Determination of atmospheric soot carbon with a simple thermal method

The dark component of carbonaceous aerosols is often referred to as “soot carbon”. Soot consists of pure elemental carbon along with highly polymerized organic matter. An accurate discrimination between the soot carbon and the other components of carbonaceous aerosols is difficult to obtain by thermal analytical processes. Here, we report an optimization of a 2-step thermal method focused on the soot carbon determination of atmospheric particles. The organic material which does not absorb visible light is removed from the collection substrate under a pure oxygen flow during a precombustion step which has been carefully optimized in terms of temperature (340°C) and duration (2 h). The remaining carbon content is determined by coulometric titration of the CO₂ evolved from the combustion of the samples. The method has been tested quantitatively for analytical artefacts (e.g., “soot” production due to the charring of organics; soot losses during the preheating step) by using various standards such as pure graphite, pure organic and natural biogenic compounds and replicates of ambient air samples collected in urban, rural and forested areas in France. The results obtained so far indicate that this approach satisfactorily distinguishes between organic and soot carbon and allows reliable soot carbon determination at the μg level in atmospheric samples from a wide variety of environments. This study confirms that soot carbon is not composed primarily of elemental carbon. It appears to be a variable mixture of highly condensed organic compounds. These compounds may be either combustion-derived material or the result of low-temperature gas-to-particle conversion processes.