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From Molecular Clusters to Nanoparticles: Second-Generation Ion-Mediated Nucleation Model

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From Molecular Clusters to Nanoparticles: Second-Generation Ion-Mediated Nucleation Model Atmos. Chem. Phys., 6, 5193–5211, 2006 www.atmos-chem-phys.net/6/5193/2006/ Atmospheric © Author(s) 2006. This work is licensed Chemistry under a Creative Commons License. and Physics From molecular clusters to nanoparticles: second-generation ion-mediated nucleation model F. Yu Atmospheric Sciences Research Center, State University of New York, 251 Fuller Road, Albany, New York, 12203, USA Received: 16 January 2006 – Published in Atmos. Chem. Phys. Discuss.: 13 April 2006 Revised: 09 October 2006 – Accepted: 31 October 2006 – Published: 15 November 2006 Abstract. Ions, which are generated in the atmosphere by 1 Introduction galactic cosmic rays and other ionization sources, may play an important role in the formation of atmospheric aerosols. Atmospheric aerosols from both natural and anthropogenic In the paper, a new second-generation ion-mediated nucle- sources indirectly impact the Earth climate through the ef- ation (IMN) model is presented. The new model explicitly fect on the cloud formation and properties, and precipitation treats the evaporation of neutral and charged clusters and (e.g., Twomey, 1977; Charlson et al., 1992). Indirect ra- it describes the evolution of the size spectra and composi- diative forcing is poorly constrained in the existing climate tion of both charged and neutral clusters/particles ranging models and this represents the single greatest uncertainty from small clusters of few molecules to large particles of in assessing the climate change (Penner et al., 2001). An- several micrometers in diameter. Schemes used to calcu- other important issue is that the particles in the atmosphere late the evaporation coefficients for small neutral and charged can cause visibility degradation and adverse health effects. clusters are consistent with the experimental data within the Climatic, health and environmental impacts of atmospheric uncertainty range. The present IMN model, which is size-, particles depend strongly on the size spectra and composi- composition-, and type-resolved, is a powerful tool for inves- tions of aerosols in the Earth atmosphere. These proper- tigating the dominant mechanisms and key parameters con- ties are governed by a number of microphysical processes trolling the formation and subsequent growth of nanoparti- (e.g. nucleation, condensational growth and coagulation) tak- cles in the atmosphere. This model can be used to analyze ing place simultaneously. New particle formation has been simultaneous measurements of the ion-mobility spectra and observed in various locations in the lower troposphere (e.g., particle size distributions, which became available only re- Kulmala et al., 2004a). Some observations have indicated cently. General features of the spectra for ions smaller than that significant numbers of newly formed particles can grow the critical size, size-dependent fractions of charged nanopar- to the size of the cloud condensation nuclei (CCN) within ticles, and asymmetrical charging of freshly nucleated par- a day under favorable conditions (Kulmala, 2003; Stanier et ticles predicted by the new IMN model are consistent with al., 2004; Zhang et al., 2004a; Wehner et al., 2005; Laak- recent measurements. Results obtained using the second sonen et al., 2005; Stolzenburg et al., 2005). A clear and generation IMN model, in which the most recent thermo- insightful understanding of the mechanisms responsible for dynamic data for neutral and charged H2SO4-H2O clusters the formation of the atmospheric particles is critically impor- were used, suggest that ion-mediated nucleation of H2SO4- tant for quantitative assessment of the climate-related, health H2O can lead to a significant production of new particles in and environmental impacts of atmospheric aerosols. the lower atmosphere (including the boundary layer) under Binary homogeneous nucleation (BHN) of sulfuric acid favorable conditions. It has been shown that freshly nucle- and water has been considered for a long time as a dominant ated particles of few nanometers in size can grow by the con- mechanism of the new particle formation. However, it has densation of low volatile organic compounds to the size of been recently found that BHN theory can’t explain the ob- cloud condensation nuclei. In such cases, the chemical com- served nucleation events in the lower troposphere (e.g, We- position of nucleated particles larger than 10 nm is domi- ber et al, 1996; Clarke et al., 1998). Ternary homogeneous nated by organics. ∼ nucleation (THN) theory, which involves H2SO4,H2O and NH3 (e.g., Coffman and Hegg, 1995; Korhonen et al., 1999; Correspondence to: F. Yu Napari et al., 2002), and ion-mediated nucleation (H2SO4- ([email protected]) H2O-Ion) (Yu and Turco, 2000, 2001) have been proposed as Published by Copernicus GmbH on behalf of the European Geosciences Union. 5194 F. Yu: Ion-mediated nucleation alternative mechanisms of the particle formation. The actual of derivations from measurements of small ion clusters, ab role of NH3 in the atmospheric nucleation remains unclear initio calculations, thermodynamic cycle, and some approx- because the difference between experimental results and pre- imations (adjustment of Gibbs free energy for neutral clus- dictions of THN is excessively large (Yu, 2006a). Prelim- ters calculated based on liquid droplet model, interpolation, inary simulations performed using a kinetically consistent etc.). Some uncertainties in the thermodynamic data used in THN model constrained using the experimental data indicate the model of Lovejoy et al. (2004) are expected. Lovejoy that the contribution of THN to the formation of new par- et al. (2004) stated that the nucleation of H2SO4 (and H2O) ticles in the boundary layer is negligible (Yu, 2006a). Other on negative ions does not generally explain the observed nu- recently proposed nucleation mechanisms include nucleation cleation events in the boundary layer. Recent observations of iodine species (O’Dowd et al., 2002) and organics com- of the overcharge of 3–5 nm particles show that ions are in- pounds (Zhang et al., 2004b). It may be possible that all the volved in the nucleation in the boundary layer (Laakso et al., above nucleation mechanisms contribute to the formation of 2004; Vana et al., 2006; Iiad et al., 2005). Sorokin et al. new particles in the atmosphere and that the atmospheric nu- (2006) reported laboratory investigation of positive and neg- cleation under different conditions may be dominated by dif- ative H2SO4-H2O cluster ions and they showed that small ferent nucleation mechanisms. The present paper is focused positive ions are markedly enriched in H2O and negative on the ion-mediated nucleation. ions are enriched in H2SO4 molecules. Sorokin et al. (2006) Yu and Turco (1997, 1998, 2000, 2001) developed a novel also developed an ion-cluster-aerosol kinetic model which approach to study the nucleation on ions. The new approach uses the thermochemistry data reported in Froyd and Love- is based on a kinetic model that explicitly solves the com- joy (2003a, b) and empirical correction terms suggested in plex interactions among ions, neutral and charged clusters Lovejoy et al. (2004). Sorokin et al. (2006) compared their of various sizes, vapor molecules, and pre-existing parti- model predictions with laboratory measurements but didn’t cles. Compared to homogeneous nucleation involving neu- discuss what their model will predict under ambient atmo- tral clusters, the nucleation on ions is favored because: (1) spheric conditions. The small charged clusters are much more stable thermody- In the last few years, we have made substantial progress namically; (2) Growth rate of small ion clusters is enhanced in understanding the molecular nature and thermodynam- by the dipole-charge interaction between charged clusters ics of small charged clusters (Nadykto et al. 2003, 2006; and strongly dipolar precursor molecules and, thus, ionic Nadykto and Yu, 2003; 2004; Yu, 2005a) and derived a mod- clusters grow faster than neutrals. Evolution of the size dis- ified Kelvin-Thomson (MKT) equation that accurately takes tributions of both charged and neutral clusters in the pres- into account the dipole-charge interactions (Yu, 2005a). The ence of ionization, neutralization, coagulation, and scaveng- thermodynamic data for small ion clusters derived from re- ing leads to a net flux of clusters crossing the critical size cent MKT equation are in much better agreement with ex- (i.e., nucleation). This mechanism is referred to as the ion- periments than those obtained using the classical Kelvin- mediated nucleation (IMN) (Yu and Turco, 2000). In recent Thomson equation (Yu, 2005a). We have also developed years there is a continuously growing interest to the nucle- a scheme to calculate the evaporation of H2SO4 molecules ation of H2SO4-H2O on ions (Yu, 2002; Carslaw et al., 2002; from neutral H2SO4-H2O clusters (Yu, 2005b, 2006b). This Eichkorn et al., 2002; Laakso et al., 2003; Lee et al., 2003; scheme, in which the most recent thermodynamic data for Harrison and Carslaw, 2003; Lovejoy et al., 2004; Wilhelm H2SO4-H2O binary system are used, explicitly includes the et al., 2004; Sorokin et al., 2006). The mobility distribution hydration effect and it predicts H2SO4-H2O homogeneous of charged clusters/particles measured with the ion mobil- nucleation rates within the uncertainty range of the experi- ity spectrometers (Tammet, 2006) provide useful information mental data (Yu, 2005b, 2006b). In this paper we present about the possible involvement of ions in the atmospheric
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