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FORMATION EVENTS OF INTERMEDIATE AIR IONS AT TAHKUSE OBSERVATORY IN
1995–2016
K. KOMSAARE1, M. VANA1, S. MIRME1, M. PALO1, A. LUTS1, H. TAMMET1, H. IHER2 and U.
HÕRRAK2
1
Faculty of Science and Technology, Institute of Physics, University of Tartu,
Ülikooli 18, 50090 Tartu, Estonia.
Keywords: AIR IONS, NUCLEATION EVENTS.
INTRODUCTION
Air ions are known as carriers of electric current and space charge in the atmosphere, but they are also
involved in the new-particle formation (NPF). The importance of new-particle formation and ionization on
the Earth's climate has been discussed in many papers (e.g. Tinsley and Yu, 2004; Carslaw et al., 2010).
The nucleation events were frequently observed at many places in Europe (Manninen et al., 2010;
Nieminen et al., 2014; Birmili et al., 2016) and in the world (Hirsikko et al., 2011), but their formation
mechanisms are still not fairly known despite of recent developments in the application of high-tech (mass
spectrometric) measuring techniques to identify the chemical precursors of particles involved in the NPF
process (Luts et al., 2011; Kourtchev et al., 2013; Bianchi et al., 2016; Kirkby et al., 2016). The robust
methods of particle and air ion mobility spectrometry can still provide valuable information to study the
NPF events on statistical bases, starting from the size range of smallest stable structures - cluster ions
(Hõrrak et al., 2008). Intermediate air ions represent the charged fraction of the smallest stable aerosol
particles in the diameter interval of 1.6–7.4 nm and can be used to study the NPF in the atmosphere
(Hõrrak et al., 2000). The mechanism and species responsible for the nucleation are not well understood.
One of the major contributor to NPF is sulphuric acid (Almeida et al., 2013) and, as recent studies show,
also biogenic volatile organic compounds (Ehn et al., 2014).
METHODS
Tahkuse Observatory (58°31'N 24°56'E) is located in a sparsely populated rural region in the southwestern part of Estonia, at eastern shore of Pärnu river. The nearest city Pärnu with 40000 inhabitants is
located about 25 km to south-west and Soomaa National Park (bogs and wetlands preserve) about 8 km
south-east from the station.
Air ion size distributions are measured by Air Ion Spectrometers (AIS), which covers size range 0.8–79
nm and Neutral cluster and Air Ion Spectrometer (NAIS), which covers size range 0.8–42 nm, according
to Millikan formula (Hõrrak et al., 2000; Mirme et al., 2007). Long term measurements took place from
March 1995 to December 2016, measurement data are available for 6765 days, which is 84.7 % of total
days (Figure 1). NAIS measurements started from July 2011.
Figure 1. Available data for years 1995–2016. Green areas indicate available data, white areas missing
data and black areas days when (class Ia) nucleation event occurred.
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We have classified the intermediate air ion formation events according to similar principles as described
by Hirsikko et al. (2007), but taking into account the specificity of NPF events at Tahkuse.
Four classes of intermediate air ion formation events are probably associated with photochemical
nucleation mechanism in fair weather conditions, that we call as “events” and one class with intermediate
ion generation during precipitation, usually rain, we call those as “precipitation events”. In the case of
nucleation “events”, a growth of newly formed particles can be seen. According to the shape of particle
growth followed by the evolution of particle size distribution, these events can be classified as follows:
Class Ia. The formation and subsequent growth of particles is well developed. The size distribution has no
deep gap between the cluster (below 1.6 nm) and intermediate ions indicating that the cluster ions are
involved in the nucleation. The growth rate of particles over entire measured size range can be calculated.
Class Ib.1. Similarly to class Ia, the formation of particles starts from the cluster ion mode, but the growth
of particles is suppressed before they reach to the sizes of 5–10 nm. The reason for this type of event
might be an insufficient concentration of nucleating vapors.
Class Ib.2. The particle formation does not start from the cluster ion mode and low concentration gap
exists between cluster and intermediate ions at about 2–3 nm.
Class II. The events are similar to a class Ia , but the growth rate of new particles is difficult to estimate or
there are deep gaps during particle growth.
Featureless. During these events the elevated concentration of intermediate ions is detected, which might
be generated due to nucleation processes, but the shape is unclear and/or duration is short (less than 1
hour) and we do not see growth of the particles
Precipitation (rain or snow). During event the elevated concentration of negative intermediate ions with
peak about 2–3 nm is detected. The balloelectric mechanism is considered to be the main reason of
intermediate ion formation (Tammet, 2009; Luts et al., 2009). The concentration of positive intermediate
ions also increases but usually it is 50% or less compared to negative ions, exception may occur in winter
during snowfall or snowdrift.
Undefined. The reason of elevated concentration of intermediate ions is not known or it might be
instrumental or local pollution effect.
Non-event. The concentration of intermediate ions at size 1.6–7 nm remains close to low background (50–
100 cm–3) during day. The situation is similar to the stationary size distributions in night to early morning
(about 0:00–6:00) shown in Figure 2, well before the nucleation onset.
The examples of days associated with different event classes are depicted in Figure 2 below.
Figure 2. Examples of NPF events (usually midday) and precipitation (rain) event (at afternoon
20120416). Color code indicates particle number concentration (cm–3).
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RESULTS
During the 21-year-long study period from 1995 to 2016, we found that the intermediate ion events and
non-events had a clear seasonal variation with the maximum frequency during spring and the second
maximum during autumn (Figure 3). Days with nucleation events (class Ia, Ib.1, Ib.2 or II) form about
14.1 % and days with one or more precipitation events 15.3 % of total measurement days respectively,
whereas 34.5 % days are non-event days with no intermediate ion formation. Class Ia form 16.5 %, class
Ib.1 24.4 %, class Ib.2 9.5 % and class II 50.1 % of total nucleation events. Growth rate was calculated
using maximum-concentration method (Hirsikko et al., 2005). Average (median) growth rates calculated
from class Ia events for three size ranges (1.6–3 nm, 3–7 nm and 7–25 nm) were 2.4 (1.9), 4.8 (3.4) and
7.6 (5.3) (nm h–1) for negative and 3.3 (2.9), 6.9 (4.3) and 9.3 (6.1) (nm h–1) for positive ions respectively.
The annual variation of negative ion growth rate values for three size ranges are depicted in the Figure 4.
Figure 3. Annual variation of nucleation (class Ia, Ib.1, Ib.2 and II) and precipitation events and nonevents. Upper numbers indicate available data in percent for given month.
Figure 4. Annual variation of negative ion growth rate in three size ranges, 1.6–3 nm (blue boxes), 3–7 nm
(green boxes) and 7–23 nm (red boxes). Boxes indicates quartile ranges (25th and 75th percentiles) and
median value at center. Upper numbers indicate total number of events (class Ia) for given month.
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ACKNOWLEDGEMENTS
This work was supported by the Estonian Research Council Projects IUT20-11 and IUT20-52, and by the
grant of Environmetal Agency number 3-8/74 „Complex studies of the air quality at Tahkuse in 2016“.
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