THE DRI THERMAL OPTICAL REFLECTANCE CARBON ANALYSIS SYSTEM - DESCRIPTION, EVALUATION AND APPLICATIONS IN UNITED-STATES AIR-QUALITY STUDIES

The thermal/optical reflectance method of carbon analysis developed by Huntzicker et al. (in Particulate Carbon, Atmospheric Life Cycle, edited by Wolff G. T. and Klimisch R. L., pp. 79-88, Plenum Press, New York, 1982) has been adapted by several laboratories for the quantification of organic and elemental carbon on quartz-fiber filter deposits. While the principle used by these laboratories is identical to that of Huntzicker et al., the details differ with respect to calibration standards, analysis time, temperature ramping and volatilization/combustion temperatures. This paper reports a variation on this method which has been applied to over 27,000 samples taken in more than a dozen urban and regional air quality studies in the U.S.A. In this variation, a 0.5 cm2 punch from a filter is submitted to volatilization at temperatures of 120, 250, 450 and 550-degrees-C in a pure helium atmosphere, then to combustion at temperatures of 550, 700 and 800-degrees-C in a 2% oxygen and 98% helium atmosphere. The carbon which evolves at each temperature is converted to methane and quantified with a flame ionization detector. The seven separate carbon fractions facilitate evaluation of the method and increase the information content concerning the samples. The reflectance from the deposit side of the filter punch is monitored throughout the analysis. This reflectance usually decreases during volatilization in the helium atmosphere owing to the pyrolysis of organic material. When oxygen is added, the reflectance increases as the light-absorbing carbon is combusted and removed. Organic carbon is defined as that which evolves prior to re-attainment of the original reflectance, and elemental carbon is defined as that which evolves after the original reflectance has been attained. By this definition, ''organic carbon'' is actually organic carbon that does not absorb light at the wavelength used (632.8 nm) and ''elemental carbon'' is light-absorbing organic and elemental carbon. Assumptions underlying the procedure are discussed, as well as comparisons with other methods and recommendations for further work.