Resolving the atmospheric sulphur budget over the Elandsfontein area of the Mpumalanga Highveld

[摘要] A novel study on the investigation of three very common atmospheric sulphur speciesrelevant to the Mpumalanga Highveld subregion was conducted. Long-term in situmeasurements were applied in the diurnal and seasonal evaluation of the observed sulphurspecies. Ambient pollutant concentrations and surface meteorological data were collectedat an air quality monitoring station at Elandsfontein. Elandsfontein air quality monitoringstation was ideal for the observations due to its high elevation within the MpumalangaProvince surrounded by few rolling hills and negligible windbreaks which easily allows forextensive plume-contact with the surface during convective daytime mixing. The temporalcharacteristics of the sulphur species have been assessed relative to one another withvarying meteorological conditions. The diurnal and seasonal concentration variations wereused to describe the physical characteristics exhibited by the compounds overElandsfontein. Pollution roses were used to target the source of the major release pointsand areas of these sulphur species relative to the Elandsfontein monitoring station. Gas andparticle concentrations were analysed in relation to varying meteorological parameters witha view to ascertaining the sulphur transformation and concentration distribution in theplanetary boundary layer. Particulate sulphate distribution has been modelled throughmultivariate regression analyses in relation to three meteorological parameters, namely,wind speed, relative humidity and ambient temperature for the various seasons observedover southern Africa.This study has shown that hydrogen sulphide, sulphur dioxide and sulphate species arepresent throughout the year in the Mpumalanga Highveld at notably significant levels. Thepresence of ambient particulate sulphate has been shown to result from the combination ofchemical interactions during long-range aerosol transport; atmospheric recirculationprocesses shown from back trajectories over the southern Africa sub-region, as well as thevariation in the removal mechanisms and rates for the different seasons throughout theyear. These transport and removal processes all contribute to the overall sulphur massbalance in the planetary boundary layer. Dosage of the three sulphur species was evaluatedto provide data for sulphur pollution loading that could form a basis for health andviienvironmental impact assessments over the area. In view of the characteristic patternsdisplayed by particulate sulphate, multivariate mathematical models have been developedon a seasonal basis with variations in meteorological parameters. This was seen to predictan accuracy of up to 70 % of the particulate sulphate loading for different seasons over theSouth African Highveld.In order to understand the chemical interactions of atmospheric sulphur species, it isimportant to be able to predict the route taken and expected products of transformation onany given condition. Theoretical analyses of the chemical thermodynamic properties of theknown reacting species and a well-established approach were used in predicting reactionpaths and establishing the possible and feasible products of chemical transformation inrelation to the ambient temperature. The determination of reaction paths and possibleproducts of chemical transformation provides a measure of the relative importance of thereacting species and the mechanism of reaction. Gas-, aqueous-phase and radical reactionsinvolving sulphur (IV) were investigated with a view to establishing their relativeimportances. Thermochemical properties of several sulphur-containing compounds notavailable in the literature have been generated for evaluation of Gibbs free energy (ΔG)and enthalpy (ΔH). An electronic energy structural approach has been applied to model forΔG and ΔH of 88 sulphur species in 90 chemical reactions comprising gas-phase, aqueousphaseand radical reactions. Modelling was evaluated for their relative importances over atemperature range of –100 °C to +100 °C. The temperature range is well above the knowntropospheric temperature range to account for variations in the atmospheric environment.To further comprehend the chemistry of sulphur with regards to distribution of the speciesin the atmosphere, a kinetic model is developed and incorporated into a dispersion model.The kinetic evaluation of the oxidation rate of SO2 to sulphate has been determined withadvection and dispersion over the Elandsfontein area. Gas-phase transformation withadvection and dispersion has been used to evaluate the extent of the distribution of SO2relative to the major contributing sources. The dry deposition was considered to be thedominant removal mechanism. It was assumed that the reaction rate was second order inconcentration and that the rate of deposition was first order. The oxidation rates obtainedfor the seasons were 10.9 % h-1 for summer; 8.83 % h-1 for autumn; 6.56 % h-1 for winter;viii10.8 % h-1 for spring, while an overall rate of 9.6 % h-1 was obtained for the one year studyperiod. The transformation rate model produced a reaction constant and an activationenergy of 4.92 x 10-6 μg m-3 s-1 and 36.54 kJ kg-1 for summer; 3.939 x 10-6 μg m-3 s-1and 43.89 kJ kg-1 for autumn; 2.90 x 10-6 μg m-3 s-1 and 115.69 kJ kg-1 for winter;4.82 x 10-6 μg m-3 s-1 and 43.29 kJ kg-1 for spring, while for the year4.29 x 10-6 μg m-3 s-1 and 34.31 kJ kg-1. A Gaussian puff unsteady state Lagrangiandispersion model with reflection at the surface and inversion layer was applied forconcentration diffusion. The Lagrangian dispersion model with dry deposition was a betterprediction of the observed data than the models from previous studies using a first orderrate constant with or without deposition rate.

[发布机构] University of the Witwatersrand