Characterization of ions at Alpine waterfalls

Abstract

During a three-year field campaign of measuring waterfall generated ions, we monitored five different waterfalls in the Austrian Alps. Most measurements were performed at the Krimml waterfall (Salzburg, Austria), which is the biggest waterfall in Europe, and the Gartl waterfall (Mölltal, Austria). We characterized spatial, time and size distributions of waterfall-generated ions under the influence of surrounding topography. The smallest ions with boundary diameters of 0.9, 1.5 and 2 nm, were measured with a cylindrical air ion detector (CDI-06), while ion sizes from 5.5 to 350 nm were measured using a modified Grimm SMPS aerosol spectrometer. High negative ion concentration gradients are detected in the vicinity of the waterfalls, whereas the increase of positive ions was only moderate. Ions in the nano range were the most abundant at 2 nm, and at 120 nm in the sub-micrometer range.

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Full text

Atmos. Chem. Phys., 12, 3687–3697, 2012
www.atmos-chem-phys.net/12/3687/2012/
doi:10.5194/acp-12-3687-2012
© Author(s) 2012. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Characterization of ions at Alpine waterfalls
P. Kolarˇ
z1,*, M. Gaisberger2,*, P. Madl3, W. Hofmann3, M. Ritter2, and A. Hartl2
1Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
2Institute of Physiology and Pathophysiology, Paracelsus Medical University, Strubergasse 21, 5020 Salzburg, Austria
3Department of Materials Research and Physics, Division of Physics and Biophysics, University of Salzburg, Hellbrunner Str.
34, 5020 Salzburg, Austria
*These authors contributed equally to this work
Correspondence to: P. Kolarˇ
z et al. (kolarz@ipb.ac.rs)
Received: 16 June 2011 – Published in Atmos. Chem. Phys. Discuss.: 9 September 2011
Revised: 28 March 2012 – Accepted: 9 April 2012 – Published: 24 April 2012
Abstract. During a three-year field campaign of measuring
waterfall generated ions, we monitored five different water-
falls in the Austrian Alps. Most measurements were per-
formed at the Krimml waterfall (Salzburg, Austria), which
is the biggest waterfall in Europe, and the Gartl waterfall
(M¨
olltal, Austria). We characterized spatial, time and size
distributions of waterfall-generated ions under the influence
of surrounding topography. The smallest ions with bound-
ary diameters of 0.9, 1.5 and 2 nm, were measured with a
cylindrical air ion detector (CDI-06), while ion sizes from
5.5 to 350 nm were measured using a modified Grimm SMPS
aerosol spectrometer. High negative ion concentration gradi-
ents are detected in the vicinity of the waterfalls, whereas the
increase of positive ions was only moderate. Ions in the nano
range were the most abundant at 2nm, and at 120 nm in the
sub-micrometer range.
1 Introduction
1.1 Air ions in the environment
The term “air ions” refers to all charged airborne particles
with electrical mobility. In general, they are continually cre-
ated by natural sources such as cosmic rays, radioactive de-
cay of noble gases (such as radon) in the air, and radioactive
minerals of the ground. Primary ions evolve after ioniza-
tion, which typically occurs within microseconds via a pro-
cess of hydration and ion cluster formation into complexes,
known as small air ions (charged nano-aerosols) with a typ-
ical lifetime of 5–60s (H˜
orrak et al., 2000). The central ion
of a cluster can also contain one inorganic molecule and can
be surrounded by one layer of water molecules. In highly
pure waters, such inoculation cores are largely absent in the
aerosols, at is the case in the waterfalls investigated in this
study.
The near-ground ionization rate, caused by background
ionization, of 10 ion pairscm−3s−1is considered as a stan-
dard in continental areas (Chalmers, 1967). The small air
ion concentration (n±) itself is determined by the following
balance equation:
dn±
dt =q−α n±n∓−n±βZ (1)
where qis the volumetric production rate, Zis the aerosol
number concentration, αcoefficient accounts for the losses
of ion-to-ion recombination and βrepresents an effective
ion-aerosol attachment coefficient, which is the integral over
the size distribution of aerosol particles (Laakso et al., 2004;
H˜
orrak et al., 2008). The atmospheric electric electrode ef-
fect in near ground layer induces an imbalance between pos-
itive and negative small air ion concentrations, and results in
a ratio of n+/n−=1.12 (Hoppel et al., 1986; H ˜
orrak, 2001).
According to the size categorization provided by H˜
orrak
et al. (2000) and H˜
orrak (2001), which uses electri-
cal mobility to mass diameter conversion provided by
Tammet (1995), air ions are classified in: small clus-
ter ions 0.36–0.85nm (3.14–1.28cm2V−1s−1), big cluster
ions 0.85–1.6nm (1.280.5cm2V−1s−1), intermediate 1.6–
7.4nm (0.034–0.5cm2V−1s−1) and large ions 7.4–79 nm
(0.034–0.00041cm2V−1s−1). Intermediate and large ions
are also called aerosol ions (H˜
orrak et al., 1994). Cluster air
Published by Copernicus Publications on behalf of the European Geosciences Union.

3688 P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls
ions are supposed to carry only a single elementary charge
that relates mobility with mass, if the carrier gas is known
(Aplin, 2008). The effect of multiple charges is assumed
to occur in low concentrations in the case of large particles
>50nm (H˜
orrak, 2001), which is in this study considered as
an artifact.
Air ion generation is caused by several, both natural and
artificial mechanisms. In this study, we investigated the
“waterfall effect”, which creates mostly negative ions, also
known as Lenard‘s ions (Lenard, 1892) or ballo-electric
ions. Investigations of the ballo-electric ion phenomenon
was started by Philipp Lenard (Lenard, 1915, 1892) and con-
tinued by Chapman (1938a, b), Blanchard (1955), Gathman
and Hoppel (1970), Levin (1971), Reiter (1994), H˜
orrak et
al. (2006), Hirsikko et al. (2007, 2011), Laakso et al. (2006,
2007), Luts et al. (2009), Tammet et al. (2009) and oth-
ers. Laakso et al. (2006) and Luts et al. (2009) proposed
and analyzed several pathways which can produce waterfall
ions. They found that combination of factors, such as auto-
ionization of water molecules, fluctuating charge rearrange-
ment, surface protrusions, evaporation of droplets formed in
collisions, and Coulomb explosion serve both as the main
source of intermediate ions as well as the extra source for
large ions. Furthermore, they computed velocities of pri-
mary waterfall droplets required to break up into two new
ones, and their velocities and dimensions.
Upon creation, waterfall droplets undergo charge redistri-
bution forming dipoles with negatively charged surfaces. The
phenomenon of charge separation occurs as a consequence of
the disruption of the water surface by splashing and bubbling,
when moving liquid is aerosolized at an obstacle, aqueous
surface or by aerodynamic break-up of water droplets during
free fall (see Fig. 1). After breaking up, smaller fragments
(or clusters) carry negatively charged OH−ions, while the
remaining bigger fragments become positive, according to:
(H2O)n→(H3O+)(H2O)n−m+OH−(H2O)m−2
where mn, where large proton clusters are known as
H3O+(H2O)20 (Hulthe et al., 1997; Laakso et al., 2006;
Vostrikov et al., 2006; Parts et al., 2007; Luts et al., 2009). A
positive value of droplet surface potential (+0.1 V) indicates
that the water dipoles are preferentially oriented with the neg-
ative pole toward the gas phase, while the positive pole is
oriented towards the liquid phase (Parfenyuk, 2002). Botti et
al. (2004) suggested that OH−ions are hydrated with 4H2O
molecules with a presence of a weakly bound fifth water
molecule, close to the hydrogen atoms. Negatively charged
particles remain in the air and become thermophoretically
dispersed, due to the cold air stream caused by the tempera-
ture difference between water and the surrounding air. The
larger positively charged droplets precipitate to the ground,
i.e. in the pond. As a result, negative air ion concentrations
near waterfalls can reach several tens of thousands per cu-
bic centimeter. Laakso et al. (2007) found that the diame-
Fig. 1. Theoretical concept of the charge separation mechanism of
a falling water droplet. Water bag formation (a–d), fracture into
micro-bubbles (e–f) (modified according to Zilch et al., 2008).
ter of waterfall-related negative air ions range between 1.5–
10nm, whereby 2nm sized negative ions were most abun-
dant. Tammet et al. (2009) investigated the generation and
mobility distribution of ballo-electric ions during rain, find-
ing that the mobilities of most of these ions were between 0.1
and 0.5cm2V−1s−1(1.6–4.0nm) with distribution maxima
between 0.20 and 0.27cm2V−1s−1(2.22–2.64nm).
The goal of this study was to investigate the spatial, time
and mobility distributions of waterfall-generated ions, and
the location-specific waterfall physics and topographic struc-
tures in a region of the Austrian Alps. The physical charac-
terization of waterfall-generated ions was part of a random-
ized controlled clinical study, which focused on the influ-
ence of these waterfall generated ions and aerosols on human
lung function. Physiological and medical effects of ions and
waterfalls has been described (Iwama, 2004; Yamada et al.,
2005; Takahashi et al., 2008), but interpretation of specific
health-related effects still require a thorough physical char-
acterization. In the present study we analyzed five waterfalls
with respect to their distribution of ions.
2 Methods
2.1 Instrumentation and measurements
Integral aspiration counter, also known as Gerdien con-
denser, is the most commonly used tool for atmospheric con-
ductivity measurements (Gerdien, 1905). Its operation is
based on the polarizing voltage that is applied between two
aspirated coaxial electrodes. Ions of the same polarity as po-
larising voltage are deflected toward the collecting electrode
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P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls 3689
depositing charge (q=1.62×10−19 C) and generating cur-
rent (I):
I=n·q·Q(2)
where Qis air-flow through the electrodes.
The limiting mobility of the aspiration condenser (µc), is
defined as a combination of polarizing voltage (U) and air
flow:
µc=VS·R2
2−R2
1·ln(R2/R1)
2·L·U(3)
where R2and R1are the radii of the polarizing and collecting
electrode; Lis the collecting electrode length and VSis the
air velocity through the electrodes (Applin, 2005). Gerdien
integral ion counters collect all ions with the mobilities above
those defined by polarizing voltage and flow rate (µc) of the
instrument, but also they collect a small fraction of excessive
ions with lower mobilities. Mobility distribution of integral
counters is commonly obtained using second derivation of
the current to voltage (I−U) characteristic curve (Tammet,
1970; Horrak et al., 2008; Flagan, 1998).
Concentrations of small and intermediate ions with mo-
bilities below 0.3 cm2V−1s−1were measured using three
identical Gerdien condenser detectors (CDI-06), constructed
at the Institute of Physics, Belgrade. Using the dynamic
property of ionic motion in an electric field, the polarizing
voltage and air flow on three identical CDI-06 detectors was
set to measure ions with the following limiting mobilities:
1.18, 0.55 and 0.3 cm2V−1s−1, which corresponds to the
boundary mass diameters of 0.9, 1.5 and 2nm (Eq. 3). Ion
mobility (measured by CDI-06) to mass diameter conversion
was based on the formulation given by Tammet (1995). Se-
lected limiting mobilities correspond to characteristic ion-
sizes originating from waterfalls (WF), which is around 2 nm
(Laakso et al., 2007), and by background ionization sources,
i.e. nuclear decay as well as cosmic rays – with the latter up
to 0.9nm (small cluster ions).
The relatively large electrode cross-section
(R1=0.028m, R2=0.005m) along with the collect-
ing electrode length (L=0.4m) provides a high air flow
(0.0048m3s−1) and intense signal to use a small measuring
resistor (RM=1G), thus resulting in a reduced mea-
suring uncertainty. The collecting electrode is at virtual
ground potential; with the measuring system, including the
polarization circuit, galvanically separated from the power
supply and the PC communications port (USB). Each of
the three detectors has its own power supply, temperature,
pressure and relative humidity sensor, and also sophisticated
features such as zeroing (eliminates zero offset drifts during
measurements), live data acquisition, with programmable
data averaging, sampling and automatic polarity change
options (Kolarˇ
z et al., 2012).
According to Eq. (2), the detector amplifier (i.e. A/D con-
verter) output signal to air ion concentration conversion fac-
tor was calibrated prior to each measurement using a Keith-
ley Picoampere 261 source. The limiting ion mobility was
calculated using Eq. (3). The instrumental uncertainties un-
der regular operational conditions are about 5% (Kolarˇ
z et
al., 2009). Taking into account the extreme and difficult
conditions during the field campaign, and particularly in the
close vicinity to the WFs, an additional uncertainty should
be added. Yet, efforts, such as correction due to repulsion of
small ions at the inlet of the detector (Anderson et al., 1991;
Misaki, 1961), maintaining constant number of revolutions
per minute of the fans, thermostating both the electronics
and electrodes, usage of additional protective case and man-
ual zeroing procedure have been employed to limit additional
uncertainties to around 10%. Uncertainty due to the collec-
tion of excessive ions with lower mobilities than limiting was
estimated up to 5%. Altogether, by using the “root-sum-of-
squares” method combined standard uncertainty amounts to
12%.
Ion concentrations in the size ranges of 0.36–0.9nm, 0.9–
1.5nm and 1.5–2nm were found from the data of three ion
detectors (CDI-06), set to measure ions at the corresponding
boundary diameters of 0.9, 1.5 and 2 nm, using a simple sub-
traction method (Figs. 9, 13). Obtaining size distribution us-
ing second derivation of I-U characteristic was not possible
using this setup, thereby increasing measuring uncertainty.
Although the determination of ion mobilities using integral
Gerdien counters is not as accurate and narrowband as for
e.g. a DMA (differential mobility analyzer), portability and
robustness compensate for that disadvantage and allow mea-
surement on nearly impassable terrains.
Further away from the WF, in conditions where mostly
small cluster ions prevail (except in cases of nucleation bursts
of intermediate ions), two detectors which are measuring
ions with lower mobilities should detect equal or slightly
higher ion concentrations compared to one detector measur-
ing small ions (Laakso et al., 2007). As stated above, similar
applies to the ratio of positive and negative background ion
concentrations on the reference sites, where n+/n−should be
about 1.12.
To test the performance of the instrumentation, an inter-
comparison between the cylindrical ion detectors was per-
formed (Fig. 2). Two detectors, with the same voltage and
amplification settings, placed side by side, were used to mea-
sure negative ion concentration gradients near the Krimml
WF. Comparison of detectors reveals the Pearson correlation
coefficient of r=0.97, which is more than satisfactory con-
sidering the sampling conditions encountered during the field
campaign. Testing was conducted also at different sites and
locations, and under different detector setups. The time res-
olution of CDI-06 detectors was set on 2s for all measure-
ments so that sufficient statistics and data analysis could be
achieved.
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3690 P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls
Fig. 2. Intercomparison of ion detectors (CDI-06) nearby Krimml
WF versus time. Correlation coefficient 0.97.
Air ion particles in the size range from 5.5 to 358nm
were measured with the Grimm SMPS (Scanning Mobility
Particle Sizer). It consists of a CPC (Condensation Parti-
cle Counter, model 5403) to quantify the particle concentra-
tion, and is attached to a middle sized M-DMA (Medium-
Differential Mobility Analyzer) column (5540 26 Reischl Vi-
enna type) for size class differentiation. Within the DMA,
the particles are selected according to their electrical mobil-
ity, which is accomplished using a 241Am radioactive source
mounted on the DMA air inlet. To avoid neutralization of the
aerosolized ions, this so-called “neutralizer” was detached
from the M-DMA resulting in an SMPS detector which is
able to count negatively charged particles. SMPS default
conversion algorithm is using Stokes Millikan mobility di-
ameter (Hirsikko et al., 2011). Instrumental losses are also
accounted by the built-in algorithm and take into account
the time response characteristic of the CPC in terms of flow
and voltage stepping, the aerosol residence time within the
DMA, and particle losses as particles pass through the DMA
as well as the CPC (Heim et al., 2004). Although the SMPS
efficiency correction acts over the full spectrum, the smaller
particle spectrum below 10nm is more affected than the scan
window above this threshold. Similarly, its impact on the
DMA-correction is larger than on the CPC-correction. Al-
together, the correction algorithm affects the overall particle
count per scan by roughly 30% for the DMA and approx-
imately 10% for the CPC. The entire system was operated
with a backup battery-operated power supply, enabling at
least 8 hours of uninterrupted operation in the field.
Due to the dynamics of waterfall generated aerosols, mon-
itoring the inventory with a SMPS is not the first choice, as
it requires a 240s interval for the completion of a full scan.
Therefore, the SMPS was operated in fast mode, which uses
only 22 voltage channels, instead of the 44 voltage channels
in the normal operational mode. In this mode, each informa-
tion from the counts detected in-between the voltage step are
utilized to calculate an additional series of 22 pseudovoltage
channels. Together, these combine again in the total number
of 44 channels (Heim et al., 2004). The ideal detector should
at least measure in second intervals, like the ELPI. However,
this instrument, due to its weight and power requirements, is
not well suited for field measurements. This shortage was
overcome by making at least three full SMPS scans at every
point, each divided into 44 size channels, and each channel
computed as an average of 9 individual recordings (out of
which the 1st and 9th are omitted as the system dynamically
changes the voltage when switching from one size bin to the
next). Due to the nature of its detection principle, the SMPS
only enables to measure negatively charged particles thus de-
priving us from monitoring positively charged aerosols in the
200nm size range and beyond, as reported by Reiter (1994).
Atmospheric data, such as temperature and wind speed,
were measured by a PCE-007 anemometer attached to a
data logger (PCE Deutschland GmbH, Germany, http://www.
pce-instruments.com). Radon concentration in the air was
measured with a continuous radon gas monitor RAD7 (Dur-
ridge Co, USA, http://www.durridge.com). Natural radioac-
tivity in the air was detected using a Gamma Scout counter
based on Geiger-M¨
uller principle (Gamma-Scout GmbH &
Co. KG, Germany, http://www.gamma-scout.com).
Measuring of ion concentrations in the vicinity of the WFs
is difficult and affected by water aerosols. Beyond contribut-
ing to the increase in relative humidity, these droplets wet
the instruments, soak the electrodes of the CDI and thus
cause current leakages in the electrode system, and ampli-
fying electronics. In addition, the temperature gradient at the
waterfalls results in a heavy surge, which is directed down-
stream away from the WF in periodical gust. These gusts
are carrying water aerosols, and can be highly loaded with
air ions. Periods of intense gusts are followed by modest
ones and typically last between 5 and 40 seconds, a phe-
nomenon which is intensified at the Krimml waterfall where
three falling cascades generate very complex air-flow pat-
terns.
We partly overcome these impairments of the measure-
ment conditions by encasing the CDI instruments in alu-
minium boxes with inlets for the probes and outlet for ex-
haust. The SMPS was also operated in a sealed housing, thus
only subjecting the inlet to excessive humidity. All instru-
ments were mounted onto a 4-wheel trolley (Fig. 3).
3 Measurements and discussion
The present study lasted from 2008 until 2010. At the very
beginning, measurements on several different waterfalls were
conducted (shown in Sect. 3.1). Early preliminary data in-
dicated significant health effects of the Krimml waterfall,
therefore, time and spatial measurements (Sect. 3.2), and
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P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls 3691
Table 1. Height of WF cascades (approximate), average and maximal water flow. Bold numbers represent the heights of the investigated
cascades.
WF name Height (m) Average water Maximal water
flow (m3s−1) flow (m3s−1)
Krimml 140×100×140 5.1 17.5
Stuiben 150 x 150 1.6 2
Bad Gastein 310 (measured cascade 50) 1.83 6.8
Gartl 100 (50×50) 0.6 0.9
Artificial (river M¨
oll) 51.33 4.66
Table 2. Overview of the ion (≤2nm) concentrations measured at closest range of the WFs and at control places (CP) during July 2008.
WF name Average WF ion concentrations Max.WF ion concentrations Average ion concentrations Conc. increase Conc. increase
(ionsm−3) (ions m−3) at ref. site, (ions m−3)n−
WF/n−
CP n+
WF/n+
CP
negative positive negative positive negative positive
Krimml 16581 1040 27540 1736 460 476 36 2
Stuiben 43913 2142 54 359 3098 546 620 80 3
Bad Gastein 24 748 1290 31 606 428 774 510 32 3
Gartl 42660 1798 57510 2400 840 760 51 2
Artificial (river M¨
oll) 3294 1446 6251 1582 306 544 11 3
Fig. 3. Air ion detectors (left), SMPS, anemometer and GPS (mid-
dle) and field strength meter (right).
characterization of ion size distribution (Sect. 3.3) were per-
formed.
3.1 Characterization of 5 different Alpine waterfalls
During July 2008, five WFs were investigated, and compared
with control sites lacking WF-related ions. In the topography
of the Austrian Alps numerous glaciers are embedded feed-
ing their liquid phase into creeks. These collecting moun-
tain torrents form little cascades which also generate WF ions
Fig. 4. Artificial WF at the river M¨
oll.
thus leading to variable ion distribution spectra in their em-
bedded areas.
Measurements at the WFs have been carried out during
midday, along existing footpaths, using a CDI-06 setting that
has been configured to measure ions with mobilities up to
1.18cm2V−1s−1(2nm in diameter).
Table 1 lists the height, average and maximal water flow of
each WF. Positive and negative air ion averages, and maximal
number concentrations of ions are given in Table 2. At all
sites ion concentrations were measured at places of maximal
ionization and allowable wetting of the instruments. Com-
pared to the control sites, the highest increase of the concen-
tration of negative ions (up to 2nm) was found to be 80-fold
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3692 P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls
(Stuiben waterfall). Ion concentrations fluctuate depending
on the amount of water flowing, which is subject to seasonal
variations and oscillates with the melting of the glaciers. In
contrast, the origin and structure of the ions generated at ref-
erence sites arise mostly from natural radioactivity, which is
subject to diurnal changes and related to the fluctuation in
concentrations of radon, thorium and their progenies (Kolarˇ
z
et al., 2009; Chandrashekara, et al., 2006). Accordingly, the
presented relations between alpine WFs and background air
ion concentration vary diurnally and seasonally.
The data in Tables 1 and 2, lead to the conclusion that the
production of WF ions not only depends on the height of the
WF and its quantity of water flowing per unit of time, but also
on the location and shape of the WF, the surrounding topog-
raphy, and of course, the location of the measuring points.
Figure 4 shows the artificial WF on the river M¨
oll with
a cascade of 5m height. This artificial WF differs from
the other WFs, in that the water pours straight past a man-
made barrier into the bottom pond, no water splashing on
rocks takes place. Although ion concentrations are 11-fold
higher than background measurements, they are much lower
compared to the other investigated WFs (Table 2). Possi-
ble mechanisms contributing to WF-related ion generation
could be bubble break-up on aqueous surfaces, and rapid de-
cline in relative humidity as the aerosolised droplets drift
further off the WF (Laakso et al., 2006). This implies that
if the particle size is larger than the Kelvin diameter, the
aerosol particles will grow, if the particle size is smaller, they
will evaporate. Hence, as one moves away from the wa-
terfall and due to the change in super-saturated conditions,
the corresponding Kelvin droplet diameter becomes larger
as under-saturated conditions are reached (decrease in rel-
ative humidity and increase in temperature), which causes
the WF-generated aerosols to shrink in size. Furthermore,
charge separation via aerodynamic break-up of micrometer-
sized water droplets into nano-sized aerosols may play a role
(Fig. 1; Zilch et al., 2008). Nevertheless, water droplets hit-
ting a surface (rock, water) represents the most prominent
mechanism of ion formation by bubble bursting, thus yield-
ing nano-sized aerosols.
3.2 Ion measurements at the Krimml waterfall
The Krimml waterfall is located in the northern region
of the “Hohe Tauern” – a part of the eastern Alpine re-
gion. It belongs to a glacial creek named “Krimmler Ache”
whose water flows according to the season. In July the
average throughput is about 5.5m3s−1, whereas it is only
0.14m3s−1in February. The WF itself consists of three con-
secutive cascades, with heights of 140m, 100 m and 140m
(Fig. 5). The base of the WF is at an altitude of around
1100 m above sea level. Like most of alpine WFs, it is sur-
rounded with high cliffs that determine local atmospheric pa-
rameters and air motion. Chemical analysis of the water re-
vealed a pH close to neutral (7.36), with very low concentra-
Fig. 5. The last of three cascades at the Krimml WF.
tions of Ca2+(3.45mgL−1), Mg2+(1.22 mg L−1) and Cl−
(0.26mgL−1). Conductivity was 30µS cm−1. A microbio-
logical analysis elicited a moderate count of colony forming
units (≤100CFUml−1).
During the 2009 field campaign, a series of 7-day mea-
surements of ions ≤2nm has been performed, including
two separate field campaigns: 10–26 July as well as 9–23
September. Measurements were carried out twice a day, and
consisted of a sampling session during the morning hours
(around noon) and one in the afternoon (around 16 h) for 1h
each. According to the data provided by the authorities of
the local hydro power plant, which is just 700m downstream
from the fall, the average water flow in June was 14.4 m3s−1
whereas in September it was 5.5m3s−1. The former is 2.6
times larger than the latter and directly correlating with the
period of glacier melting. Daily variations of negative WF
ion concentrations were within the 8% range. However, the
average ion concentration in July (10944 ions cm−3) was 1.5
times higher than in September (7502 ionscm−3). While
daily differences in ion concentrations are the consequence
of local meteorological parameters, the significant difference
between July and September obviously correlates with the
water flow.
Average spatial distributions of ions ≤2nm were mea-
sured during the summer seasons 2007–2010 on both sides
of the WF (Fig. 6). Distances were measured from the
base of the lowest cascade (using Google earth coordinates:
N47.208283, E12.170859). The reference site is located
547 m away from the falls (see orographic right side), assum-
ing that no WF-related ions are present there. The reason for
the relatively short gradient distance on the orographic left
side is due to the dense forest cover that dominates the far-
ther site some 228m from the WF. All ion measurements
were carried out at points that ensure that the instruments
were not subject to instant flooding. Accordingly, the nearest
measuring points were 57m and 75m on the orographic left
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P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls 3693
Fig. 6. Average concentrations of ions ≤2nm measured in the summer period from 2008 to 2010. on both sides of the Krimml WF. The
distances from the WF were measured via GPS and visualized on a Google Earth map.
and right side of the Krimml WF, respectively. Both measur-
ing points are slightly shifted out of the main axis, which are
directed towards the left orographic uphill side (Fig. 5 and
aerial view of Fig. 6). The differences between the maximal
values on the left and the right side of the WF, i.e. the creek,
arise from the topographic configuration, the time variations
when measurements were performed, complex turbulent air-
flow generated by the huge amount of water pouring into the
pond and varied solar flux within the upper Krimml valley.
In comparison with the other WFs that were measured
(Table 2), the maximal ion concentrations generated near
the Krimml WF were apparently low, i.e. only about 20
times higher than the measured background, while maximal
ion concentrations at the Gartl WF (D¨
ollach) amounted to
6×104ionscm−3, i.e. 120 times higher than the measured
background. Noteworthy for our field observation is the fol-
lowing: the Krimml WF generates much more aerosols but
fewer in the sub-nano-size range, and wetting of the instru-
mentation was much more prevalent than at the other WFs
(Fig. 5). Due to nice weather conditions, the relative hu-
midity did not exceed the critical limit (>80%) for a proper
function of the instrumentation.
Figure 7 shows integral values of ions with boundary size
limits of 0.36–0.9 nm, 0.36–1.5 nm and 0.36–2nm on oro-
graphic right side of the Krimml WF. As mentioned before,
the non-linear decay function of the negative ion concentra-
tion is caused by the local topography and wind patterns. An
obvious decrease of ion concentration with distance from the
base of the WF is visible in Figs. 6 and 7, likewise there
is an increase of the unipolarity coefficient at the measuring
point 290 m from the base of the WF (Fig. 8). Both are at-
tributable to the shift in location, i.e. moving away from the
river (58m) (Fig. 6). On the other hand, the measuring point
222 m far on the orographic left side showed significant pres-
ence of WF-generated ions although it is almost 200 m away
from the river. This is probably due to orientation of the
WF, which upon hitting the ground, bends slightly towards
the orographic right, leaving part of the mainstream aerosol
plume in this direction. Detection of positive ions did not
reveal significant increases in concentrations, and only 0.36–
2 nm-sized ions showed a correlation (r=0.6) between pos-
itive and negative ion gradients. Interestingly, all three di-
mensions of positive ions were rising with distance within
the first hundred meters from the WF and decreased further
afield. This is probably due to very high concentrations of
negative ions, as well as aerosols, in the vicinity of the WF.
This indicates that higher coefficients of αand β(see Eq. 1),
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3694 P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls
Fig. 7. Integral ion concentration distribution by ion boundary di-
ameter and polarity versus distance, measured at the Krimml WF,
orographic right side.
amplified by strong turbulent air flow patterns must be taken
into consideration. The average wind speed from the first
to the last point measured on the riverbanks was 4.6m s−1.
Approximate calculations revealed that under these circum-
stances, WF-related ions could persist up to 120s.
Equalization of the smallest positive and negative air ion
concentrations is a signal that almost all WF-related negative
ions are in some way neutralized. The remaining ones can be
traced back to the formation mechanisms usually associated
to natural radioactivity and cosmic radiation. Air ions created
by mentioned ionizing sources are smaller than WF-related
ions. Their size range is shown by the reference point data
series (547m) in which the ions with boundary diameter of
0.9nm (0.36–0.9nm) were the most abundant that is visible
in Figs. 7, 8 and 9.
Supplementary measurements of radon dissolved in water,
assessed with the RAD7 instrument showed radon concentra-
tions in the water above and below the Krimml WF to be 141
and 148Bqm−3, respectively, hence they remain negligible
for the overall ion production rate. Also, natural radioactiv-
ity in the air was measured with a Gamma Scout counter,
eliciting typical background values of ∼0.15mSv h−1.
As outlined in the “Methods” section, the ion diameter
range from 0.36 to 350.4nm was measured using CDI-06
and modified SMPS with M-DMA at five measuring points
(Fig. 9). By operating the SMPS with and without the 241Am
neutralizer, it was possible to uncover the differences in the
spectra. Besides the sharp increase in sub-20 nm size range,
when the SMPS was operated without the neutralizer, the re-
mainder of the spectra (20–350 nm) still reveal similar trends,
regardless of whether the neutralization source was attached
or not.
According to Laakso et al. (2007), the ion-effect of the
WF is visible only in ions smaller than approximately 30nm
with a maximal rise of negative ion concentration near the
Fig. 8. Unipolarity coefficients (u±) for 3 different ion sizes versus
distance measured at the Krimml WF, orographic right side.
Fig. 9. Ion concentration distribution by size measured at differ-
ent distances on 1 July 2010 at Krimml WF, orographic right side.
Shaded area indicates interpolated values (cubic B-spline) between
CDI-06 (0.36-2.0nm) and SMPS (5.5–350nm) measurements. The
547 m data series (magenta curve) represents the reference site
where no WF-generated ions were expected to occur.
WF in the size range of 1.5 to 10nm, which is also in agree-
ment with our data. In the size range of heavy large ions
between 25 and 40nm there was no increase in observable
WF-generated ions, whereas larger ions are present from 40
to above 350nm, whereby clustering took place into a size
of approximately 120nm (their concentration directly at the
WF was three-fold higher than at the reference site). Further-
more, ions with a boundary diameter up to 0.9nm increased
in the close vicinity of the falls, indicating the extended spec-
trum of the WF-related ion size range along with its mobility.
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P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls 3695
Fig. 10. Gartl WF with surroundings.
Fig. 11. Integral ion concentration distribution by ion boundary
diameter and polarity versus distance measured at the Gartl WF.
3.3 Ion measurements at the Gartl waterfall
The Gartl WF, as shown in Fig. 10, is located in the south-
ern sector of the “Hohe Tauern” region and is also part of the
Alpine region. It is composed of 2 cascades in series. To-
gether they are about 50m high and steeply plunge into the
pond at the base of the falls. This WF is tightly embedded
into a narrow topographic setting, encompassing a small val-
ley opening closely surrounded with high cliffs, which are
directing the air stream downhill.
The generated ion gradients at Gartl waterfall show similar
characteristics as observed at the Krimml waterfall. How-
ever, this WF produces significantly higher ion concentra-
tions. Unlike the Krimml waterfall, the positive ion concen-
trations of this WF were low in the vicinity of the fall itself
(Figs. 11, 12). Strong correlation coefficients between pos-
itive and negative ions (r∼0.8) of the same size were ob-
Fig. 12. Negative and positive integral ion concentration ratios (u∓)
versus distance at the Gartl WF.
Fig. 13. Ion concentration distribution by size, measured at different
distances on 3 July 2010 at Gartl WF, D¨
ollach. Shaded area indi-
cates interpolated values (cubic B-spline) between CDI-06 (0.36–
2.0nm) and SMPS (5.5–350 nm) measurements. The reference site
data series (magenta curve) represents the reference site where no
WF-generated ions were expected to occur.
tained (Fig. 11), but with huge deviations in the overall con-
centration.
Figure 13 demonstrates the distribution of the negative ion
concentration by their size. It reveals a similar size distri-
bution as observed at the Krimml waterfall (compare with
Fig. 9). This observation is interesting, because there is a
clear difference in both, the topography as well as the height
of the falls. The ion concentration-peak in the size range
of 0.36–0.9nm at the reference points of both WFs (Fig. 9
and 13) is a consequence of different source and formation
mechanisms as outlined above. Due to orographical reasons
(steep cliff), the last point approachable for the experimental
measurements was 120m away from the WF. The selected
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3696 P. Kolarˇ
z et al.: Characterization of ions at Alpine waterfalls
reference site was near the village of D¨
ollach (Carinthia), a
few kilometres from the falls on the opposite side of the val-
ley.
4 Conclusion
In this study, spatial, time and size distributions of WF-
generated ions on several alpine WFs are presented. The size
range of ions from 0.36 to 350nm was measured at 2 WFs
in the Austrian Alps, using a Gerdian-type ion counter and
a Scanning Mobility Particle Sizer, adapted for negative ion
counting purposes.
Although the daily as well as seasonal variations of WF-
related water masses (maxima versus minima differ several
times) is reflected in the fluctuating ion concentrations, con-
centration maxima of generated ions are more related to the
geological structure of the WF than on the amount of water
pouring down. Nonetheless, the combined mix of parame-
ters such as topographic configuration (height and shape of
the WFs and surrounding rock topography), quantity of wa-
ter and local meteorological conditions are reflected in the
detected ion concentrations of the fall. These parameters also
determine the quantity of water aerosols dispersed in the air,
which in turn, can increase the ion sink rate.
Spatial distribution of aerosolized ions of alpine WFs is
attributed to air flow trajectories, which typically follow the
course of the river bed. In the case of the Krimml water-
fall, generated ions reached distances up to 500 m which
implies maximal WF-related ion lifetime of approximately
120s. Compared with the measurements from the artificial
WF, we can conclude that bubble break-up on aqueous sur-
faces along with splashing of water on solid surfaces are the
major sources of WF-related ions.
The investigated WFs are embedded in very different to-
pographical settings, yet their ion size distribution was found
to be similar in shape thus confirming the data of Laakso
et al. (2007). The abundance of negative small and inter-
mediate ions generated by these WFs was found to be in the
size range of 0.36-25 nm, with a maximal concentration peak
measured at the size window of 1.5-2nm, i.e. in the mobility
range of 0.55-0.3 cm2V−1s−1. On the other hand, heavier
sub-micrometer ions, in the range of 40 to 350 nm, peaked
in a size window at around 120nm. The generation of ions
within the range from 0.9 to 1.5nm was less dominant than
those at 0.36 to 0.9 and 1.5 to 2.0 nm, which is again in agree-
ment with the results as reported by Laakso et al. (2007). Av-
erage negative ion concentrations have been found to be 11 to
80-times higher in the vicinity of the WFs, compared to the
control sites. Positive ion generation by WFs could also be
detected, but their concentration never exceeded triple fold,
as compared to the control sites, and their spatial distribution,
as well as their concentration differed greatly (Fig.6).
Acknowledgements. The study was funded by the “Salzburger
Nationalparkfonds” with leader funds of the EU program for rural
development. It was partially also supported by MES Serbia No:
171020, 45003.
Edited by: M. Kulmala
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