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Atmospheric Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Characteristic features of air ions at Mace Head on the west coast of Ireland
Marko Vana a,d,⁎, Mikael Ehn a, Tuukka Petäjä a,e, Henri Vuollekoski a, Pasi Aalto a,
Gerrit de Leeuw a,b, Darius Ceburnis c, Colin D. O'Dowd c, Markku Kulmala a
a
b
c
d
e
Department of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland
Finnish Meteorological Institute, Research and Development, P. O. Box 503, FI-00101 Helsinki, Finland
School of Physics and Centre for Climate and Air Pollution Studies, Environmental Change Institute, National University of Ireland, Galway, Ireland
Institute of Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia
Earth and Sun Systems Laboratory, Atmospheric Chemistry Division, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-5000, USA
a r t i c l e
i n f o
Article history:
Received 21 November 2007
Received in revised form 18 April 2008
Accepted 22 April 2008
Available online xxxx
Keywords:
Nucleation
Atmospheric ions
Marine aerosols
Particle formation and growth
a b s t r a c t
Coastal nucleation events and behavior of cluster ions were characterized through the
measurements of air ion mobility distributions at the Mace Head research station on the west
coast of Ireland in 2006. We measured concentrations of cluster ions and charged aerosol
particles in the size range of 0.34–40 nm. These measurements allow us to characterize freshly
nucleated charged particles with diameters smaller than 3 nm. The analysis shows that bursts
of intermediate ions (1.6–7 nm) are a frequent phenomenon in the marine coastal environment.
Intermediate ion concentrations were generally close to zero, but during some nucleation
episodes the concentrations increased to several hundreds per cm3. Nucleation events occurred
during most of the measurement days. We classified all days into one of seven classes according
to the occurrence and type of new particle formation. Nucleation events were observed during
207 days in 2006, most prominently in the spring and summer months. Rain-induced events, in
turn, were observed during 132 days. Particle formation and growth events mostly coincided
with the presence of low tide. Also small cluster ions (0.34–1.6 nm) were characterized. Average
concentrations of small ions were 440 cm− 3 for the negative ions and 423 cm− 3 for the positive
ions. Average mean mobilities of small ions were 1.86 cm2V− 1s− 1 and 1.49 cm2V− 1s− 1 for the
negative and positive polarities, respectively. Concentrations of small ions were observed to be
strongly dependent on the variations of meteorological parameters including wind speed and
direction.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The importance of different new particle formation mechanisms in the lower atmosphere has been discussed for a long
time. Binary nucleation of H2O and H2SO4, and ternary
nucleation of H2O, H2SO4 and NH3 (Kulmala et al., 2000 and
Kumala, 2003; Korhonen et al., 1999) has been assumed as
probable nucleation mechanisms in the atmosphere. However,
it has become known in recent years that these theories cannot
explain observed nucleation events in the lower troposphere
⁎ Corresponding author. Institute of Physics, University of Tartu, Ülikooli
18, 50090 Tartu, Estonia.
E-mail address: [email protected] (M. Vana).
(Yu, 2006b). Due to the fact that the classical theory of sulfuric
acid–ammonia–water predicts too high nucleation rates,
recently, the effect of stable ammonium bisulfate formation
was included into calculations (Vehkamäki et al., 2004;
Merikanto et al., 2007). Predicted nucleation rates lowered by
many orders of magnitude, bringing them close to agreement
with available experimental results. Other possible mechanisms for aerosol formation, such as ion-induced nucleation (Yu
and Turco, 2001; Lovejoy et al., 2004; Laakso et al., 2004; Eisele
et al., 2006; Yu, 2006a; Yu et al., 2007), and activation of neutral
or ion clusters (Kulmala et al., 2006), have also been suggested
as viable alternatives.
Air ions (charged clusters and aerosol particles) have been
measured at different sites around the world (Kulmala and
0169-8095/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2008.04.007
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
Res. (2008), doi:10.1016/j.atmosres.2008.04.007
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M. Vana et al. / Atmospheric Research xxx (2008) xxx–xxx
Tammet, 2007). They participate in different atmospheric
processes and their measurement provides indirect information on atmospheric aerosols and air chemistry. Tropospheric
ions make up a very small fraction of ambient air, typically
only reaching concentrations of 102–103 cm− 3 (Hõrrak et al.,
2003). However, even very low concentrations of atmospheric
ions could initiate the production of a significant number of
ion clusters that could eventually grow into new particles.
Measurements of air ions have shown the existence of ioninduced nucleation in the atmosphere (Vana et al., 2006; Iida
et al., 2006; Laakso et al., 2007a). The relative role of ioninduced nucleation mechanism is still unclear and measurements of air ions contribute to elucidating their role in this
process.
Knowledge about the formation of 3 nm particles and their
following growth has been developed in recent years
(Kulmala et al., 2004a). However, observations and theory
suggest that the initial nucleation produces particles smaller
than 3 nm in diameter (Kulmala et al., 2004b). Therefore, to
understand particle nucleation mechanisms, measurements
of particles smaller than 3 nm are important. This can be
accomplished with air ion mobility spectrometers which
measure the mobility (size) distribution of charged particles
for the smallest thermodynamically stable particles with
diameters of the order of 1–2 nm (Kulmala et al., 2000;
Hõrrak et al., 1998, 2000). Experimental evidence on the
existence of a pool of neutral clusters in the sub-3 nanometer
size range was reported by Kulmala et al. (2007).
Studies of the coastal new particle formation have been
reviewed by O'Dowd and Hoffmann (2005). Coastal new particle formation has already been observed more than 100 years
ago. Investigations of elevated particle concentrations in the
coastal environment started in the 1890s when the phenomenon was observed to occur in clear air on the west coast of
Scotland (Aitken, 1897). Coastal regions are places where new
particle formation takes place frequently (O'Dowd, et al., 1999,
2002a; Wen et al., 2006). Therefore, coastal aerosols can significantly contribute to the natural background aerosol population. Observations have shown that these nucleation events
usually coincide with the occurrence of low tide and solar
irradiation (O'Dowd et al., 2002b). Though the older version of
classical ternary homogeneous nucleation theory suggests that
the concentrations of sulfuric acid and ammonia are sufficient
for nucleation of thermodynamically stable clusters, this
doesn't explain the rapid growth of the freshly formed particles
(Kulmala et al., 2002). Furthermore, Yu (2006b) presented a
kinetic ternary homogeneous nucleation model which constrained by experimental results indicated a negligible contribution of ternary homogeneous nucleation to new particle
formation in the boundary atmosphere. O'Dowd et al. (2002a)
showed that biogenic iodine oxides can participate in coastal
new particle production and growth. In general, coastal
nucleation events can be driven by emissions of iodine vapours
that undergo rapid chemical reactions to produce condensable
iodine oxides leading to nucleation and growth of new particles
(O'Dowd and Hoffmann, 2005). However, the detailed nucleation mechanisms in coastal aerosol formation are still unknown. Nucleation episodes at Bodega Bay, California reported
by Wen et al. (2006) did not correlate with tidal height. Instead,
the seasonal and inter-annual variations of ultrafine particle
number concentration appear to correlate with ocean upwel-
ling. During upwelling the biogenic activity peaks as more
nutrients are available for the coastal biota growth. Thus, the
results of Wen et al. (2006) suggest that nucleation is correlated
with coastal biogenic activity and consequently elevated production of aerosol precursor gases.
In studies of new particle formation, important questions
are the frequency of occurrence and the spatial extent of this
phenomenon. The classification of new particle formation
events contributes to answering these questions. Detailed
classifications of these events have been introduced for the
boreal forest ecosystem at the SMEAR II station at Hyytiälä,
Finland based on measurements of aerosol and air ion size
distributions (Dal Maso et al., 2005; Hirsikko et al., 2007). At
the Mace Head research station, nucleation events were
previously classified according to air mass trajectories, corresponding to different distances from the tidal source region,
and aerosol size distribution measurements using Differential
Mobility Particle Sizers (DMPS) (O'Dowd et al., 2002c).
In this work we utilized an air ion mobility spectrometer
(AIS) to measure the mobility (size) distribution of charged
particles at the Mace Head research station on the west coast of
Ireland. One of our aims was to obtain more information on the
behavior of particles with diameters smaller than 3 nm and also
to detect possible seasonal variation in the particle formation.
These are new insights our measurements can offer with
regards to the investigation of coastal aerosol formation. We
introduce a classification of the formation events of intermediate air ions (1.6–7 nm in diameter) suitable for coastal areas. The
knowledge about the behavior of ion clusters, particles and
their charged fraction during nucleation events could help
elucidating the importance of different nucleation mechanisms.
2. Instrumentation and data acquisition
We measured air ion mobility distributions with an Air Ion
Spectrometer (AIS), designed by the University of Tartu, and
built by Airel Ltd., Estonia (Mirme et al., 2007). The AIS is a
multi-channel, parallel-principle device, measuring simultaneously ion concentrations in 27 mobility fractions of both
positive and negative ions. The AIS consists of two identical
cylindrical aspiration-type Differential Mobility Analyzers
(DMA), one for positive and one for negative ion measurements. A radial electrical field is applied to separate naturally
charged particles (cluster ions and aerosol particles) which
are deposited on different electrodes of the DMA depending
on their electrical mobility. Both mobility analyzers have 21
insulated collector electrodes. Electrometrical amplifiers connected to these collectors measure the electric current carried
by the ions. The measurement range of ion mobility is from
0.0013 to 3.2 cm2V− 1s− 1. The corresponding diameter range of
singly-charged particles is from 0.34 to 40 nm. Particle diameter and mobility are uniquely related through the modified
Millikan formula (Tammet, 1995, 1998).
As a part of the European Union project MAP (Marine Aerosol
Production), we deployed an AIS at the Mace Head Atmospheric
Research Station (53°19′N, 9°54′W) on the west coast of Ireland
throughout 2006, and continuing in 2007. Air ions were continuously measured during one year with the aim to detect a
possible seasonal variation in the particle formation. The location of the monitoring station provides a good opportunity to
study particle formation events at different distances from the
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
Res. (2008), doi:10.1016/j.atmosres.2008.04.007
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M. Vana et al. / Atmospheric Research xxx (2008) xxx–xxx
tidal source regions. In this paper we present the data base
which spans the period from 8 January to 31 December 2006.
Supporting basic meteorological data (temperature, pressure, relative humidity, precipitation, wind speed and direction), solar radiation, air mass trajectories and modeled data
of the tidal height for the measurement location were included in the data analysis.
3. Classification of particle formation events
Coastal areas are somewhat different from other places
around the world from the point of view of new particle formation and therefore a classification is introduced which slightly differs from that applied to studies over the boreal forest. Dal
Maso et al. (2005) and Hirsikko et al. (2007) presented a detailed visual method for the distinction of different particle
formation periods. These classifications apply at sites where
particle formation occurs over a geographically wide area. In
this study we introduce a modification of the method applicable
for coastal sites such as Mace Head, where new particle
formation events frequently happen during low tide when the
algae on the beach are exposed and can be observed during
3
different air mass trajectory regimes which correspond to
different distances from the tidal source regions. (O'Dowd et al.,
2002a,b). This means that usually particle formation happens
practically at a point or line source and often the growth of the
nucleated particles cannot be followed.
We analyzed the one-year data set of daily surface plots of
both positive and negative air ion size distributions. We divided all measurement days into seven classes depending on
whether new particle formation occurred or not. For a day to
be classified as including a new particle formation event, a
distinct new mode must appear in the diameter range of
intermediate air ions (1.6–7 nm). Also the mode must prevail
over a time interval of at least couple of hours. An exception to
this is rain-induced events since their duration depends on
the persistence of the precipitation. Sometimes the new mode
also shows signs of growth. From visual inspection of the
concentration vs time plots we classified each day in the year
2006 into one of the following seven classes.
• Class I-type events: Days when the distinct new mode of
particles appears in the size range of intermediate air ions.
The formation of particles and following growth towards
Fig. 1. Examples of classes for intermediate ion formation events: class I, a banana-type event (a); class II, a hump-type event (b); class III, an apple-type event (c);
class IV, a mixed-type nucleation event (d); class V, a rain-induced event (e); class VI, a non-event day.
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
Res. (2008), doi:10.1016/j.atmosres.2008.04.007
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M. Vana et al. / Atmospheric Research xxx (2008) xxx–xxx
Fig. 2. The formation of intermediate air ions usually happened during low tide, but rainfall also generated large numbers of ions in the intermediate size range.
During the rain-induced event the concentration of negative ions was much higher than the concentration of positive ions.
larger sizes continues during several hours. These are usual
events observed in many places around the world and the
shape of the concentration vs time plot resembles a banana
(Fig. 1a). In such cases polluted air is either advected over
tidal regions far from the measurement station or has not
passed over tidal regions. However, this type of event is not
likely associated with low tide because the events usually
last longer than one tidal cycle. To follow the class I events
by an Eulerian experiment, the particle formation has to
occur over a geographically wide area. The present study
and earlier observations by Vana et al. (2002) show that
clear class I events occur within a certain airflow regime. In
this case in Arctic air coming from the north which picks up
moderate pollution over Great Britain and Ireland.
• Class II, hump-type events: Days when the distinct new mode
of particles appear in the size range of intermediate air ions
and the nucleation burst starts directly from the cluster ions
region. However, particles do not usually grow larger than
10 nm in diameter. These events usually occur coinciding with
low tide in the presence of solar radiation. And the shape of the
concentration vs time plot resembles a hump (Fig. 1b). Class II
events usually occur in clean marine air advected over sparsely
populated land in the northwest-to-north wind sector.
• Class III, apple-type events: Days when the distinct new mode
of particles appears in the size range of intermediate air ions.
However, the characteristic feature here is that the particle
formation events of charged particles did not start from the
cluster ion mode and a clear gap in the ion distribution is
observed between the cluster and the intermediate ion
modes. Like for class II events, the presence of apple-type
events mostly coincides with low tide. And the shape of the
concentration vs time plot resembles an apple (Fig. 1c).
• Class IV, mixed-type nucleation events: Days were classified
as mixed-type nucleation days when formation of intermediate air ions was clearly observed but we were not able
to classify such events as banana-, hump-, or apple-type (Fig.
1d). During class IV events the wind direction was often
variable. In this case air can pass over many point or line
sources causing difficulties to determine the type of the
event.
• Class V, rain-induced events: Days were classified as raininduced when the formation of intermediate air ions
occurred during rain (Fig. 1e). A characteristic feature of
this type is that the concentration of negatively charged
particles is much higher than that of positively charged
particles. Fig. 2 shows the formation of intermediate air ions
during rainfall. As precipitation is a common phenomenon
at Mace Head, we observed rain-induced events during
many days in 2006. Therefore, precipitation data is important when analyzing the AIS data and classifying events.
Table 1
Statistics of the classification for different new particle formation types in 2006, median particle concentrations during event periods and non-event days
Event class
Number Median ion
of days concentration
(cm− 3), 1.6–3 nm
Median ion
concentration
(cm− 3), 3–7 nm
Negative Positive Negative Positive
ions
ions
ions
ions
Class I, banana-type events
25
Class II, hump-type events
34
Class III, apple-type events
45
Class IV, mixed-type nucleation events
103
Class V, rain-induced events (in brackets days with a rain-induced event together with 132
an ordinary nucleation event from classes I–IV)
(62)
Class VI, non-event days
58
Class VII, unclassified days (in brackets days with completely missing data)
30 (22)
Median total
particle
concentration
(cm− 3), 3–1000 nm
63
52
24
45
53
34
36
18
29
18
129
15
108
86
45
123
21
121
80
35
23,010
3110
9340
7430
490
2
–
2
–
2
–
3
–
730
–
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
Res. (2008), doi:10.1016/j.atmosres.2008.04.007
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Fig. 3. The monthly distribution of the number of non-event, event, rain-induced event and unclassified days (a); the subdivision of new particle formation events
into four classes (b). The distribution of the different classes shows that most events occur during spring and summer.
tively charged ones. During some days we clearly had both
an ordinary new particle formation event (classes I–IV) and
a rain-induced event.
• Class VI, non-event days: Days when particle formation is
not detected (Fig. 1f). Measured mobility distributions show
a deep depression between the cluster ion mode and large
ions with a nearly zero concentration of intermediate ions.
• Class VII, unclassified days: Days when due to low signal-noise
ratio or other uncertainties we were not able to identify
whether particle formation happened or not, and also days
The lifetime of the rain-induced particles is relatively small,
their concentration decreases quickly after the rain due to
the coagulation with larger particles. In general, the breaking or splashing of water drops can generate negative space
charge in the atmosphere during rainfall and close to lakes
and waterfalls (Laakso et al., 2007b). The drops of pure fresh
water can break up into numerous fine negative droplets
and some remaining larger positively charged drops. As a
result the concentrations of positively charged particles can
be many times smaller than the concentrations of nega-
Fig. 4. The time variation of the concentration of small ions (0.34–1.6 nm in diameter) during measurement period in the year 2006.
^^
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Table 2
Median and average concentrations of small cluster ions, intermediate ions
and large ions in the nucleation mode size range, and standard deviations
Diameter range
(nm)
0.34–1.6
1.6–3
3–7
7–22
Concentration of negative
ions (cm− 3)
Concentration of positive
ions (cm− 3)
Median
Average
Std
Median
Average
Std
403
3
5
25
444
14
27
69
233
42
80
187
384
3
6
29
423
10
33
83
196
35
102
227
with missing data. This class increases the uncertainty in the
statistics of events.
Table 1 shows the statistics of the classification for the
different new particle formation types observed during the
year 2006 at Mace Head. Also the median concentrations of
positive and negative particles in the size ranges of 1.6–3 nm
and 3–7 nm measured by AIS, and median concentrations of
aerosol particles in size range 3–1000 nm measured by
Scanning Mobility Particle Sizer (SMPS) during the event
periods for the first five classes are given as well as the
concentrations for non-event days. According to our classification 58 non-event days, 207 event days, 132 rain-induced
event days, and 30 unclassified days occurred. This adds to
more than 365 days, because during 62 days a rain-induced
event occurred together with an ordinary nucleation event
from classes I–IV (see Table 1). The event days comprised of
25 banana-type event days, 45 apple-type event days, 34
hump-type event days and 103 mixed-type nucleation event
days. The latter class, as well as unclassified days, increased
the uncertainty in classification of nucleation burst events.
Yoon et al., 2006 reported similar frequency of coastal nucleation events at the Mace Head Atmospheric Research Station. They observed nucleation events during 58% of the days
over a 2-year period from August 2002 to July 2004.
Median concentrations of intermediate air ions during
event periods are mostly higher in the size range of 3–7 nm
compared to the size range of 1.6–3 nm (see Table 1). Ex-
ceptions are hump-type events and rain-induced events where
considerably higher concentrations of air ions in the size range
of 1.6–3 nm were measured. We can see more pronounced
differences between concentrations of air ions with different
polarity for the smaller size range, the concentrations of
negative ions are considerably higher than the concentrations
of positive ions. This is not usually the case for the size range of
3–7 nm. Elevated concentrations of negative air ions in the size
range of 1.6–3 nm imply the significance of negative ions in new
particle formation processes. The character of particle formation events in different classes shows that ions seem to be more
involved in the nucleation for class I and class II but probably
less involved in class III. Rain-induced events seem to be
connected to evaporation of ions from rain droplets and not to
any nucleation mechanisms.
We compared days with and without events separately for
each month. Fig. 3a shows the monthly distribution of the
number of non-event, event, rain-induced event and unclassified days. The analysis showed particle formation during
more than half of the days over the period from March to
September, and less than half of the days during the other
months excluding rain-induced event days. The unclassified
days added uncertainty to this analysis. In Fig. 3b the
subdivision of new particle formation events into four classes
is depicted. Most of the events occurred during the warm
season from April to September.
As the measurements of air ions continued in 2007, the
future work will include a specification of the statistics of event
classification, and also comparison with aerosol size distribution data to find out the charging state of newly formed
particles at Mace Head.
4. Characteristics of small ions
A mode of small cluster ions (0.4–1.6 nm) in the size
distribution of air ions always exist in the atmosphere. These
particles play a crucial role in the initial steps of nucleation and
in the formation of thermodynamically stable clusters. Therefore, it is important to study the character of small ions in the
Fig. 5. A typical time variation of the mean mobility of small ions at Mace Head and correlation with temperature.
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
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Fig. 6. Scatterplots showing the correlation between the concentration of small ions and wind speed when wind direction is from the clean/ocean sector (180–310°)
(a) and from the polluted/land sector (all other directions) (b).
^^ ^
Fig. 7. Scatterplots showing the correlation between (a) the concentration of small ions and wind direction, and (b) wind speed and wind direction.
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
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M. Vana et al. / Atmospheric Research xxx (2008) xxx–xxx
atmosphere. Our measurements show that the concentrations
of small ions with one polarity were typically between 200
and 800 cm− 3. Fig. 4 illustrates the time variation of the
concentration of small ions throughout 2006. The concentrations of intermediate ions (1.6–7 nm) of one polarity were on
average about 40 cm− 3 but increased to 500–1000 cm− 3
during some nucleation events. Outside periods with rain or
new particle formation, intermediate ion concentrations were
close to zero. The concentrations of light large ions (7–22 nm)
with one polarity were on average approximately 70 cm− 3.
Table 2 shows median and average concentrations, and their
standard deviations for different size classes of air ions during
the measurement period in the year 2006.
To study the evolution of the mobility (size) distribution an
integral parameter, the mean mobility can be used instead of
spectral presentation. We calculated the mean natural mobility
of small ions by averaging over the mobility interval from 0.5 to
3.2 cm2V− 1s− 1. Fig. 5 shows a typical example of the time
variation of the mean mobility of small ions at Mace Head
together with the air temperature. The regular diurnal variation
of the mean mobility and the air temperature shows negative
correlation. Fig. 5 also clearly shows the alternation of air masses
and the associated variation of the mean mobility. The decrease
in mean mobility was connected with advection of cold air
masses. Average values of the mean mobility and its standard
deviations in 2006 were 1.86 ± 0.21 cm2V− 1s− 1 for negative
cluster ions and 1.49 ±0.14 cm2V− 1s− 1 for positive cluster ions.
These values are slightly higher than previously obtained in
more continental rural areas such as at Tahkuse (Estonia), where
average values of the mean mobility were 1.53 ±0.10 cm2V− 1s− 1
and 1.36 ±0.06 cm2V− 1s− 1, respectively (Hõrrak et al., 2003).
We also studied the dependence of the concentration of
small ions on various meteorological parameters. In this data
set we observed a correlation between the small ion concentrations and both wind speed and direction (see Figs. 6 and 7).
As Mace Head is located on the coast, the wind speed and
direction are also correlated, which complicates the interpretation. Fig. 6a shows that the concentration of small ions decreases with increasing wind speed when wind direction is
from the clean/ocean sector (180–310°). However, this dependence is more pronounced when wind direction is from the
polluted/land sector (all other direction) (see Fig. 6b). The
reason for the observed high concentrations can be the accumulation of radon near the ground during low wind speeds.
The main sources for small ions are cosmic radiation and radon.
Radon is emitted from the soil, and thus an air mass coming
directly from the ocean to the measurement site is expected to
have a lower small ion concentration. Similarly, higher wind
speeds will cause more turbulence, and vertical mixing, in
addition to faster advection, diluting the radon concentrations
at ground level more efficiently. However, it is possible that the
observed high concentrations at low wind speed are associated
with wind direction from polluted sector, which usually occur
at Mace Head during anti-cyclonic conditions with low wind
speed. This is further suggested by Fig. 7a which shows that the
concentration of small ions depends on wind direction. The
concentration of small ions increases considerably when the
wind direction turns from the clean marine sector (180–310°) to
the sector where air is advected over sparsely populated land in
the northwest-to-north direction. At the same time Fig. 7b
shows that wind speed is typically higher when airflow comes
from the clean ocean sector, which adds uncertainties to the
interpretation of these results.
5. Conclusions
We deployed an Air Ion Spectrometer for continuous measurements at Mace Head Research Station on the west coast of
Ireland. The analysis of the size distributions of atmospheric air
ions relied on a classification of the data according to the
characteristic features of the size distribution evolution of the
intermediate ions. We classified all the measurement days into
one of seven classes depending on whether new particle formation occurred or not.
From the analysis we can draw the following conclusions:
i. Classification of nucleation events showed 207 event
days in 2006. Most of the events occurred during spring
and summer. At Mace Head wind direction often varies
with time, which caused about half of the observed new
particle formation events being classified as mixed-type
nucleation events. In this case air can pass over many
point or line sources causing difficulties to determine the
type of the event. The hump- and apple-type events,
specific events for coastal areas, were observed during
34 and 45 days, respectively.
ii. Formation of intermediate air ions was observed during rainfall, and seems to be a common phenomenon
at Mace Head. We detected 132 days with rain-induced
nucleation events in 2006. During 62 days out of 132
rain-induced event days and 207 ordinary nucleation
event days, we clearly had both an ordinary new
particle formation event and a rain-induced event.
iii. Small air ions were characterized. Calculated values of
mean mobility of small ions turned out to be relatively
high and the time variation of this parameter clearly
depends on temperature and character of the air mass.
The concentrations of small ions clearly depend on
wind speed and wind direction. The concentration of
small ions can be several times higher during low wind
speed compared to that of during high wind speed.
Acknowledgements
This work was supported by EU (FP6, MAP project number
018332), the Nordic Center of Excellence (BACCI), the University
of Tartu research project PP1FY07913 and by the Estonian
Science Foundation under grants no. 6988 and 6223.
References
Aitken, J.A., 1897. On some nuclei of cloudy condensation. Trans. R. Soc. Edinb.
XXXIX.
Dal Maso, M., Kulmala, M., Riipinen, I., Wagner, R., Hussein, T., Aalto, P.P.,
Lehtinen, K.E.J., 2005. Formation and growth of fresh atmospheric
aerosols: eight years of aerosol size distribution data from SMEAR II,
Hyytiälä, Finland. Boreal Environ. Res. 10, 323–336.
Eisele, F.L., Lovejoy, E.R., Kosciuch, E., Moore, K.F., Mauldin III, R.L., Smith, J.N.,
McMurry, P.H., Iida, K., 2006. Negative atmospheric ions and their
potential role in ion-induced nucleation. J. Geophys. Res. 111. doi:10.1029/
2005JD006568.
Hirsikko, A., Bergman, T., Laakso, L., Dal Maso, M., Riipinen, I., Hõrrak, U., Kulmala, M.,
2007. Identification and classification of the formation of intermediate ions
measured in boreal forest. Atmos. Chem. Phys. 7, 201–210.
Hõrrak, U., Salm, J., Tammet, H., 1998. Bursts of intermediate ions in
atmospheric air. J. Geophys. Res. 103, 13909–13915.
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
Res. (2008), doi:10.1016/j.atmosres.2008.04.007
ARTICLE IN PRESS
M. Vana et al. / Atmospheric Research xxx (2008) xxx–xxx
Hõrrak, U., Salm, J., Tammet, H., 2000. Statistical characterization of air ion
mobility spectra at Tahkuse Observatory: classification of air ions. J. Geophys.
Res. 105, 9291–9302.
Hõrrak, U., Salm, J., Tammet, H., 2003. Diurnal variation in the concentration
of air ions of different mobility classes in a rural area. J. Geophys. Res. 108.
doi:10.1029/2002JD003240.
Iida, K., Stolzenburg, M., McMurry, P., Dunn, M.J., Smith, J.N., Eisele, F., Keady, P.,
2006. Contribution of ion-induced nucleation to new particle formation:
methodology and its application to atmospheric observations in Boulder,
Colorado. J. Geophys. Res. 111. doi:10.1029/2006JD007167.
Korhonen, P., Kulmala, M., Laaksonen, A., Viisanen, Y., McGraw, R., Seinfeld, J.H.,1999.
Ternary nucleation of H2SO4, NH3, and H2O in the atmosphere. J. Geophys. Res.
104, 26349–26353.
Kulmala, M., 2003. How particles nucleate and grow. Science 302, 1000–1001.
Kulmala, M., Tammet, H., 2007. Finnish–Estonian air ion and aerosol
workshop. Boreal Env. Res. 12, 237–245.
Kulmala, M., Pirjola, L., Mäkelä, J.M., 2000. Stable sulphate clusters as a source
of new atmospheric particles. Nature 404, 66–69.
Kulmala, M., Korhonen, P., Napari, I., Karlsson, A., Berresheim, H., O'Dowd, C.D.,
2002. Aerosol formation during PARFORCE: ternary nucleation of H2SO4,
NH3, and H2O. J. Geophys. Res. 107. doi:10.1029/2001JD000900.
Kulmala, M., Vehkamäki, H., Petäjä, T., Dal Maso, M., Lauri, A., Kerminen, V.-M.,
Birmili, W., McMurry, P.H., 2004a. Formation and growth rates of ultrafine
atmospheric particles: a review of observations. J. Aerosol Sci. 35,
143–176.
Kulmala, M., Laakso, L., Lehtinen, K.E.J., Riipinen, I., Dal Maso, M., Anttila, T.,
Kerminen, V.-M., Hõrrak, U., Vana, M., Tammet, H., 2004b. Initial steps of
aerosol growth. Atmos. Chem. Phys. 4, 2553–2560.
Kulmala, M., Lehtinen, K.E.J., Laaksonen, A., 2006. Cluster activation theory as
an explanation of the linear dependence between formation rate of 3 nm
particles and sulphuric acid concentration. Atmos. Chem. Phys. 6,
787–793.
Kulmala, M., Riipinen, I., Sipilä, M., Manninen, H.E., Petäjä, T., Junninen, H.,
Dal Maso, M., Mordas, G., Mirme, A., Vana, M., Hirsikko, A., Laakso, L.,
Harrison, R.M., Hanson, I., Leung, C., Lehtinen, K.E.J., Kerminen, V.-M.,
2007. Towards direct measurement of atmospheric nucleation. Science
318, 89–92. doi:10.1126/science.1144124.
Laakso, L., Anttila, T., Lehtinen, K.E.J., Aalto, P.P., Kulmala, M., Hõrrak, U.,
Paatero, J., Hanke, M., Arnold, F., 2004. Kinetic nucleation and ions in
boreal forest particle formation events. Atmos. Chem. Phys. 4,
2353–2366.
Laakso, L., Gagne, S., Petäjä, T., Hirsikko, A., Aalto, P.P., Kulmala, M.,
Kerminen, V.-M., 2007a. Detecting charging state of ultra-fine particles: instrumental development and ambient measurements. Atmos.
Chem. Phys. 7, 1333–1345.
Laakso, L., Hirsikko, A., Grönholm, T., Kulmala, M., Luts, A., Parts, T.-E., 2007b.
Waterfalls as sources of small charged aerosol particles. Atmos. Chem.
Phys. 7, 2271–2275.
Lovejoy, E.R., Curtius, J., Froyd, K.D., 2004. Atmospheric ion-induced
nucleation of sulphuric acid and water. J. Geophys. Res. 109.
doi:10.1029/2003JD004460.
Merikanto, J., Napari, I., Vehkamäki, H., Anttila, T., Kulmala, M., 2007. New
parametrization of sulfuric acid–ammonia–water ternary nucleation
9
rates at tropospheric conditions. J. Geophys. Res. 112. doi:10.1029/
2006JD007977.
Mirme, A., Tamm, E., Mordas, G., Vana, M., Uin, J., Mirme, S., Bernotas, T.,
Laakso, L., Hirsikko, A., Kulmala, M., 2007. A wide-range multi-channel air
ion spectrometer. Boreal Env. Res. 12, 247–264.
O'Dowd, C.D., Hoffmann, T., 2005. Coastal new particle formation: a review of
the current state-of-the-art. Environ. Chem. 2, 245–255.
O'Dowd, C., McFiggans, G., Creasey, D.J., Pirjola, L., Hoell, C., Smith, M.H.,
Allan, B.J., Plane, J.M.C., Heard, D.E., Lee, J.D., Pilling, M.J., Kulmala, M.,
1999. On the photochemical production of new particles in the coastal
boundary layer. Geophys. Res. Lett. 26, 1707–1710.
O'Dowd, C.D., Jimenez, J., Bahreini, R., Flagan, R., Seinfeld, J., Hämeri, K.,
Pirjola, L., Kulmala, M., Jennings, S.G., Hoffmann, T., 2002a. Marine
aerosol formation from biogenic iodine emissions. Nature 417, 632–636.
O'Dowd, C.D., Hämeri, K., Mäkelä, J.M., Pirjola, L., Kulmala, M., Jennings, S.G.,
Berresheim, H., Hansson, H.-C., de Leeuw, G., Kunz, G.J., Allen, A.G.,
Hewitt, C.N., Jackson, A., Viisanen, Y., Hoffmann, T., 2002b. A dedicated
study of New Particle Formation and Fate in the Coastal Environment
(PARFORCE): overview of objectives and achievements. J. Geophys. Res.
107. doi:10.1029/2001JD000555.
O'Dowd, C.D., Hämeri, K., Mäkelä, J., Väkevä, M., Aalto, P., de Leeuw, G., Kunz, G.J.,
Becker, E., Hansson, H.-C., Allen, A.G., Harrison, R.M., Berresheim, H.,
Kleefeld, C., Geever, M., Jennings, S.G., Kulmala, M., 2002c. Coastal new
particle formation: environmental conditions and aerosol physicochemical
characteristics during nucleation bursts. J. Geophys. Res. 107. doi:10.1029/
2001JD000206.
Tammet, H., 1995. Size and mobility of nanometer particles, clusters and ions.
J. Aerosol Sci. 26, 459–475.
Tammet, H.,1998. Reduction of air ion mobility to standard conditions. J. Geophys.
Res. 103, 13933–13937.
Vana, M., Jennings, S.G., Kleefeld, C., Mirme, A., Tamm, E., 2002. Small-particle
concentration fluctuations at a coastal site. Atmos. Res. 63, 247–269.
Vana, M., Tamm, E., Hõrrak, U., Mirme, A., Tammet, H., Laakso, L., Aalto, P.P.,
Kulmala, M., 2006. Charging state of atmospheric nanoparticles during
the nucleation burst events. Atmos. Res. 82, 536–546.
Vehkamäki, H., Napari, I., Kulmala, M., Noppel, M., 2004. Stable ammonium
bisulphate clusters in the atmosphere. Phys. Rev. Lett. 93, 148 501.
Wen, J., Zhao, Y., Wexler, A., 2006. Marine particle nucleation: observation at
Bodega Bay, California. J. Geophys. Res. 111 doi:10129/2005JD006210.
Yoon, Y.J., O'Dowd, C.D., Jennings, S.G., Lee, S.H., 2006. Statistical characteristics
and predictability of particle formation events at Mace Head. J. Geophys.
Res. 111 doi:10.1029/2005JD006284.
Yu, F., 2006a. From molecular clusters to nanoparticles: second-generation
ion-mediated nucleation model. Atmos. Chem. Phys. 6, 5193–5211.
Yu, F., 2006b. Effect of ammonia on new particle formation: a kinetic H2SO4–H2O–
NH3 nucleation model constrained by laboratory measurements. J. Geophys.
Res. 111. doi:10.1029/2005JD005968.
Yu, F., Turco, R.P., 2001. From molecular clusters to nanoparticles: role of
ambient ionization in tropospheric aerosol formation. J. Geophys. Res.
106, 4797–4814.
Yu, F., Wang, Z., Luo, G., Turco, R., 2007. Ion-mediated nucleation as an important
global source of tropospheric aerosols. Atmos. Chem. Phys. Discuss. 7,
13597–13626.
Please cite this article as: Vana, M., et al., Characteristic features of air ions at Mace Head on the west coast of Ireland, Atmos.
Res. (2008), doi:10.1016/j.atmosres.2008.04.007