and the airflow (typical flow rate ca.
0.7 m3 h-1, filter face velocity ca. 0.4 m s-1) recorded
by means of a mass flow controller, assuming equilibrium between 222Rn
and its progeny. The WLM uses a mathematical calibration method. There is no
explicit mathematical formula available because an iterative method is
applied. The sampling and the decay of the filter activities are described by
differential equations:
dA(218Po)/dt=C(218Po)⋅V-λ(218Po)⋅A(218Po)dA(214Pb)/dt=C(214Pb)⋅V-λ(214Po)⋅(A(218Po)-A(214Pb))dA(214Bi)/dt=C(214Bi)⋅V-λ(214Bi)⋅(A(214Pb)-A(214Bi))dN(218Po)/dt=η⋅A(218Po)dN(214Po)/dt=η⋅A(214Po)with:A(214Po)=A(214Bi),
where A represents filter activities of the Rn progeny,
C is the activity concentration of the Rn progeny in air,
V is the online measured volume air flow rate,
λ are the decay constants,
N is the number of α counts, and
η is the counting efficiency of the detector-filter system.
The microcomputer of the WLM integrates in real time the differential
equations for 20 different initial sets of the air activity concentrations
C(218Po), C(214Pb), and C(214Bi). That is, the collection and the
decay of the filter activity is simulated during the measurement. The result
of the 20 simultaneous simulations are 20 pairs of calculated counts
N(218Po) and N(214Pb).
The used sets of air activities are distributed over the range
from
C(218Po) : C(214Pb) : C(214Bi) = 26.34 : 1.862 : 0.132
to
C(218Po) : C(214Pb) : C(214Bi) = 3.766 : 3.766 : 3.766.
The calculated α counts of 218Po and 214Po for each of the
20 sets are compared with the real α counts seen by the detector.
The ratio of the air activities, which fits best is taken to calculate the
calibration factors for the potential α energy and the Rn progeny.
The activity concentration of 218Po and the concentration ratio
218Po and 214Bi are used to estimate the radon gas concentration
at equilibrium according to
C(222Rn)=C(218Po)⋅(C(218Po)/C(214Bi))⋅kwithk=0.3.
Cycle time is 1 h, and the filter is changed every 24 h. The
manufacturer describes the detection limit of this instrument as
0.2 Bq m-3, the uncertainty of measured activity with ±5 %, and
the uncertainty of estimated 222Rn assuming equilibrium with
±25 %. (Method description from the operating manual of “Tracerlab WLM
ASF 200” by TRACERLAB GmbH, Aachener Str. 1354, 50859 Cologne, Germany.)
Comparison of 214Po activity
concentrations of two Heidelberg radon monitors. HD-R is the monitor
routinely running at the Heidelberg measurement site. 1_HD
(uncal) is a monitor which had not been calibrated with HD-R before. All
monitors that were used for the comparison campaigns were calibrated against
HD-R.
Results from comparisons performed with the Heidelberg
radon monitor (HRM) run at different European stations. The slopes
(correction factors) are defined as (routine station
monitor) / HRM (see Figs. S1 – S12).
ANSTO monitors
Period
Activity range
Slope
Offset
Cabauw: 200/180 m
10 July–26 August 2012
0–8 Bq m-3
1.11 ± 0.04
0.11 ± 0.06
Cabauw: 20 m
27 June 2012–10 January 2013
0–12 Bq m-3
1.30 ± 0.01
0.21 ± 0.03
Lutjewad: 60 m
1 January–1 October 2007
0–6 Bq m-3
1.11 ± 0.02
0.11 ± 0.02
Heidelberg: 35 m
25 April–31 July 2015
0–15 Bq m-3
1.22 ± 0.01
0.42 ± 0.04
Other monitors
Pallas: FMI-1 2014
14 June–15 September 2014
0–6 Bq m-3
1.45 ± 0.05
0.18 ± 0.06
Helsinki: FMI-2 May 2014
22 May–10 June 2014
0–6 Bq m-3
1.04 ± 0.06
-0.03 ± 0.11
Helsinki: FMI-2 October 2014
1–22 October 2014
0–10 Bq m-3
1.02 ± 0.03
-0.03 ± 0.09
Mace Head: LSCE 2013
4 March–20 May 2013
0–3.5 Bq m-3
0.95 ± 0.07
-0.06 ± 0.06
GIF: LSCE 2014
27 February–28 April 2014
0–9 Bq m-3
0.68 ± 0.03
-0.18 ± 0.09
SIL: BfS 2013 vs. 5_SIL2
24 September–10 December 2013
0–8 Bq m-3
1.12 ± 0.02
0.24 ± 0.04
SIL: BfS 2013 vs. 9_InGOS
24 September–10 December 2013
0–8 Bq m-3
1.12 ± 0.02
0.24 ± 0.04
HPB: Tracerlab 2014
1 January 2014–30 April 2014
0–12 Bq m-3
1.03 ± 0.02
0.26 ± 0.05
Method of comparison between radon monitors
As an example of the comparison method used throughout this study, here we
compare observations between an original HRM (i.e. our reference monitor,
called HD-R (Heidelberg reference), that is used as reference throughout the
comparison project to calibrate all other monitors that were sent to the
various stations) and a modernized HRM in Heidelberg. A typical comparison
period is displayed in Fig. 1. The upper panel of Fig. 1 shows the
atmospheric 214Po activity concentrations measured over 6 weeks in
spring 2012 with two Heidelberg monitors (HD-R and the first prototype of the
modernized version called “1_HD”). For a quantitative evaluation of the
compatibility of measurements between the two monitors we first calculate the
half-hourly activity ratios. The mean of these ratios (Fig. 1b) was
1.012 ± 0.127 in the concentration range 1 to 15 Bq m-3, which
is typical for the Heidelberg measurement site, sampling air from about
30 m a.g.l. The half-hourly activity ratios show increasing scatter when
ambient concentrations decrease. Linear regression of the half-hourly
activity concentration data is displayed in Fig. 1c. The slope of the York
fit (York et al., 2004), taking into account errors in both the x and y
components, is 1.021 ± 0.016, i.e. not significantly different from
unity and the intercept is very close to zero. The uncertainty of the slope
is very small and may be used as an approximation of the mean compatibility
of long-term measurements with different Heidelberg instruments. Likewise,
the standard deviation of the activity concentration ratios allows an
estimate of the typical measurement repeatability in the concentration range
at the observational site. The respective standard deviation of ca. 13 %
for the half-hourly ratios of the two Heidelberg data sets from Fig. 1b is at
the upper end of our monitor comparability (generally between 7 and
14 %). From this we can estimate a typical uncertainty of half-hourly
atmospheric 214Po data of about 10 %. This is in accordance with
uncertainty estimates reported by Levin et al. (2002).
Similar comparison evaluations to those shown in Fig. 1 were made for a pair
of monitors at Heidelberg and for detector pairs (mobile HRMs and routine
station monitors) at the other sites included in the European Radon
Comparison Project. It should be noted that in all comparisons presented here
(see Supplement Figs. S1–S12, Schmithüsen et al., 2017), we do not correct for disequilibrium but
directly compare the 214Po or other 222Rn progeny activity
concentrations (in the case of one-filter systems) or to 222Rn activity
concentrations (in the case of two-filter systems, i.e. from ANSTO).
Map of European stations where 222radon comparison campaigns were conducted. This
map was created with Google Earth (http://earth.google.com).
Site descriptions and 222Rn instrumentation
at the comparison stations
Between 2007 and 2015, different HRMs were sent from Heidelberg to eight
stations in Europe for comparison with the local radon measurement systems
(for station locations, see map Fig. 2). In addition, comparison between the
HRM and a newly installed ANSTO monitor in Heidelberg was made. Before and
after each measurement campaign, the mobile HRM was calibrated against our
original reference monitor HD-R in the Heidelberg laboratory. All comparison
periods at the remote stations covered at least 4 weeks, to obtain sufficient
data and sample different meteorological conditions. The stations, campaign
dates, concentration ranges covered, as well as slopes and y intercepts of
the regression lines are summarized in Table 1. A brief description of the
station characteristics and routine measurement systems used at these sites
is given in the following sections.
Pallas (FI, 67∘58′ N, 24∘07′ E; 565 m a.s.l.)
The WMO/GAW station Pallas is located in Northern Finland ca. 170 km north
of the Arctic Circle. The station lies on top of a treeless subarctic hill
(fell), Sammaltunturi, at an elevation of 565 m a.s.l., and some
200–300 m higher than the surrounding area. Routine radon measurements at
this site are conducted using a simplified FMI β-activity monitor.
This monitor has only one filter-holder/GM-tube assembly through which air is
continuously drawn. This simplified monitor does not take into account
possible beta activity from artificial (i.e. long-lived) radio nuclides or
220Rn progeny. It was adapted for Pallas because the station is not
part of the national radiation surveillance network. Most of the year,
220Rn progeny cannot be transported from the local soil to the
atmosphere due to frozen ground and snow cover. However, at times of the year
when local soils do emit 220Rn to the atmosphere it rarely influences
observations since the station, due to its elevation, predominantly samples
free tropospheric air. Therefore, 220Rn progeny have a negligible
contribution to the total beta count rate (Mattsson et al., 1996; Paatero et
al., 1998). For the same reason 222Rn and its short-lived progeny at
this site predominantly arise through long-range transport and are
consequently close to equilibrium during most meteorological situations (see
below).
Ambient air is collected via an inlet 5 m a.g.l. Due to its elevation, the
station is in cloud from time to time, ca. 10 % of the time during summer
and up to 40 % of the time during autumn (Hatakka et al., 2003). For this
reason the sampling line inlet is warmed during the seasons when the
temperature can drop below freezing (ca. October–May). A rough estimation of
the 1σ counting statistics of the Pallas monitor is ±20 %,
assuming a stable 222Rn activity concentration of 1 Bq m-3. The
comparison campaign at Pallas was conducted during summer and autumn, i.e.
from 14 June to 15 September 2014. The activity concentration range covered
during this campaign (as measured by the HRM) was between 0.05 and
6 Bq m-3.
As with all systems that measure aerosol-bound 222Rn progeny, there are
uncertainties associated with estimating atmospheric 222Rn activity
concentration due to potential disequilibrium between 222Rn and its
progeny. This is particularly the case when the air is humidity saturated.
According to Gründel and Porstendörfer (2004) over 80 % of the
short-lived radon progeny are attached to accumulation-mode particles. If the
monitor is sampling in cloud or fog, these particles can form cloud droplets.
Komppula et al. (2005) have reported that at Pallas on the average 87 %
of the accumulation-mode particles and 30 % of Aitken-mode particles grow
to cloud droplets. The system does not collect these droplets due to the
sampling line design. However, the comparison at this station is between a
pair of one-filter systems, and both instruments encounter this problem.
Helsinki (FI, 60∘12′ N, 24∘58′ E; 26 m a.s.l.)
The FMI's head office is located on top of a small hill at Kumpula campus,
Helsinki, about 4 km NNE of the city centre. Routine radon measurements are
conducted using a standard FMI one-filter β-activity monitor. Ambient
air is collected at 27 m a.g.l. The estimated counting uncertainty is
±20 %, assuming a stable 222Rn activity concentration of
1 Bq m-3. The comparisons were conducted in two periods, i.e. from
22 May to 10 June and from 1 to 22 October 2014. The activity concentrations
covered ranges in the first campaign from almost zero to 6 Bq m-3 and in the second
campaign from almost 0 up to ca. 10 Bq m-3.
Mace Head (IR, 53∘20′ N, 9∘54′ W;
15 m a.s.l.)
The WMO/GAW and AGAGE station Mace Head is located at the west coast of
Ireland, about 10 m away from the coastline (Fig. 2). 222Rn and
220Rn progeny have been monitored at Mace Head since June 1995. Routine
measurements at this site are conducted using an active deposit moving-filter
monitor, built and run by LSCE (Polian, 1986; Biraud, 2000). The detection
limit of the LSCE measurement system is 0.3 mBq m-3. The statistical
error for a 2 h measurement period at ambient activity concentrations of
about 1 mBq m-3 is close to 10 %, and the total error including
uncertainties on flow rate and filtering efficiency is estimated to
±20 %.
The comparison measurements at Mace Head were made at the occasion of a
comparison campaign performed for greenhouse gases measurements in the
framework of the InGOS project (Vardag et al., 2014) from 4 March to
20 May 2013. Ambient air for 222Rn progeny comparison measurements was
collected here using 11 m of standard Decabon tubing (10 mm inner diameter)
with the air intake at ca. 5 m a.g.l., the same height and type of tubing
as for the routine measurements.
Cabauw (NL, 51∘58′ N, 4∘56′ E;
-0.7 m a.s.l.)
The instrument tower at Cabauw is 213 m high, built specifically for
meteorological research to establish relations between the states of the
atmospheric boundary layer, land surface conditions, and the general weather
situation for all seasons. The tower is located in the western part of the
Netherlands in a polder 0.7 m below average sea level. This site was chosen
because it is representative for this part of the Netherlands. The North Sea
is more than 50 km away to the WNW. Routine radon measurements at this site
are conducted using two 1500 L ANSTO two-filter detectors operating at two
heights: 20 and 200 m a.g.l. Air for each monitor is drawn at approximately
6 m3 h-1 through 7 cm outer diameter terylene fiber water pipes
by a stack blower and pushed through the radon monitor.
Uncertainty of the calibrated hourly radon concentrations depends upon a
combination of calibration source accuracy (±4 % for both detectors),
statistical counting error (which decreases with increasing radon activity
concentration: e.g. 30 % at ∼ 0.03 Bq m-3, 13 % at
0.1 Bq m-3, 3 % at 1 Bq m-3), the coefficient of
variability of valid monthly calibration coefficients (2.1 % at 20 m and
2.4 % at 200 m), and the background count variability (σ≈7 mBq m-3). Therefore, at radon concentrations of around
100 mBq m-3, the uncertainty would be of order 26 %, but this
reduces to ∼ 10 % at a concentration of 1 Bq m-3.
As two ANSTO monitors are continuously analysing 222Rn in air from the
20 and the 200 m level at Cabauw, this provided the opportunity to compare
both instruments with the HRM. Two Heidelberg radon monitors were used, and
for some time they were run in parallel for comparisons at both levels. However,
as there was no possibility to install the HRM filter head directly at the
200 m level close to the ANSTO intake, it was set up at the 180 m platform,
i.e. 20 m below the intake of the ANSTO system. This platform is accessible
via stairs and/or an elevator, so that the HRM filter changes were easy to
perform.
At Cabauw two tests could be conducted: (1) HRMa collected air directly at
the 180 m level through a short (0.5 m) Teflon tubing (10 July–26 August
2012) and (2) HRMb collected air directly from the 20 m level also through a
short (0.5 m) Teflon intake line (27 June 2012– 10 January 2013).
Lutjewad (NL, 53∘24′ N, 6∘21′ E; 1 m a.s.l.)
Lutjewad station is located directly on the sea dike at the Dutch North Sea
coast in the so-called Julianapolder reclaimed in 1923. The location allows
for the sampling of continental air masses with southerly winds, as well as
nearly undisturbed marine air masses with a long North Sea fetch from the
north.
Routine radon measurements at Lutjewad have been conducted since August 2005,
using a 1500 L ANSTO two-filter monitor located in the Lutjewad station
building. Sample air is drawn at a flow rate of 4.8 m3 h-1 from
the top of a 60 m tower through 100 m of 100 mm internal diameter PVC pipe
(van der Laan et al., 2010). As for the Cabauw monitors, the uncertainty of
calibrated radon concentrations is a combination of source accuracy
(±4 %), the detector's counting error (which decreases with
increasing radon concentration, i.e. 30 % at 0.04 Bq m-3, 15 %
at 0.1 Bq m-3, 3.5 % at 1 Bq m-3), the coefficient of
variability of valid monthly calibration coefficients (2 %), and the
background count variability (σ≈10 mBq m-3). Therefore,
at concentrations of around 100 mBq m-3, the uncertainty would be of
order 31 %, but this reduces to around 11 % at a concentration of
1 Bq m-3.
For the comparison campaign at Lutjewad, the filter holder of the HRM was
mounted also at the 60 m level of the tower with a 0.5 m Teflon inlet pipe
and a funnel to prevent rainwater intake. The comparison was conducted from
1 January to 1 October 2007.
Heidelberg (DE, 49∘25′ N, 8∘41′ E;
116 m a.s.l.)
Heidelberg is a medium size city located in the Upper Rhine valley in
south-west Germany. Monitoring of air constituents such as greenhouse gases
(Levin et al., 2011) is conducted from the roof of the institute's building
on the university campus. 222Rn has been measured at this station since
1999 with an original HRM. Since April 2015 radon has also been monitored
simultaneously with an ANSTO 1500 L two-filter radon monitor and a second
HRM. Both detectors sample from a height of ca. 35 m a.g.l., through short
co-located intake lines. The ANSTO monitor samples at a flow rate of ca.
3 m3 h-1.
As for the Cabauw detector, the two-filter monitor at Heidelberg is
calibrated at about monthly intervals by introducing an air stream with
222Rn from a 226Ra calibration source of known emission rate (Pylon
model 2000A passive radon source). Background measurements are performed
about every 3 months. Uncertainty of the calibrated hourly radon
concentrations depends upon a combination of calibration source accuracy
(±4 %), statistical counting error (which decreases with increasing
radon concentration: e.g. 30 % at ∼ 0.034 Bq m-3, 13 %
at 0.1 Bq m-3, 3.2 % at 1 Bq m-3), the coefficient of
variability of valid monthly calibration coefficients (3.5 %), and the
background count variability (σ≈5 mBq m-3). Therefore,
at radon concentrations of around 100 mBq m-3, the uncertainty would
be of order 26 %, but this reduces to ∼ 11 % at a concentration
of 1 Bq m-3. The results from the HRM–ANSTO comparison in
Heidelberg are presented for the period May–July 2015.
Gif-sur-Yvette (F, 48∘25′ N, 02∘05′ E; 167 m a.s.l.)
Gif-sur-Yvette station is located approximately 20 km southwest of Paris.
Routine radon measurements have been performed at this station since 2002
using an LSCE active deposit moving filter detector. However, unlike the LSCE
monitor configuration at Mace Head, the sampling period at this site lasts
only 1 h before the filter is placed under an α spectrometer to
measure the radioactive decay of the 222Rn and 220Rn progeny. The
inlet line at Gif-sur-Yvette station is located only 2 m a.g.l., where the
short-lived 222Rn progeny are not in equilibrium with the gaseous
222Rn. However, as we compare only 214Po activity concentrations,
this is not relevant here. The comparison campaign at Gif-sur-Yvette was
conducted from 27 February to 28 April 2014, with an activity concentration
range of about 0–9 Bq m-3.
Schauinsland (DE, 47∘55′ N, 07∘54′ E;
1205 m a.s.l.)
The measurement station of BfS at Schauinsland in the Black Forest in
south-western Germany is located on a mountain ridge at an elevation of about
1000 m above the Upper Rhine Valley. During daytime in summer the station is
frequently influenced by upslope winds, while at night, and also in winter,
it is often isolated from the valley meteorology by an inversion layer and
samples free tropospheric air. Routine radon measurements are conducted at
Schauinsland by the BfS using the
P3 α/β monitor. Ambient air is drawn in continuously from ca.
2.5 m a.g.l. and pumped through the membrane filter (mixed cellulose ester,
1.2 µm, 250 × 150 mm2 ME 28, Schleicher &
Schuell,
until April 2010; afterwards cellulose nitrate filters from Sartorius Stedim
Biotech GmbH) for 1 week. After this sampling time the pump is switched
off, a 1 h calibration check is performed using a 241Am / 90Sr
source, the filter is replaced with a new one, the background is measured
with a new filter for an additional hour, and then the air flow is started
again. The sensitivity for 222Rn is 3.367 Bq cps-1 (counts per
second) or 0.0673 Bq m-3 cps-1 for an airflow rate of about
50 m3 h-1. The background count rate used for data evaluation is
0.043 cps and was determined during a period of several days with no
airflow. The temporal resolution of 222Rn and progeny measurements is
10 min. Stockburger (1960) estimated an uncertainty of 3–4 % for a
typical 222Rn measurement at Schauinsland at activity concentrations of
1–4 Bq m-3 (not taking into account uncertainties in the
disequilibrium). More realistically, we assume an overall uncertainty (of
214Po) to be comparable to that of the Heidelberg system, i.e. around
5–10 % for the activity concentration range of 0–8 Bq m-3, as
measured during the comparison of the two detection systems.
At Schauinsland, comparison with two HRMs was conducted in parallel from
24 September to 10 December 2013. An earlier comparison study with an ANSTO
system had been performed in 2007–2008 by Xia et al. (2010). For this
comparison the authors reported mean ratios between the ANSTO and the BfS
system of BfS / ANSTO = 0.74 to 0.87, depending on meteorological
conditions. Still, during dry weather situations with potentially small
aerosol loss processes being active, the ANSTO system was measuring at least
14 % higher activity concentrations than the BfS system. We will discuss
the results from this study to evaluate possible calibration differences
between the one-filter systems and the ANSTO detectors, as well as for
estimating 214Po / 222Rn disequilibrium at the Schauinsland
station.
Hohenpeißenberg (DE, 47∘48′ N,
11∘01′ E;
985 m a.s.l.)
The GAW station HPB is located on a small mountain
ridge in the pre-Alps in southern Germany. It is run by the German Weather
Service (DWD). Radon progeny measurements started here in 1999 with the data
being available at WMO/GAW World Data Centre for Greenhouse Gases (WDCGG)
(http://ds.data.jma.go.jp/gmd/wdcgg/cgi-bin/wdcgg/accessdata.cgi?index=HPB647N00-DWD¶=222Rn&select=inventory).
Routine radon measurements at this station are made using a Tracerlab WML
monitor. The inlet line for the HPB 222Rn monitor consists of a ca.
0.4 m, 6 cm inner diameter PVC tubing. For comparison measurements with the
HRM its 0.4 m long PFA tubing was mounted inside the HPB PVC tubing,
ensuring the same air is sucked into the respective instruments. Both
monitors were located at the HPB-GAW lab on the fourth floor of the building
with the air intake directly at the window, ca. 10 m a.g.l. As no
disequilibrium between atmospheric 222Rn and 214Po is taken into
account in the data evaluation, we assume equal activity concentration of
214Po and estimated 222Rn of the Tracerlab WML detector in our
comparison.
The last calibration of this instrument took place at the BfS, Berlin, Germany, with calibration mark
612/D-K-15063-01-00/2013-03 in March 2012 at activity concentrations measured
by the reference monitor with 10.1×10-6 J m-3 with an
extended uncertainty of 1.2×10-6 J m-3. The HPB Tracerlab
WLM measured an activity of 9.2×10-6 J m-3 with an extended
uncertainty of 1.4×10-6 J m-3, leading to a ratio between
reference and examinee analyser of 1.09±0.21 on the 95 % confidence
level. The comparison campaign with the HRM at HPB
observatory was conducted from 1 January to 30 April 2014.