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Atmos. Chem. Phys., 11, 767798, 2011www.atmos-chem-phys.net/11/767/2011/doi:10.5194/acp-11-767-2011 Author(s) 2011. CC Attribution 3.0 License.
Atmospheric ions and nucleation: a review of observations
A. Hirsikko1, T. Nieminen1, S. Gagne1,2, K. Lehtipalo1, H. E. Manninen1, M. Ehn1, U. Horrak3, V.-M. Kerminen1,4,L. Laakso1,4,5, P. H. McMurry6, A. Mirme3, S. Mirme3, T. Petaja1, H. Tammet3, V. Vakkari1, M. Vana1,3, andM. Kulmala1
1Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland2Helsinki Institute of Physics and University of Helsinki, Department of Physics, P.O. Box 64,00014 University of Helsinki, Finland3Institute of Physics, University of Tartu, 18 Ulikooli Str., 50090 Tartu, Estonia4Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101 Helsinki, Finland5School of Physical and Chemical Sciences, North-West University, Potchestroom, Republic of South Africa6Particle Technology Laboratory, University of Minnesota, Minneapolis, Minnesota, USA
Received: 20 September 2010 Published in Atmos. Chem. Phys. Discuss.: 19 October 2010Revised: 14 January 2011 Accepted: 16 January 2011 Published: 26 January 2011
Abstract. This review is based on ca. 260 publications, 93of which included data on the temporal and spatial varia-tion of the concentration of small ions (
768 A. Hirsikko et al.: Atmospheric ions and nucleation
Air ions larger than 1.6 nm in mass diameter are definedas charged aerosol particles according to their physical na-ture (Tammet, 1995), whereas smaller air ions are chargedmolecules or molecular clusters (e.g. Ehn et al., 2010; Junni-nen et al., 2010). The atmospheric electricity measurementcommunity divides the air ion population into small, interme-diate and large ions. These ion modes were already observedin the early decades of electrical aerosol measurements (El-ster and Geitel, 1899; Langevin, 1905; Pollock, 1915; Israel,1970; Flagan, 1998). The same classification scheme will beused in this paper. However, in some studies the term clusterion has been used to describe the whole small ion population(e.g. Horrak et al., 2000; Hirsikko et al., 2005). The air ionclassification was further developed based on long term mea-surements at Tahkuse Observatory, Estonia, and the bound-ary between the intermediate and large ions was found to beat 7.4 nm in mass diameter (Horrak et al., 2000, 2003).
The primary sources of air ions are radon decay, gammaradiation, and cosmic radiation (Israel, 1970; Bazilevskayaet al., 2008). Other ionisation mechanisms are negligiblesources of air ions in the lower atmosphere (Harrison andTammet, 2008). Other known sources for small ions aretraffic, corona dischargers (e.g. power lines, and point dis-chargers in the case of enhanced atmospheric electric field,like thunder storms) and splashing water (e.g. Chalmers,1952; Eisele 1989a,b; Haverkamp et al., 2004; Tammet etal., 2009). Primary ions (singly charged positive ions andfree electrons) form via ionisation of air molecules, and theybecome small ions in less than a second. During their lifetime of ca. 100 s, small ions undergo a series of ion-moleculereactions and continuously change their chemical identity.Thus, the chemical composition of small ions depends onthe age of the ions and on the trace gas concentration in theair (Mohnen, 1977; Keesee and Castleman, 1985; Viggiano,1993; Luts and Parts, 2002; Parts and Luts, 2004). Air ionsare redistributed into particles of different sizes by coagula-tion with pre-existing aerosol particles and by their growthto larger sizes, or they can be lost by ion-ion recombinationand dry deposition (Hidy, 1984; Hoppel and Frick, 1990; Se-infeld and Pandis, 1998; Tammet et al., 2006).
Small ions exist practically all the time and throughout thetroposphere, as evidenced by a large number of observationsmade both close to the Earths surface and at various altitudesup to several kilometres (e.g. Arnold et al., 1978; Eichkornet al. 2002; Dhanorkar and Kamra, 1993a; Vartiainen et al.,2007; Hirsikko et al., 2005, 2007c; Virkkula et al., 2007;Vana et al., 2008, Suni et al., 2008, Laakso et al., 2008; Ven-zac et al., 2007, 2008; Mirme et al., 2010). In contrast, in-termediate ions are usually detected only during periods ofnew-particle formation, snowfall or falling water droplets, orat high-wind speed conditions (snowstorm) in winter (e.g.Horrak et al., 1998b; Hirsikko et al., 2005, 2007a; Virkkulaet al, 2007; Laakso et al., 2007b; Tammet et al., 2009; Man-ninen et al., 2010). When new aerosol particles are formed bynucleation, intermediate ions are produced by ion-mediated
pathways (e.g. Froyd and Lovejoy, 2003a,b; Lovejoy et al.,2004; Yu and Turco, 2000, 2008; Yu, 2010; Yu et al., 2010)or via the attachment of small ions to newly-formed neutralparticles (e.g. Iida et al., 2006).
Several review articles that discuss various aspects of airions have recently been published. These reviews show thatwhile atmospheric new-particle formation is a frequent phe-nomenon taking place almost all over the world (Kulmala etal., 2004a; Smirnov, 2006; Kulmala and Kerminen, 2008;Kerminen et al., 2010), the role of ions in this process is notwell quantified (e.g. Kulmala and Tammet, 2007; Enghoffand Svensmark, 2008; Arnold, 2008; Laakso et al., 2007a;Yu and Turco, 2008; Manninen et al., 2010). In a globalperspective, ion-aerosol-cloud interactions have potential cli-matic implications (Harrison and Carslaw, 2003; Kazil et al.,2008).
Despite the increasingly active research on air ions duringthe last decade, no comparative studies on the evolution ofsmall ion concentrations in different environments have beenpublished to date. The main focus of this review is to providea comprehensive overview of the spatial and diurnal varia-tions of the naturally created small ion concentrations in thelower troposphere, along with the dependence of these con-centrations on the ion sources and sinks. Another focus is tolook at the connection between air ions and atmospheric new-particle formation. We begin our analysis by introducing the-oretical considerations on the mobility-diameter conversionand the balance equation for small ion population (Sect. 2),we continue with the relevant measurement devices (Sect. 3),small ion observations (Sect. 4) and connections betweenions and atmospheric new-particle formation (Sect. 5), andcomplete by presenting some concluding remarks (Sect. 6).
2 Air ions
In this section we discuss the conversion of mobility to diam-eter, since typically mobility distributions are measured butsize distributions are presented (Sect. 2.1). In Sect. 2.2 wediscuss the balance equation for small ions, and in Sect. 2.3,the connection between air ions and conductivity is dis-cussed.
2.1 Physical parameters of air ions
The physical parameters of an air ion are its mass m, diameterd, density of ionic matter , electric charge qe, and electricalmobility Z. The electric charge of small ions in the atmo-sphere is always one elementary charge qe = e (e.g. Hinds,1999) and the diffusion coefficient D is related to the mobil-ity according to the Einstein relation
D =kT Z
where k is the Boltzmann constant and T is temperature inKelvin. The mass of an ion can be unambiguously measured
Atmos. Chem. Phys., 11, 767798, 2011 www.atmos-chem-phys.net/11/767/2011/
A. Hirsikko et al.: Atmospheric ions and nucleation 769
Fig. 1. Mobility versus mass- and mobility diameter calculated according to Tammet (1995) and modified Stokes-Millikan (Ku and de laMora, 2009) using three different densities.
by means of a mass spectrometer and the electrical mobilitycan be measured by means of a mobility analyser. Thus onlytwo parameters, diameter (d) and density () of particles, arenot unambiguously defined.
The term mobility size is often used in aerosol research.Unfortunately, this term is ambiguous, because differentauthors use different models of size-mobility relation (seeMakela et al., 1996; Li and Wang, 2003; Ehn et al., 2011).The traditional macroscopic model that assumes a particleas a sphere with an exactly determined geometric surface isnot adequate in the nanometer size range. In atomic physics,the microscopic particles are characterised by the interactionpotential and collision cross-sections. The concept of thesurface is sometimes not applicable and particles are char-acterised by continuous coordinate functions. To correct thesituation, Mason (1984) proposed the mass diameter
as a size parameter of ions. The concept of size of an aerosolparticle was analysed by Tammet (1995) and the mass di-ameter was recommended as a preferable universal modelparameter for ions or aerosol particles.
Tammet (1995) derived a semi-empirical size-mobilitymodel, which approaches the Chapman-Enskog equationwhen the particle size decreases, and the Millikan equa-tion when the particle size increases (Chapman and Cowl-ing, 1970; Millikan, 1923). The concept of size was pre-served using the (-4) potential from the kinetic theory andthe transfer between the microscopic and macroscopic lim-its was accomplished by accounting for the dependence of
the law of the reflection of gas molecules on the particle sizeand considering of the collision distance as a function of theinteraction energy. The result was formulated as a comput-ing algorithm given by Tammet (1995) and later corrected byTammet (1998). Another sophisticated model (Li and Wang,2003) is based mostly on theoretical calculations.
Unfortunately, the sophisticated models are in some re-spects inconvenient in the practice of air ion and aerosol mea-surements. If the size range below 1.5 nm is not important,the models can be essentially simplified. In this case the maindifference b