Technical Note: The single particle soot photometer failsto reliably detect PALAS soot nanoparticles

Abstract

The single particle soot photometer (SP2) useslaser-induced incandescence (LII) for the measurement of at-mospheric black carbon (BC) particles. The BC mass con-centration is obtained by combining quantitative detectionof BC mass in single particles with a counting efficiency of100 % above its lower detection limit. It is commonly ac-cepted that a particle must contain at least several tenths ofa femtogram BC in order to be detected by the SP2.Here we show the result that most BC particles froma PALAS spark discharge soot generator remain undetectedby the SP2, even if their BC mass, as independently de-termined with an aerosol particle mass analyser (APM),is clearly above the typical lower detection limit of theSP2. Comparison of counting efficiency and effective densitydata of PALAS soot with flame generated soot (combustionaerosol standard burner, CAST), fullerene soot and carbonblack particles (Cabot Regal 400R) reveals that particle mor-phology can affect the SP2’s lower detection limit. PALASsoot particles are fractal-like agglomerates of very small pri-mary particles with a low fractal dimension, resulting in avery low effective density. Such loosely packed particles be-have like “the sum of individual primary particles” in theSP2’s laser. Accordingly, most PALAS soot particles remainundetected as the SP2’s laser intensity is insufficient to heatthe primary particles to their vaporisation temperature be-cause of their small size (Dpp≈5–10 nm). Previous knowl-edge from pulsed laser-induced incandescence indicated thatparticle morphology might have an effect on the SP2’s lowerdetection limit, however, an increase of the lower detection limit by a factor of∼5–10, as reported here for PALAS soot,was not expected.In conclusion, the SP2’s lower detection limit at a cer-tain laser power depends primarily on the total BC massper particle for compact particles with sufficiently high ef-fective density. By contrast, the BC mass per primary par-ticle mainly determines whether fractal-like particles withlow fractal dimension and very small primary particles aredetectable, while their total BC mass has only a minor in-fluence. This effect shifts the lower detection limit to muchhigher BC mass, or makes them completely undetectable.Consequently, care has to be taken when using the SP2 inapplications dealing with loosely packed particles that havevery small primary particles as building blocks.