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KINETICS AND MECHANISMS OF ACID GENERATION IN CLOUDS AND PRECIPITATION

Objective


The transition metal (specially manganese) catalysed mechanism may play a major role in the oxidation of sulphur dioxide in polluted boundary layer clouds, but uncertainties exist concerning the mechanisms and rates of these processes.
Research was carried out in order to characterize the reactions of the free radicals involved in the chain mechanism and to investigate the photolysis of iron (III) hydrated complexes as a possible source of hydroxyl radicals. A good understanding of these processes is needed to assess their actual impact on sulphur dioxide oxidation in clouds over continents.

A large number of reactions of the SO3(-), SO4(-) and SO5(-) radicals relevant to the oxidation of sulphur dioxide and sulphuric acid in cloud water chemistry have been investigated, using pulse radiolysis and laser photolysis methods.

The data obtained have contributed to the identification and characterization of the individual reactions participating in the radical induced chain oxidation of sulphite to sulphate in aqueous solution.

The mechanism for the oxidation of formaldehyde to formic acid by the hydroxyl radical has been established. The primary reaction between the hydroxyl radical and hydrated formaldehyde yields the hydrated formyl radical. The subsequent reactions of this radical, self reaction, reaction with oxygen and H2O2 were found to yield formic acid. This in cloud formation of formic acid could significantly contribute to the acidity of precipitation at remote sites.
The reaction of formaldehyde with hydroxyl and sulphate radicals may play an important role in inhibiting the chain process of converting sulphur (IV) into sulphur (VI).

The possibility of hydroxyl radical production from photodecomposition of FE (III) complexes in polluted boundary layer clouds has been further investigated. Almost exclusively, the hydroxyl radicals were found to arise from photodecompositon of FeOH(2+), and absolute quantum yields could be determined for this spec ies.
Quantum yields were also determined for the production of sulphate radicals from the iron sulphate complex FeSO4(+). The implications are that FeOH(2+) is potentially significant to the chemistry of clouds, whereas FeSO4(+) is not.
THE RESEARCH PROPOSED HEREIN WILL BE CONDUCTED IN COLLABORATION WITH RESEARCH GROUPS AT LEEDS UNIVERSITY AND AT C.E.R.L. WHERE PULSE TECHNIQUES ARE IN USE FOR THE STUDY OF RADICAL REACTIONS. THE OVERALL AIM IS TO UNRAVEL THE DETAILS OF THE RADICAL CHAIN OXIDATION MECHANISM FOR SO2 IN AQUEOUS SOLUTION. IT WILL BE ASSUMED THAT OH RADICALS ARE GENERATED BY PHOTOLYSIS OF IRON (III)-HYDROXYL ION COMPLEXES. THE REACTION OF OH WITH SULFITE AND BISULFITE THEN INITIATES THE CHAIN. OXY-SULFUR RADICALS SO3, SO4 AND SO5 HAVE BEEN PROPOSED AS INTERMEDIATES IN THE OVERALL REACTION AND THEIR ROLES WILL HAVE TO BE ELUCIDATED.

THE WORK AT MAINZ WILL CONCENTRATE ON THE IDENTIFICATION OF REACTION INTERMEDIATES BY RADICAL SCAVENGING TECHNIQUES, USING CHROMATOGRAPHIC EQUIPMENT FOR THE DETERMINATION OF PRODUCTS. THIS INVOLVES THE DEVELOPMENT OF NEW ANALYTICAL PROCEDURES. TWO LINES OF RESEARCH WILL BE PERSUED : ONE IS TO QUANTIFY THE EFFECTIVENESS OF IRON (III) ION COMPLEXES AS A PHOTOLYTIC SOURCE OF OH BY MEASURING THE ASSOCIATED QUANTUM YIELDS. THE OTHER LINE OF WORK WILL BE DEVOTED TO THE OCCURENCE OF SO4 AS AN INTERMEDIATE IN THE OXIDATION CHAIN AND CONCEIVABLE REACTIONS GENERATING IT.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

MAX-PLANCK-GESELLSCHAFT ZUR FOERDERUNG DER WISSENSCHAFTEN E.V.
Address
Johann Joachim-becher-weg 27
55020 Mainz
Germany

Participants (2)

Central Electricity Generating Board (CEGB)
United Kingdom
Address

EC1A 7A4 London
UNIVERSITY OF LEEDS
United Kingdom
Address
Woodhouse Lane
LS2 9JT Leeds