Laboratory evidence for surface nucleation of solid polar stratospheric cloud particles

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.