Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling
Activation energies Δ<i>G</i><sub>act</sub> for the nucleation of nitric acid dihydrate (NAD) in supercooled binary HNO<sub>3</sub>/H<sub>2</sub>O solution droplets were calculated from volume-based nucleation rate measurements using the AIDA...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2006-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/6/3035/2006/acp-6-3035-2006.pdf |
| Summary: | Activation energies Δ<i>G</i><sub>act</sub> for the nucleation of nitric acid dihydrate (NAD) in supercooled binary HNO<sub>3</sub>/H<sub>2</sub>O solution droplets were calculated from volume-based nucleation rate measurements using the AIDA (Aerosol, Interactions, and Dynamics in the Atmosphere) aerosol chamber of Forschungszentrum Karlsruhe. The experimental conditions covered temperatures T between 192 and 197 K, NAD saturation ratios <i>S</i><sub>NAD</sub> between 7 and 10, and nitric acid molar fractions of the nucleating sub-micron sized droplets between 0.26 and 0.28. Based on classical nucleation theory, a new parameterisation for Δ<i>G</i><sub>act</sub>=<i>A</i>×(<i>T</i> ln <i>S</i><sub>NAD</sub>)<sup>−2</sup>+<i>B</i> is fitted to the experimental data with <i>A</i>=2.5×10<sup>6</sup> kcal K<sup>2</sup> mol<sup>−1</sup> and <i>B</i>=11.2−0.1(T−192) kcal mol<sup>−1</sup>. <i>A</i> and <i>B</i> were chosen to also achieve good agreement with literature data of Δ<i>G</i><sub>act</sub>. The parameter <i>A</i> implies, for the temperature and composition range of our analysis, a mean interface tension σ<sub><i>sl</i></sub>=51 cal mol<sup>−1</sup> cm<sup>−2</sup> between the growing NAD germ and the supercooled solution. A slight temperature dependence of the diffusion activation energy is represented by the parameter <i>B</i>. Investigations with a detailed microphysical process model showed that literature formulations of volume-based (Salcedo et al., 2001) and surface-based (Tabazadeh et al., 2002) nucleation rates significantly overestimate NAD formation rates when applied to the conditions of our experiments. |
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| ISSN: | 1680-7316 1680-7324 |