PIXE investigation of aerosol composition over the Zambian Copperbelt

Atmospheric sulphate aerosol concentrations are of interest in climate chan ge studies because of their negative climate forcing potential. Quantificat ion of their forcing strength requires the compilation of global sulphur em ission inventories to determine the magnitude of regional sources. We repor t on measurements of the ambient aerosol concentrations in proximity to a c opper refinery in the central African Copperbelt, along the border of Zambi a and the Democratic Republic of the Congo. This region is historically reg arded as one of the largest African sources of sulphate aerosols. Sulphate is produced by oxidation in the atmosphere of SO2 emitted during the pyrome tallurgical processing of Cu-Co sulphide ores. Since the last quantificatio n of sulphur emissions (late 1960s), there has been large-scale reduction i n copper production and more frequent use of the leaching technique with ne gligible sulphur emissions. Samples were collected over four weeks, November-December 1996, at Kitwe, Z ambia. A low volume two-stage time-resolving aerosol sampler (streaker) was used. Coarse and fine mode aerosols were separated at >2.5 and >10 mu mad. Hourly elemental concentrations were determined by 3.2 MeV PIXE, and routi nely yielded Si, S, K, Ca, Ti, Mn, Fe, Cu and Zn, above detection limits. S i, K, Ca and Fe (major crustal components) dominated the coarse elemental m ass. In the fine stage, S and Si accounted for up to 80% of the measured ma ss, and S alone up to 60%. Time series analysis allowed the division of sul phur and crustal elements (Si, K, Ca, Fe) between (i) background concentrat ions representative of synoptic scale air masses and (ii) contributions fro m local sources, i.e., copper smelter and re-suspended soil dust. Short dur ation episodes of S concentrations, up to 26 mu g/m(3), were found simultan eously with enhanced Cu, Fe and Zn. Contributions from individual pyrometal lurgic processes and the cobalt slag dump could be distinguished from the e lemental signatures. Periods of diminished sulphur concentrations were also identified, indicating a well-mixed regional air mass. These results will contribute towards validating global climate model predictions of aerosol f orcing over central Africa. (C) 1999 Elsevier Science B.V. All rights reser ved.