Black carbon or 'soot', generated by combustion, harms climate and health. Traditional filter-based sensors are prone to artefacts and need frequent human intervention. Our optoacoustic illumination-detection separating sensor (IDSS) is filterless and requires minimal cleaning. Flows of clean air shield the sensor's cavity, preventing contamination. We demonstrate the stable performance of the IDSS and estimate a cleaning cycle of 1.5 years in ship-board applications.