| 10 Jun 2026
Traceable calibration of in-situ aerosol absorption instruments with monodisperse nigrosin
Abstract. The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO2. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.
Competing interests: LD, GM and JYD are employed by Haze Instruments d.o.o., the PTAAM manufacturer. GM is an editor for AMT.
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