Revisiting thermal-optical analyses of carbonaceous aerosol using a physical model

Thermal-optical analysis (TOA) has been widely used to separate carbonaceous aerosols from ambient and source samples into two components, organic and elemental carbon. This method uses volatility to separate groups of carbon, and laser monitoring to correct for the transformation of non-absorbing carbon into pyrolytic carbon that absorbs light. However, assumptions inherent in this method have proven incorrect, leaving interpretation of the results open to question. We present a framework for interpreting TOA results based on the optical and carbon-release signals recorded by the instrument, which accounts for co-evolution of different groups of carbon (organic carbon, light-absorbing carbon [LAC] native to the sample, and pyrolytic carbon). Optical cross-sections of carbon groups for use in this model are derived from measurements, and depend on filter transmittance for LAC but not for pyrolytic carbon. We constrain temperatures of carbon evolution by examining samples from controlled aerosol generation and model organic compounds. The system of equations describing the analyzer's response is underdetermined during portions of the analysis, with one fewer equation than needed to quantify all the evolving groups. Our model, REACTO (REAnalyzing Carbon Traces Optically) identifies the range of possible sample compositions consistent with the analyzer's output. We also demonstrate the utility of the "thermabsgram," which identifies formation and loss of absorbing carbon by taking the derivative of the change in filter transmission.