22 Feb 2022
22 Feb 2022
Status: this preprint is currently under review for the journal AMT.

A dual-wavelength photothermal aerosol absorption monitor: design, calibration and performance

Luka Drinovec1,2,3, Uroš Jagodič1,2, Luka Pirker2,4, Miha Škarabot2, Mario Kurtjak5, Kristijan Vidović6, Luca Ferrero7, Bradley Visser8, Jannis Röhrbein8, Ernest Weingartner8, Daniel M. Kalbermatter9, Konstantina Vasilatou9, Tobias Bühlmann9, Celine Pascale9, Thomas Müller10, Alfred Wiedensohler10, and Griša Močnik1,2,3 Luka Drinovec et al.
  • 1Haze Instruments d.o.o., Ljubljana, Slovenia
  • 2Department of Condensed Matter Physics, Jozef Stefan Institute, Ljubljana, Slovenia
  • 3Center for Atmospheric Research, University of Nova Gorica, Nova Gorica, Slovenia
  • 4Faculty for mathematics and physics, University of Ljubljana, Ljubljana, Slovenia
  • 5Advanced Materials Department, Jozef Stefan Institute, Ljubljana, Slovenia
  • 6Deparment for Analytical Chemistry, National Institute of Chemistry, Ljubljana, Slovenia
  • 7GEMMA center, University of Milano-Bicocca, Milano, Italy
  • 8Institute for Sensors and Electronics, University of Applied Sciences Northwestern Switzerland, Windisch, Switzerland
  • 9Federal Institute of Metrology METAS, Bern, Switzerland
  • 10Leibniz Institute for Tropospheric Research, Leipzig, Germany

Abstract. There exists a lack of aerosol absorption measurement techniques with low uncertainties and without artefacts. We have developed a two-wavelength Photothermal Aerosol Absorption Monitor (PTAAM-2λ), which measures the aerosol absorption coefficient at 532 and 1064 nm. Here we describe its design, calibration and mode of operation and evaluate its applicability, limits and uncertainties. The 532 nm channel was calibrated with ~1 µmol/mol NO2, whereas the 1064 nm channel was calibrated using measured size distribution spectra of nigrosin particles and a Mie calculation. Since the aerosolized nigrosin used for calibration was dry, we determined the imaginary part of the refractive index of nigrosin from the absorbance measurements on solid thin film samples. The obtained refractive index differed considerably from the one determined using aqueous nigrosin solution. PTAAM-2λ has no scattering artefact and features very low uncertainties: 4 % and 6 % for the absorption coefficient at 532 nm and 1064 respectively; and 9 % for the absorption Ångström exponent. The artefact-free nature of the measurement method allowed us to investigate the artefacts of filter photometers. Both the Aethalometer AE33 and the CLAP suffer from the cross-sensitivity to scattering – this scattering artefact is most pronounced for particles smaller than 70 nm. We observed a strong dependence of the filter multiple scattering parameter on the particle size in the 100–500 nm range. The results from the winter ambient campaign in Ljubljana showed similar multiple scattering parameter values for ambient aerosols and laboratory experiments. The spectral dependence of this parameter resulted in AE33 reporting the absorption Ångström exponent for different soot samples with values biased 0.17–0.3 higher than the PTAAM-2λ measurement. Photothermal interferometry is a promising method for reference aerosol absorption measurements.

Luka Drinovec et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-21, Drinovec et al. 2022', Anonymous Referee #2, 09 Mar 2022
  • RC2: 'Comment on amt-2022-21', Anonymous Referee #3, 15 Mar 2022

Luka Drinovec et al.

Luka Drinovec et al.


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Short summary
A new photothermal interferometer (PTAAM-2λ) for artefact-free determination of the aerosol absorption coefficient at two wavelengths is presented. The instrument is calibrated with NO2 and polydisperse nigrosin, resulting in very low uncertainties of the absorption coefficients: 4 % at 532 nm and 6 % at 1064 nm. The instrument’s performance makes it a strong candidate for the reference absorption measurement.