Evaluating the importance of innovative heterogeneous chemistry to explain observed stratospheric ozone depletion

Abstract

In re-analyzing total ozone data from a NASA satellite for the period 1978 -1990 found unexpectedly high ozone reductions at mid to high-latitudes of both hemispheres, for example a 8% ozone reduction at 60"N between 1980 and 1990. The Junge layer of particulates in the lower stratosphere is composed primarily of super-cooled liquid sulfuric acid with dissolved water and trace amounts of other species. Rodriquez et al. [1991] demonstrated with model calculations that the heterogeneous reaction of N2O5 with water in sulfate aerosols may contribute to the high ozone reductions at midlatitudes. Recently, the 15 June 1991 volcanic eruption of Mt. Pinatubo resulted in large amounts of sulfur containing particles in the stratosphere. Complete conversion of SO2 to H2SO4 aerosols occurred within a few months after the eruption. Balloon sondes and satellite data suggest that stratospheric ozone decreased 6-10% in the tropics by October 1991. Ground based measurements along with ozone sonde and satellite measurements show above average loss of ozone in 1992 and 1993. Currently, there is a widespread search for additional heterogeneous reactions or combination of heterogeneous and homogeneous (gas-phase) reactions that could catalytically reduce ozone to observed levels. Burley and Johnston [1992a,b], of the University of California at Berkeley, proposed that nitrosyl sulfuric acid (NSA) NOHS04, is a promising heterogeneous reactant for activating HC1 in sulfuric acid particles. They list several sources for producing it in the stratosphere and they carried out thermodynamic and chemical kinetic calculations at one stratospheric altitude and at one latitude. NSA has been overlooked in+all previous stratospheric model calculations, even though it has been observed in stratospheric sulfate aerosols (Farlow et al. 1978). This study makes large scale atmospheric model calculations to test the proposal by Burley and Johnston [ 1992a,b] that a promising heterogeneous process for activating HC1 in sulfuric acid particles is a catalytic couple based on nitrosyl sulfuric acid (NSA). This mechanism would convert HCl, which is a reservoir species for chlorine free radicals, into a form that catalytically reduces ozone. These calculations used the LLNL 2-D zonally averaged chemical-radiative-transport model of the troposphere and stratosphere.These calculations set firm limits on the range of kinetic parameters over which this heterogeneous reaction would be important in the global ozone balance, and thus be a guide for where laboratory work is needed.