Particulate Carbon Speciation by MnO2Oxidation

Abstract

A selective thermal oxidation method was developed for the speciation of carbonaceous aerosols collected on filters into organic carbon (OC) and elemental carbon (EC). The technique is based on studying the thermal oxidation of microcrystalline graphite by MnO2as well as various organic compounds. The procedure uses a modified Dohrmann DC-52 carbon analyzer with a flame ionization detector to detect the CO2resulting from the oxidation as methane after catalytic conversion. The results led to the selection of 525 °C as the optimal temperature for the oxidation of OC while leaving EC intact. After the organic oxidation, the sample is heated at 850° C, at which EC is oxidized rapidly and completely by MnO2. Carbonates that may be present in either the particles or the filter medium are removed by acidification and heating to ∼ 120°C prior to performing the organic and EC measurements. Analysis of split ambient particulate samples in which the OC levels had been reduced by solvent extraction produced EC results statistically the same as the original untreated samples. These results suggest that the speciation is not sensitive to the level of organics in the sample. During the Carbonaceous Species Methods Comparison Study (CSMCS) in which the participants analyzed 20 blind samples, with four being triplicates, this technique yielded results in good agreement with the average results of the participants, with coefficients of variation (CV) derived from the triplicate analysis being 2.1%, 2.6%, and 8.1%, respectively, for total, organic, and elemental carbon.