Toward a standardized thermal-optical protocol for measuring atmospheric organic and elemental carbon : the EUSAAR protocol

Thermal-optical analysis is a conventional method for classifying carbonaceous aerosols as organic carbon and elemental carbon. Unfortunately, different thermal evolution protocols result in a wide elemental carbon-to-total carbon variation up to a factor of five. In Europe, there is currently no standard procedure for determining carbonaceous aerosol fraction which implies that data from different laboratories at various sites are most likely not comparable and affected by unknown errors. In the framework of the European project EUSAAR (European Supersites for Atmospheric Aerosol Research), a comprehensive work has been carried out to investigate the causes of differences in the EC measured using different thermal evolution protocols and attempts have been devoted to assess and mitigate major positive and negative biases affecting thermal-optical analysis. Our approach to improve the accuracy of the thermal-optical discrimination between organic carbon and elemental carbon was essentially based on four goals. First, as charring correction relies on assumptions proven to be generally not true −e.g. pyrolic carbon is considered to evolve completely before native elemental carbon throughout the analysis−, we sought to reduce pyrolysis to minimum levels in favour of a maximum volatilisation of organic carbon. Second, we sought to minimize the potential negative bias in EC determination caused by early release of light-absorbing species at high temperature in the He-mode, including native EC or a combination of EC and pyrolitic carbon with potentially different specific cross section values. Third, we sought to minimize the potential positive bias in EC determination resulting from the slipping of residual organic carbon into the He/O2-mode and its potential evolution after the split point. Finally, we sought to reduce the uncertainty arising from the position of the OC/EC split point on the FID profile through multiple desorption steps in the He/O2-mode. Based on different types of carbonaceous PM encountered across Europe, we defined an optimised thermal evolution protocol, the EUSAAR_2 protocol, as follows: step 1 in He, 200 °C for 120 s; step 2 in He 300°C for 150s; step 3 in He 450°C for 180s; step 4 in He 650°C for 180s. For steps 1-4 in He/O2, the conditions are 500°C for 120s, 550°C for 120s, 700°C for 70s, and 850°C for 80s, respectively.