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

O. Möhler (1) , H. Bunz (1) , O. Stetzer (1)

1 Forschungszentrum Karlsruhe

Activation energies ?G_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 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 lnS_NAD)^-2+B is fitted to our experimetnal 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 data of ?G_act. The parameter A implies a constant interfacial tension ?_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 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.

Domaines: Océan, Atmosphère

O. Möhler, H. Bunz, O. Stetzer. Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions ? Part II: Modelling. Atmospheric Chemistry and Physics Discussions, 2006, 6 (2), pp.2119-2149. ⟨hal-00301081⟩