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
Authors: Modini Rob L. , Corbin Joel C. , Brem Benjamin T. , Irwin Martin , Bertò Michele , Pileci Rosaria E. , Fetfatzis Prodromos , Eleftheriadis Kostas , Henzing Bas , Moerman Marcel M. , Liu Fengshan , Müller Thomas , Gysel-Beer Martin 
Published: February 2021

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 near-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.