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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  06 Aug 2020

06 Aug 2020

Review status
This preprint is currently under review for the journal AMT.

Detailed characterization of the CAPS single scattering albedo monitor (CAPS PMssa) as a field-deployable instrument for measuring aerosol light absorption with the extinction-minus-scattering method

Rob L. Modini1, Joel C. Corbin2, Benjamin T. Brem1, Martin Irwin1,a, Michele Bertò1, Rosaria E. Pileci1, Prodromos Fetfatzis3, Kostas Eleftheriadis3, Bas Henzing4, Marcel M. Moerman4, Fengshan Liu2, Thomas Müller5, and Martin Gysel-Beer1 Rob L. Modini et al.
  • 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, Switzerland
  • 2Metrology Research Centre, National ResearchCouncil Canada, 1200 Montreal Road, Ottawa K1A 0R6, Canada
  • 3Institute of Nuclear and Radiological Science & Technology, Energy & Safety N.C.S.R. "Demokritos", Attiki, Greece
  • 4Netherlands Organisation for Applied Scientific Research (TNO), Princetonlaan 6, 3584 Utrecht, the Netherlands
  • 5Leibniz Institute for Tropospheric Research (TROPOS), Permoserstrasse 15, 04318, Leipzig,Germany
  • anow at: Catalytic Instruments GmbH, Zellerhornstrasse 7, Rosenheim, 83026, Germany

Abstract. The CAPS PMssa monitor is a recently commercialized instrument designed to measure aerosol single scattering albedo (SSA) with high accuracy (Onasch et al., 2015). The underlying extinction and scattering coefficient measurements made by the instrument also allow calculation of aerosol absorption coefficients via the extinction-minus-scattering (EMS) method. Care must be taken with EMS measurements due to the occurrence of large subtractive error amplification, especially for the predominantly scattering aerosols that are typically found in the ambient atmosphere. Practically this means that although the CAPS PMssa can measure scattering and extinction coefficients with high accuracy (errors on the order of 1–10 %), the corresponding errors in EMS-derived absorption range from ~ 10 % to greater than 100 %. Therefore, we examine the individual error sources in detail with the goal of constraining these as tightly as possible.

Our main focus is on the correction of the scattered light truncation effect (i.e., accounting for the near-forward and -backward scattered light that is undetectable by the instrument), which we show to be the main source of underlying error in atmospheric applications. We introduce a new, modular framework for performing the truncation correction calculation that enables the consideration of additional physical processes such as reflection from the instrument’s glass sampling tube, which was neglected in an earlier truncation model. We validate the truncation calculations against comprehensive laboratory measurements. It is demonstrated that the process of glass tube reflection must be considered in the truncation calculation, but that uncertainty still remains regarding the effective length of the optical cavity. Another important source of uncertainty is the cross calibration constant that quantitatively links the scattering coefficient measured by the instrument to its extinction coefficient. We present measurements of this constant over a period of ~ 5 months that demonstrate that the uncertainty in this parameter is very well constrained for some instrument units (2–3 %), but higher for others.

We then use two example field datasets to demonstrate and summarize the potential and the limitations of using the CAPS PMssa for measuring absorption. The first example uses mobile measurements on a highway road to highlight the excellent responsiveness and sensitivity of the instrument, which enables much higher time resolution measurements of relative absorption than is possible with filter-based instruments. The second example from a stationary field site (Cabauw, the Netherlands) demonstrates how truncation-related uncertainties can lead to large biases in EMS-derived absolute absorption coefficients. Nevertheless, we use a subset of fine-mode dominated aerosols from the dataset to show that under certain conditions and despite the remaining truncation uncertainties, the CAPS PMssa can still provide consistent EMS-derived absorption measurements, even for atmospheric aerosols with high SSA. Finally, we present a detailed list of recommendations for future studies that use the CAPS PMssa to measure absorption with the EMS method. These recommendations could also be followed to obtain accurate measurements (i.e., errors less than 5–10 %) of SSA, and scattering and extinction coefficients with the instrument.

Rob L. Modini et al.

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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

Rob L. Modini et al.

Rob L. Modini et al.


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Latest update: 27 Nov 2020
Publications Copernicus
Short summary
Extinction-minus-scattering is an important method for measuring aerosol light absorption, but its application in the field presents a number of challenges. A recently developed instrument based on this method – the CAPS PMssa – has the potential to overcome some of these challenges. We present a compilation of theory, lab measurements, and field examples to characterize this instrument and show the conditions under which it can deliver reliable absorption measurements for atmospheric aerosols.
Extinction-minus-scattering is an important method for measuring aerosol light absorption, but...