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Aerosol-Induced Lightning Activation in Thunderclouds Andrei P. Sommer* Central Institute of Biomedical Engineering, University of Ulm, Albert Einstein Allee 47, 89081 Ulm, Germany Received February 11, 2002. In Final Form: April 23, 2002 There are a large number of observations describing aerosol-induced lightning activation in clouds. However, little is known on the precise mechanism aerosols have on atmospheric electrification processes. Aerosols could modify the topography of ice crystals grown by riming, generating nanoscopic curvature asymmetries between colliding ice particle surfaces. By application of the FERMIC (free energy reduction by molecular interface crossing) model, the curvature asymmetries and their effect on mass and charge transfer could be analyzed. Operating prior to contact, the model predicts an elevated transfer of mass and charge when solid aerosols are incorporated in the water drops collected by ice crystals in thunderclouds.
Introduction Aerosols could strongly influence climate development, locally and globally. The most prominent effects induced by aerosols are rain suppression1 and enhancement of lightning activity,2,3 both over polluted areas. As cloud condensation nuclei (CCN), aerosols primarily increase the total number of small droplets that carry more water high into the cloud and limit formation of larger drops required for precipitation.4 In contrast to the physicochemical mechanisms responsible for aerosol-induced rain suppression,5 the effects by which aerosols could boost lightning activity are not very well understood. By exploitation of first principles, it has been shown how the free energy reduction by molecular interface crossing (FERMIC) mechanism,6 a transport mechanism operating prior to contact and generated by thermodynamic instabilities at curvature asymmetries between proximal cloud particle surfaces, could be complementarily implemented in existing atmospheric electrification concepts.7-11 Both the estimated order and the calculated sign of the charge transferred between ice crystals and graupel pellets were * E-mail:
[email protected]. (1) Rosenfeld, D. Suppression of Rain and Snow by Urban and Industrial Air Pollution. Science 2000, 287, 1793. (2) Orville, R. E.; Huffines, G.; Nielsen-Gammon, J.; Zhang, R.; Ely, B.; Steiger, S.; Phillips, S.; Allen, S.; Read, W. Enhancement of Cloudto-Ground Lightning over Houston, Texas. Geophys. Res. Lett. 2000, 28, 2597. (3) Lyons, W. A.; Nelson, T. E.; Williams, E. R.; Cramer, J. A.; Turner, T. R. Enhanced Positive Cloud-to-Ground Lightning in Thunderstorms Ingesting Smoke from Fires. Science 1998, 282, 77. (4) Crutzen, P. J.; Andreae, M. O. Biomass Burning in the Tropics: Impact on Atmospheric Chemistry and Biogeochemical Cycles. Science 1990, 250, 1669. (5) Ramanathan, V.; Crutzen, P. J.; Kiehl, J. T.; Rosenfeld, D. Aerosols, Climate, and the Hydrological Cycle. Science 2001, 294, 2119. (6) Sommer, A. P.; Ro¨hlke, W.; Franke, R. P. Free Energy Reduction by Molecular Interface Crossing: Novel Mechanism for the Transport of Material Across the Interface of Nanoscale Droplets Induced by Competing Intermolecular Forces for Application in Perfluorocarbon Blood Substitutes. Naturwissenschaften 1999, 86, 335. (7) Williams, E. R. The Electrification of Thunderstorms. Sci. Am. 1988, Nov, 88. (8) Saunders, C. P. R. A Review of Thunderstorm Electrification Processes. J. Appl. Meteorol. 1993, 32, 642. (9) Saunders, C. P. R.; Avila, E. E.; Peck, S. L.; Castellano, N. E.; Aguirre Varela, G. G. A laboratory study of the effects of rime ice accretion and heating on charge transfer during ice crystal/graupel collisions. Atmos. Res. 1999, 51, 99. (10) Vonnegut, B. Jovian Lightning After Comet Impacts? Science 1995, 268, 1829. (11) Baker, M. B.; Dash, J. G. Mechanism of charge transfer between colliding ice particles in thunderstorms. J. Geophys. Res. 1994, 99, 10621.
in agreement with data from laboratory experiments and values expected from real processes in thunderclouds.12 Here, it is attempted to extend the application of the FERMIC model to the theoretical description of aerosolinduced lightning activation in thunderclouds (AILAT). Due to the small cross section of the aerosols suspended in the atmosphere, the likelihood for direct collisions between aerosols and cloud particles could be regarded as negligible. However, water vapor condensing on aerosol surfaces could create a large number of microscopic supercooled water drops containing soluble and insoluble CCN, which in turn could collide with ice crystals, freezing on their surface. In this simple model of growth by riming via heterogeneous nucleation, ice crystals will show surface structures corresponding to the morphology of the collected droplets deposited onto their surface. In case of a high concentration of solid aerosols, high ice crystal concentrations, and a retarded freezing process, the CCN granules could dominate the topography of the ice crystal surface: Especially at elevated cloud temperatures, the collected water drops attached to the ice crystal surface may uniformly disperse. As a result, high-curvature aerosols carrying a liquidlike layer would directly participate in interfacial mass and charge-transfer processes. Probably we could assume the presence of a liquidlike water layer on the surfaces exchanging mass and charge. There is clear experimental evidence for the existence of a liquidlike nanolayer on solid surfaces exposed to atmospheric humidity, including ice 38 °C below zero, and local surface melting.13-17 Supposing the aerosols concentrated close to their source,18 the ascending ice crystals may collide with the nucleated cloud droplets containing solid aerosols prior to the graupel. Thus, ice crystal growths by CCNinduced riming will presumably increase the number of high-curvature zones on the ice crystal surface. Consider(12) Sommer, A. P.; Levin, Z. Charge transfer in convective thunderclouds induced by molecular interface crossing and free energy reduction. Atmos. Res. 2001, 58, 129. (13) Do¨ppenschmidt, A.; Butt, H. J. Measuring the Thickness of the Liquid-Like Layer on Ice Surfaces with Atomic Force Microscopy. Langmuir 2000, 16, 6709. (14) Sommer, A. P.; Franke, R. P. Hydrophobic optical elements for near-field optical analysis (NOA) in liquid environment - a preliminary study. Micron 2002, 33, 227. (15) Holden, C. Ice’s Tricky Surface. Science 1998, 280, 385. (16) Wettlaufer, J. S.; Dash, J. G. Melting Below Zero. Sci. Am. 2000, Feb, 34. (17) Baker, M. B.; Dash, J. G. Charge transfer in thunderstorms and the surface melting of ice. J. Cryst. Growth 1989, 97, 770. (18) Toon, O. B. How Pollution Suppresses Rain. Science 2000, 287, 1763.
10.1021/la025619o CCC: $22.00 © 2002 American Chemical Society Published on Web 05/31/2002
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Figure 1. Calibration particles (60 ( 2.5 nm diameter) attached to the tapered part of a fiber optic sensor. Nanoparticles and fiber are homogeneously coated by a thin polymer film (bar ) 1 µm).
ing that aerosols have diameters ranging from a few nanometers to several micrometers,19 the topographic effect of solid aerosols deposited on ice crystal dendrites could be simulated by dispersing calibration particles (60 ( 2.5 nm) in a polymer fluid: By immersion of the tapered part of a fiber optic sensor into the fluid, the arrangement of the solid nanoparticles on the surface of the quartz fiber could be visualized by scanning electron microscopy (SEM), demonstrated in Figure 1. Application of the FERMIC Model For differently curved liquid drops or liquid-coated solid surfaces in interfacial proximity, it was possible to derive a mechanism pumping molecules and ions from highcurvature surfaces to low-curvature surfaces.6 In the simplest formulation, maximum molecular instability (fluctuations, probable molecular transfer) is expected to occur at the point of contact where surface molecules are attracted anisotropically and by a lesser number of closest neighbors than in the bulk liquid. Similar concepts are applied in calculating total intermolecular interactions and surface tensions in liquids. It can be easily realized that the coordination number of a surface molecule decreases as the curvature of the surface increases. Denoting the difference in the interaction energies of two proximal surface molecules belonging to two curved surfaces of radius R1 and R2 by ∆E0s ) Es01 - Es02, it can be shown that this energy can be described by the 2D analogue of the 3D model of a spherical object of radius R (Figure 2), where the range of the attractive intermolecular forces centered around a surface molecule is rw. The 2D model gives ∆E0s ) (1/4π)zsrwWAA[(1/R2) - (1/R1)], where WAA denotes the pair interaction energy and zs is the coordination number of a surface molecule at the plane liquid surface.6 This equation has been experimentally verified for nanoscale liquid drops in interfacial contact. By noting that the pair interaction energy WAA is a negative energy, it is clear that the interaction energy ∆E0s becomes negative for R1 > R2. Following the definition of ∆E0s , a negative ∆E0s is equivalent to ∆Es02 > ∆Es01. Thus, molecules at the surface of the smaller object should be less stable, having a higher probability for fluctuations than the corresponding surface molecules from the larger object. For two curved surfaces approaching each other to distances of the order of magnitude of rw , R1, R2, the (19) Schwartz, S. E.; Buseck, P. R. Absorbing Phenomena. Science 2000, 288, 989.
Figure 2. A schematic representation of the asymmetric spatial distribution of molecules affected by interfacial forces at curved, liquid, or liquid-coated surfaces. As can be seen in the 3D model, only three out of eight equidistantly distributed molecules (red spheres) are located inside the arc limiting the larger sphere (blue).
equation predicts the transfer of molecules from the smaller to the larger object. This is because the system tends to reduce its potential energy through its evolution toward the highest possible thermodynamic stability. Thus, with more numerous high-curvature zones covering the ice crystal surfaces, augmenting curvature asymmetries existing between ice crystals and graupel particles, there could also be a higher probability for transferring molecules and ions from ice crystals to graupel particles. The transfer is expected to occur prior to contact.12 Differences in topography and an additional temporal asymmetry assumed to occur in the lifetime of H3O+ and OH- ions in water20 could favor the transfer of negative ions from the ice crystal to the graupel. This picture emerges from associating the asymmetric lifetimes of the ions with their mean free path, suggesting that there is a higher probability for the charged molecule with the longer lifetime (OH-) to reach the graupel surface than for a molecule carrying a positive charge (H3O+). The resulting charge distribution is in agreement with observations in nature in which most frequently the lower (containing larger particles) and upper (containing smaller ice crystals) cloud charge centers are negatively and positively charged, respectively. The FERMIC model predicts that the presence of solid aerosols attached to ice particle surfaces could be accompanied by an enhanced charging and an increased lightning activity in thunderclouds. The model also predicts that the charging could be more pronounced at elevated cloud temperatures. This picture is in agreement with observations in the clouds over the Brazilian Amazon and over Houston, TX,21 where aerosols from burning vegetation and fossil fuel, including petroleum refineries, are the major air pollution sources, respectively. The physicochemical nature and the chemical composition of the aerosol particles could influence processes in the atmosphere by stimulating the condensation mechanisms in the cloud. In particular, the surface polarity could determine the preference for vapor deposition, while soluble components are likely to have a sensitive impact on the condensation process22 and on the speed at which collected water could freeze. Surface-active compounds, lowering the surface tension of the wetted (20) Halle, B.; Karlstro¨m, G. Prototropic Charge Migration in Water. J. Chem. Soc., Faraday Trans. 1983, 79, 1047. (21) Cole, S. Bright Sky, Dirty City. Sci. Am. 2001, May, 14.
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aerosols, have been supposed to initiate the formation of a larger number of cloud drops of smaller size.23 Both the composition and the size of the aerosols are important parameters which could be critical for the formation and the growth of cloud drops and ice crystals in the cloud,24 implicitly influencing the transfer of OH- and hydrated protons between the hydrometeors. Conclusions By realizing that all the physical and chemical properties of aerosols, including the environmental conditions, could practically equally influence atmospheric processes, it is clear that reliable large-scale extrapolations of aerosol(22) Levin, Z.; Ganor, E.; Gladstein, V. The Effects of Desert Particles Coated with Sulfate on Rain Formation in the Eastern Mediterranean. J. Appl. Meteorol. 1996, 35, 1511. (23) Rhode, H. Atmospheric chemistry: Clouds and climate. Nature 1999, 401, 223. (24) Yin, Y.; Levin, Z.; Reisin, T. G.; Tzivion, S. The effects of giant cloud condensation nuclei on the development of precipitation in convective clouds - a numerical study. Atmos. Res. 2000, 53, 91.
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induced phenomena require extended models and additional data. Recent advances in atomic force microscopy and related optical technologies facilitating operations in water and on liquid layers on ice could provide valuable information regarding the relevant topographic situation on ice surfaces with differently sized aerosols attached. Similarly, imaging optical differences between chemically different aerosol particles attached to ice surfaces via nearfield optical analysis, a method allowing the simultaneous mapping of associated topographies, could be beneficial for a better understanding of the cloud microphysical processes. This should motivate further implementation of nanoscale imaging techniques25 in atmospheric sciences and could represent a major challenge to the experimental side. LA025619O (25) Sommer, A. P.; Franke, R. P. Near-Field Optical Analysis of Living Cells in Vitro. J. Proteome Res. 2002, 1, 111.
