Index Geophysics

Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling

Activation energies &Delta;<i>G</i>_act for the nucleation of nitric acid dihydrate (NAD) in supercooled binary HNO₃/H₂O 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>_NAD 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 &Delta;<i>G</i>_act=<i>A</i>&times;(<i>T</i> ln <i>S</i>_NAD)^&minus;2+<i>B</i> is fitted to the experimental data with <i>A</i>=2.5&times;10⁶ kcal K² mol^&minus;1 and <i>B</i>=11.2&minus;0.1(T&minus;192) kcal mol^&minus;1. <i>A</i> and <i>B</i> were chosen to also achieve good agreement with literature data of &Delta;<i>G</i>_act. The parameter <i>A</i> implies, for the temperature and composition range of our analysis, a mean interface tension &sigma;_<i>sl</i>=51 cal mol^&minus;1 cm^&minus;2 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.

Möhler, O.

Bunz, H.

Stetzer, O.

2018-06-28

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10.5194/acp-6-3035-2006

https://acp.copernicus.org/articles/6/3035/2006/

eISSN: 1680-7324

eng