Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part I: Experimental results O. Stetzer1,2, O. Möhler2, R. Wagner2, S. Benz2, H. Saathoff2, H. Bunz2, and O. Indris3 1Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland 2Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany 3Max-Planck-Institut für Kernphysik, Abt. Atmosphärenphysik, Heidelberg, Germany
Abstract. The low temperature aerosol chamber AIDA was used to study the nucleation of
nitric acid dihydrate (NAD) in super-cooled nitric acid aerosols
under simulated stratospheric conditions in the temperature range 192 K–197 K.
The nucleating solution droplets had median diameters between 225 and 290 nm
and molar fractions of nitric acid between 0.26 and 0.28.
Nucleation of solid particles was unambiguously observed in
two out of three experiments during time periods of up to five hours.
The newly formed crystals could be clearly distinguished from the remaining
liquid droplets by their increasing size with an optical particle spectrometer.
The solid particles could be unequivocally identified as strongly aspherical
nitric acid dihydrate crystals (α-NAD) by in-situ FTIR-spectroscopy.
From our experimental data set there is no indication of direct nucleation of NAT or
a conversion of NAD into NAT while having saturation ratios with respect to
NAT of about 20–26.
The temporal evolutions of the NAD particle concentrations were
used to derive individual nucleation rates for NAD.
The measured volume nucleation rates ranged from
3.9×105 cm−3 s−1 at 195.8 K
and XNA=0.27 to
1.9×107 cm−3 s−1 at 192.1 K and
The corresponding hypothetical surface nucleation rates of
2×100 to 1×102 cm−2 s−1
are smaller than the parameterization
of Tabazadeh et al. (2002) by factors between 25 and >103.
Citation: Stetzer, O., Möhler, O., Wagner, R., Benz, S., Saathoff, H., Bunz, H., and Indris, O.: Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part I: Experimental results, Atmos. Chem. Phys., 6, 3023-3033, doi:10.5194/acp-6-3023-2006, 2006.