Atmospheric Photolysis of Sulfuric Acid

Henrik G. Kjaergaard Joseph R. Lane, Anna L. Garden, Daniel P. Schofield, Timothy W. Robinson and MichaelJ. Mills  

Abstract We describe theoretical methods for the calculation of vibrational and 

Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zeaiand 

Lundbeck Foundation Center for Theoreticai Chemistry, Department of Chemistry, Aarhus University, DK-8000 

LASP/PAOS, University of Coiotado, Bouider, Coiorado, 80309, USA 

---

The fate of sulfuric acid in the upper stratosphere and mesosphere is key to understanding the role of sulfur in the upper atmosphere [10]. In this regard the photolysis of sulfuric acid (H2SO4) via the reaction... 

We have shown that modern theoretical methods are capable of calculating cross sections for sulfuric acid purely ab initio. This includes vibrational transitions, low lying electronic transitions and transitions to high energy Rydberg states. The results obtained are of an accuracy that is suitable as input parameters for atmospheric modeling of the photolysis of sulfuric acid. 

Leung, Colussi and Hoffmann have used isotopic analysis in an attempt to constrain the amount of sulfate that could be produced by Crutzen s mechanism [129,130]. The first study retrieved the concentration profiles of OC S and OC S from infrared transmission spectra of the atmosphere recorded by the NASA MkIV balloon-borne interferometer. They derived an enrichment factor of 73.8 zb 8.67oo defined such that photolytically generated sulfur would be enriched in An isotopic budget based on this result shows that OCS photolysis cannot be a significant source of sulfate aerosol, since the enrichments of OCS, sulfuric acid aerosol and SO2 are known to be small [131]. A later laboratory study by the same group came to the conclusion that stratospheric photolysis results in an enrichment of 67 zb 77oo. 

The aqueous sulfuric acid droplets in the clouds result from UV sunlight photolysis of SO2, which reduces the SO2 abundance from 150 ppmv below the clouds to —10 ppbv at the cloud tops. The photochemistry of SO2 and CO2 is closely coupled. The O2 produced from CO2 photolysis is used to convert SO2 to SO3, which then forms sulfuric acid. Spectroscopic observations show temporal trends in the SO2 abundance at the cloud tops. These observed variations are probably due to atmospheric dynamics. 

The SO2 abundance in Venus atmosphere decreases at high altitudes due to the formation of the global sulfuric acid cloud layer (45-70 km), which is produced through SO2 photolysis via the net photochemical reaction ... 

The essential oxidation rate steps in the environment involve reactive species such as free radicals (RO2RO, HO ), ozone, and 02 (singlet oxygen). For predictive purposes, it is important to identify the important oxidants and their concentrations in the environmental compartments. Values of many oxidation rate constants are known reliably (Hendry etal, 1974). In the atmosphere, oxidation by the OH radical is significant, whereas ozone is important in the oxidation of some olefins and possibly some sulfur or phosphorus compounds. The RO2 radical can be important in sunlight photolysis in natural waters. Competitive kinetic techniques could be used to evaluate the relative loss of two chemicals, one of which is a standard of known reactivity to a specific oxidant (Mill etal., 1978). Azo compounds could be used to generate RO2 radical in water, nitrous acid to form the HO radical in air, and a dye to generate 02 in water. 

---