Ozone Observations and Models

Prior to the eruption of Mount Pinatubo, Stolarski et al. [1991] reported ozone trends determined from data obtained by the Total Ozone Mapping Spectrometer (TOMS) on the Nimbus 7 satellite. Several-percent per-decade losses in column were detected at nearly all latitudes, with the exception of the tropics. These reductions have occurred in the lower stratosphere [ Hood et al., 1993 and Reinsel et al, 1994], and appear to be most severe in the southern hemisphere polar region in springtime (i.e., the ozone hole) and in the northern hemisphere at middle to high latitudes in winter and spring. Rodriguez et al. [1991] showed that models could account better for the mid-latitude trends (although they still underestimated the observed losses) if they included the heterogeneous hydrolysis of NO on sulfate aerosols. Calculations of ozone loss rates under background aerosol conditions [ McElroy et al., 1992, Avallone et al., 1993a, Weisenstein et al., 1993, and Garcia and Solomon, 1994], indicated that the contributions from ClO, BrO, and HO radicals exceeded that due to NO below about 25 km. These results have led to a reevaluation of the impact on ozone of NO emitted into the stratosphere by aircraft [ Weisenstein et al., 1993].

Initial losses of ozone following the eruption of Mount Pinatubo were reported by Schoeberl et al. [1993a] and Grant et al. [1992] and occurred in the tropics in regions of greatest aerosol loading. Models have attributed these losses to changes in circulation [ Kinne et al., 1992] and to decreasing photolysis of O due to absorption of solar ultraviolet by SO in the plume [ Bekki et al., 1993]. Bojkov et al. [1993] and Gleason et al. [1993] showed that global ozone columns following the eruption were lower than in any previous year on record. Since then, there has been some recovery of ozone; therefore, it is likely that the post-Pinatubo losses were the result of changes in chemistry and transport due to enhanced sulfate aerosols.

Two-dimensional model studies published shortly after the eruption [ Brasseur et al., 1992] predicted that the enhancements of chlorine due to heterogeneous reactions on sulfate could induce chemical loss of about 10%\ globally. The model predicted the largest changes to be at high latitudes, presumably the result of increased efficiency of the ClONO hydrolysis reaction. However, the predicted latitudinal gradient in ClO was nearly opposite to that observed [ Avallone et al., 1993b] based on a comparison of ER-2 aircraft observations from before and after the eruption at similar seasons. More recently, Rodriguez et al. [1994] have shown that increases in the HO contribution to ozone loss (with a smaller contribution from halogens) were greater than the decreases in the contribution from NO. Their conclusions agree well with a study of ozone loss rates based on aircraft measurements [ Wennberg et al., 1994]. These results have bolstered confidence in model treatments of heterogeneous chemistry on sulfate aerosol at mid-latitudes. The importance of aerosol-induced transport changes has not yet been studied in detail.