Analysis of Organic and Elemental Carbon in Ambient Aerosols by a Thermal-Optical Method

Abstract

A thermal-optical method has been developed for the analysis of organic and elemental carbonaceous aerosol on glass or quartz fiber filters. Organic carbon is volatilized in two steps: at 350°C in an O₂ (2%)-He mixture and at 600°C in He. The volatilized organic carbon is oxidized to CO₂, reduced to CH₄, and measured by a flame ionization detector. Elemental carbon is combusted to CO₂ in O₂ (2%)-He at 400, 500, and 600°C, and the CO₂ is measured as above. The reflectance of the filter segment, which is continuously monitored with a He-Ne laser system, decreases during the organic analysis because of pyrolytic conversion of organic to elemental carbon and increases during the combustion of elemental carbon. Correction for pyrolytic production of elemental carbon is accomplished by measuring the amount of elemental carbon oxidation necessary to return the filter reflectance to the value it had before pyrolysis occurred. This is facilitated by the slow, three step elemental carbon combustion process. All switching of gas flows, timing, temperature control, pyrolysis correction, analog to digital conversion electronics, electrometer functions, signal integration, data storage, and data outputs are controlled by a microcomputer system built around a Motorola 6802 microprocessor. The instrument has been used to measure organic and elemental carbon concentrations at 42 urban sites in the United States.