A Photothermal Sensor Concept for Black Carbon Mass Concentration Measurement

The impact of black carbon (BC) on human health and its role as potential accelerator of climate change place it among major scientific and policy concerns. Quantifying BC with adequate sensitivity and selectivity is challenging and yet requires expensive and heavy equipment. A highly promising concept for reliable, low-prized and miniaturized sensing devices resides in the concept of photothermal spectroscopy (PTS). To demonstrate this, we have developed a sensor concept for BC mass concentration by exploiting a fiber-coupled Fabry-Pérot interferometer (FPI) combined with a low-cost laser diode as excitation source. Here, periodic irradiation of the BC particles caused a periodic change in temperature and thus in the refractive index, monitored with the FPI. The FPI featured a 1 x 2 mm (width x length) air spaced etalon, through which an air flux with a specified BC mass concentration was guided. The excitation was realized by focusing a laser beam with a wavelength of 860 nm and an optical peak-to-peak power of 1.5 W onto the particles upstream of the etalon. A modulation frequency of 300 Hz and a gas flow of 0.34 l/min was chosen, offering a favorable signal-to-noise ratio while providing a good balance between flow noise and gas exchange time. BC particles were generated with a commercially available soot generator and the adjusted mass concentration in each case was compared to a reference instrument. The concentrations were quantified up to 1.2 mg/m³, and the background corrected signal amplitude yielded a linear correlation to the reference instrument. A limit of detection of approximately 15 µg/m³ (3σ) was determined. BC particles that were not thermally excited (= excitation beam off) did not exhibit a detectable signal in the FPI etalon. Our results provide a promising sensing method for monitoring BC mass concentrations that impresses through its ability of miniaturization.