Review
Polar stratospheric cloud microphysics and chemistry

https://doi.org/10.1016/j.jastp.2007.09.011Get rights and content

Abstract

The solid and liquid particles which constitute polar stratospheric clouds (PSCs) are of manifold importance to the meteorology of the stratosphere. The heterogeneous reactions which take place on and within these particles release halogens from relatively inert reservoir species into forms which can destroy ozone in the polar spring. In addition, solid PSC particles are instrumental in the physical removal of nitrogen oxides (denitrification) and water (dehydration) of regions of the polar stratosphere. Denitrification, in particular, allows extended ozone destruction by slowing the conversion of chlorine radicals back into reservoir species.

We review the historical development of PSC studies, with particular emphasis on results from the last decade, encompassing developments in observations, in laboratory experiments, and in theoretical treatments. The technical challenge of measuring sufficient of the parameters describing any given PSC, to allow its microphysics to be understood, has driven forward balloon-borne, aircraft, and satellite instrumentation. The technical challenge of finding suitable laboratory proxies for PSCs, in order to observe the microphysics under controlled conditions, has resulted in a wide variety of experimental designs, some of which maximise the probability of observing phase change, others which mimic the surface–volume ratios of PSCs more closely. The challenge to theory presented by PSCs has resulted in improvements in the thermodynamics of concentrated inorganic solutions of volatile compounds, and a new general theory of freezing of water ice from concentrated aqueous solutions. Of the major processes involving PSCs, heterogeneous reaction probabilities for ternary HNO3/H2SO4/H2O solutions, and heterogeneous freezing to produce nitric-acid hydrates, are the least well understood.

Section snippets

Historical overview

The first polar stratospheric clouds (PSCs) observed were the beautiful nacreous, or mother-of-pearl, clouds which are quite commonly visible above the mountains of Scandinavia and the Antarctic peninsula, but are also occasionally seen in the middle latitudes (Stanford and Davis, 1974; simply type “nacreous cloud” into an internet search engine for recent images). For example, as pointed out to us by an anonymous reviewer, PSCs were recorded in 1901 by the Danish painter Jorgensen. By the

Fieldwork instrumentation

The data used for PSC studies range from retrieved atmospheric composition over synoptic scales by satellite-mounted instruments to the in-situ measurement of particle compositions and gas-phase mixing ratios by balloon and aircraft mounted instruments. Most aspects of the stratosphere, from air temperature and the mixing ratios of individual components, to the size, number, and composition of aerosol particles, can be studied. However, it is not always possible to measure all the atmospheric

Particle compositions

Since the discovery of PSCs, a number of different types of particles have been postulated as being important in the formation and lifecycles of these aerosols. Observations of PSCs have been split into Types Ia, Ib, and II based on classifications of lidar measurements (Poole and McCormick, 1988, Toon et al., 1990). However, these divisions are based on the optical parameters of the aerosols, which, while providing an idea of typical shapes and sizes of particles, contain very little

The effects of PSCs

Soon after the discovery of the ozone hole PSCs were found to be important both for aiding the release of active chlorine species as well as extending the lifetimes of these species by removing HNO3 (Toon et al., 1986, Solomon et al., 1986, Solomon, 1990, Solomon, 1999).

Below we review how the microphysical properties of the PSCs determine the magnitude and extent of these effects, and how the microphysical properties are, in turn, dependent on the climate of the polar vortex. The Antarctic

Summary

To summarise: (1) in situ instrumentation has improved to the point where we can measure the amounts of the major constituents of the condensed phase and their ratios—allowing for the positive identification of STS and NAT particles in the Arctic stratosphere. (2) Satellite instruments can now be used to identify the category of PSCs as well as their synoptic extent—improving the temporal and spatial coverage of observations. (3) The major types of PSC particles have been identified and

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