Laboratory Evidence for Surface Nucleation of Solid Polar Stratospheric Cloud Particles

Abstract

Nitric acid-containing cloud particles, known as polar stratospheric clouds, play an important role in the springtime ozone destruction over the polar regions. Nitric acid initially condenses in the polar stratosphere to form supercooled solution droplets of mainly nitric acid and water with trace amounts of sulfuric acid. Nitric acid dihydrate (NAD) and nitric acid trihydrate (NAT) later crystallize from this supercooled solution phase to form solid polar stratospheric cloud particles. Until now, experimental data on this crystallization process has been analyzed under the assumption that NAD and NAT nucleation took place in the interior volume of a cloud droplet. However, in this paper, reanalysis of experimental data on the homogeneous freezing rates of concentrated aqueous nitric acid solution droplets provides substantial support for the occurrence of nucleation "pseudoheterogeneously" at the air-aqueous nitric acid solution interface of the droplet. Furthermore, in a following paper, theory that provides compelling evidence for such interfacial nucleation is developed. Together, the reanalysis of laboratory data in this paper and the supporting theoretical arguments in the following paper suggest that the homogeneous nucleation process occurring in atmospheric droplets may be a surface-rather than a volume-related rate process.

A 3D-CTM with detailed online PSC-microphysics: analysis of the Antarctic winter 2003 by comparison with satellite observations

“…Finally it is interesting to mention the actual freezing rates we have found in the simulations, using the rates of Tabazadeh et al (2002) reduced by 100. The highest values we obtain are of order ∼10 m −2 s −1 × surface area.…”

“…homogeneous volume dependent nucleation of ICE out of STS 3-4 K below the ice frost point temperature T ice (Koop et al, 2000), and homogeneous surface dependent nucleation of nitric acid dihydrate (NAD) out of STS (Tabazadeh et al, 2002) above T ice , followed by (assumed) instantaneous conversion of NAD to NAT. The NAD homogeneous nucleation rates have been reduced by a factor of 100 following recent studies (Knopf et al, 2002;Voigt et al, 2005;Höpfner et al, 2006b).…”

“…However, we cannot say if this occurs through heterogeneous nucleation, through NAD and (rapid) conversion to NAT, or directly to NAT. We use the parametrisation of Tabazadeh et al (2002) with the applied corrections because this is presently the only available parametrisation, based on laboratory measurements. The fact that MIPAS does not see NAD, as found by Höpfner et al (2006b), could be due either to the absence of NAD formation, or to a very fast transition from NAD to NAT -too quick to be detected.…”

“…Besides ice particles, which freeze out of STS at very low temperatures, nitric acid trihydrate (NAT) has progressively been regarded as a second important type of solid PSC particle (Voigt et al, 2000;Fahey et al, 2001). Not only is NAT a remnant of evaporation of ice particles, but recent research indicates that NAT can also form out of metastable nitric acid dihydrate (NAD) which may freeze out of STS at temperatures above the ice frost point (Tabazadeh et al, 2002). This means that NAT may nucleate and grow at higher temperatures than ice.…”

Does the Homogeneous Ice Nucleation Initiate in the Bulk Volume or at the Surface of Super-Cooled Water Droplets? A Review

“…Tabazadeh et al [43] re-analyzed the experimental data on the homogeneous freezing rates of concentrated aqueous nitric acid solution droplets, which constitute polar stratospheric clouds. They found that surface-based rate expressions are consistent with all the considered laboratory data [44]- [48] and that the homogeneous nucleation process occurring in atmospheric droplets may be a surface-rather than a volume-related rate process.…”