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

International audienceActivation energies ?Gact 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 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 ?Gact=A×(T lnSNAD)-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 ?Gact. 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

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