Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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6
2
2006
10.5194/acpd-6-2119-2006
http://www.atmos-chem-phys-discuss.net/6/2119/2006/
http://www.atmos-chem-phys-discuss.net/6/2119/2006/acpd-6-2119-2006.html
http://www.atmos-chem-phys-discuss.net/6/2119/2006/acpd-6-2119-2006.pdf
2119
2149
2006-03-28
Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling
O. Möhler
H. Bunz
O. Stetzer
Forschungszentrum Karlsruhe, Institute for Meteorology and Climate Research (IMK-AAF), Germany
Activation energies Δ<i>G</i><sub>act</sub> for the nucleation of nitric acid
dihydrate (NAD) in supercooled binary HNO<sub>3</sub>/H<sub>2</sub>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 K and 197 K, NAD saturation ratios <i>S</i><sub>NAD</sub>
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 Δ<i>G</i><sub>act</sub>=<i>A×(T </i>ln<i>S</i><sub>NAD</sub>)<sup>-2</sup>+<i>B</i>
is fitted to our experimetnal data with <i>A</i>=2.5×10<sup>6</sup> kcal K<sup>2</sup> mol<sup>-1</sup>
and <i>B</i>=11.2−0.1(<i>T</i>−192) kcal mol<sup>-1</sup>.
<i>A</i> and <i>B</i> were chosen to also achieve good agreement with literature data
of Δ<i>G</i><sub>act</sub>. The parameter <i>A</i> implies a constant interfacial tension
σ<sub><i>sl</i></sub>=51 cal mol<sup>-1</sup> cm<sup>-2</sup> 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.