On a possible bias in elemental carbon measurements with the Sunset thermal/optical carbon analyser caused by unstable laser signal

We present results that demonstrate a possible bias in the fractioning of total carbon (TC) into elemental carbon (EC) and organic carbon (OC) for measurements with the Sunset Laboratory Inc. Thermal/Optical Carbon Aerosol Analyser. The bias is caused by an unstable laser transmission signal. The transmission signal during the analysis of an instrument blank filter can give an indication of the possible bias. If the transmission signal around the OC/EC split point deviates from its initial value, the EC attribution is altered. In a sensitivity study, we show that for a deviation of 10% the EC content is substantially biased. •Unstable laser transmission signal in Sunset can lead to a bias in EC attribution.•An instability of more than 10% around the split point leads to a substantial bias.•An experiment checks the relevance of an instability during the cooling phase.•Systematic monitoring of the laser transmission signal is strongly recommended.