Vehicle Contribution to Ambient Carbonaceous Aerosols by Scaled Thermograms

Carbon thermograms, which classify carbon aerosol according to its volatility, were obtained for fine-particle samples from an isolated highway vehicle source and a vehicle-dominated ambient site. The thermograms from the sites were compared after scaling by the carbon monoxide concentration. The high- and low-volatility carbon fractions in the ambient sample agreed to within 10% of the corresponding fractions in the highway vehicle sample. Excess carbon in the range of intermediate volatility comprised 15 to 19% of the ambient carbon mass and is attributed to aerosols from secondary processes and nonvehicular primary sources. When lead was used as a tracer to scale the thermograms, the high- and low-volatility ambient carbon fractions were underestimated by a factor of 2. The low volatility fraction (“black carbon”) present in the atmospheric sample was evolved at lower temperatures than the equivalent fraction in the isolated highway vehicle sample. This creates an ambiguity in defining the low-volatility fraction, which is a problem if black carbon is used as a tracer. The scaling technique described in this work avoids the problem because it does not require an estimation of the low-volatility carbon fraction.