It is believed that heterogeneous chemical reactions, particularly those associated with ozone depletion, occur on stratospheric particles. The stratospheric aerosol consists of three types. Sulfate aerosols are believed to originate from transport of long-lived sulfur gases from the troposphere to the stratosphere, where they are oxidized to sulfate, and from periodic injection of volcanic emissions. A number of observations of the characteristics of the Pinatubo aerosol have been reported (e.g., Brock et al. [1993] and Pueschel et al. [1994]). The background distribution of aerosol was modified by the addition of a large particle mode which increased aerosol surface area by more than a factor of 10, although the Pinatubo cloud varied temporally and spatially. There is evidence that Pinatubo aerosols affected the characteristics of polar stratospheric clouds (PSCs) over Antarctica ( Deshler et al. [1994]), both in an increased frequency of occurrence of cloud and a higher concentration of smaller particles than observed in other years.
Polar stratospheric cloud particles comprise the other types of
stratospheric aerosol. Type II PSCs are thought to be composed
primarily of water ice. Type I PSCs are composed primarily of HNO
and water. The temperature at which they are observed to form is
consistent with a composition of nitric acid trihydrate (NAT) (
Kawa et al. [1992]), although recent evidence suggests that nitric
acid dihydrate may also form ( Worsnop et al. [1993]). A
discussion of the formation of Type I PSCs is presented in
Tabazadeh et al. [1994]. The mechanism by which these particles form
is not well understood; a key question is whether sulfuric acid nuclei
upon which the PSCs form must first freeze, or whether other
mechanisms exist. The pathways are temperature dependent and have
different selectivity. As a result, the formation of PSCs and the
surface area ultimately available for heterogeneous chemistry are
sensitive to the formation path assumed.
The increase in aerosol surface area after a major volcanic eruption
has important implications for heterogeneous chemistry ( Granier
and Brasseur [1992]). Midlatitude stratospheric ozone depletions
observed after previous eruptions had been partially attributed to
heterogeneous chemistry occurring on sulfate aerosol. Pinatubo
provided a unique opportunity for further study of correlations
between aerosol and ozone concentrations in the tropics as well as at
higher latitudes. Increases in chlorine dioxide observed over
Antarctica the year after the eruption ( Solomon et al. [1993])
were consistent with the proposed mechanisms. Comparison of
measurements of stratospheric ozone before and after the eruption of
Mt. Pinatubo showed that large ozone decreases in the tropics were
also correlated with enhanced aerosol loading between 16 and 28 km
altitude ( Grant et al. [1994]). Some of the ozone destruction
is likely attributable to heterogeneous chemistry, although radiative
perturbations to ozone photochemistry are caused by the presence of
the aerosol itself. Evidence for heterogeneous chemisty involving
nitrogen species was also found in Pinatubo-perturbed regions of the
stratosphere ( Rinsland et al. [1994]). The relevant reaction is
the conversion of N
O
into nitric acid, which has been shown to
occur on sulfuric acid surfaces. The perturbation of odd-nitrogen
partitioning will of course affect ozone, since the chemical cycles
are linked through NOx-catalyzed ozone losses.
Periodic stratosphere-troposphere exchange occurs via tropopause folding. Sassen [1992] reports an instance of liquid-phase cirrus cloud formation that apparently occurred on particles of volcanic origin. This observation implies that aerosol in the upper troposphere and stratosphere can have indirect climate effects by modifying cirrus cloud formation and radiative properties.