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Activation energies ΔGact for the nucleation of nitric aciddihydrate (NAD) in supercooled binary HNO3/H2O solution dropletswere calculated from volume-based nucleation rate measurements using the AIDA(Aerosol, Interactions, and Dynamics in the Atmosphere) aerosol chamber ofForschungszentrum Karlsruhe. The experimental conditions covered temperaturesT between 192 and 197 K, NAD saturation ratios SNAD between7 and 10, and nitric acid molar fractions of the nucleating sub-micronsized droplets between 0.26 and 0.28. Based on classical nucleationtheory, a new parameterisation for ΔGact=A×(T ln SNAD)−2+B is fitted to the experimental data with A=2.5×106 kcal K2 mol−1 and B=11.2−0.1(T−192) kcal mol−1.A and B were chosen to also achieve good agreement with literature dataof ΔGact. The parameter A implies, for the temperature andcomposition range of our analysis, a mean interface tensionσsl=51 cal mol−1 cm−2 between the growing NAD germ andthe supercooled solution. A slight temperature dependence of the diffusionactivation energy is represented by the parameter B. Investigations with adetailed microphysical process model showed that literature formulations ofvolume-based (Salcedo et al., 2001) and surface-based (Tabazadeh et al., 2002) nucleation ratessignificantly overestimate NAD formation rates when applied to the conditionsof our experiments

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