Atmospheric nucleation: highlights of the EUCAARI project and future directions

Tags: nucleation, nucleation rate, Aerosol Sci, particle formation, Atmos, NAIS, experiments, atmospheric nucleation, formation rate, charged particles, J. Geophys, Kulmala, homogeneous nucleation, measurements, sulphuric acid, Integrated project, nm particles, nucleation rates, atmospheric models, Kerminen, size distribution, neutral particles, total concentration, aerosol formation, Laboratory experiments, Heterogeneous nucleation, Atmospheric Chemistry Division, Atmospheric Chemistry and Physics Atmospheric, Paul Scherrer Institut, Atmospheric Chemistry, Carnegie Mellon University
Content: Atmos. Chem. Phys., 10, 10829­10848, 2010 www.atmos-chem-phys.net/10/10829/2010/ doi:10.5194/acp-10-10829-2010 © Author(s) 2010. CC Attribution 3.0 License.
Atmospheric Chemistry and Physics
Atmospheric nucleation: highlights of the EUCAARI project and future directions V.-M. Kerminen1,2, T. PetaЁjaЁ1, H. E. Manninen1, P. Paasonen1, T. Nieminen1, M. SipilaЁ1, H. Junninen1, M. Ehn1, S. Gagneґ1, L. Laakso12,1, I. Riipinen1,13, H. VehkamaЁki1, T. Kurten1, I. K. Ortega1, M. Dal Maso1,6, D. Brus2, A. HyvaЁrinen2, H. Lihavainen2, J. LeppaЁ2, K. E. J. Lehtinen2,11, A. Mirme3, S. Mirme3, U. Ho~rrak3, T. Berndt4, F. Stratmann4, W. Birmili4, A. Wiedensohler4, A. Metzger5,*, J. Dommen5, U. Baltensperger5, A. Kiendler-Scharr6, T. F. Mentel6, J. Wildt6, P. M. Winkler7,**, P. E. Wagner7, A. Petzold8, A. Minikin8, C. Plass-DuЁ lmer9, U. PoЁschl10, A. Laaksonen1,11, and M. Kulmala1 1Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland 2Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101 Helsinki, Finland 3Institute of Physics, University of Tartu, UЁ likooli 18, 50090, Tartu, Estonia 4Leibniz-nstitute fuЁr TroposphaЁrenforschung, Permoserstrasse 15, Leipzig 04318, Germany 5Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland 6Institut fuЁr Chemie und Dynamik der GeosphaЁre (ICG), Forschungszentrum JuЁlich, 52425 JuЁlich, Germany 7FakultaЁt fuЁr Physik, UniversitaЁt Wien, Boltzmanngasse 5, 1090 Wien, Austria 8Deutsches Zentrum fuЁr Luft- und Raumfarhr, Institut fuЁr Physik der AtmosphaЁre, Oberpfaffenhofen, Germany 9Hohenpeissenberg Meteorological Observatory, Deutscher Wetterdienst, Germany 10Max Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55128 Mainz, Germany 11Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland 12School of Physical and Chemical Sciences, North-West University, Potchefstroom, South Africa 13Department of Chemical engineering, Carnegie Mellon University, 15213, Pittsburgh, PA, USA *now at: Ionicon Analytik GmbH, 6020 Innsbruck, Austria **now at: Atmospheric Chemistry Division, National Center for Atmospheric Research, 1850 Table Mesa Dr., Boulder, CO-80305, USA Received: 24 May 2010 ­ Published in Atmos. Chem. Phys. Discuss.: 2 July 2010 Revised: 28 September 2010 ­ Accepted: 8 November 2010 ­ Published: 18 November 2010
Abstract. Within the project EUCAARI (European Integrated project on Aerosol Cloud Climate and air quality interactions), atmospheric nucleation was studied by (i) developing and testing new air ion and cluster spectrometers, (ii) conducting homogeneous nucleation experiments for sulphate and organic systems in the laboratory, (iii) investigating atmospheric nucleation mechanism under field conditions, and (iv) applying new theoretical and modelling tools for data interpretation and development of parameterisations. The current paper provides a synthesis of the obtained results and identifies the remaining major knowledge gaps related to atmospheric nucleation. The most important technical achievement of the project was the development of new in- Correspondence to: V.-M. Kerminen ([email protected])
struments for measuring sub-3 nm particle populations, along with the extensive application of these instruments in both the laboratory and the field. All the results obtained during EUCAARI indicate that sulphuric acid plays a central role in atmospheric nucleation. However, also vapours other than sulphuric acid are needed to explain the nucleation and the subsequent growth processes, at least in continental boundary layers. Candidate vapours in this respect are some organic compounds, ammonia, and especially amines. Both our field and laboratory data demonstrate that the nucleation rate scales to the first or second power of the nucleating vapour concentration(s). This agrees with the few earlier field observations, but is in stark contrast with classical thermodynamic nucleation theories. The average formation rates of 2-nm particles were found to vary by almost two orders of magnitude between the different EUCAARI sites, whereas the formation rates of charged 2-nm particles varied very
Published by Copernicus Publications on behalf of the European Geosciences Union.
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V.-M. Kerminen et al.: Highlights of the EUCAARI project and future directions
little between the sites. Overall, our observations are indicative of frequent, yet moderate, ion-induced nucleation usually outweighed by much stronger neutral nucleation events in the continental lower troposphere. The most concrete outcome of the EUCAARI nucleation studies are the new semiempirical nucleation rate parameterizations based on field observations, along with updated aerosol formation parameterizations. 1 Introduction The recent decade of atmospheric measurements demonstrated nucleation to be a frequent phenomenon in the continental boundary layer, as well as in the free troposphere (Kulmala et al., 2004; Kulmala and Kerminen 2008, and references therein). direct observational evidence was further received that particles nucleated in the atmosphere are able to grow into cloud condensation nuclei (CCN) sizes (O'Dowd, 2001; Lihavainen et al., 2003; Kuwata et al., 2005; Laaksonen et al., 2005; Whitehead et al., 2009; Wiedensohler et al., 2009) and ultimately to form cloud droplets (Kerminen et al., 2005). Model simulations suggest that nucleation is very likely the dominant source of the particle number concentration in the global atmosphere (Spracklen et al., 2006; Yu and Luo, 2009), and that it is a significant contributor to global CCN concentrations (Spracklen et al., 2008; Merikanto et al., 2009; Pierce and Adams, 2009; Yu and Luo, 2009) and cloud droplet number concentrations (Makkonen et al., 2009; Wang and Penner, 2009; Kazil et al., 2010). In spite of its evident importance in the global aerosol system, climatic and other influences of atmospheric nucleation have turned out to be very difficult to quantify. Several reasons for this can be identified. First of all, our inability to measure neutral sub-3 nm diameter particles, until very recently, has hampered the interpretation of both field and laboratory experiments (e.g., SipilaЁ et al., 2008, 2009, 2010). Second, beside sulphuric acid, it is still not known which vapours take part in atmospheric nucleation and to which extent (e.g., Smith et al., 2008; Claeys et al., 2009). Third, the role of ions in atmospheric nucleation has remained ambiguous (e.g., Iida et al., 2006, Kazil et al., 2008; Yu and Turco, 2008; Yu, 2010). The lack of a proper mechanistic understanding of atmospheric nucleation has made it difficult to develop reliable aerosol formation parameterisations for large-scale modelling frameworks ­ the ultimate tools to address the role of nucleation in climate and air quality issues. Due to the reasons highlighted above, nucleation studies were given a high priority in the ongoing project EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality interactions; Kulmala et al., 2009). The overall goal of these studies was to produce parameterised representations of nucleation processes for sulphuric acid-ammonia-
water, organic and iodine oxide systems based on combined information from nucleation theories, modelling and experimental studies, to be used in regional and global scale models. The problem was approached by (i) developing and testing new ion and cluster spectrometers, (ii) conducting homogeneous nucleation experiments for sulphate and organic systems in the laboratory, (iii) investigating atmospheric nucleation mechanism under field conditions, and (iv) applying new theoretical and modelling tools for data interpretation and development of parameterisations. In the following sections we will summarize our main results from nucleation studies conducted within the EUCAARI project, after which a brief scientific synthesis with concluding remarks will be presented. 2 Development of instrumentation The main emphasis in the instrumental development within EUCAARI was put on the detection of sub-3 nm neutral particles/clusters. For this purpose, an entirely new air ion spectrometer was designed, built, tested and calibrated. Major developments were also achieved with regard of measurement capabilities and application of various condensation particle counters, and a new way of applying the mobility size spectrometer technique for obtaining information about ioninduced nucleation was introduced. Finally, we were able to measure the chemical composition of atmospheric ions with high-resolution mass spectrometric methods. 2.1 Neutral cluster and Air Ion Spectrometer (NAIS) During EUCAARI, a new prototype ion spectrometer, termed NAIS (Neutral cluster and Air Ion Spectrometer, Kulmala et al., 2007a; Manninen et al., 2009a) was developed. The NAIS builds on the Air Ion Spectrometer (AIS, Mirme et al., 2007), following the principle of multi-channel parallel electrical aerosol spectrometry. The NAIS is able to measure the concentrations and size distributions of both neutral and charged particles in 21 size fractions (channels). The mobility range of the NAIS is 3.2­0.0013 cm2 V-1 s-1, corresponding to a mobility diameter (Millikan-Fuchs equivalent diameter) range of 0.8­42 nm. In case of neutral particles, the lowest measurable size is in practice about 2 nm due to the presence of charger ions with mobilities of 1.3­ 1.6 cm2 V-1 s-1 (Wiedensohler, 1988; Asmi et al., 2009). The NAIS operates at a five-minute time resolution in order to optimize the sensitivity and signal-to-noise ratio. The overall performance of the NAIS was tested under both laboratory and field condition in Tartu, Estonia. In the laboratory tests, well-defined cluster ions, aerosol ions and neutral aerosol particles were used. The first air ion spectrometer calibration and inter-comparison workshop was then organised in Helsinki, Finland. The workshop took place in January­February, 2008, just prior to the EUCAARI
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Intensive Observation Period (Asmi et al., 2009). In the workshop, ten ion spectrometers, including four NAIS instruments, were compared and calibrated. Calibrations were made with mobility standards (see Ude and de La Mora, 2005) and silver particles by using high-resolution differential mobility analysers (HDMA, Hermann et al., 2007). The monodisperse mobility distribution broadened to approximately 3­5 size channels when measured by the ion spectrometers due to the strong diffusion of these small ions. Particle sizes detected by the ion spectrometers were slightly smaller (30­50% larger mobilities) than those characterized with the HDMA. Excluding some overestimation at the smallest sizes, ion concentrations measured by the spectrometers were in good agreement with those measured by the aerosol electrometer and condensation particle counter. The NAIS was developed further in order to extend its operation to variable altitudes, including its airborne operation. The development aimed at the improved control and automatic tuning of air flows and other instrument operation parameters following the variations of ambient conditions. The NAIS was tested in airborne measurements during the EUCAARI long-range experiment EUCAARI LONGREX 2008. The NAIS performed very well during the flights (Mirme et al., 2010). The effects of varying pressure and temperature as a function of flight height were taken into account by automatically adjusting the sheath flow of the instrument, which kept the volumetric sampling flow rate constant. Furthermore, the variability of the charger ion mobility was compensated by adjusting the corona current. The NAIS was capable of automatically adapting to variations of barometric pressure from the surface level up to the 8-km altitude without any additional corrections. At higher altitudes, the measured size distribution was corrected in the post-processing phase (Mirme et al., 2010). The NAIS is the first instrument, from which the formation rates of both neutral and charged sub-3 nm diameter particles can be determined. By writing the balance equation for 2­ 3 nm particles and rearranging the terms, the total formation rate of 2-nm particles (J2) is obtained from (Kulmala et al., 2007a; Manninen et al., 2009b):
J2
=
dN2-3 dt
+ CoagS2
Ч N2-3
+
f 1nm
GR3N2-3.
(1)
Here N2-3 is the total concentration of particles in the size range 2­3 nm, CoagS2 is the coagulation sink of 2-nm particles, and GR3 is the particle growth rate at 3 nm and f presents a fraction of 2­3 nm particles that has been activated for the growth (assumed equal to unity without a better knowledge). Since the particle number size distribution in the size range 2­3 nm is not known, we cannot calculate the exact rate at which particles in this size range are lost by coagulation with pre-existing larger particles. As a result, we use CoagS2 as an approximation to this loss rate, which may lead to a slight over-prediction of the value of J2.
In case of charged particles, the ion-ion recombination and charging of 2­3 nm neutral particles need to be taken into account, after which their formation rate at 2 nm becomes:
J2±
=
dN2±-3 dt
+ CoagS2
Ч N2±-3
+
f 1nm GR3N2±-3
+ N2±-3N<3 - N2-3N<±2.
(2)
Here the superscript ± refers to positively and negatively charged particles and N2±-3, N<±2 and N<±3 are the corresponding ion concentrations in the size range 2­3 nm, below 2 nm and below 3 nm, respectively. The ion-ion recombination coefficient, , and the ion-neutral attachment coefficient, , can be assumed to be equal to 1.6 Ч 10-6 cm3 s-1 and 0.01 Ч 10-6 cm3 s-1, respectively (e.g. Tammet and Kulmala, 2005). The last two terms in the right hand side of Eq. (2) are not exact but rather provide a first order correction for the formation rate due to ion-ion recombination and ion-aerosol attachment, respectively. Used together, Eqs. (1) and (2) make it possible to estimate the contribution of ioninduced nucleation to the total nucleation rate, as will be demonstrated in Sect. 4.2.
2.2 Condensation particle counters
A Condensation Particle Counter (CPC) is a widely-used instrument to detect the number concentration of aerosol particles too small to be observed with optical techniques (McMurry, 2000). The CPC is able to monitor concentrations of both charged and neutral particles, although the experiments show that the charge carried by the particle enhances the detection efficiency (Winkler et al., 2008a, b). The instrumental development has improved the detection efficiency (D50) of the CPCs defined as the size, where 50% of the sampled particles are detected. In the CPC design, the work by Stolzenburg and McMurry (1991) was a milestone, as they presented a counter capable of detecting particles down to 3 nm in diameter. The detection efficiency of a CPC depends in general on the generated supersaturation inside the CPC, which determines the smallest particle size that is activated to growth. Already Mertes et al. (1995) showed that for a butanol based CPC, the value of D50 can be decreased by increasing the supersaturation inside the CPC. PetaЁjaЁ et al. (2006) showed that this applies also to a waterbased CPC (Hering et al., 2005). The limiting factor is the onset of homogeneous nucleation of the CPC working fluid. The supersaturation at the onset of homogeneous nucleation depends on thermodynamic properties of the working fluid, and the detection efficiency of the CPC can be improved by selection of the working fluid (Iida et al., 2009). The CPC performance can also be improved by minimizing the losses of the small particles. As part of EUCAARI, Vanhanen et al. (2010) combined the rapid mixing type CPC (Sgro and de la Mora, 2004) with a diethylene glycol based CPC and showed that in their design the diffusion losses dominate the detection efficiency down to molecular sizes
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(diameter 1 nm). In other words, given that the sampled particles are not lost during the sampling process, the instrument developed by Vanhanen et al. (2010) is able to detect particles down to 1 nm in size. The onset of homogeneous nucleation does not necessarily restrain the use of a CPC in atmospheric measurements. Kulmala et al. (2005) used a UF02-proto CPC (Mordas et al., 2005, 2008) as a nucleation chamber. By subtracting the contribution of homogeneous nucleation inside the CPC, they were able to probe the ambient sub-3 nm particle concentration. During EUCAARI, this approach was further developed by SipilaЁ et al. (2008, 2009), who utilized the pulse-height (PH) analysis (Saros et al., 1996; Weber et al., 1996) in differentiating the signals originating from the homogeneously nucleated working fluid and the ambient sample. Lehtipalo et al. (2009) measured the concentration of sub-3 nm particles in a boreal forest by using a tuned PHCPC. The estimated concentrations varied from 5 Ч 102 to 5 Ч 104 clusters cm-3, which is more than what one would expect from the recombination of ion clusters. The detection efficiency of the CPC depends also on the chemical composition of the sampled particles. For example, water solubility and wetability increase the detection efficiency of inorganic salt particles compared with nonhygroscopic silver particles (PetaЁjaЁ et al., 2006; Herman et al., 2007). In terms of reliable and reproducible number concentration measurements, this is a drawback, especially in environments where a lot of nucleation mode particles are present. This disadvantage was turned into a benefit by Kulmala et al. (2007b) who applied a battery of CPCs (CPCB) with different working fluids in parallel. Furthermore, Riipinen et al. (2009) utilized the CPCB in the boreal forest to probe the composition of 2­9 nm particles by looking into their water solubility. The results showed that during newparticle formation, the initially more hygroscopic particles grew in size by condensation of less water soluble material. 2.3 Ion-Differential Mobility Particle Sizer (Ion-DMPS) Traditionally, aerosol number size distributions are measurements with mobility size spectrometers such as the DMPS or SMPS (Differential/Scanning Mobility Particle Sizer; Hoppel, 1978; Wang and Flagan, 1990; Aalto et al., 2001). The mobility size spectrometer relies on the fact that the sampled particles have a known charge distribution (Wiedensohler, 1988). This can be acquired with a radioactive source, which provides an excess amount of both negative and positive ions that either charge or neutralize the sampled particle population depending on the initial charging state. The residence time of the air sample in the bipolar charger is long enough for the sample to reach the known, steady-state charge distribution. In EUCAARI, a new instrument called the Ion-DMPS was introduced and also applied in field (Laakso et al., 2007a). The radioactive source of the Ion-DMPS can be by-passed on
demand, which enables the measurement of either the atmospheric ion number size distribution or of the corresponding size distribution of a neutralized aerosol sample. By comparing these two modes of operation, a size-dependent charging state is obtained (Laakso et al., 2007a; Gagneґ et al., 2008). The value of the charging state is larger than unity when the population of particles of a given size is more charged than in the stationary state corresponding to the neutralized aerosol sample. In such a case the particle population is called overcharged. Similarly, an undercharged particle population has a charging state smaller than unity. The Ion-DMPS measures the charging state for both negative and positive polarities. The charging state obtained from the Ion-DMPS provides information about the participation of ion-induced nucleation in new-particle formation: a measured charging state >1 suggests at least some contribution by ion-induced nucleation, whereas a charging state <1 indicates no or minor contribution by ion-induced nucleation (Laakso et al., 2007a). For a more quantitative statement, we need to apply the theoretical framework developed by Kerminen et al. (2007). According to this work, the charging state of a growing nucleation mode is governed by its initial charging state, atmospheric cluster ion concentration, and the growth rate of the nucleation mode. In practice this can be interpreted as follows: regardless of the nucleation mechanism, growing nuclei are exposed to collisions with atmospheric cluster ions, as a result of which the charging state of the growing nuclei population is changed. If the nuclei growth rate is slow, e.g. due to a low concentration of condensable vapours, the information on the initial charging state will be lost by the time the IonDMPS detects the growing clusters. On the other hand, if the nuclei growth is rapid enough and the atmospheric cluster ion concentration is low enough, the analytical formulae presented by Kerminen et al. (2007), together with Ion-DMPS data, can be used to extract the relative roles of ion-induced and neutral nucleation mechanisms in observed new-particle formation events. The application of the Ion-DMPS under field conditions will be discussed in Sect. 4.2. 2.4 Atmospheric Pressure Interface Time of Flight Mass Spectrometer (APi-TOF) Mass spectrometric techniques can provide insights into the composition of atmospheric ions and clusters (Eisele, 1989a, b; Eichkorn et al., 2002; Junninen et al., 2010; Zhao et al., 2010). Within EUCAARI, we tested an Atmospheric Pressure Interface Time-of-Flight Mass Spectrometer (APi-TOF, Tofwerk AG), the mass/charge (in unit Th) range of which extends up to 2000 Th (Junninen et al., 2010). With the high mass accuracy (<20 ppm) and mass resolving power (3000 Th/Th), the APi-TOF makes it possible to determine the composition of small atmospheric ions. The ions were identified based on their high-resolution masses, isotopic patterns and peak-to-peak correlograms. Potential candidates were also judged based on proton affinities and quantum
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chemical considerations (Junninen et al., 2010; Ehn et al., 2010). The operation of the APi-TOF at an urban site in Helsinki and in a rural environment in HyytiaЁlaЁ, Finland revealed a considerable diurnal variability in the chemical composition of ions and their clusters (Junninen et al., 2010; Ehn et al., 2010). The driving factors were photochemical production of various ions and their proton affinity. In the atmospheric ion population the charge is transferred to molecules with the highest (positive ions) and lowest proton affinities (negative ions). Thus, during daytime the negative ions were dominated by strong acids (e.g. sulfuric and malonic acid) and their clusters. During night, nitric acid and organic acids were the dominant peaks in the negative spectrum. For the first time an organo-sulphate (glycolic acid sulphate) was detected in the gas phase (Ehn et al., 2010). The diurnal cycle was less pronounced in the positive spectrum, which was dominated by strong bases (alkyl pyridines, quinolines and amines). A detailed description of the API-TOF and first results can be found in Junninen et al. (2010) and Ehn et al. (2010). 3 Laboratory experiments Within EUCAARI, homogeneous nucleation experiments were conducted in three laboratories using two different flow tubes and a smog chamber. Homogeneous nucleation experiments were made for the binary sulphuric acid-water system (H2SO4-H2O), for the ternary sulphuric acid-waterammonia system (H2SO4-H2O-NH3), and for various systems including both sulphuric acid and organic compounds (H2SO4-Org). The binary H2SO4-H2O system was given a high priority because (i) it is the most widely-studied atmospheric nucleation mechanism, (ii) even today only few large-scale atmospheric models include any other nucleation mechanism in their simulations, and (iii) laboratory experiments concerning this system have turned out to be very difficult to conduct and interpret (Berndt et al., 2005, 2008; Benson et al., 2008). The ternary H2SO4-H2O-NH3 system was selected because ammonia, being the dominant base to neutralize atmospheric sulphate particles (Bowman et al., 1997), is also the most obvious candidate for enhancing sulphuric acid-water nucleation (see Merikanto et al., 2007, and references therein). The H2SO4-Org system was selected (i) because organic compounds are known to play a significant role in nuclei growth (e.g. Smith et al., 2008), and thus one might expect them to be involved in the nucleation process as well, and (ii) because very few laboratory experiments on this system have been conducted so far (Zhang et al., 2004). In addition to homogenous nucleation, a series of heterogeneous nucleation experiments were made for both neutral and charged particles and clusters.
3.1 Binary sulphuric acid-water nucleation The binary sulphuric acid-water nucleation experiments were performed in the Leibniz Institute for Tropospheric Research laminar flow tube (IfT-LFT) and in the Finnish Meteorological Institute (FMI) laminar flow tube (SipilaЁ et al., 2010). Two types of experiments were conducted: (i) experiments where H2SO4 was produced in situ via the reaction of OH radicals with SO2 ("photolysis" experiments), and (ii) experiments where H2SO4 was taken from a liquid sample ("liquid-sample" experiments). In both cases, the H2SO4 concentration was measured directly using a chemical ionization mass spectrometer (PetaЁjaЁ et al., 2009). Another specific feature associated with these experiments was that nucleated particles were measured down to 1.3­1.5 nm in mobility diameter. This was achieved with the help of a modified pulse height analyzing ultrafine condensation particle counter and, in some experiments, with a mixing type particle size magnifier (SipilaЁ et al., 2010). Earlier studies on H2SO4-H2O nucleation reported a clear disagreement between the photolysis and liquid-sample experiments, being several orders of magnitude in the nucleation rate and a couple of orders of magnitude in the onset H2SO4 concentration required for a nucleation rate of 1 cm-3 s-1 (Benson et al., 2008; Berndt et al., 2008). The experiments conducted within EUCAARI demonstrate that this disagreement is largely a measurement artifact arising from the high sensitivity of the measured "nucleation rate" to: (i) the temporal and spatial profile of the gaseous H2SO4 concentration inside the measurement device and (ii) the detection efficiency of the instrument used to measure nucleated particles. When minimizing the influence of these two effects in the experiments, practically no difference in the nucleation rate between the photolysis and liquid-sample experiments was observed any more (SipilaЁ et al., 2010). The new H2SO4-H2O nucleation experiments are in line with EUCAARI field observations (Fig. 1). They both predict a slope between about 1 and 2 in a plot of the nucleation rate versus gaseous H2SO4 concentration, and require roughly the same amount of H2SO4 to initiate the nucleation process. These findings indicate (1) that particles are very likely formed via a similar H2SO4-driven nucleation mechanism in both the laboratory and the ambient atmosphere, and (2) that according to the nucleation theorem, critical clusters formed in the nucleation process contain only one or two H2SO4 molecules. Both laboratory and field measurements could be explained by either activation-type (Kulmala et al., 2006) or kinetic (McMurry and Friedlander, 1979) nucleation, but not by the thermodynamic binary H2SO4-H2O nucleation which predicts more than five H2SO4 molecules in a critical cluster under typical ambient conditions (e.g. Yu, 2008). This does not necessarily mean that existing binary H2SO4-H2O nucleation theories are wrong by themselves: it is possible that H2SO4-H2O nucleation is affected by the presence of impurities like ammonia, amines and various
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V.-M. Kerminen et al.: Highlights of the EUCAARI project and future directions
r.h.: 47% r.h.: 22%
104
r.h.: 13%
particle number (cm-3)
103
102
101
100
open: w/o NH (<2.5·109 cm-3) 3
full: [NH ] = 1.2·1012 cm-3
3
107
108
109
[H SO ] (molecule cm-3) 24
FigFurieg1. .1M. eaMsuereadsuforremdatfioornmraatetioofn2-rnamtepoarftic2l-ens m(J2)pianrdtiicffleersen(tJa2tm)oisnphderiifcferent atmospheric locations (HyytiaЁlaЁ, Melpitz and San Pietro Capofi- locations (Hyytiдlд, Melpitz and San Pietro Capofiume) and in a laminar flow reactor ume) and in a laminar flow reactor (IfT-LFT) as a function of mea(IfTs-uLrFeTd)sauslafufurnicctiaocnidofcmoenacseurnetdrastuilofunr.ic acid concentration. organic compounds. Such compounds are practically always
FigFHui2rgeS. O22.4.TcoTotoantlcaeplnaptrrataircttiilocelnneuinnmunbmuebcrelceroanctiocoennncteerxnapttrieaortniimoanesnaatssfuamnacfudtineocnatitoodfnitfhofeefrHtehn2etSO4 concentra nucrelelativoen heuxmpeirdiimtiesnt(sRmHa)d. eThatedeixffpeerreinmternetlsatwiveerehucmoniduitcietesd(rb.ho.t)h. Tinhe experiment conthdeucptreedsebnocteh oinf tNhHe p3r(efsuelnl cceirocfleNs)Ha3n(dfuwllitchirNclHes3) acnondcwenitthraNtioHn3 rceo-ncentration maining below the detection limit (open circles). remaining below the detection limit (open circles).
present in the atmosphere, and low level of these compounds increased slightly the mean diameter of nucleated particles,
(impurities) cannot be excluded from current laboratory ex- as well as their total number concentration (Fig. 3).
periments.
The above experiments show a clear, yet moderate, en-
hancing effect of ammonia on sulphuric acid-water nucle-
3.2 Influence of ammonia and amines on sulphuric
ation. While this is qualitatively similar to what has been
acid-water nucleation
observed in other laboratory experiments (e.g., Ball et al.,
1999; Benson et al., 2009), a quantitative comparison be-
The ternary sulphuric acid-water-ammonia and sulphuric tween these experiments, or between the experiments and
acid-water-amine nucleation experiments were performed in available theories, is not possible at the moment. One reason
the Leibniz Institute for Tropospheric Research laminar flow for this is that the different experimental studies have been
tube (IfT-LFT; Berndt et al., 2005, 2010) at a temperature of 293 ± 0.5 K by producing H2SO4 via the reaction of OH
made at different NH3 and H2SO4 concentration levels. The second reason is that none of the laboratory experiments have
radicals with SO2. NH3 (Merc5k3, >99.9%) was added to the carrier gas stream using a diluted sample from a gas meter-
been made at low (ing unit. NH3 concentrations were measured at the inlet and outlet of the IfT-LFT by means of an OMNISENS TGA310 system (detection limit 2.5 Ч 109 molecules cm-3).
ing ternary H2SO4-H2O-NH3 nucleation theories, although deviating quite a lot from each other, are most sensitive to changes in NH3 at concentration levels less than a few ppt
Figure 2 shows measured total particle number con- (e.g., Napari et al., 2002, Anttila et al., 2005, Merikanto et
centrations (TSI 3025) as a function of H2SO4 con- al., 2007).
centration in the absence (NH3 concentration below 2.5 Ч 109 molecules cm-3, i.e. below about 100 ppt) and
A series of experiments were con5d4ucted, in which tertbutylamine (as an arbitrary sample amine) instead of am-
presence of NH3, and for different values of rel- monia was added into the system (Berndt et al., 2010).
ative humidity. The inlet NH3 concentration was 1.2 Ч 1012 molecules cm-3 in these experiments and, after an
Measurements at a relative humidity of 13% with a tertbutylamine addition of about 1010 molecules cm-3 showed
equilibration time of about one hour, the corresponding outlet concentration was 1.1 Ч 1012 molecules cm-3. A distinct increase of the total particle number concentration with in-
an enhancement of produced particles by about two orders of magnitude, whereas extrapolation of the NH3 data down to concentrations of about 1010 molecules cm-3 suggested
creasing relative humidity was observed when the NH3 concentration was below the detection limit. The enhancing ef- fect of NH3 addition on the nucleation was found to be more pronounced under drier conditions, i.e. a factor of about 20
only a small or negligible effect of NH3. This finding in- dicates a very strong effect of amines for nucleation even for atmospheric amine concentrations in the range of 108­ 109 molecule cm-3. Therefore, amines are probably promis-
at a relative humidity of 13% and only a factor of about 2 ing candidates explaining existing discrepancies between bi-
at a relative humidity of 47%. Experiments with different nary nucleation theory and observations in the field and the
inlet NH3 concentrations showed that the presence of NH3 laboratory.
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105
of sufficiently low volatility to participate in the particle for-
mation process (called NucOrg) was calculated based on the
decay of the TMB concentration and assuming the same loss
104
rates as sulphuric acid (for details see Metzger et al., 2010).
dN/d log d / cm-3 p
The isopleth plot of Fig. 4 clearly shows that the data can
103
only be explained with a dependence of the nucleation rate
on both sulphuric acid and a nucleating organic (see Metzger
102
[NH ] / molecule cm-3 3
w/o (< 2.5·109)
1.3·1011 (8.5·1010)
101
3.8·1011 (2.1·1011)
1.2·1012 (8.9·1011)
100 1
d / nm p
et al., 2010, Supporting Information). Thus, the slope of 2
mentioned above is rather a result of highly correlated con-
centrations of sulphuric acid and NucOrg.
This result was also implemented in a global model.
Parameterising this process in the global aerosol model
4
GLOMAP resulted in substantially better agreement with
ambient observations compared to control runs. It can there-
fore be speculated that in many locations, the new-particle
formation is influenced not only by the sulphuric acid con-
FigFuirge. 33.. MMeaesausurereddpapratritcilcelenunmumbebrersisziezedidsitsritbriubtuiotinosnsovoevretrhtehesizseizreange ocfe1n.t5r­a4tionnm binut also by the concentrations of co-nucleating
range of 1.5­4 nm in nucleation experiments conducted at differ- species. The chemical nature of these species remains, how-
nucelnetaatimonmeoxnpiaercimonecnetnstcraotniodnuclteevdelast. dTifhfeerHen2tSaOm4mcoonnicaecnotrnactieonntrwataiosn leveelvs.eTr,hteoHb2eSiOd4entified.
con1c.2enЧtr1a0ti8onmwolaescu1l.e2sЧc1m08-m3 oanledcuthleesreclmat­i3veanhdumthiedriteylawtiavse2h2u%m.idTihtye was 22%T.hTehJePgAivCensetup at JuЁlich (Mentel et al., 2009) was used
given NH3 concentrations refer to those inside the inlet and the corNHre3scpoonncdeinntgraotuiotlnest rceofnecretnotrtahtoiosensinasriedpertehseeninteledtianntdhethberaccokrreetss.ponding
outtolesttudy the effect of organics with realistic mixtures of organic emissions. The real plant emissions were introduced
concentrations are presented in the brackets.
to air containing atmospheric levels of ozone, and the pro-
3.3 Influence of organics
duction of OH radicals was induced by the UV light. In experiments with a constant OH radical production rate and a
The role of organic compounds in nucleation was investigated in Paul Scherrer Institute (PSI) by using an environmental chamber, and in JuЁlich using a Plant Aerosol Atmosphere Chamber (JPAC) setup.
varying organic vapour emission rate, it was found that both the mass and number production rates of >5 nm particles increased with an increasing organic vapour source. For individual tree species, the emitted volatile carbon was the main predictor of formed aerosol number and mass, even though
A series of photo-oxidation experiments was performed in the 27-m3 Paul Scherrer Institute environmental chamber
large variations between the different tree species were observed. Threshold concentrations of organic compounds ini-
investigating new particle formation in the presence of 1,3,5- tiating particle formation were lower for emissions from all
trimethylbenzene (TMB), NOx and SO2 at various mixing the tree species than for the reference compound, -pinene.
ratios (Metzger et al., 2010). After irradiation of this mixture The differences between individual species could possibly be
OH radicals oxidized SO2 and TMB producing H2SO4 and explained by oxidized VOC concentrations; sesquiterpenes
a variety of organic products. The production of low volatil- were not found to play a specific role (Mentel et al., 2009).
ity products lead to formation of secondary organic aerosols In another experiment series, plant emissions were studied
(SOA). The importance of sulphuric acid was clearly seen, in the presence and absence of added isoprene. From 3 to 4
as with increasing SO2 mixing ratio nu5c5leation occurred earlier and the particle number concentration (diameter>3 nm) increased from 103 to 105 cm-3. After reaching the peak
OH radical reactions were needed to induce nucleation and the addition of isoprene suppressed aerosol number formation while having a negligible effect on particle growth. The
concentration, the particle number concentration decreased suppression could be parameterised using a model that had
due to wall loss and coagulation.
particle number formation depending on the OH oxidation of
Plotting the nucleation rate of 1.5-nm particles (J1.5) ver- other plant VOCs, with isoprene acting as an OH scavenger
sus the concentration of sulphuric acid yielded a slope close (Kiendler-Scharr et al., 2009).
to 2 (Metzger et al., 2010). This would indicate that the crit-
In both JPAC experiments, a clear positive correlation be-
ical cluster contains two sulphuric acid molecules. However, tween the amount of non-isoprene organic emissions and the
this only applies when other variables of influence (temper- rate of particle formation points towards an enhancing effect
ature and gas phase concentrations of other species partici- by organics. If particle formation was controlled by some
pating in the nucleation process) remain constant. However, inorganic vapour formed by OH oxidation, nucleation sup-
within an individual experiment H2SO4 and organic photo- pression by all OH-reactive organics would be expected due
oxidation products are expected to be highly correlated since to the competition for OH radicals. However, because the
their formation and loss processes are highly similar. For size cutoff in the measurements was significantly larger than
the further analysis, the concentration of a first order product the expected size of the newborn CN, the exact nature of the
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1.2
Heterogeneous nucleation probability
1.0
0.8
0.6
0.4
T = 275 K
d = 2.0 nm
seed
negative
0.2
positive
neutral
0.0
2.0
2.5
3.0
3.5
n-Propanol saturation ratio
FFiigg.ur5e. 5.HHeetteerrooggeenneeoouussnunculecaletiaotniopnrobparboibliatybicluitryvecsufrovr ethsefaocrtivthaetioancotif-differently vation of differently charged tungsten oxide (WOx) particles with mcheaarngeedletucntgriscteanl omxoidbeil(iWtyOdx)iapmaretitcelersowfit2h.0mneamn.eleNcetrgicaatlivmeolybilcithyadrgiaemdeter of 2.0 nm.
pNaertgiactlievselayrcehaarcgteivdapteardtiacltelsoawreeascttvivaapteodr astaltouwraetsitovnapraotrisoastufroaltlioonwreadtiobsyfollowed by
ppoossiitivellyycchharagregdedparptiacrlteisclaensd anneudtrnaleountreaslcloenarelsy icnldeiacraltyinginadcihcaartgineganad sign
Isopleth pFliog.t o4.f J1Is.5op(lcemth ­p3lost­1o)faJs1a.5 f(ucnmc-t3ios-n1)ofasloag[fHun2ctSioOn 4o]fverscpupharsaretrfilegcoreleegnacsn[eidNzfeos.urigtchneOphrreetgefer]or.egnenceeofuosrntuhcelehateiotenroatgtehnisepoaursticnluecsliezea.tion at this
log[H2SO4] versus log[NucOrg]. The iso-lines are drawn to guide ines are drtahewenyet.oIfgJu1i.5dedetphenedeedyeon. IefithJe1r.5Hd2SeOp4enordNeudcOorng ealiotnheetrheH2SO4 or NucOrg
iso-lines
iso-lines wdiaoguonladl
would need to be horizontal or vertical, respectively. The insoe-elidnetsoclbeaerlyhsohroiwzothnattathleodratva ecarntiocnally, breeesxppelacintievdely.
ttThheehnneethgdeaitapivogesoliytni-vacehllayirs-gcoeh-darpgaerdticolneesso, rancldufistnearsllywialllsaoctthiveanteeufitrrsatl,
rly show tMwhieathttzgatehdreeeptdeanla.d,tea2n0cc1e0a,onSf uoJp1np.5olyrftrinobgmeInebfoxotrhpmlHaat2iionSnOe)4d. awnditNhuacOdrgep(feronmdenceonoefs.J1T.5hifsrokimndboof tbhehaviour was evident in the sub-4 nm
size range, and the effect was more pronounced for smaller
nd NucOrg (from Metzger et al., 2010, Supporting Information)p.article sizes (Winkler et al., 2008a).
formation enhancement mechanism remains unknown. The isoprene effect demonstrates that organics influence nucleation indirectly via effects on the gas phase oxidation; direct effects may include modifications of early growth or even
Heterogeneous nucleation of clusters and particles can be described using the concept of activation (or nucleation) probability, P , which has been widely applied in the theory of heterogeneous nucleation (e.g., L5a7zaridis et al., 1992):
participation in nucleation itself. Based on the JPAC measurements, particle formation is induced exclusively by (multiple) OH oxidation rather than ozonolysis of organic precursors.
P = Nacti = 1 - exp(-It),
(3)
Nc
Here Nact is the number concentration of activated clusters (aerosol particles), Nc is the total cluster concentration be-
3.4 Heterogeneous nucleation experiments
fore activation, I is the heterogeneous nucleation rate (per aerosol particle and time), and t is nucleation time. Acti-
Laboratory experiments on the effect of charge (both negative and positive) on the heterogeneous nucleation probability were performed at University of Vienna (Winkler et al., 2008a). In those experiments, the condensing vapours used were n-propanol, water and n-nonane, i.e. one water-soluble and one non-soluble organic substance. Both unary and binary nucleation were investigated. An example of the conducted nucleation experiments can
vation of pre-existing clusters by sulphuric acid could explain the linear dependence of the nucleation rate to the sulphuric acid concentration (Kulmala et al., 2006), as seen in the recent laboratory experiments (Sects. 3.1 and 3.3) and many field observations (Sect. 5.3). Preferential activation of charged clusters over neutral ones might also explain the apparently larger contribution by ion-induced nucleation in the beginning of atmospheric nucleation events (see Sect. 4.2).
be seen in Fig. 5. This example illustrates clearly that when
the saturation ratio of the vapour responsible for heteroge-
neous nucleation (here n-propanol) is gradually increased,
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4 Field observations The specific feature of EUCAARI field measurements was the extensive use of various ion and cluster spectrometers. Prior to the EUCAARI Intensive Observation Period (IOP) that took place between March 2008 and April 2009, the Neutral cluster and Air Ion Spectrometer (NAIS; see Sect. 2.1) was operated intermittently in HyytiaЁlaЁ, Finland (Kulmala et al., 2007a; Manninen et al., 2009a, b). During the IOP, five NAIS instruments and eight other ion spectrometers were continuously operated for roughly a full year at 13 field sites (Manninen et al., 2010). These sites included HyytiaЁlaЁ and Pallas (Finland), Vavihill (Sweden), Mace Head (Ireland), Cabauw (The Netherlands), K-Puszta (Hungary), Hohenpeissenberg and Melpitz (Germany), San Pietro de Capofiume (Italy), Jungfraujoch (Schwitzerland), Puy de Do^me (France), Finokalia (Greece) and Marikana village (South Africa). Finally, free-tropospheric cluster measurements were conducted by operating the airborne NAIS in an aircraft during the EUCAARI LONGREX experiment in May 2008 (Mirme et al., 2010). In the following we summarise the main findings from the NAIS measurements, along with additional information obtained from the IonDMPS measurements. 4.1 Detection of neutral and charged clusters and particles Prior to EUCAARI, experimental information on sub-3 nm atmospheric aerosol populations was based almost entirely on air ions, i.e. measuring charged molecular clusters and aerosol particles. The NAIS instrument made it possible to detect neutral atmospheric aerosol particles down to about 2 nm diameter. As a result, the first quantitative estimates on the concentrations of neutral sub-3 nm particles were obtained for both continental boundary layer (Kulmala et al., 2007a) and the free troposphere (Mirme et al., 2010). Size distributions of neutral and naturally charged particles/clusters provide further insight into the origin and dynamics of nucleated particles. In practically all lowertropospheric environments, naturally charged particles were found to have an almost persistent and narrow concentration band, or mode, close to the mobility diameter of 1 nm (e.g. Ho~rrak et al., 2003; Hirsikko et al., 2005; Vartiainen et al., 2007; Manninen et al., 2009a). The distinct presence of this cluster ion mode was perturbed only (i) by clouds, inside which the smallest ions are effectively scavenged by cloud droplets (Lihavainen et al., 2007; Venzac et al., 2007), (ii) by rain events that typically produced additional sub-10 nm ions (see Tammet et al., 2009), and (iii) by some nucleation events (e.g. Ho~rrak et al., 2003; Vana et al., 2008). The aircraft measurements made during the LONGREX experiment, along with ground-based measurements at high altitudes, revealed that the cluster ion mode can be seen in the free troposphere as well (Fig. 6; Venzac et al., 2007; Boulon
FFiiggu. r6e .6: AAvveerraaggeepaprtaicrlteic(lbelac(kb)laacndk)ioann(pdosiiotinve(:preods;itnievgea:tivree:db;lune)enguamtibveer:size blue) number size distributions at different height levels during the EdUistCribAuAtioRnsI aLt OdiNffeGreRntEhXeig2h0t 0le8veclsamduprianiggtnh.e TEUheCAshAaRdIeLdOaNreGaRrEeXpr2e0s0e8nctsampaign. vTahreiasbhaildietdy a(r5eatroep9re5sepntesrvcaerniatbilielisty) (o5ftoth95e pceorcrernetsipleos)nodfitnhge cnourrmesbpeonrdsinizgenumber dsiiszetrdibisutrtiibountiso.ns. et al., 2010; Mirme et al., 2010).58Concentrations of charged particles displayed usually a minimum just above the cluster ion mode and then a broader secondary maximum above 10 nm (e.g. Komppula et al., 2007). This latter mode results from the attachment of cluster ions with pre-existing neutral particles (see the simulations in Sect. 5.2), being most pronounced in polluted continental boundary layers loaded with Aitken mode particles (see Fig. 6). Due to instrumental limitations, the NAIS cannot provide quantitative information about the total concentration of neutral sub-3 nm particles, nor about the exact shape of the corresponding size distribution down to 1 nm. The existing NAIS data demonstrates, however, that neutral sub-3 nm particles clearly dominate overcharged ones in the lower troposphere (Kulmala et al., 2007a; Manninen et al., 2009a). During the LONGREX measurements, concentrations of neutral particles in the diameter range 2-10 nm were, on average, roughly two orders of magnitude larger than those of charged particles throughout the tropospheric column (Fig. 6 and Mirme et al., 2010).
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4.2 Contribution of ion-induced nucleation The NAIS and Ion-DMPS provide complementary information about the role of ion-induced nucleation in atmospheric new-particle formation. In HyytiaЁlaЁ, Finland, these two instruments were operated in parallel for several months. An example of the resulting measurements during one of the nucleation event days is depicted in Fig. 7. Both instruments showed a clear increase in the charged fraction (CF) of 2.8nm particles at the beginning of the event, with subsequent decrease of the CF toward the end of the event. Such behaviour indicates that the contribution of the ion-induced nucleation to the total nucleation is at its highest during the initial stages of new-particle formation. Above 5 nm, the value of the CF increased with increasing particle size, which can be explained by evolution of the nuclei toward charge equilibrium during their growth (Kerminen et al., 2007). The ratio of the apparent formation rate of charged particles to that of total particles is in line with these views (Fig. 7, bottom). The somewhat smaller values of the CF for the smallest particles recorded by the NAIS, as compared with the Ion-DMPS, are probably due to the background caused by charger ions inside the NAIS. Both the NAIS and Ion-DMPS data indicate that ioninduced nucleation contributes, on average, less than 10% of the total nucleation rate in HyytiaЁlaЁ (Gagneґ et al., 2008, 2010; Manninen et al., 2009b). However, the fraction of nucleation explained by ion-induced nucleation varied considerably between the different days, with larger fractions favoured by warmer and sunnier days. On most of the days, both neutral and ion-induced nucleation seemed to occur simultaneously, but with temporally varying portions (Laakso et al., 2007b; Gagneґ et al., 2010). The multi-site operation of NAIS and other ion spectrometers (Manninen et al., 2010) revealed that the average formation rate of charged 2-nm particles (0.1­0.2 cm-3 s-1) varied surprisingly little between the different measurement sites, whereas the average total formation rate of 2-nm particles varied from below 1 to more than 30 cm-3 s-1. This indicates that neutral nucleation pathways become increasingly important when the total nucleation rate is higher. In more general terms, these results might be interpreted as a frequent, yet moderate, ion-induced nucleation taking place in the lower troposphere, outweighed by usually much stronger neutral nucleation that is sensitive to local atmospheric conditions. According to theoretical arguments, the most favourable location for ion-induced nucleation is the upper part of the troposphere (Kazil et al., 2008; Yu, 2010). If the contribution of ion-induced nucleation to total nucleation were to increase considerably when going from the boundary layer toward the upper troposphere, one would expect to see a corresponding increase in the concentration ratio between charged and neutral clusters. During the air craft measurements conducted within the EUCAARI LONGREX campaign, no sign of such
an increase was observed. More airborne measurements of charged and neutral sub-3 nm clusters in different environments are clearly needed to address the role of ion-induced nucleation in free-tropospheric aerosol formation. 5 Nucleation theory, modelling and parameterisations 5.1 Quantum chemical calculations Quantum chemical methods have become a powerful tool to study the molecular mechanism behind new-particle formation and composition of molecular clusters that are always present in the atmosphere (Kurteґn and VehkamaЁki, 2008; Nadykto et al., 2008). Most importantly, such a high-level theory can complement, guide and help to interpret experimental work, especially since experimental techniques detecting the composition of small molecular clusters present in the atmosphere are rapidly developing at the moment. By using different Quantum Mechanics methods, atmospherically relevant molecular clusters were studied in EUCAARI, with the final aim of elucidating the molecular mechanism behind observed atmospheric nucleation. Quantum chemical calculations provide evaporation rates, or equivalently formation free energies, of different clusters that can be involved in nucleation. Evaporation rates are needed to assess the stability of various clusters and to identify the pathways through which clusters nucleate. As part of EUCAARI, the evaporation rates of a wide variety of clusters were calculated, ranging from clusters containing only sulphuric acid to clusters containing complex molecules like amines or large organic acids (Fig. 8). Our main findings can be summarized as follows: (i) ammonia can enhance neutral sulphuric acid-water nucleation to some extent, but has a smaller role in corresponding ion-induced nucleation (Ortega et al., 2008), (ii) dimethylamine enhances neutral and ion-induced sulphuric acid-water nucleation in the atmosphere more effectively than ammonia (Kurteґn et al., 2008; Loukonen et al., 2010), (iii) some of the organic acids resulting from monoterpene oxidiation can form very stable clusters with sulphuric acid, being good candidates to explain the pool of neutral clusters found in field measurements (see Sect. 4.1), and (iv) organo-sulphates can be involved in ion-induced nucleation. Note that indications of the presence of gaseous organo-sulfates ions were obtained, for the first time, by using the new APi-TOF instrument in EUCAARI (Sect. 2.4). 5.2 Ion-UHMA A new modelling tool, called Ion-UHMA, which assists the interpretation of ion spectrometer measurements, was developed (LeppaЁ et al., 2009). The Ion-UHMA is a sectional box model that simulates the dynamics of neutral and electrically charged aerosol particles under atmospheric conditions. It builds on the aerosol dynamical model UHMA (Korhonen et
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Fig. 7. Time evolution of the charged fraction of particles in three different size bins (top panels) based on NAIS and Ion-DMPS measurements at HyytiaЁlaЁ on 30 April 2007. The lower panels depict the ratio between the charged and total particle production rate determined from the NAIS data at theFciogrureresp7o.nTdiinmgesiezve obliunsti.oTnhoefptehrieodchdaurrginegdwfrhaichtiothneonfucplaerattiicolnesevienntherfefeecdtivifefleyreafnftecstiszeabcihnssize bin is separated by vertical bars in the fi(gtuorpesp. anels) based on NAIS and Ion-DMPS measurements at Hyytiдlд on 30 April 2007.
The lower panels depict the ratio between the charged and total particle production rate
al., 2004) and moddeelteArmERinOedIOfrNom(LtahaekNsoAeItSadl.a,t2a0a0t2t)h.eTchoerresposnudbi-n2gnsmizeclbuisntse.rsThanedpeprairotdicldeusrilnarggwerhtihcahn 20 nm were used Ion-UHMA includes the basic aerosol dynamical processes as model inputs. Averaged over the particle formation event, (condensation, coatghuelnatuicolne,adtiroyndeevpeonstiteifofne)c,tiavloelnygawffiethctsioena-ch siztehebimneiasssuerpeadraftoerdmbaytiovnerrtaicteasl boafrtsoitnalt,hneegative and positive aerosol attachmenftigaunrdesio. n-ion recombination. The forma- particles were equal to 1.14, 0.08 and 0.09 cm-3 s-1, retion of new aerosol particles is treated as an input to the spectively, being indicative of the dominance of neutral nu-
model or, alternatively, the model can be coupled with an ex- cleation. On the other hand, the fraction of 2-nm particles
isting mechanistic nucleation model. The size range covered formed as charged was above the charging probability of
by the Ion-UHMA is adjustable, but typically ranges from particles of that size, so it is likely that ion-induced nucle-
1­2 nm up to 1000 nm.
ation was operating as well during this day. Two condens-
The technical performance of the Ion-UHMA was tested, and its ability to simulate atmospheric nucleation events was evaluated (LeppaЁ et al., 2009). Most importantly, it was shown that when the formation rates of neutral and charged 2 nm particles, as obtained from NAIS measurements (see Eqs. 1 and 2 in Sect. 2.1), are used as model inputs, the Ion-UHMA successfully reproduces the observed dynamics
ing vapours were assumed in the simulation: sulphuric acid with a sinusoidal concentration pattern peaking at local noon (PetaЁjaЁ et al., 2009), and an organic compound with a temporally constant and 1­2 orders of magnitude higher concentration than sulphuric acid. The exact concentration levels of these vapours were chosen such that the simulated Growth Rates of sub-20 nm particles were close to observations.
of both charged and neutral particles over the size range 2­
The simulation produced a new particle formation event
20 nm. This means that (i) the model correctly captures the 59 that was qualitatively similar to the measured one (Fig. 9). aerosol dynamics taking place in this size range, and (ii) The formation of neutral particles dominated over that of
the formation rates of 2-nm particles determined from NAIS charged particles, even though increased concentrations of
measurements are reliable.
charged particles at sizes of around 2­2.5 nm could be ob-
As an example of Ion-UHMA simulations, we investi- served in both the simulation and measurements. These par-
gated how well it can reproduce the time evolution of particle ticles were formed as charged and their concentrations de-
number distribution measured in HyytiaЁlaЁ on 15 April 2007. creased with an increasing particle diameter due to their neu-
Measured values of temperature, relative humidity, forma- tralization by ion-ion recombination. At sizes of around
tion rates of 2 nm particles, and concentrations of charged 4 nm, concentrations of charged particles began to increase
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Fig. 8. Relative stability of sulfuric acid clusters with different stabilizing compounds (water, organic acids, ammonia and dimethyl amine) based on the evapoFraigtiuonrera8t:esRkeelvaaptivcaelcsutalabteilditwyiothfqsuualnfuturmiccahceimdisctlruys. tTehrse awtoitmhsdairfefecroelonrt-csotadbediliazsifnogllocwoms: pyoelulonwds­ sulfur, red ­ oxygen, grey ­ hydrogen, blue ­ nitrogen and green ­ carbon. Dashed lines depict hydrogen bonds. (water, organic acids, ammonia and dimethyl amine) based on the evaporation rates kevap
calculated with quantum chemistry. The atoms are color-coded as follows: yellow- sulfur, again due to the increasing efficiency of ion-aerosol attach- growth rates of sub-5 nm particles can be estimated from ion ment. The combreinde­d oexffyegcet no,fgtrheeyse­ thwyodrpohgeenno,mbleunea­wnaistraogen asnpdecgtrroeemne­tercamrbeaosnu.rDemasehnetdusliinnegsthdeepaivcatilable methods (Hir-
concentration gahpyodfrochgaerngbedonpdasr.ticles at around 2­4 nm, ob- sikko et al., 2005), and whether new methods to determine
served both in the simulation and in the measurements. A gap the formation and growth rates of freshly-nucleated particles
in the number size distribution of charged particles below di- from measurement data are needed.
ameters of a few nm is frequently seen in association with measured new-particle formation events (e.g., Komppula et 5.3 Nucleation rate parameterisations
al., 2007; Suni et al., 2008; Vana et al., 2008; Manninen et
al., 2009a).
Over the years, nucleation parameterisations have been de-
veloped for binary H2SO4-H2O nucleation (Russell et al.,
While the overall evolution of the particle number size dis- 1994; VehkamaЁki et al., 2002; Yu, 2008), ternary H2SO4-
tribution was quite similar between the simulation and mea- H2O-NH3 nucleation (Napari et al., 2002; Merikanto et al.,
surements, also some differences can be observed. For ex- 2007), and ion-induced nucleation (Turco et al., 1998; Mod-
ample, simulated concentrations of 10­20 nm particles were gil et al., 2005; Yu, 2010). While all these parameterisations
somewhat smaller than those observed. A probable reason reproduce quite accurately the nucleation rates predicted by
for this is the slight underestimation of particle formation corresponding nucleation theories, they all have problems
or growth rates from the measurement data. In this respect, when applied to large-scale atmospheric modelling. The ex-
the growth rates of the smallest particles are of specific im- isting binary H2SO4-H2O nucleation theories are not able to
portance, since these particles are most vulnerable to scav- reproduce nucleation events observed in continental bound-
enging by coagulation into larger particles (Kerminen et al., ary layers (e.g., Spracklen et al., 2006; Jung et al., 2008;
2004). Measurements at a fixed location are always affected Chang et al., 2009), in addition to which they are not consis-
by transport phenomena, including the diurnal evolution of 60 tent with the most recent laboratory findings (see Sects. 3.1
the mixed layer height and advection of air masses with dif- and 3.3). Ternary H2SO4-H2O-NH3 nucleation mechanisms
ferent aerosol characteristics. The former was apparently ac- may work reasonably well in sulphur-rich urban environ-
tive prior to local noon, whereas the latter may have caused ments (Jung et al., 2008), but probably not in the global at-
the minor but rapid change in the measured particle number mosphere (e.g., Lucas and Akimoto, 2004). In case of ion-
size distribution between about 14:00 and 15:00 LT. Our box induced nucleation the main problem is the scarcity of suit-
model is unable to imitate such transport phenomena.
able measurement data, which so far has hindered the proper
In addition to assisting the interpretation of field mea- testing of this mechanism.
surements, ion-UHMA can be used for many other pur-
In EUCAARI, we concentrated on developing semi-
poses. One such application is to estimate how accurately the empirical nucleation parameterizations, in which the
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Fig. 9. Simulated (left panels) and measured (right panels) number size distribution of all particles (top row), negative ions (middle row) and positive ions (bottom row). The data represents the time evolution during a new particle formation event day (17 April 2007) in HyytiaЁlaЁ, Finland.
Figure 9. Simulated (left panels) and measured (right panels) number size distribution of
npcouenncldceeeannttciroeantoironantt.ehadeliaTlsgthapaaeassrsererutoepimaucrsseleoessdnseutnl(foptotohsfrpuottrlhhlrieocioswwaticsm)ai,dtsenhi(meeaegnvpadaocltecloiuuvrptgmeoiaowuinnoleiacnrdt-sivlunaa(rgwpminoeidguvderida)--lneerwoJwp2)a=+ratKnKicdssl31ep[oNfHosu2irctSmiOvOare4tgi]io2o2n+n, seKv(sbe2no[ttHtdo2amSyOr(4o1]w7Ч)A.NTpurhicelOrg
(11)
dence that such relations appear to mimic atmospheric nucleation much2b0e0tt7e)r itnhaHn ypyretidдilcдti,oFnisnblaansedd. on classical nu-
Here, J2 is the formation rate of 2-nm particles, [NucOrg] refers to the concentration of organic vapour(s) participating
cleation theories (Sect. 3.1; Weber et al., 1996; Sihto et al., 2006; Riipinen et al., 2007; Kuang et al., 2008; Paasonen
in nucleation, and Ai and Ki are the first and second order nucleation coefficients, respectively. At all the four sites, the
et al., 2009). By combining measurement data from four sites (HyytiaЁlaЁ in Finland, Hohenpeissenberg and Melpitz
H2SO4 concentration was obtained directly from measurements, whereas the organic vapour concentration was derived
in Germany, San Pietro Capofiume in Italy), the following from the closure of 2­4 nm particle growth rates. The val-
eight candidate mechanisms were investigated (Paasonen et al., 2010):
ues of the coefficients Ai and Ki were determined for each site separately, as well as for the whole data set together, by
J2 = A[H2SO4],
fitting the regression formulae in question to the measure(4) ment data points. The success of the fittings was evaluated by
J2 = K [H2SO4]2 ,
looking at how well the measurement data points correlated (5) with the fittings and how scattered they were with respect to
J2 = Aorg NucOrg ,
the fitting.
(6)
The analysis showed that of the two mechanisms based
J2 = Korg NucOrg 2 ,
solely on the H2SO4 concentration, Eq. (5) was clearly the (7) better one and worked reasonably well for HyytiaЁlaЁ, Melpitz
J2 = As1 [H2SO4] + As2 NucOrg ,
61 and San Pietro Capofiume. However, the values of K giving (8) the best prediction for J2 differed by more than a magnitude between these three sites. Neither Eq. (4) nor Eq. (5) worked
J2 = Khet [H2SO4] Ч NucOrg ,
(9) for the Hohenpeissenberg data. Of the two mechanisms
based solely on organic vapour concentrations, Eq. (7) was J2 = KSA1 [H2SO4]2 + KSA2 [H2SO4] Ч NucOrg , (10) the best one in Hohenspeissenberg, whereas neither Eq. (6)
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nor Eq. (7) worked in the three other sites. The parameterisations relying on different combinations of H2SO4 and organic vapours (Eqs. 8­11) displayed a variable success between the four sites. When trying to predict the value of J2 by using a single set of nucleation coefficient for all the sites together, Eqs. (10) and (11) appeared to work the best, even though none of the equations showed a superior performance over the others (Paasonen et al., 2010). There are two issues worth mentioning here. First of all, the derivation of semi-empirical parameterizations, like Eqs. (4) to (11), is always subject to uncertainties in measured quantities. For example, there is up to 50% uncertainty in measured H2SO4 concentrations and even a slightly larger one in [NucOrg] due to uncertainties related to determining the growth rate of 2­4 nm particles (Paasonen et al., 2010). Likewise, formation rates of 2-nm particles (J2) may be up to a factor two lower or higher than the estimated ones due to uncertainties in measurements and data analysis (Manninen et al., 2010). Second, it is clear that Eqs. (4) to (11) are oversimplifications of the physical and chemical factors influencing the nucleation rate. These include the ambient temperature and relative humidity and the stabilizing effect of vapors other than H2SO4 and NucOrg, causing additional scatter in measured data points. Such factors need to be investigated more thoroughly in the future, along with the applicability of the current parameterizations for conditions other than the continental boundary layers. All of the nucleation rate parameterisations presented above (Eqs. 4­11) are similar to the standard formalisms of chemical kinetics describing of second-order or pseudo-first order reactions of atmospheric gases and aerosols (PoЁschl et al., 2007). Rate equations like Eqs. (10) and (11) are characteristic for processes that can proceed via different mechanistic pathways and can be described by a linear combination of the rates of each pathway. This approach is consistent with recent developments in the modelling of aerosol chemical transformation and aging by multi-component and multiphase processes (Shiraiwa et al., 2009, 2010). Equation (11) is the most general formulation and seems most promising as a basis for future developments aimed at a universally applicable parameterisation of aerosol nucleation rates across different regions and regimes. Knowledge of both sulphuric acid and organic vapour concentrations appears necessary to explain and parameterise atmospheric nucleation rates, and the field measurements are in line with the most recent laboratory experiments discussed in Sects. 3.1 and 3.3. The strong interplay between sulphuric acid and low-volatile organics in atmospheric nucleation and subsequent particle growth is also apparent when looking at long-term changes in aerosol concentrations over Central Europe due to concomitant reductions in SO2 emissions (Hamed et al., 2010).
5.4 Parameterising the apparent particle formation rate
Direct application of nucleation rate parameterisations in large-scale models is not possible, or at least not desirable, for two reasons. First of all, most of the current global models simulating aerosol dynamics do not explicitly cover particle sizes relevant to nucleation. Second, the dynamics of freshly-nucleated particles depends in a complicated way on the interplay between their formation rate, their condensation growth and their scavenging by coagulation (Kerminen et al., 2004; McMurry et al., 2005; Pierce and Adams, 2007). Such interplay cannot be accurately handled in a large-scale modelling framework due to excessive computational costs. For the reasons highlighted above, the early dynamics of nucleated clusters is usually parameterised in large-scale atmospheric models. In EUCAARI, a parameterisations that relates the formation rate of particles of diameter dp (J (dp), i.e. the apparent formation rate of particles at size dp) and the nucleation rate (Jnuc) was derived (Lehtinen et al., 2007):
J (dp) = Jnuc exp
dnuc 1 - m+1
dp m+1 dnuc
CoagS(dnuc) GR
. (12)
Here dnuc is the size of nucleated clusters, CoagS(dnuc) is their coagulation sink, i.e. the rate at which they coagulate with pre-existing aerosol particles, GR is their growth rate, and m (1.5­2) is a constant that depends on the shape of the particle number size distribution. Predictions by Eq. (12) are similar to those by the widely-applied formulae proposed by Kerminen and Kulmala (2002). A drawback of Eq. (12), like in All Other corresponding parameterisations developed until now, is the neglect of nuclei self-coagulation. This process accelerates nuclei growth and reduces their number concentration. Anttila et al. (2010) derived an iterative procedure by which the effect of nuclei self-coagulation on GR and CoagS can be taken into account when applying Eq. (12). Comparisons to detailed numerical simulations showed that the apparent particle formation rate is affected by nuclei self-coagulation only when atmospheric nucleation rates are exceptionally high (>10 cm-3 s-1 in the free troposphere and >104 cm-3 s-1 in the polluted boundary layer). In order to apply the parameterisations by Lehtinen et al. (2007) and Anttila et al. (2010) in atmospheric models, the following quantities need to be known or derived from other model variables: (i) the nucleation rate, (ii) the particle number size distribution, and (iii) the concentrations of vapours that cause the fresh nuclei to grow in size. The first of these requirements means simply that the aerosol formation rate parameterisation cannot be used without a nucleation rate parameterisation. The second one implies that the representation of the particle size distribution in the model must allow for determination of the coagulation sink. The third requirement is perhaps the toughest one: the model needs to have some way of estimating the sulphuric acid concentration or,
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preferably, concentrations of all the vapours that contribute significantly to the nuclei growth. Potential ways to deal with condensing vapour concentrations in a large-scale modelling framework have been discussed lately by Chang et al. (2009). 6 Concluding remarks Our understanding of atmospheric nucleation relies essentially on four very different sources of information: field measurements, laboratory experiments, theoretical calculations and model studies. Until very recently, these approaches have not been able to provide a consistent picture on atmospheric nucleation. Perhaps the most important problem in this regard has been the relation between the nucleation rate and the identity and concentrations of nucleating vapours. For example, the functional dependence of the nucleation rate on the gaseous sulphuric acid concentration, as observed in the ambient atmosphere, appeared very different from that seen in most laboratory experiments, and neither field nor laboratory data could be reconciled with existing classical nucleation theories. As demonstrated in this publication, the EUCAARI project has significantly reduced the gap between the different approaches used to tackle atmospheric nucleation. The most important reason for this development has been the enhanced capabilities to measure sub-3 nm particle populations, along with the extensive application of the new instruments in both laboratory and field. From a theoretical point of view, quantum chemical calculations have eventually evolved to a stage, at which they can provide useful information to guide measurements and to constrain model approaches. All the results obtained during EUCAARI indicate that sulphuric acid plays a central role in atmospheric nucleation. However, our most recent laboratory experiments and field measurements show that also vapours other than sulphuric acid are needed to explain the nucleation process. Such vapours might be of organic origin, at least in continental boundary layers. By stabilizing molecular clusters containing sulphuric acid, it has been speculated for quite some time that basic vapours like ammonia would participate in atmospheric nucleation. The laboratory experiments and quantum chemical calculations made within EUCAARI give support for the moderate involvement of ammonia in nucleation, and indicate further that amines might be even more important than ammonia in assisting atmospheric nucleation. The field and laboratory data obtained during EUCAARI demonstrate that the nucleation rate scales to the first or second power of the nucleating vapour concentration(s). This agrees with the few earlier field observations, but is in stark contrast with classical thermodynamic nucleation theories, such as binary sulphuric acid-water nucleation or ternary sulphuric acid-water-ammonia nucleation. The new findings, while suggesting that the formation of very small molecu-
lar clusters drives atmospheric nucleation, are not sufficient enough to reveal the actual nucleation mechanism. The EUCAARI field measurements brought plenty of new insight into the role of ions in atmospheric nucleation. One important finding was that the average formation rate of charged 2-nm particles varied very little, by roughly a factor two, between the different measurement sites. This contrasts to the average total formation rate of 2-nm particles which varied by almost two orders of magnitude between the sites. The contribution of charged particles to the total formation rate of 2-nm particles was usually well below 10%, but it showed substantial temporal variability both during a nucleation event and between the different event days. In general, our observations are indicative of frequent, yet moderate, ion-induced nucleation usually outweighed by much stronger neutral nucleation in the continental lower troposphere. No evidence on the enhanced role of ion-induced nucleation in the upper free troposphere, as suggested by some theoretical studies, was obtained from our air craft measurements. The most concrete outcome of the EUCAARI nucleation studies are the new semi-empirical nucleation rate parameterisations, along with updated aerosol formation parameterisations. Although these parameterisations require theoretical improvements, as well as intensive testing against both laboratory and field data, we recommend that they should gradually replace the traditional binary and ternary nucleation parameterisations currently used in most atmospheric models. From a global and Earth System modelling point of view, the new semi-empirical nucleation parameterisations provide a simple and effective tool, by which one can investigate the sensitivity of the global aerosol system to atmospheric nucleation and related emissions of precursor gases and primary particles. Several open questions remain that should be addressed in the future. First of all, we do not really know whether atmospheric nucleation is dominated by a single nucleation pathway, or whether multiple different mechanisms are competing with each other. Second, the relative importance of the kinetic and thermodynamic factors controlling the nucleation rate is unclear. Third, the identity and role of organic vapours in the nucleation process are still unknown. Finally, although ion-induced nucleation appears to be of minor significance in continental boundary layers, this is not necessarily the case in the free troposphere or above the oceans. In this regard, there are very little experimental data on how ions interact with neutral particles and clusters in the sub-2 nm size range. In order to address the remaining knowledge gaps and to quantify the relevant nucleation mechanisms, we need to find out how the actual nucleation rate is connected with the dynamics of the smallest atmospheric clusters. This requires information on the chemical composition, Physical Properties and evaporation rates of these clusters. Essential tools to tackle the problem are highly sensitive and selective new instruments capable of operating at the sub2 nm size range, kinetic molecular-scale models, laboratory
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experiments, and various theoretical approaches relying on both quantum chemistry and classical thermodynamics. Acknowledgements. This work has been supported by European Commission 6th Framework program projects EUCAARI (contract no. 036833-2) and EUSAAR (contract no. 026140). The support from the Academy of Finland Centre of Excellence program (project no. 211483, 211484 and 1118615) is also gratefully acknowledged. Edited by: E. Swietlicki References Aalto, P. P., HaЁmeri, K., Becker, E., Weber, R., Salm, J., MaЁkelaЁ, J. M., Hoell, C., O'Dowd, C. D., Karlsson, H., Hansson, H.-C., VaЁkevaЁ, M., Koponen, I. K., Buzorius, G., and Kulmala, M.: Physical characterization of aerosol particles during nucleation events, Tellus B, 53, 344­358, 2001. Anttila, T., VehkamaЁki, H., Napari, I., and Kulmala, M.: Effect of ammonium bisulphate formation on atmospheric water-sulphuric acid-ammonia nucleation, Boreal Environ. Res., 10, 511­523, 2005. Anttila, T., Kerminen, V.-M., and Lehtinen, K. E. J.: Parameterizing the formation rate of new particles: the effect of nuclei self-coagulation, J. Aerosol Sci., 41, 621­636, 2010. Asmi, E., SipilaЁ, M., Manninen, H. E., Vanhanen, J., Lehtipalo, K., Gagneґ, S., Neitola, K., Mirme, A., Mirme, S., Tamm, E., Uin, J., Komsaare, K., Attoui, M., and Kulmala, M.: Results of the first air ion spectrometer calibration and intercomparison workshop, Atmos. Chem. Phys., 9, 141­154, doi:10.5194/acp-9-141-2009, 2009. Ball, S. M., Hanson, D. R., Eisele, F., and McMurry, P. H.: Laboratory studies of particle nucleation: Initial results for than H2SO4, H2O, and NH3 vapors, J. Geophys. Res., 104, 23709­23718, 1999. Benson, D. R., Young, L.-H., Kameel, F. R., and Lee, S.-H.: Laboratory-measured nucleation rates of sulfuric acid and water binary homogeneous nucleation from the SO2 + OH reaction, Geophys. Res. Lett., 35, L11801, doi:10.1029/2008GL033387, 2008. Berndt, T., BoЁge, O., Stratmann, F., Heintzenberg, J., Kulmala, M.: Rapid formation of sulfuric acid particles at near-atmospheric conditions, Science, 307, 698­700, 2005. Berndt, T., Stratmann, F., BraЁsel, S., Heintzenberg, J., Laaksonen, A., and Kulmala, M.: SO2 oxidation products other than H2SO4 as a trigger of new particle formation. Part 1: laboratory investigations, Atmos. Chem. Phys., 8, 6365­6374, doi:10.5194/acp8-6365-2008, 2008. Berndt, T., Stratmann, F., SipilaЁ, M., Vanhanen, J., PetaЁjaЁ, T., Mikkil, J., GruЁner, A., Spindler, G., Lee Mauldin III, R., Curtius, J., Kulmala, M., and Heintzenberg, J.: Laboratory study on new particle formation from the reaction OH + SO2: influence of experimental conditions, H2O vapour, NH3 and the amine tert-butylamine on the overall process, Atmos. Chem. Phys., 10, 7101­7116, doi:10.5194/acp-10-7101-2010, 2010. Bowman, A. F., Lee, D. S., Asman, W. A. H., Dentener, F. J., Van Der Hoek, K. W., and Olivier, J. G. J.: A global high-resolution
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