Ion-mediated nucleation in the atmosphere Raffaella D'Auria and Richard P. Turco Department of Atmospheric Sciences, University of California, Los Angeles (dauria@atmos.ucla.edu) Abstract: In situ formation of new particles in the troposphere is well-documented observationally [e.g., Clarke et al. 1998]. However, a theory that successfully explains the process of particle formation from precursor vapors, is consistent with basic physical and chemical principles, and explains existing data, is still unresolved [e.g., Clarke et al., 1998]. Recently, massive molecular ionic clusters have been detected in the upper troposphere [Eichkorn et al., 2002; Heitmann and Arnold, 1983]. Similar observations have also been made in the lower troposphere [Eisele, 1988; Tanner and Eisele, 1991], stratosphere and in the mesosphere [e.g.; Arnold et al., 1977, 1982]. Ionic clusters have the property of stabilizing the formation of condensation nuclei through the strong interaction of a charge center with polar molecules at the atomic scale. Hence, ion production in the atmosphere can influence the abundances of cloud condensation nuclei that ultimately impact Earth's climate. The primary ionization source throughout the lower atmosphere (excluding the first few kilometers over land) are galactic cosmic rays (GCRs). Because the intensity of galactic cosmic radiation penetrating into the middle and lower atmosphere is significantly modulated by solar activity, a potentially critical connection may exist between solar activity and changes in climate forcing as mediated by ionic clusters. In this paper we present results from an ongoing research effort to determine the atmospheric roles of ionic clusters with respect to heterogeneous chemistry and cloud microphysics. We focus on the properties and growth of small molecular clusters that form on hydronium, nitrate and sulfate core ions with water, nitric acid, and sulfuric acid ligands. We present results that describe the sensitivity on cluster properties and behavior to changes in ionization levels and ambient environmental conditions. Our Approach: Ion Cluster Nucleation Kinetics kf,(n-1) /k 1. Uses available experimental data on ion cluster equilibria: kf,(n-1) + ligand molecule A Hybrid Approach to Determine the Ion Cluster Thermodynamics logK(n-1),n = -DG(n-1),n kr,n r,n= ( K(n-1),n = exp - DG(n-1,n) (n-1),n we have used density functional theory (DFT) 3. Uses the classical Thomson liquid droplet model for the larger clusters (once the microscopic results have converged to the macroscopic ones): /RT) Using thermodynamic data (DG0,n), and noticing that the forward rate coefficient, kf,n, typically fall within a narrow range of value (~10-9 cm3/s), the reverse rate coefficients, kr,n, can be estimated. DG0,n = -nRTlnS + 4pr2sNA + (NAq2/8pe0)(1-1/er)(1/r - 1/r0) A Comparison of the Different Approaches: Gibbs Free Energy Change for Hydronium Ion Clusters, H3O+(H2O)n at 300 K Nucleation kinetics Taking into account ion sources and sinks, ion concentrations are governed by: Some example of ion cluster structures obtained from the quantum mechanical simulations: Hydronium ion-water clusters . c0 = -kf,0 l c0 + kr,1 c1 - arec C c0 + Q0 ... . cn = -kf,(n-1) l c(n-1) + kr,n cn - kf,n l cn - arec C cn + kr,(n+1) c(n+1) ... { (n-1),n 2. Uses quantum mechanical simulations to infer the structure and the thermochemistry of ion clusters: ion cluster, n ligands ion cluster, (n-1) ligands /RT = -DH /RT + DS /R . cn = -kf,(N-1) l c(N-1) + kr,N cN - arec C cN where: c0 is the core ion, cn is the ion cluster containing n ligands, C is the total concentration of the ion clusters of opposite charge, and arec (~ 10-6 cm3s-1) is the ion recombination coefficients and Q0 (~ 10 ion-pair cm-3) is the ionization rate The general behavior of the Gibbs free energy change associated with the formation of a ionic cluster composed of n ligands, DG0,n (Note that from n=1-10 the curves are built from actual data or quantum mechanical simulations as oppose of being derived from the classic liquid drop Thomson model). The number of ligands corresponding to the minimum on the DG0,n is plotted as a function of temperature, T, and water concentration, [H2O] (cm-3). The parameters span the entire range of the atmospheric conditions (i.e., troposphere, stratosphere and mesosphere). Where are the ion clusters nucleating in the atmosphere? The following graphs shows in blue areas where nucleation is not occurring, in green areas for which there is a finite energy barrier to nucleation and in red areas where ions nucleate freely. The plot represents 100 H3O+(H2O)n clusters. H O (H O) 190 K Note that in most of p = 100 mbar 50 the likely atmospheric 220 K 180 K situations ions are 210 K 0 in the green area or 200 K close to it. Upon 175 K condensation of 50 170 K other ligands (i.e., 100 H2SO4, HNO3, NH3, etc.) the hydrated hydronium 10 10 10 10 10 n, # of ligands per cluster ions are likely to grow and undergo ion-mediated nucleation. Furthermore massive ion can act as Changing the ionization source strength in response to solar variation (either in the form of freezing nuclei thus modifying the particle altering the GCRs flux or, in the mesosphere, by directly changing the ionization source) microphysics. changes the concentration of ion clusters throughout the atmosphere. Studies of such variations together with studies of the effects of different ligands is under way. 300 280 + 3 2 260 n 240 T (K) 220 DG0,n [kcal/mol] 1 nmin 10 200 180 100 160 0 10 150 15 14 13 140 200 12 11 10 [H2O] (cm-3) 250 9 8 300 120 T (K) 100 7 8 9 10 11 12 13 14 15 16 [H2O] (cm-3) 0 0 1 Acknowledgments: 2 3 5 10 15 4 We thank Professor Kendall Houk for his support in the implementation of the quantum mechanical simulations. This work has been funded by NASA under grant NAG1-1899. RD is also supported by the NASA Earth System Fellowship ESS/00-0000-0080.