Atmospheric Ion Clusters: Properties and Size Distributions
Abstract
Ions are continuously generated in the atmosphere by the action of galactic cosmic radiation. Measured charge concentrations are of the order of 103 ~ {cm-3} throughout the troposphere, increasing to about 5 x 103 ~ {cm-3} in the lower stratosphere [Cole and Pierce, 1965; Paltridge, 1965, 1966]. The lifetimes of these ions are sufficient to allow substantial clustering with common trace constituents in air, including water, nitric and sulfuric acids, ammonia, and a variety of organic compounds [e.g., D'Auria and Turco, 2001 and references cited therein]. The populations of the resulting charged molecular clusters represent a pre-nucleation phase of particle formation, and in this regard comprise a key segment of the over-all nucleation size spectrum [e.g., Castleman and Tang, 1972]. It has been suggested that these clusters may catalyze certain heterogeneous reactions, and given their characteristic crystal-like structures may act as freezing nuclei for supercooled droplets. To investigate these possibilities, basic information on cluster thermodynamic properties and chemical kinetics is needed. Here, we present new results for several relevant atmospheric ion cluster families. In particular, predictions based on quantum mechanical simulations of cluster structure, and related thermodynamic parameters, are compared against laboratory data. We also describe a hybrid approach for modeling cluster sequences that combines laboratory measurements and quantum predictions with the classical liquid droplet (Thomson) model to treat a wider range of cluster sizes. Calculations of cluster mass distributions based on this hybrid model are illustrated, and the advantages and limitations of such an analysis are summarized. References: Castelman, A. W., Jr., and I. N. Tang, Role of small clusters in nucleation about ions, J. Chem. Phys., 57, 3629-3638, 1972. Cole, R. K., and E. T. Pierce, Electrification in the Earth's atmosphere for altitudes between 0 and 100 kilometers, J. Geophys. Res. 70, 2735-2749, 1965. D'Auria, R. and R. P. Turco, Ionic clusters in the polar winter stratosphere, Geophys. Res. Lett., 28, 3871-3874, 2001. Paltridge, G. W., Experimental measurements of the small-ion density and electrical conductivity of the stratosphere, J. Geophys. Res. 70, 2751,2761, 1965. Paltridge, G. W., Stratospheric small-ion density measurements from a high-altitude jet aircraft, J. Geophys. Res. 71, 1945-1952, 1966.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.A71F..10D
- Keywords:
-
- 0305 Aerosols and particles (0345;
- 4801);
- 0335 Ion chemistry of the atmosphere (2419;
- 2427);
- 0340 Middle atmosphere: composition and chemistry;
- 0355 Thermosphere: composition and chemistry