Homogeneous nucleation of NAD

Activation energies ∆G act for the nucleation of nitric acid dihydrate (NAD) in super-cooled binary HNO 3 /H 2 O solution droplets were calculated from volume-based nucle-ation rate measurements using the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) aerosol chamber of Forschungszentrum Karlsruhe. The experimental 5 conditions covered temperatures T between 192 K and 197 K, NAD saturation ratios S 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 ∆G act =A×(T ln S NAD) −2 + B is fitted to our experimetnal data with A=2.5×10 6 kcal K 2 mol −1 and B=11.2−0.1(T −192) kcal mol −1. A and B were chosen 10 to also achieve good agreement with literature data of ∆G act. The parameter A implies a constant interfacial tension σ sl =51 cal mol −1 cm −2 between the growing NAD germ and the supercooled solution. A slight temperature dependence of the diffusion activation energy is represented by the parameter B. Investigations with a detailed micro-physical process model showed that literature formulations of volume-based (Salcedo 15 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.