Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling O. Möhler, H. Bunz, and O. Stetzer Forschungszentrum Karlsruhe, Institute for Meteorology and Climate Research (IMK-AAF), Germany
Abstract. Activation energies ΔGact 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 SNAD 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 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 data
of ΔGact. The parameter A implies, for the temperature and
composition range of our analysis, a mean interface tension
σsl=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.
Citation: Möhler, O., Bunz, H., and Stetzer, O.: Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling, Atmos. Chem. Phys., 6, 3035-3047, doi:10.5194/acp-6-3035-2006, 2006.