The process by which liquid cloud droplets homogeneously crystallize into ice is still not well understood. The ice nucleation process based on the standard and classical theory of homogeneous freezing initiates within the interior volume of a cloud droplet. Current experimental data on homogeneous freezing rates of ice in droplets of supercooled water, both in air and emulsion oil samples, show considerable scatter. For example, at ؊33°C, the reported volume-based freezing rates of ice in supercooled water vary by as many as 5 orders of magnitude, which is well outside the range of measurement uncertainties. Here, we show that the process of ice nucleus formation at the air (or oil)-liquid water interface may help to explain why experimental results on ice nucleation rates yield different results in different ambient phases. Our results also suggest that surface crystallization of ice in cloud droplets can explain why low amounts of supercooled water have been observed in the atmosphere near ؊40°C. For almost 200 years, persistent (liquid) fogs have been observed at temperatures well below the frost point, and there has been a continued vigorous interest in understanding why, and how far, water droplets can supercool in the atmosphere. The presence of heterogeneous ice nuclei in supercooled water droplets has been shown to be necessary for glaciating clouds at temperatures above about Ϫ30°C (1). However, ice particle number densities in clouds below Ϫ30°C are often observed to exceed the ice nuclei number densities (2-7). This finding suggests that, in clouds, some supercooled water droplets freeze homogeneously.The conversion of supercooled water droplets and͞or droplets of aqueous salt solutions into ice below Ϫ30°C can occur anywhere in the atmosphere from the surface layer (resulting in ice fogs) (8) to the upper troposphere (in cirrus clouds) (4-6). In addition, ice freezing in the polar stratosphere has been shown to occur via a homogeneous nucleation process (9, 10). Therefore, it is important to elucidate the actual physical process by means of which clouds glaciate in the atmosphere, particularly at cold temperatures and in situations where ice nuclei become less abundant and less effective in promoting the freezing of cloud droplets into ice (1).Radiative properties of ice clouds and their subsequent effect on climate depend strongly on the size of the cloud particles (11), a property closely linked to the rate at which ice particles in the cloud nucleate and grow. In addition, the rates of chemical reactions, which occur in cloud droplets or on cloud surfaces, depend on the phase of the cloud particle (12). Thus it is important to be able to predict under what set of environmental conditions supercooled cloud droplets freeze into ice. Other significant natural processes, which are affected by the physical state of clouds, include lightning (13) and precipitation. The occurrence of lightning can increase the concentration of reactive nitrogen oxides in air, thereby affecting the rate of gas-phase chem...

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.