Ion clusters in the lower stratosphere and upper troposphere
Abstract
Ions are continuously generated in the atmosphere under the action of galactic cosmic rays. Strong electrostatic interactions between the ions and surrounding neutral molecules lead to the formation of large ionic clusters. Recent measurements by Arnold and coworkers [ Eichkorn et al., 2002] by means of airborne ion mass spectrometry reveal the existence of positive ion cluster families reaching masses of up to 2500 amu in the upper troposphere. Ion clusters have also been detected in the stratosphere and throughout the troposphere [ e.g., Arnold et al., 1982; Eisele, 1988]. In all cases, positive ion families containing hydronium ions clustered with water and other trace gases such as acetone, acetonitrile and ammonia are detected. Negative ion cluster families are found consisting mainly of nitrate anions clustered with water and nitric acid, and sulfate anions associated with sulfuric acid and nitric acid ligands, with various degrees of hydration [e.g., Arnold et al., 1977]. Evidence also exists for very massive ions [Eichkorn et al., 2002; Hõrrak et al., 1998]. Ion clustering could play a role in aerosol formation through ion mediated nucleation [Turco and al., 1998; Yu and Turco, 2000; 2001], and could affect the microphysics of freezing in clouds. We have developed a ``hybrid'' model that characterizes the distributions of ionic clusters based on their structure and thermodynamics [D'Auria and Turco, 2001; 2002]. Here, we present results that describe the interaction of nitric acid vapor with hydronium/water clusters. We show, consistent with previous laboratory analyses [Zhang et al., 1994; Gilligan and Castleman, 2001], that the uptake of nitric acid on such clusters is a selective phenomenon, depending on the degree of cluster hydration. The clusters appear to take up nitric acid preferentially in ratios of nitric acid to water similar to those of solid nitric acid hydrates (particularly the trihydrate). It has been speculated that such stable nitric acid/water embryos can act as freezing nuclei for supercooled polar stratospheric clouds [Hamill and Turco, 2000; D'Auria and Turco, 2001]. Is likely that similar processes occur throughout the troposphere. We discuss the techniques used to define ion cluster properties, and potential applications of our hybrid thermodynamic model to study aerosol formation. References: begin{description
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EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....7816D