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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-325
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-325
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Sep 2020

08 Sep 2020

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This preprint is currently under review for the journal AMT.

A comparison of three optical absorption photometers at a boreal forest site – effects of different correction algorithms

Krista Luoma1, Aki Virkkula2, Pasi Aalto1, Katrianne Lehtipalo1,2, Tuukka Petäjä1, and Markku Kulmala1 Krista Luoma et al.
  • 1Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki, 00014, Finland
  • 2Atmospheric Composition Research, Finnish Meteorological Institute, Helsinki, 00560, Finland

Abstract. We present a comparison of three absorption photometers that measured the absorption coefficient (σabs) of ambient aerosol particles in 2012–2017 at SMEAR II, a measurement station located in a boreal forest in southern Finland. The comparison included an Aethalometer (AE31), a Multi Angle Absorption Photometer (MAAP), and a Particle Soot Absorption Photometer (PSAP). These optical instruments measured particles collected on a filter, which is a source for systematic errors, since in addition to the particles, also the filter fibers interact with the radiation. To overcome this problem, several algorithms have been suggested to correct the data measured by the AE31 and the PSAP. Our aim is to study how the different correction algorithms affected the derived optical properties. We applied different correction algorithms to the AE31 and PSAP data, and compared the results against the reference measurements conducted by the MAAP. The comparison between the MAAP and AE31 resulted to a multiple scattering correction factor (Cref) used in the AE31 correction algorithms to compensate for the scattering by the filter fibers. The Cref varies between different environments, and our results are applicable for measurements conducted in a boreal environment. We observed a clear seasonal cycle of Cref, which was probably due to the variations in aerosol optical properties, such as the backscatter fraction and single-scattering albedo, and also due to the variations in the relative humidity (RH) even though the RH in the instruments were kept below 40 %. The results showed that the filter measurement methods seemed to be rather sensitive to the RH even if the RH was below the recommended value of 40 %. The instruments correlated well (R ≈ 0.98) but the slopes of the regression lines varied between the instruments and correction algorithms: compared to MAAP, the AE31 underestimated the σabs (the slopes varied between 0.93–0.97) and the PSAP overestimated the σabs (the slopes varied between 1.07–1.24). The instruments and correction algorithms had a notable influence on the absorption Ångström exponent: the median absorption Ångström exponent varied between 0.93–1.54 for the different algorithms and instruments.

Krista Luoma et al.

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Short summary
The study presents a comparison of three different absorption photometers that measured ambient aerosol particles at a boreal forest site. The study aims to better understand problems related to filter-based measurements. Results show how different correction algorithms, which are used to produce the data, affect the derived optical properties of aerosol particles.
The study presents a comparison of three different absorption photometers that measured ambient...
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