Analysis of Organic, Elemental, and Carbonate Carbon in Ambient Aerosols

A method has been developed for the analysis of organic, elemental, and carbonate carbon in ambient aerosols collected on glass-fiber or quartz-fiber filters. The organic analysis proceeds in two steps. The filter is first heated in an oxygen (2 percent)-helium atmosphere at 340°C to volatilize the bulk of the organic carbon. The volatilized carbon passes through a manganese dioxide (MnO₂) bed at 900°C and is converted to carbon dioxide (CO2), which is reduced to methane (CH₄) and measured in a flame ionization detector. After the oven has been purged with helium, the oven temperature is raised to 600°C, and a secondary volatilization of organic carbon results. This is measured by the technique just described. The purpose of the two-step organic measurement is to minimize the pyrolytic conversion of organic to elemental carbon during the organic carbon analysis. For typical filter samples, approximately two thirds of the organic carbon is removed in the 340°C oxygen-helium step. The elemental carbon is then measured by combusting the sample in an oxygen-helium atmosphere at 600°C and measuring the evolved CO₂ as described. A second combustion at 700°C in the oxygen-helium atmosphere is included to ensure complete recovery of the elemental carbon. The carbonates are measured by acidification of the filter and measurement of the resultant CO₂. Because there is still some conversion of organic to elemental carbon during the organic analysis, the values for organic carbon and elemental carbon must be considered as lower and upper limits, respectively. An optical technique is being refined to permit a quantitative determination of the degree of conversion.