MICROPHYSICS AND HETEROGENEOUS CHEMISTRY OF POLAR STRATOSPHERIC CLOUDS

Liquid and solid particles in polar stratospheric clouds are of central importance for the depletion of stratospheric ozone. Surface-catalyzed reactions on these particles, and diffusion-controlled processes in the bulk of the particles, convert halogens, which derive from compounds of mainly anthropogenic origin, from relatively inert reservoir species into forms that efficiently destroy ozone. The microphysics of these particles under cold stratospheric conditions is still uncertain in many respects, in particular concerning phase transitions such as freezing nucleation and deposition nucleation. Furthermore, there are indications that the rates of key heterogeneous reactions have not yet been established with sufficient accuracy to enable a reliable diagnosis of observed ozone losses by means of global models. The present paper reviews the current (late 1996) knowledge of the physico-chemistry of polar stratospheric clouds and evaluates the remaining uncertainties with respect to their ozone depletion potential.

Chemical Loss of Ozone in the Arctic Polar Vortex in the Winter of 1991-1992

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

Heterogeneous Atmospheric Reactions: Sulfuric Acid Aerosols as Tropospheric Sinks

The collisional reaction probabilities of several atmospheric species on bulk sulfuric acid surfaces indicate that heterogeneous processes may be important in tropospheric chemistry.

Volcanically Related Secular Trends in Atmospheric Transmission at Mauna Loa Observatory, Hawaii

Twenty years of atmospheric transmission data from Mauna Loa Observatory show secular decreases at irregular intervals. In addition, a regular annual variation is present during unperturbed as well as perturbed periods. These variations in transmission can be measured to a few tenths of a percent from the data record. Transient decreases in transmission are strongly correlated with explosive volcanic eruptions that inject effluent into the stratosphere. Recovery from these ejections takes as much as 8 years and the recovery curve is linear. Observations in 1977 at Mauna Loa show that, for the first time since the Mount Agung eruption in 1963, the atmospheric transmission of direct-incidence solar irradiation at Mauna Loa returned to values measured in 1958 to 1962.