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

Abstract

Abstract. Activation energies G act for the nucleation of nitric acid dihydrate (NAD) in supercooled binary HNO 3 /H 2 O 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 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 for G act =A×(T ln S NAD ) −2 +B is fitted to the experimental 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 to also achieve good agreement with literature data of G act . The parameter A implies, for the temperature and composition range of our analysis, a mean interface 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 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 I: Experimental results

“…This finding is also supported by simulations of our experiments using a microphysical process model. The description and the results of these simulations where different parameterizations are compared to experimental data is the subject of the second part of this paper (Möhler et al, 2006). The findings in this study complement the results by Duft and Leisner (2004) where droplets with diameters of 40 µm and larger were used to study the homogeneous nucleation of ice.…”

“…To overcome this difficulty the discussion of results can be moved from nucleation rates to the Gibbs free energies G act of critical cluster formation. This discussion is done thoroughly in the companion paper (Möhler et al, 2006). The surface nucleation rates which we obtained from our measurements are compared with the theory discussed in Tabazadeh et al (2002).…”

“…Note that the vast majority of these data were measured below 190 K. Because of the complex and non-linear relation between nucleation rates, temperature and molar composition of the nucleating droplets a better approach to compare our results with literature data is to deploy nucleation theory and use the relation between temperature and the Gibbs free energy of activation G act . This is done thoroughly in the second part of this publication (Möhler et al, 2006) and is thus not discussed here. However, the hypothetical surface based nucleation rates we derive from our measurements do not agree with parameterizations published by Tabazadeh et al (2002).…”

“…Since the TDL spectrometer failed to operate in experiment E1, the vapor concentrations from the model simulation are taken instead (cf. companion paper Möhler et al, 2006). Due to losses of the trace gases to the chamber walls the total aerosol mass decreases with time (panel d) since equilibrium vapor pressures are reestablished continuously by the evaporation from the aerosol particles.…”

Ice structures, patterns, and processes: A view across the icefields

“…In contradiction to CNT, Tabazadeh et al (2002) suggested that homogeneous nucleation is initiated close to surfaces. There is an ongoing debate on the validity of these mechanisms and the currently available data may not be sufficiently detailed to distinguish between the two hypotheses (Möhler et al, 2006a;Stetzer et al, 2006;Tabazadeh, 2005).…”