Hydrogen soil deposition at an urban site in Finland

Abstract

Hydrogen deposition velocities (v_d) were estimated by field chamber measurements and model simulations. A closed-chamber method was used for soil deposition studies in Helsinki, Finland, at an urban park inhabited by broad-leaved trees. Radon tracer method was used to estimate the v_d in nighttime when photochemical reactions were minimal and radon gas was concentrated to shallow boundary layer due to exhalation from soil. A two-dimensional atmospheric model was used for calculation of respective v_d values and radon exhalation rate. v_d and radon exhalation rates were lower in winter than in summer according to all methods. The radon tracer method and two-dimensional model results for hydrogen deposition velocity were in the range of 0.13 mm s−1 to 0.90 mm s−1 (radon tracer) and 0.12 mm s−1 to 0.61 mm s−1 (two-dimensional). The soil chamber results for v_d were 0.00 mm s−1 to 0.70 mm s−1. Both models and chamber measurements revealed relation between one week cumulative rain sum and deposition velocity. Lower v_d values were usually measured in high soil moisture conditions. Precipitation occurring a few days before chamber measurements decreased v_d values. The snow cover also lowered v_d.

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Atmos. Chem. Phys., 9, 8559–8571, 2009
www.atmos-chem-phys.net/9/8559/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Hydrogen soil deposition at an urban site in Finland
M. Lallo, T. Aalto, J. Hatakka, and T. Laurila
Finnish Meteorological Institute, Helsinki, Finland
Received: 10 June 2009 – Published in Atmos. Chem. Phys. Discuss.: 9 July 2009
Revised: 17 October 2009 – Accepted: 28 October 2009 – Published: 11 November 2009
Abstract. Hydrogen deposition velocities (vd) were esti-
mated by field chamber measurements and model simula-
tions. A closed-chamber method was used for soil depo-
sition studies in Helsinki, Finland, at an urban park inhab-
ited by broad-leaved trees. Radon tracer method was used
to estimate the vd in nighttime when photochemical reac-
tions were minimal and radon gas was concentrated in the
shallow boundary layer due to exhalation from soil.
two-dimensional atmospheric model was used for the cal-
culation of respective vdvalues and radon exhalation rates.
The vd and radon exhalation rates were lower in winter
than in summer according to all methods. The radon tracer
method and the two-dimensional model results for hydro-
gen deposition velocity were in the range of 0.13mms−1to
0.93mms−1(radon tracer) and 0.12mms−1to 0.61mms−1
(two-dimensional). The soil chamber results for vd were
0.00mms−1to 0.70mms−1.
measurements revealed a relation between one week cumu-
lative rain sum and deposition velocity. When precipitation
events occurred a few days before the chamber measure-
ments, lower vdvalues were observed. A snow cover also
lowered vd.
A
Both models and chamber
1Introduction
There is a need to better understand interactions of molecular
hydrogen in the atmosphere. The interest to develop carbon
dioxide free energy production methods promotes the hydro-
gen economy goals to utilize hydrogen as an energy transport
media. Energy produced using cleaner methods (e.g. wind
power) can be used to split water to hydrogen (and oxygen)
electrochemically. The interest in hydrogen in the past few
Correspondence to: M. Lallo
(marko.lallo@fmi.fi)
years has accelerated and steered research to find out pro-
cesses, where hydrogen is participating, these include strato-
spheric (Rahn et al., 2003; R¨ ockmann et al. 2003) and tro-
pospheric studies (Barnes et al., 2003). Several authors have
reported that soil uptake is the largest sink, which is respon-
sible for 73–82% of total hydrogen turnover (Novelli et al.,
1999; Hauglustaine and Ehhalt, 2002; Rhee et al., 2006).
The soil uptake is higher in Northern Hemisphere due to
larger land coverage. An extensive review of tropospheric
hydrogen cycle was made by Ehhalt and Rohrer (2009). A
companion article by Aalto et al. (2009) is focused on at-
mospheric variations and to the traffic emission of hydrogen.
Recent field measurements to estimate soil uptake of hydro-
gen were made in agricultural area in Heidelberg (Schmitt
et al., 2009), in forest, marsh and desert area in Califor-
nia (Smith-Downey et al., 2008), in northern boreal zone
(Lallo et al., 2008) in Alaska (Rahn et al., 2002). Yone-
mura et al. (1999, 2000) made field studies in a temperate
forest and in an arable field in Japan. Earlier field measure-
ments were made by Conrad and Seiler (1980, 1985) in Ger-
many and in Africa. Soil microbes and free soil enzymes
are responsible for hydrogen uptake (Conrad, 1996; Constant
et al., 2008, 2009), and soil hydrogenases that are respon-
sible for hydrogen uptake have recently been extracted by
Guo and Conrad (2008) and Constant et al. (2008). Photo-
chemical production and destruction of atmospheric hydro-
gen is hydroxyl radical controlled. (Schmidt, 1974; Sanders,
2006; Simmonds et al., 2000). Radon tracer method is suit-
able for estimating various greenhouse gas emissions (Za-
horovski et al., 2006) including nitrous oxide (Schmidt et
al., 2001), methane (Levin et al., 1999) and carbon diox-
ide (Langend¨ orfer et al., 2002), and has recently been ap-
plied for hydrogen by Hammer and Levin (2009). The re-
gional representativeness of radon tracer method depends on
an integration time of fluxes, site topography and meteoro-
logical parameters (Levin et al., 1999). The results are de-
pendent on radon exhalation rate, which depends mainly on
Published by Copernicus Publications on behalf of the European Geosciences Union.

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8560M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland
a grain size distribution and a soil porosity. A high soil mois-
ture/water content is also known to hinder radon exhalation
(Levin et al., 2002). The latitudinal distribution of222Rn flux
is examined by Conen and Robertson (2002). A simple two-
dimensional atmospheric model, based on earlier work by
Aalto et al. (2006) and Lallo et al. (2009), was utilized to es-
timate local radon exhalation and hydrogen uptake rates by
inverting soil fluxes from atmospheric measurements. These
two models and chamber measurements were used to eval-
uate the soil sink strength and were compared against each
other. Ehhalt and Rohrer (2009) reported based on the find-
ings of Price et al. (2007) and Novelli (1999). The hydrogen
mixing ratio is asymmetrically distributed between the hemi-
spheres. In Northern Hemisphere (NH) it reaches 490ppb
at 80◦N. In Southern Hemisphere (SH) mixing ratio reaches
535 ppb at 88◦S. Barnes et al. (2003) observed a seasonal cy-
cle for mixing ratio of atmospheric hydrogen in Harvard for-
est with the maximum in winter and spring months (February
to June) and a short minimum in September–October. Sim-
monds et al. (2000) observed a similar seasonal cycle with
thehighestmixingratioduringthespringmonthsofMarchto
May and the lowest values in September to November. Rhee
et al. (2006) estimated a total budget for NH, where the to-
tal hydrogen sources were 69Tg(H2)a−1and 62Tg(H2)a−1
of the total sources were consumed by soils. For the SH
total hydrogen sources was estimated to be 38Tg(H2)a−1
and 26Tg(H2)a−1being consumed by the soils. Globally
the atmospheric sink and source budget vary between 136
and 155Tg(H2)a−1(Hauglustaine and Ehhalt, 2002; Nov-
elli, 1999).
A soil moisture is coupled with a soil temperature. The
high H2deposition velocities are usually associated with the
low soil moisture (Schmitt, 2009), when the soil temperature
is high. A thick snow layer hinders the gas permeation into a
soil surface lowering the H2deposition velocity values (Lallo
et al., 2008).
2Materials and methods
The urban park near the center of Helsinki in Kumpula
(60◦12?13??00N, 24◦57?40??00E) was selected for studies of
soil uptake of molecular hydrogen. The vegetation consists
mainly of broad-leaved trees and low grass species. The
location is under the influence of local traffic and adjacent
sea. The closest roads with a high traffic volume H¨ ameentie
(44700 cars per day) and M¨ akel¨ ankatu (45000 cars per day)
were in a minimum distance of 350m and 700m, respec-
tively.
2.1Soil chamber measurements
The soil texture of the measurement site including surface
vegetation is fine sandy till (sandy loam) according to maps
provided by the Geological Survey of Finland, (a geological
map available at: http://geomaps2.gtk.fi/geo/, 2009).
the south direction the soil texture changes to clay 100m
away from the measurement site.
was determined in laboratory studies (Soil Analysis Service
in Mikkeli, Finland) to be gravely sandy loam (fraction-
ated soil type) in the first 7cm. The 7cm to 20cm layer
was determined as fine sandy till (coarse soil type). The
field measurement setup included a stainless steel chamber
(60cm×60cm) fixed into ground about 5cm in depth. One
chamber was normally used, except on 30 October 2007,
when two chambers were used for comparison. The low
grass species vegetation was removed inside the chamber.
Both the chamber and aluminium cover was 20cm in height.
A small battery-operated fan was attached inside the cover to
ensure mixing in the closed-chamber. The first sample was
taken immediately after lowering the cover. The following
samples were taken after 2 to 5min intervals. The sampling
from the closed chamber was made through a silicon tube
mounted on the top of aluminium cover. A 20cm3plastic
syringe with a three-way stop-cock valve was attached to a
silicon tube during the sampling. The total length of one
measurement cycle was 15 to 20min, which included five
samples. The cover was opened after a cycle and cham-
ber was ventilated for a few minutes. Three to four cy-
cles were made in sequence. One ambient air sample was
taken at 2m height for reference purposes during one ar-
bitrarily chosen cycle. The effect of pressure differentials
were minimized using the short measurement time and the
large chamber volume (Davidson et al., 2002). The soil tem-
perature was measured using thermistors and thermocouples.
The volumetric soil moisture concentration was measured by
ThetaProbe ML2x sensor. Both the soil temperature and the
moisture was measured four times outside of the chamber.
The air samples were analyzed during the same day. The
closed chamber setup was tested in a laboratory. The cham-
ber was filled with the hydrogen in air gas, with the concen-
tration two times higher than the ambient hydrogen mixing
ratio. The hydrogen mixing ratio inside the closed chamber
did not change during the 20min test. The leakage from the
syringes was measured and it was found to be 4ppbh−1. The
analysis sequence differed from the method used in the atmo-
spheric samples. The sample inlet line was detached before
syringe samples. The syringe samples were injected directly
to the RGA5. At least three working standard samples were
analyzed before and after the syringe samples. A response
curve was made in the concentration range between the 200–
2000ppb. By plotting the mixing ratio against peak area, it
is possible to construct a linear or second order fitting, which
can be used for a non-linearity correction. The concentration
in the chamber decreased under 200ppb usually after 15min.
A difference between the linear and the non-linear fit var-
ied in the range 0.02mms−1–0.05mms−1. The correspond-
ing difference value between the non-corrected value and the
second order fit was in a range 0.03mms−1–0.06mms−1.
In
The detailed soil type
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M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland 8561
2.2 Atmospheric measurements
The sampling inlet was 2m above the roof of FMI (Finnish
Meteorological Institute) institute building and the roof was
25m above the ground level (53ma.s.l.). The sample air
was first transferred through a plastic tubing at a flow speed
10ms−1, with a residence time ca. 1s. A side flow to a hy-
drogen analyzer was filtered with a 1.0µm Gelman filter and
flushed through a stainless steel tubing to a flow restrictor
and a pressure relief valve, which was adjusted to pass about
200cm3min−1to the analyzer. A modified RGA5 instru-
ment with RGD detector is used for the detection of molecu-
lar hydrogen. After a chromatographic separation of sample
air, the molecular hydrogen passes through a mercury oxide
(HgO) bed. H2reduces HgO to gaseous Hg, which is then
detected by UV absorption. The same detector is also able to
detect CO. Four ambient air samples were measured during
one measurement cycle, after which a working standard was
included in the cycle. Each analysis took 5min. The sys-
tem was calibrated according to four standards (scale 400–
700ppb) acquired from Max-Planck Institute in Jena. The
reproducibility at ambient levels was obtained using calibra-
tion samples. The maximum standard deviation during the
calibration was 1.5%. The reproducibility working standard
samples (range 915–950ppb) of RGA5 instrument was esti-
mated by taking into account ten consecutive samples, and
it was found to be 1.1%. The linearity of the RGA5 instru-
ment was checked using a series of known mixing ratios over
the atmospheric range, resulting in R2of 0.97. A correlation
was calculated based on the peak area versus mixing ratio.
The quality of measurements was verified by the intercom-
parison samples of the EU-project EUROHYDROS. The de-
scription of the ambient air measurement is given in a com-
panion article by Aalto et al. (2009). The sampling line was
checked using two flask samples, taken on the roof of the
institute building. Normal ambient air samples (through the
inlet line) and flask samples were compared with each other.
The mean of three samples was close to flask samples (dif-
ference: 2ppb). The radioactive radon isotope222Rn is mea-
sured on the roof of FMI (Finnish Meteorological Institute)
building. The sampling inlet is the same as used for hydro-
gen. The determination of radon is based on the short-lived
222Rn progeny assumed to be in radioactive equilibrium. The
air samples are collected onto a filter and one hour means are
calculated. The analysis method is similar as described in
Paatero et al. (1998) and Hatakka et al. (2003). The weather
parameters were monitored with an automated weather sta-
tion MILOS 500. Wind parameters were measured with a
two component ultrasonic anemometer on a 32m high mast
next to the FMI institute building and the temperature with a
shielded Pt100 detector 2.5m above ground.
2.3Analysis of results
2.3.1Soil chamber method
The hydrogen concentration decrease inside a closed-
chamber follows an exponentially decreasing function. The
hydrogen uptake into the soil follows first-order kinetics as
mentioned by Yonemura et al. (2000). An exponential fit in
Eq. (1) was applied to the concentration values.
C(t)=(C0−yτ)exp(−t/τ)+yτ,
where t is time, C0is the hydrogen concentration at time
zero. y is a production term and τ is a decay term. The
deposition velocity is calculated as vd=h/τ, where h is the
chamber height. Hydrogen emission from the soil is taken
into account in the production term y. The hydrogen mixing
ratio decreased to 10–20ppb, when the chamber was kept
closed for three hours in a field test, thus the zero hydrogen
production can be used. There was not any clover vegetation
in the field site, which could produce hydrogen.
(1)
2.3.2Radon tracer method
The radon tracer method (Levin et al., 1999; Schmidt et al.,
2001) is suitable for the tropospheric determination of emis-
sion rates of trace gases (e.g. H2, CH4, N2O). There is a
strongcovariancebetween222Rn, CH4andalsoCO2summer
and autumn nighttime 5 mixing ratios (Levin et al., 1999).
This indicates that the changes in trace gas mixing ratios
originates from the variability of diurnal atmospheric con-
ditions rather than the short term changes of trace gas emis-
sions (Levin et al., 1999). The radon tracer method was used
only during the nights, when a stable nocturnal boundary
layer was formed. The height of nocturnal boundary layer
is usually a few hundreds of meters, in which radon is ac-
cumulating. During the nighttime, photochemical processes
affecting the hydrogen concentration are minimal since the
intensity of solar irradiation in summer in Helsinki is less
than 5% of the daytime values. For the method, those hours
were selected when the difference between maximum and
minimum hydrogen concentration was at least 5ppb. The
possibility for H2emissions is then lowest.
The photochemical reactions, e.g. due to the hydroxyl rad-
ical formation from ozone, are not significant in the low ir-
radiance conditions during nighttime, thus the major sink of
hydrogen is soil and hydrogen is consumed in the first few
centimeters of the soil (Schmitt et al., 2008), while the only
source process for222Rn is the exhalation from the soil. The
H2flux can be calculated using Eq. (2).
?
if the radon flux jRnis known (Schmidt et al., 2001). The
?cH2/?cRnis a ratio between the hydrogen concentration
difference, ?cH2and radon concentration difference, ?cRn.
jH2=jRn?cH2
?cRn
1−
λRncRn
?cRn/?t
?
(2)
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8562M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland
λRncRn(t) is the decay rate for radon including the radioac-
tive decay constant λRnand the radon concentration cRn. For
the radon tracer method calculations, only those nights were
accepted when the hydrogen mixing ratio decreased more
than 5ppb and the radon activity concentration increased
more than 0.6Bqm−3between 23:00 and 05:00LT. Only
high correlation R2>0.8 events, which had five or six hours
of data were selected.
2.3.3Two-dimensional model
The hydrogen deposition velocities were also determined by
a two-dimensional model. The model was built for support,
comparison and verification of other methods described ear-
lier. It has been applied for hydrogen soil deposition stud-
ies in Northern Finland by Lallo et al. (2009). The model is
based on the three-dimensional atmospheric model described
in Aalto et al. (2006). The variation in vegetation, land use
and topography is suppressed to one specified type to gain
faster simulation run times, while the surface fluxes are in-
vertedfromatmosphericconcentrationobservations. Theun-
certainty of fixing the vegetation to one specific type is small
compared to the uncertainty which is related to the estima-
tion of the boundary layer in the two-dimensional model. A
5m thick surface layer was built in the model, where the soil
acted as a passive solid and fluxes were defined only in the
air-soil interface of the model. The purpose of the 5m sur-
face layer is created for the calculations of the energy bal-
ance. This requires realistic values for the soil density and
the thermal conductivity. The modeled layer could also be
thinner. The model was run with setup of a 3km vertical
extent (12 layers) and a horizontal extent of 10km (10 grid
boxes), which gives a resolution of 1km. During a simula-
tion run, a user is not allowed to make any adjustment, all
changes are made by the model itself. The commercial fluid
dynamics software Fluent©(CFD Flow Modeling Software
and Solutions from Fluent, 2009) was used to solve fluxes
and concentrations of radon and hydrogen. The mass and en-
ergy exchange formulations in the soil-air surface were mod-
eled by the user defined codes added to the model (Aalto
et al., 2006). The necessary boundary conditions were also
given based on the local observations. The following bound-
ary conditions are included in the two-dimensional model:
A hydrogen and a radon outflow profile is transferred to the
inflow profile (a periodic boundary condition). Weather pa-
rameters (air temperature, humidity, atmospheric pressure,
wind speed) are controlled in the inflow, where adjustments
were made in 5min steps. These were also local observa-
tions. The two-dimensional model is used to estimate hy-
drogen deposition velocity using the meteorological input
values and hydrogen mixing ratio. In addition same two-
dimensional model is used separately for the estimation of
radon exhalation. Only the radon flux is estimated based on
the radon concentration measurements (initial profile). Af-
ter that, model changes the radon concentrations according
to model controls.
The turbulence inside the domain was simulated using the
standard K-epsilon theory by Launder and Spalding (1972).
The model equations for energy, turbulence, fluid and species
transport were solved in a segregated mode. The model al-
lowed changes to the initial value of meteorological pres-
sure i.e. the model was non-hydrostatic. As in the case of
radon tracer method, the nighttime simulations were made to
avoid the indirect photochemical degradation of hydrogen.
The model was initialized few hours before the selected time
range to achieve a balanced state. During the simulation, the
hydrogen and radon outflow vertical profile from the pre-
ceding time step was used as a new input to the following
time steps. The deposition velocities and the radon exhala-
tion rates were solved at every time step to let the modeled
outflow concentration meet the observed concentrations at
Kumpula site. The hydrogen and radon surface fluxes were
thereby inverted from the concentration observations. Simu-
lation results consist of stabilized flux values obtained at the
endofeachsimulationhour. Themodelminimizesthediffer-
ence between the modeled and observed values. For hydro-
gen, only those nights were accepted when there was at least
four hours of monotonous increase in radon and decrease in
hydrogen mixing ratios. In the case of radon, also nights with
no hydrogen decrease were accepted. These selection criteria
are somewhat different from radon tracer method and result
in a larger number of events. Our aim was to obtain an exten-
sive data set especially for radon, so that we could estimate
the radon exhalation rate separately for winter and summer.
The boundary layer height simulated by the two-
dimensional model was compared to the ceilometer data at
about 2km distance from the Kumpula site. The model pro-
duced a turn in the potential temperature profile, which was
interpreted as the top of the boundary layer. On 13 July 2007,
at 02:00 to 04:00LT the turn occurred at 124m height, while
on 18 July 2007 at 00:00 to 05:00LT it appeared at the next
model level, 198m above ground. The ceilometer bound-
ary layer estimate showed high variability for these nights,
which could not be reproduced by the model. This was ob-
served on 13 July 2007, 80–240m, and on 18 July 2007,
310–590m, respectively. On the contrasting winter night
conditions (−15◦C) in 10 February 2007, the model simu-
lated the first inversion at 75m model level, and second at
198m level, while the ceilometer results indicated 70–200m
for a boundary layer (BL) height. Generally, when the ob-
servations showed lower BL height, also the model indicated
a shallow BL. However, the model results were in the lower
endoftherangegivenbyobservations, andthereforethesim-
ulated BL height may be somewhat too low. This would re-
sult in an underestimation of the inverted hydrogen and radon
fluxes.
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M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland8563
Fig. 1. Hydrogen deposition velocity (vd) mean values from the chamber measurements, and using radon tracer method and two-dimensional
model. The vdvalues are presented as mean ±standard error of the mean (SE).
3 Results
3.1Hydrogen deposition velocities from soil chamber
measurements
The chamber measurements were performed between 28 Oc-
tober 2005 and 30 October 2007 at the urban park site in
Kumpula. The automatic weather system (AWS) data col-
lected from Helsinki area (Kaisaniemi and Kumpula) was
used in the interpretation of chamber and model results. Hy-
drogen vdvalues are shown in Fig. 1 (blue stars). The low-
est close to zero vd values were measured in January to
March2006andhighest0.70±0.02mms−1on15June2006.
The winter (13 field days from November to April) vdvalues
ranged from 0.00mms−1to 0.45mms−1with mean value of
0.20±0.05mms−1and in summer (8 field days from May to
October) from 0.13mms−1to 0.70mms−1(Table 1) with
mean value of 0.38±0.07mms−1. The lowest values oc-
curred when snow covered the ground, as indicated by Lallo
et al. (2008). The highest vd value (0.18±0.04mms−1)
with permanent snow cover was measured on 2 Febru-
ary 2007 (Fig. 2). Snow depth was then 16cm, soil tempera-
ture +0.5◦C and soil volumetric water content 0.27m3m−3.
When snow depth exceeded 20cm, vdvalues were close to
zero. When snow layer thickness was between 10cm and
20cm, vdmean values were lower than 0.20mms−1.
Hydrogen vdvalues are shown together with correspond-
ing air and soil temperatures in Fig. 3a and b (stars).
There is a large scatter in vd, but it tends to get lower
values in freezing temperatures. In Fig. 3c hydrogen vd
is plotted against soil volumetric water content showing
lower values at high soil moistures.
(above 0.50mms−1) were only recorded in drought con-
ditions, when soil moisture was between 0.10–0.25 below
and soil temperature was 10◦C or higher.
ues (below 0.20mms−1) were often associated with sub-
zero or close to zero values and snow cover (see Fig. 2).
The soil temperature was then usually below 4◦C. Accord-
ing to measurement records, heavy rain showers decreased
soil uptake when there had been precipitation within three
days before. An intensive thundershower (13.8mm) two
days before hindered the soil uptake on 24 August 2007
(Fig. 3b), lowering vdto 0.13±0.01mms−1, which was the
lowest summer time vdvalue in the whole measurement pe-
riod.On 2 August 2007 weather conditions were favor-
ing strong soil uptake. The soil temperature and the vol-
umetric water content was 15◦C and 0.24m3m−3respec-
tively, but only 0.33±0.01mms−1was recorded (Fig. 3b).
Three days before 20.4mm rain was recorded. The total
amount of 24.1mm precipitation was recorded on 31 Oc-
tober 2006 with vd value of 0.34mms−1.
high summer values recorded in 2006, soil uptake was sig-
nificantly reduced. The soil volumetric water content was
0.41m3m−3. There was a four-month drought in 2006 in
Helsinki area, when high vdvalues of 0.509±0.004mms−1
(2 August), 0.55±0.02mms−1(31 May) 0.70±0.02mms−1
(15 June) were measured (Fig. 3b and c, red stars). Soil vol-
umetric water content values were between 9m3m−3and
25m3m−3, while the typical soil volumetric water content
Higher vd values
Low vd val-
Compared to
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8564M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland
Table 1. Hydrogen deposition velocities measured and modeled with different methods.
Reference MethodComments
vd(mms−1)
this paperchamberMay to October
November to April
drought (2006)
snow
0.13–0.7
0.00–0.45
0.50–0.70
<0.20
this paperradon tracer May to October
November to April
0.14–0.93
0.13–0.27
this papertwo-dimensional modelMay to October
November to April
0.13–0.61
0.12–0.33
Hammer and Levin,
2009
radon tracerurban/suburban
environment
0.1–0.8
Rahn et al., 2002 concentration
gradients
burn boreal forest
mature boreal forest
0.44a
0.73a
Smith-Downey et al.,
2008
chamberforest
desert
marsh
0.63a
0.51a
0.35a
amean deposition velocities
Fig. 2. The effect of snow depth to soil uptake rate. R2, i.e. squared correlation coefficient, is 0.62.
values obtained in the field measurement were higher than
0.29m3m−3. The cumulative rain sum between 26 May and
29 September 2006 was only 51.6mm (287.3mm in 2007
and 265.2mm in 2008) in Helsinki area. In May 2008 the
rain sum was significantly lower (7.5mm) than in May 2006
(41.6mm) and 2007 (58.9mm) (Fig. 4).
All error bars were calculated as the standard error of the
mean (σ/√n), except otherwise mentioned. The chamber er-
ror bars were calculated by averaging all three or four repe-
titions. Sensitivity studies related to syringe sampling from
the chamber have been made in a 3-h field test. Three con-
secutive samples were taken to represent one sampling time
point. According to this test all points were well distributed
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M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland8565
Air?temperature?(°C)
-15 -10-5051015 20 25
Hydrogen?deposition?velocity?(mm?s )
-1
0.000
0.200
0.400
0.600
0.800
Chamber
Chamber?-?drought
Chamber?-?rain
Chamber?-?snow
2D?model
Rn?tracer
A
Soil?temperature?(°C)
-2024681012 14 1618
Hydrogen?deposition?velocity?(mm?s )
-1
0.000
0.200
0.400
0.600
0.800
Chamber
Chamber?-?drought
Chamber?-?rain
Chamber?-?snow
2D?model
Rn?tracer
B
24 Aug?2007
2 Aug?2007
30?Oct?2007
Fig. 3. The dependency of measured and four modeled hydrogen deposition velocity values (a) to air (b) soil temperature of chamber
measurements and (c) the dependency of measured hydrogen deposition velocity to soil volumetric water content of chamber measurements.
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8566M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland
Month
123456789 1011 12
Cumulative rain sum (mm)
0
200
400
Global radiation (W m-2)
0
200
400
600
Mean 2007-2008
Mean globrad
2006
2007
2008
Fig. 4. Biannual global radiation average obtained at 12:00UTC and biannual cumulative monthly precipitation 2007–2008 and monthly
rain sum for years 2006 to 2008.
along an exponentially decreasing curve. Soil temperature is
a slowly changing feature and it takes usually several days
to see the changing trend. This is also the case with the soil
moisture.
3.2Hydrogen deposition velocities and radon
exhalation rates from two-dimensional model
simulations
The two-dimensional model results covered the time pe-
riod between June 2007 and July 2008, when atmo-
spheric hydrogen mixing ratios were continuously mea-
sured at Kumpula. The modeled winter (13 nights) vdval-
ues ranged from 0.12mms−1to 0.33mms−1with mean
value of 0.22±0.02mms−1and in summer (35 nights)
from 0.13mms−1to 0.61mms−1with mean value of
0.35±0.02mms−1(Table 1).
exhalation rate in winter (from November to April) was
22±1Bqm−2h−1and in summer (from May to October)
45±3Bqm−2h−1with highest values occurring in Au-
gust 2007 (Fig. 5). The highest exhalation rates were ob-
served during northerly winds when air masses traveled
above continent and captured high radon activities before
arrival to the site. The northerly-northeasterly winds dom-
inated during August 2007 high radon exhalation nights.
Generally, theconditions were favorableforradonexhalation
in summer when the soil was not saturated with water despite
of short rain periods which lowered radon exhalation rates.
Radon exhalations are depicted against one week cumulative
precipitation in Fig. 6a. Pearson product moment correlation
for Fig. 6a is −0.321. The correlation is significant at the
The modeled mean radon
0.01 level (2-tailed). Over 100Bqm−2h−1emission values
were reached in dry conditions, while 25mm precipitation
lowered the radon emission down to 50Bqm−2h−1. Corre-
spondingly, one week dry period raised the hydrogen soil up-
take to hydrogen deposition velocity values up to 0.7mms−1
and 25–30mm precipitation decreased the vdvalues down to
about 0.3–0.4mms−1(Fig. 6b) according to all three meth-
ods.Pearson product moment correlation for Fig. 6b is
−0.271. Correlation is significant at the 0.05 level (2-tailed),
when outliers were excluded according to standard deviation.
The wintersoils were typicallymoister than the summer soils
due to a lower evapotranspiration regulated by a global irra-
diation, which had a maximum of over 600Wm−2in June,
while in December less than 100Wm−2(Fig. 4) was reached
(see also Vesala et al., 2006). This can be seen in the yearly
cycle of radon exhalation rates in Fig. 5, as well as in the hy-
drogen results in Fig. 1 (all methods), where lower vdvalues
were recorded during winter.
3.3Hydrogen deposition velocities from radon tracer
method
The radon tracer method results covered the time period
between June 2007 and July 2008 (Table 1).
to the radon tracer method, in winter (November to April,
5 nights), the modeled vdvalues ranged from 0.13mms−1
to 0.27mms−1with a mean value of 0.22±0.02mms−1and
in summer (May to October, 32 nights), the modeled vdval-
ues ranged from 0.14mms−1to 0.93mms−1with a mean
value of 0.43±0.04mms−1(Fig. 1, dark pink circles). The
radon tracer method is dependent on the radon exhalation
According
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M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland 8567
Month
123456789 1011 12
Modeled radon exhalation rate (Bq m-2 h-1)
10
20
30
40
50
60
70
80
2D model
Fig. 5. Two-dimensional model results for222Rn exhalation rate jRn.
rate, which was estimated by the two-dimensional model. A
monthly mean was calculated by fitting a smooth curve to
modeled radon exhalation rates (Fig. 5). The annual cycle of
hydrogen deposition velocities was similar than the one sim-
ulated with the two-dimensional model, but there were cases
when the radon tracer method indicated higher values than
the two-dimensional model, which can be seen in Figs. 1 and
6b. The range of both the radon tracer method and the two-
dimensional model results was in good agreement with the
chamber results.
All methods delivered results on 30 October 2007, the
radon tracer method and the two-dimensional model vd
values were 0.55±0.16mms−1and 0.39±0.18mms−1re-
spectively, while the soil chambers indicated a vd of only
0.19±0.01mms−1. The soil water content was 0.33m3m−3
and the soil temperature was 9◦C (Fig. 3b). There was a
rain during 30 October 2007 which may have affected the
soil chamber result, measured at midday, while the other
estimates refer to earlier nighttime observations from drier
soils. In general, the October 2007 rain sum was significantly
lower, 56mm, than in October 2006, 183.6mm and in 2008,
166.6mm. The results also had a lot of statistical variation.
The difference between the radon tracer method and the two-
dimensional model was probably due to the choice of data.
For example, if only three hours with the largest change in
hydrogen and radon are selected from the two-dimensional
modelsimulations, theresultingvdincreasesto0.65mms−1.
4 Discussion
4.1Radon exhalation rate
The radon tracer method is dependent on the pre-calculated
radon exhalation rate jRn, which is based on regional
radon emission estimates.
were measured by D¨ orr and M¨ unnich (1990) in culti-
vated fields and undisturbed forest soils. The fluxes were
in the range 500dpmm−2h−1and 6500dpmm−2h−1(8–
108Bqm−3, 1dpm=1/60Bq) in West Germany and the av-
erage value was 3200dpmm−2h−1(53Bqm−2h−1). Szeg-
vary et al. (2007) measured exhalation rates at 8 loca-
tions in southern Finland by using soil chambers. There
were significant variations in radon exhalation rates, which
were between 51Bqm−2h−1and 134Bqm−2h−1includ-
ing one peak value of 189Bqm−2h−1.
value of 100±17Bqm−2h−1was obtained when eight lo-
cations in southern Finland was included to calculations
(88Bqm−2h−1if peak value was excluded).
al. (2002) measured radon exhalation rates for moist boreal
forest using closed-chamber technique at Fyodorovskoye
in Russia.
3.3Bqm−2h−1to 7.9Bqm−2h−1, while water table depth
were from 5cm to about 70cm. Strong relation between
water table depth and radon fluxes was found. (Levin et
al., 2002; Conen and Robertson, 2002). Conen and Robert-
son (2002) measured222Rn flux using a closed-chamber
technique.They found that radon flux was at the high-
est in mineral soil type with no humic layer, decreasing to-
wards more humic and organic soil type. A modeled radon
exhalation rate can also be used as an input to the radon
The radon exhalation rates
An average
Levin et
222Rn exhalation rates were in the range
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8568M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland
One week cumulative rain (mm)
0 10 203040 50
Radon exhalation rate (Bq m-2 h-1)
0
50
100
150
200
2D model
A
One week cumulative rain (mm)
01020304050 60
Hydrogen deposition velocity (mm s-1)
0.200
0.400
0.600
0.800
1.000
Chamber
2D model
Rn tracer
B
Fig. 6. The effect of one week cumulative rain (summer points) to (a) the radon exhalation (p<0.01) and (b) the modeled and measured
hydrogen deposition velocities (p<0.05).
tracer method. Our radon fluxes simulated with the two-
dimensional model were lower in comparison to the southern
Finland and Germany chamber results, but higher in compar-
ison to Fyodorovskoye. The highest values were obtained
during northerly winds from the continent. Thus the proxim-
ity of the sea may have a lowering effect on the mean values.
Also, the two-dimensional model can provide only a crude
estimate of the boundary layer height which has a direct ef-
fect on the magnitude of the inverted flux. However, it is not
probable that the mean radon fluxes are heavily underesti-
mated.
When a nocturnal nighttime layer is formed, radon accu-
mulates in the layer. In daytime, the boundary layer height
increases and radon is mixed in to larger volume and it is
more unevenly distributed due to solar heating. Most prob-
ably slow inversion layer development enables conditions,
where radon had aged couple of hours and thus equilibrium
condition could be assumed. According to the radon tracer
method the hydrogen deposition velocity values are propor-
tional to the radon exhalation rate (Eq. 2) and the method
wouldyieldunrealisticallyhighdepositionvelocitiesifradon
fluxes were multiplied by e.g. a factor of two. The hydrogen
flux calculation in the two-dimensional model is dependent
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M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland8569
on the boundary layer height estimation. This could produce
error of 10% to a few tens. A minor source of error is the
vegetation layer, which is modeled as a single uniform layer.
4.2Comparison between the methods
The results of chamber measurements were in the same
range with the earlier measurements made in the boreal
zone (Rahn et al., 2002; Smith-Downey et al., 2008). Ac-
cording to the chamber measurement results, hydrogen soil
deposition increased on May to August 2006 after a long
drought. During that time the soil volumetric water con-
tent varied between 0.09m3m−3and 0.25m3m−3, which
was significantly lower than typical values (0.29m3m−3to
0.41m3m−3) recorded in 2005 and 2007–2008. The highest
vdvalues 0.5mms−1to 0.7mms−1were measured in high
soil temperature conditions from 10◦C to 17◦C, which was
usually recorded from May to August. Negligibly small vd
values were measured, when air and soil temperatures were
near-zero and the soil surface was snow covered. The soil
volumetric water content was usually higher in winter than in
summer. The dependency of vdto the soil volumetric water
content (in this study R2was 0.47) might be more important
than to the soil temperature (Schmitt et al., 2009). The dry-
ness of soil is in correlation with the high soil temperature.
Thehighsoilwatercontenteffectivelyhinderedthehydrogen
diffusion intoground. The modeled hydrogen deposition val-
ueswerecomparedagainstchambermeasurementsandusing
several parameters, such as air/soil temperature and soil vol-
umetric water content (Fig. 3a, b, c). The modeled and mea-
sured vdvalues were in good agreement with each other and
their annual cycles were similar (Fig. 1). The radon tracer vd
values were distributed more evenly in summer time (May to
October), yielding values from 0.14mms−1to 0.93mms−1.
The vdvalues of the two-dimensional model were distributed
to more narrow range (0.13mms−1to 0.61mms−1) than the
radon tracer method (Fig. 1). In winter time both the radon
tracer and the two-dimensional model produced vd values
lower than 0.33mms−1. Within the modeled period the low-
est air temperature was −5◦C and highest 21◦C. Among both
models, the results did not show clear temperature depen-
dency above zero temperatures. Hammer and Levin (2009
and references therein) used also the radon tracer method
for the estimation of nocturnal soil uptake rate and respec-
tive hydrogen deposition velocities in urban/suburban envi-
ronment. The estimated vdvalues ranged from 0.1mms−1
to 0.8mms−1, which is close to our results. The disturbance
of pressure differentials cannot totally avoid, but the effect
of these can be minimized using a short measurement time
and a large chamber volume (Davidson et al., 2002). For
this reason, chamber was kept closed only 15min and a large
closed-volume (about 100L) chamber was used. During the
sampling, only about 0.4dm3is removed from the chamber
headspace. When examining the deposition velocity values
of consecutive measurements, all the values were scattered
randomly and usually all values were close to each other.
Systematic trends between the repetitive measurements were
not observed. In addition, according to Hutchinson and Liv-
ingston (2001) repetitive chamber measurements is not a ma-
jor source of error.
4.3Precipitation and vd
Astrongsolarirradiationduringsummer(MaytoJuly)above
600Wm−2is capable of drying the top soil layer allowing
higher soil uptake. In fall, low solar irradiation and more
frequent rain periods keep the soil volumetric water con-
tent level high. The chamber measurements made on rainy
weather conditions indicate low hydrogen deposition rates.
The field measurement made on 24 August 2007 was af-
fected by intensive thunderstorm two days before. All field
measurements where vdwas less than 0.2mms−1were snow
results, (Fig. 3a, b) except 24 August and 30 October 2007.
However, the soil volumetric water content at 24 August was
not higher than typical values. There is not a single factor,
which fully explains the lowest vdvalue, but the soil surface
may have compacted after rain, hindering gas diffusion into
ground. On the other hand high vdvalues were occasionally
measuredinhighsoilvolumetricwatercontentconditions. In
these cases, the probe may have overestimated the soil vol-
umetric water content, especially when the soil surface was
moist due to small amount of rain. In snowy winter condi-
tions, vdvalues were small at sub-zero temperatures (Fig. 2).
This is supported by results of Lallo et al. (2008), who made
field measurements in winter with snow cover. Snow cover
hinders the gas diffusion into ground resulting in lower vdas
shown in Fig. 2. Also when thicker snow layer is formed, it
becomes more layered due to changing weather conditions.
The permeability of snow differs from the gas permeability
of thinner snow layer (Albert and Schultz, 2002). Yonemura
et al. (2000) found, based on modeling and measurements,
that diffusion into soil is an important factor controlling hy-
drogen and carbon monoxide soil uptake. The comparison
of vd values and radon exhalation rates with rain records
showed a decreasing trend towards increased one week cu-
mulative precipitation. The decreased soil uptake rate and
radon exhalation was possibly due to higher soil volumetric
water content. Precipitation has effects to short-term vari-
ations to222Rn flux (Szegvary et al., 2007). Szegvary et
al. (2007) found during the long-term measurements (June to
November 2006) in Basel, that while the prolonged dry pe-
riod decreased the soil volumetric water content, the222Rn
flux increased about 100% until the beginning of August.
The222Rn flux was enhanced due to increased diffusion and
air-filled porosity and decreased soil volumetric water con-
tent. Later measurements on September in three rainy days
showed that222Rn flux decreased immediately with the be-
ginning of precipitation, preventing222Rn diffusion into at-
mosphere (Szegvary et al., 2007). In Finland there was a dry
period from June to August 2006. The lowest soil volumetric
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8570 M. Lallo et al.: Hydrogen soil deposition at an urban site in Finland
water content in conjunction with high temperature increased
the hydrogen soil uptake, vdvalues were >0.5mms−1, due
toenhanceddiffusionintothesoillayers. Scheryetal.(1984)
found also that radon flux and its diffusion into ground is re-
duced in rainy conditions due to capping effect of the top
soil layer. The water-filled porosity is increased, when the
soil is moistened by water leading to the retarded gas emis-
sions. The soil exhalation values show significant spatial
variation, correlated to soil water content. The radon tracer
method is sensitive to changes in radon emissions and its ap-
plicability is regional. Radon gas is accumulating in to a
nighttime boundary layer. For this reason a stable noctur-
nal layer and low wind conditions are needed. Typical range
is about tens of kilometers to over one hundred. The two-
dimensional model covers also much larger area (horizontal
extend 10km) than the chamber measurements, which de-
termines the soil uptake rate at a specific point. The uncer-
tainty range of the radon tracer and the two-dimensional re-
sults in hydrogen vdvalues are larger than the chamber vd
values. The results of simulation methods are between the
range 0.00mms−1to 0.93mms−1, which follows also the
results of chamber based vd values. Only the two highest
radon tracer points (0.93mms−1) are higher than the cham-
ber results. Although error bars are larger in both models,
model points are comparable with the chamber results. The
two-dimensional model estimates the boundary layer height.
The uncertainty of flux values calculated by the model varies
from 10% to a few tens. This affects to the hydrogen flux es-
timation. The model estimates smoothly the boundary layer
height. The variability of the two-dimensional model results
are acceptable, when compared to the radon tracer and the
chamber results and also due to the integrative nature of the
atmospheric methods. The uncertainties related to the fixed
vegetation layer is small compared to the boundary layer es-
timation.
5Conclusions
Hydrogen deposition velocities in urban environment were
measured. The field measurements were made using the
closed-chamber technique. The results calculated from the
field measurements were further compared and verified with
the modeled hydrogen deposition velocity values applying
the two-dimensional model and the radon tracer method. The
hydrogen deposition velocity values obtained from all three
methods were in good agreement with each other. Based on
the chamber measurements in rainy conditions the decreased
deposition velocity suggests that the increased soil volumet-
ric water content hinders the gas diffusion into ground lead-
ing to decreased hydrogen deposition velocity rate. The soil
volumetric water content values did not vary enough to see
clear moisture dependency among the hydrogen deposition
velocity values. However a good agreement was found be-
tween the modeled and the measured hydrogen deposition
values compared to one week cumulative rain.
Acknowledgements. We would like to thank Noora Eresmaa for
the analysis of ceilometer results. This work was supported by the
Tor and Maj Nessling Foundation, the Academy of Finland and by
EU-project EUROHYDROS.
Edited by: M. Petters
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