Analysis of atmospheric neutral and charged molecular clusters in boreal forest using pulse-height CPC

Abstract

We measured the size distribution of atmospheric neutral and charged clusters and particles down to mobility diameter around 1.5 nm by applying pulse-height CPC technique at SMEAR II station in Hyytiälä, southern Finland during spring 2007 and May 2008. The concentration of molecular clusters smaller than 3 nm seems to be highly variable in boreal forest environment. The concentration varied typically between 500–50 000 cm−3, the medians being 8060 cm−3 in year 2007 and 3380 cm−3 in 2008. By comparing to concentrations measured with ion spectrometers, we conclude that ion clusters and neutral clusters produced by ion-ion recombination are usually not sufficient to explain all of the observed clusters; the median fraction of recombination products from all neutral clusters was 4.9%. Before and during most new particle formation events the cluster formation rate rose only slightly, or remained close to stable. Nocturnal formation of clusters was also frequently observed.

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Atmospheric
Chemistry
and Physics
Analysis of atmospheric neutral and charged molecular clusters in
boreal forest using pulse-height CPC
K. Lehtipalo1, M. Sipil¨ a1,2, I. Riipinen1, T. Nieminen1, and M. Kulmala1
1Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
2Helsinki Institute of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
Received: 22 September 2008 – Published in Atmos. Chem. Phys. Discuss.: 10 December 2008
Revised: 26 May 2009 – Accepted: 10 June 2009 – Published: 23 June 2009
Abstract. We measured the size distribution of atmospheric
neutral and charged clusters and particles down to mobility
diameter around 1.5nm by applying pulse-height CPC tech-
nique at SMEAR II station in Hyyti¨ al¨ a, southern Finland dur-
ing spring 2007 and May 2008. The concentration of molec-
ular clusters smaller than 3nm seems to be highly variable in
boreal forest environment, the concentration varied typically
between 500–50000cm−3. By comparing to concentrations
measured with ion spectrometers, we conclude that ion clus-
ters and neutral clusters produced by ion-ion recombination
are usually not sufficient to explain all of the observed clus-
ters; the median fraction of recombination products from all
neutral clusters was 4.9%. Before and during most new par-
ticle formation events the cluster formation rate rose only
slightly, or remained close to stable. Nocturnal formation
of clusters was also frequently observed.
1Introduction
Aerosol particles affect the climate both directly by scatter-
ing and absorbing solar radiation and indirectly via cloud
processes. The net radiative forcing of particles is currently
believed to be negative (i.e. cooling), but the level of scien-
tific understanding is still relatively low (IPCC, 2007). De-
tailed knowledge of nucleation process is needed to include
secondary aerosol particles to climate models. Nucleation
events have been proven to affect the regional and global
particle budget significantly (Spracklen et al., 2006). Addi-
tionally, aerosolparticlesenterthehumanrespiratorysystem,
Correspondence to: K. Lehtipalo
(katrianne.lehtipalo@helsinki.fi)
where particles may cause adverse immediate and chronic
health effects (e.g. Brunekreef and Holgate, 2002).
The existence of stable neutral atmospheric clusters and
their role in new particle formation is the key question in
understanding atmospheric nucleation process. New particle
formation has been observed on numerous locations around
the world (Kulmala et al., 2004a), but the exact mechanisms
how particles are formed from precursor vapors are still un-
der debate. The prevailing nucleation mechanism, at least
in boreal forest environment, is suggested to be activation
of neutral atmospheric clusters approximately 1–2nm in di-
ameter (Kulmala et al., 2000, 2006, and 2007a). Theoreti-
cal arguments and laboratory experiments support the exis-
tence of neutral clusters (e.g. Kulmala et al., 2005; Hanson
and Eisele, 2002). The proposed candidates for the chemical
compounds involved in forming clusters include sulphates
(e.g. Vehkam¨ aki et al., 2004; Kulmala et. al., 2004b), am-
monia (Torpo et al., 2007) and organics (Bonn et al., 2008).
Ion-ion recombination is also known to produce neutral clus-
ters (Turco et al., 1998).
Due to instrumental limitations direct observations of neu-
tral clusters in the atmosphere have been sparse. Weber et
al. (1995) and Kulmala et al. (2005) showed that clusters
were present during nucleation events. Ion clusters in the
size range below 1.5nm can be easily detected and have
been proven to exist practically all time in the atmosphere
(H˜ orrak et al., 1998). The concentration of ion clusters is,
however, typically not high enough to explain the observed
atmosphericparticleformationrates(e.g.Laaksoetal., 2004;
Iida et al., 2006). The contribution of ion-induced nucleation
to new particle formation in Hyyti¨ al¨ a seems to be less than
10% (Gagn´ e et al., 2008). A significant effort has recently
been put into developing instruments capable of measuring
neutral particles below 3 nm, which is the lower detection
Published by Copernicus Publications on behalf of the European Geosciences Union.

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4178 K. Lehtipalo et al.: Analysis of atmospheric neutral and charged molecular clusters
limit of the current commercial instruments. With newly de-
veloped measurement techniques, like Neutral Cluster and
Air Ion Spectrometer (NAIS) and CPC-based applications,
Kulmala et al. (2007a) and Sipil¨ a et al. (2008, 2009) ob-
served a persistent pool of neutral sub-3nm clusters in boreal
forests.
In this work we will, for the first time, present a continu-
ous time series of atmospheric neutral cluster concentrations.
Wealso studythediurnalvariationof clusterconcentrationin
boreal forest, which gives insight into their production mech-
anisms. By measuring simultaneously both charged and neu-
tral clusters with various independent instruments we aim at
estimating the significance of ions in producing neutral clus-
ters.
2Material and methods
2.1Site description and instrumentation
The measurements took place at SMEAR II station
(61◦51?N, 24◦17?E, 181ma.s.l.) in Hyyti¨ al¨ a, southern Fin-
land. The station is surrounded by coniferous Scots pine-
dominated forest. For detailed site description see e.g. Hari
and Kulmala (2005).
The particle size-distribution from 3 to 1000nm was mea-
sured with a basic twin-DMPS system (Aalto et al., 2001).
For measuring the ion mobility distribution a Balanced Scan-
ning Mobility Analyzer (BSMA, Tammet, 2006) and an Air
Ion Spectrometer (AIS, Mirme et al., 2007) were used. Both
of the ion spectrometers are manufactured by Airel Ltd.,
and their mobility ranges are 0.032–3.2cm2V−1s−1(∼0.8–
7.6nm in mobility diameter) and 0.0013–2.4cm2V−1s−1
(∼0.9–42nm), respectively.
Spectrometer (NAIS) is based on the AIS, but equipped with
a unipolar charger to measure also neutral particles. The size
range of the charger ions sets the lower size limit of the NAIS
to about 2nm depending on polarity and cluster concentra-
tion (Asmi et al., 2009). The size distribution of neutral and
charged clusters ∼1.2–5nm (equivalent ion mobility diam-
eter) was measured with a pulse-height CPC (PH-CPC, dis-
cussed in Sect. 2.2).
During spring 2007 (15 March–26 June) all of the above
mentioned instruments were measuring simultaneously in
Hyyti¨ al¨ a as a part of the EUCAARI campaign (Kulmala et
al., 2009). From spring 2008 (1–31 May) PH-CPC, DMPS
and BSMA data were available. All particle measurements
were conducted inside the forest canopy from a height of
c. 2m above the ground. All reported diameters are non-
reduced mobility diameters, and all times are local winter-
time.
Neutral Cluster and Air Ion
2.2PH-CPC
The pulse height analysis technique (Saros et al., 1996) re-
lies on detecting the intensity of light scattered by particles
after their condensational growth in the CPC. Due to super-
saturation gradient inside the condenser, particles activate for
growth at different axial positions depending on their size.
The smaller the particle, the later it will be activated leading
to smaller final droplet sizes. Clearly there is an upper size
limit, after which only total concentration, but no size infor-
mation of particles can be achieved, since all bigger particles
are activated almost simultaneously. Pulse height analysis
method has been used in size distribution measurements be-
tween 3 and 10nm (Weber et al., 1995, 1998) as well as to
determine the composition of freshly nucleated nanoparticles
(O’Dowd et al., 2002; Hanson et al., 2002). Recently Sipil¨ a
et al. (2008) applied this method to detect atmospheric clus-
ters.
The PH-CPC used in this study comprises a TSI-3025A
ultrafine CPC with modified optics (Dick et al., 2000) and
a multichannel analyzer. For increasing the detection effi-
ciency of small particles, the supersaturation inside the con-
denser was increased from nominal until homogenous nucle-
ation appeared. The pulse height analysis technique allowed
us to distinguish homogenous nucleation from activation of
clusters and resolve the size distribution of particles below
5nm. Detailed description of data inversion is published by
Sipil¨ a et al. (2009).
The PH-CPC’s size response and detection efficiency
has been calibrated using positive ions (silver and 241-Am
charger generated ions) only, due to lack of reference instru-
ment for neutral particles below 3nm. Winkler et al. (2008)
showed that charged clusters are activated in lower supersat-
urations than neutral ones, and negative ones before positive.
When PH-CPC is used for measuring atmospheric aerosol,
we therefore tend to slightly underestimate the size and con-
centration of neutral particles. The activation probability of
particles depends also on their composition (e.g. O’Down et
al., 2002, 2003; Kulmala et al., 2007b). For these reasons
the size scale of our results should not be considered as ac-
tual diameters, but rather the equivalent activation mobility
diameter of insoluble positive ions.
Furthermore, the activation probability depends on the to-
tal particle concentration activated inside the condenser due
to the vapour consumption of the growing droplets. At con-
centrations larger than 4000–5000cm−3homogenous nu-
cleation is almost completely suppressed compromising the
method. Cluster concentrations measured during strong nu-
cleation events and pollution episodes are therefore not con-
sidered reliable, and normally excluded from the analyses.
The factors affecting the size and concentration response of
the PH-CPC are further discussed by Sipil¨ a et al. (2009).
From spring 2007 to 2008 the measurement method was
modified in a way that instead of assuming a symmetrical ho-
mogenous nucleation mode, we measured the homogenous
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K. Lehtipalo et al.: Analysis of atmospheric neutral and charged molecular clusters4179
Fig. 1. Total number concentration of <3nm clusters measured
with PH-CPC in Hyyti¨ al¨ a 14 March–26 June 2007.
nucleation level after each ambient measurement. For re-
taining a similar background concentration, a diffusion tube
(c. 2m, i.d. 8mm copper tubing, flow rate of 3lpm) was ap-
plied for filtering the clusters and smallest particles away.
Additionally, an ion trap (7cm coaxial tube i.d. 8mm with
a voltage of 32V between the electrodes) was applied after
each diffusion tube measurement to get rid of charged clus-
ters. In 2007 the ion trap and a diffusion battery were applied
irregularly.
2.3Data analysis
2.3.1Neutral clusters
In 2007 the concentration of neutral particles was calculated
by subtracting the ion clusters of corresponding size range
measured with the BSMA from the total cluster concentra-
tion measured with the PH-CPC. As the concentration of
intermediate ions larger than 1.5nm is typically very low,
<200cm−3except during nucleation events (Hirsikko et al.,
2005), the effect should be minor. The neutral cluster con-
centration in 2008 can be achieved solely from PH-CPC data
by subtracting the diffusion tube measurement from the ion
trap measurements.
2.3.2 Recombination products
A neutral cluster may result if two ions of opposite charge
collide and form a stable entity; this is called recombination.
An estimate for the steady-state concentration of these re-
combination products can be obtained by setting the balance
equation
dNn,rec
dt
−CoagS(Nn,rec)Nn,rec
to zero. The subscript n refers to neutral clusters and i to ion
clusters. a is the fraction of stable recombination products, α
the ion-ion recombination coefficient and β the aerosol-ion
=aαN+
iN−
iβNn,rec(N+
i+N−
i)
(1)
Fig. 2. Total number concentration of <3nm neutral clusters mea-
sured with PH-CPC in Hyyti¨ al¨ a 1–31 May 2008.
attachment coefficient. Thus the first term on the right hand
side represents recombination source, the second and the
third loss of neutral particles by charging and coagulation,
respectively. For α we used a value of 1.6·10−6cm3s−1and
for β 0.01·10−6cm3s−1(Tammet and Kulmala, 2005). To
obtain a maximum estimate, a was set to unity. The ion clus-
ter concentrations were taken from the BSMA data. As we
were interested in recombination products in the size range
of the PH-CPC, only collisions between ions large enough to
produceparticlesabout1.5–2nmindiameterwereaccounted
for. The corresponding BSMA channels were chosen using
mass-mobility relation by M¨ akel¨ a et al. (1996, based on ex-
perimental data by Kilpatrick, 1971).
2.3.3Formation rates
The formation rate of 1.5nm clusters can be calculated from
(as in Kulmala et al., 2007a, supporting material)
J1.5=dN1.5−2
The coagulation loss is calculated for 2nm particles from the
DMPS data. We left the last term out, reaching again a min-
imum estimate for cluster formation rate. The equation can
be written separately for neutral particles, ion clusters and for
particles formed by recombination.
dt
+CoagS1.5−2·N1.5−2+GR1.5−2
0.5
N1.5−2
(2)
3Results and discussion
3.1Cluster concentrations
According to PH-CPC data from spring 2007 and May 2008,
there is a pool of molecular clusters present in Hyyti¨ al¨ a prac-
tically all the time. Due to uncertainties regarding the size
of these clusters discussed in Sect. 2.2, we hereafter usu-
ally refer to total concentration below 3nm, even though
our measurements inevitably have a lower size limit corre-
sponding to activation of positive 1.2nm ions. Most prob-
ably we were able to see only the upper size section of
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4180K. Lehtipalo et al.: Analysis of atmospheric neutral and charged molecular clusters
Fig. 3. Total number concentration of (a) clusters 1.5–3nm and (b)
particles 3–5nm measured with PH-CPC (black stars) and NAIS
(gray dots) on 31 March–3 April 2007 in Hyyti¨ al¨ a.
Fig. 4. (a) Number concentration of <3nm neutral clusters (black
dots) and 1.5–2nm recombination products (gray line), (b) concen-
tration of ions 0.8–1.3nm (black dots) and 1.3–3.2nm (gray line).
Data are calculated from PH-CPC and BSMA measurements in
March–June 2007 in Hyyti¨ al¨ a.
the cluster band. Figure 1 presents the total cluster con-
centration from year 2007 and Fig. 2 neutral cluster con-
centration from 2008. The concentration of these clusters
ranged from about 500cm−3to 50000cm−3, the lowest val-
ues usually coinciding with high coagulation sink. The me-
dian (and 5- to 95-percentile) concentration of clusters below
3nm was 8060cm−3(1510–31100cm−3) in spring 2007
and3380cm−3(220–36300cm−3)inMay2008. In2008the
homogenous nucleation level inside the PH-CPC was lower
than in 2007 indicating lower supersaturation, which could
partly explain the difference in median concentration, as we
might have failed to activate the smallest clusters. When the
temperature difference between saturator and condenser was
increased on 15 May, we began to see slightly higher clus-
ter concentrations. The median concentration on the second
half of May (16–31 May 2008) is much closer to the value
Fig. 5. Neutral <3nm clusters compared to 1.5–2nm recombina-
tion products. Data are calculated from PH-CPC and BSMA mea-
surements in April 2007 in Hyyti¨ al¨ a.
of 2007, 9350cm−3(910–45000cm−3). However, as the
concentration rises slowly rather than stepwise towards end
of May, this is maybe only a part of the explanation. The
variation in cluster concentration level might depend also on
weather conditions, biological activity etc. Resolving the an-
nualvariationofclusterswould, however, requirelongercon-
tinuous data sets.
In Fig. 3 we compared the total concentrations measured
with the PH-CPC to the ones measured with the NAIS on
31 March–3 April 2007. In the size range 3–5nm the mea-
surement agree remarkably well, especially on the negative
charging side of the NAIS. In the 1.5–3nm range concentra-
tions are of similar order, but either the NAIS is not sensitive
enoughtovariationsinclusterconcentrationorthenagainthe
PH-CPC detection efficiency is too sensitive to background
aerosol concentration.
In Fig. 4 the fraction of neutral clusters from 2007 is com-
pared to recombination products (∼1.5–2nm) and ion clus-
ter concentrations. It seems that the concentration of neutral
clusters is one order of magnitude higher than recombina-
tion product and cluster ion (∼0.8–1.3nm) concentrations,
and two orders of magnitude higher than the concentration
of ions of the corresponding size (∼1.3–3.2nm). The scatter
plot of neutral clusters as a function of recombination clus-
ters, Fig. 5, further affirms, that we are able to see the 1.5–
2nm neutral particles produced by recombination, but that
is not the single source of neutral clusters. The values be-
low the unity line are explained by measurement noise and
overestimation of recombination products. The median frac-
tion of recombination clusters from all neutral clusters was
4.9%, but the values varied from less than 0.1% to more than
100%. This is in accordance with Kulmala et al. (2007a),
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K. Lehtipalo et al.: Analysis of atmospheric neutral and charged molecular clusters4181
Fig. 6. Example of a new particle formation event day in Hyyti¨ al¨ a, 10 May 2008. (a) Number size distribution measured with PH-CPC, (b)
total number concentration of <3nm neutral clusters (red circles), 3–5nm particles (blue circles) and >5nm particles (black dots) measured
with PH-CPC, (c) number size distribution measured with DMPS, (d) total number concentration measured with DMPS.
who calculated that recombination should usually account
for less than 10% of neutral clusters in Hyyti¨ al¨ a conditions.
It is hard to give a certain accuracy estimate for our mea-
surements, since clearly the uncertainties regarding size and
concentration increase with decreasing particle size. As dis-
cussed in Sect. 2.2, most instrumental factors tend to lower
the detection efficiency of clusters, so the concentration can
be considered a minimum estimate. The measured cluster
concentrations are indeed a bit lower than the ones predicted
for Hyyti¨ al¨ a from formation rates of 2–3nm particles, 7000–
50000cm−3(Kulmala et al., 2007a). Sipil¨ a et al. (2009)
stated that the error in diameter due to unknown cluster com-
position and charge can be as large as ∼0.5nm and the un-
certainty in detection efficiency several percents.
3.2 Particle formation
An example of a new particle formation event on
10 May 2008 seen by the PH-CPC and DMPS is presented
in Fig. 6. The number concentration of clusters starts to rise
a little before 9 o’clock, and 3nm particles appear about two
hours later both in DMPS and PH-CPC. When total particle
concentration reaches c. 4000cm−3, the cluster concentra-
tionstartsdecliningduetoincreasedcoagulationsinkandde-
clining supersaturation inside the instrument. In the evening
a second process seems to take place, as the concentration
of clusters rises again strongly peaking around 20 o’clock
at 15000cm−3. These nocturnal clusters reach the size of
∼3nm, but disappear gradually as they are scavenged mainly
by coagulation (Kerminen et al., 2001). The upper range of
these clusters is also visible in the DMPS figure just above
the detection limit of the instrument.
No consistent pattern of cluster concentrations during nu-
cleation events can be attained from the data. Clear growth
from cluster band towards bigger particles was seen only
rarely. Often the particles seemed to emerge at sizes close to
3–4nm, and the cluster concentration rose simultaneously, or
remained close to stable. This indicates that the actual par-
ticle formation might not happen in the near vicinity of our
measurement place, but the air mass with newly formed par-
ticles is brought to the site by vertical or horizontal mixing.
However, as strong particle formation changes the supersat-
uration inside the instrument rapidly, these events cannot be
further studied using the present method.
The median diurnal variation of cluster concentration and
neutral cluster formation rate is presented in Fig. 7 for spring
2007. Figure 8 presents median concentration of neutral
clusters and 3–5nm particles in May 2008. Days are clas-
sified into event, non-event and undefined days according to
Dal Maso et al. (2005), excluding strong events with back-
ground aerosol concentration reaching over 5000cm−3. In-
terestingly, the highest cluster concentrations were not mea-
sured during daytime nucleation events, but in the evenings
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