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. 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 andscattering coefficient measurements made by the instrument also allowcalculation of aerosol absorption coefficients via theextinction-minus-scattering (EMS) method. Care must be taken with EMSmeasurements due to the occurrence of large subtractive error amplification,especially for the predominantly scattering aerosols that are typicallyfound in the ambient atmosphere. Practically this means that although theCAPS PMssa can measure scattering and extinction coefficients with highaccuracy (errors on the order of 1 %–10 %), the corresponding errors inEMS-derived absorption range from ∼10 % to greater than100 %. Therefore, we examine the individual error sources in detail withthe 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 ofunderlying error in atmospheric applications. We introduce a new, modularframework for performing the truncation correction calculation that enablesthe consideration of additional physical processes such as reflection fromthe instrument's glass sampling tube, which was neglected in an earliertruncation model. We validate the truncation calculations againstcomprehensive laboratory measurements. It is demonstrated that the processof glass tube reflection must be considered in the truncation calculation,but that uncertainty still remains regarding the effective length of theoptical cavity. Another important source of uncertainty is the cross-calibration constant that quantitatively links the scattering coefficientmeasured by the instrument to its extinction coefficient. We presentmeasurements of this constant over a period of ∼5 months thatdemonstrate that the uncertainty in this parameter is very well constrainedfor some instrument units (2 %–3 %) but higher for others. We then use two example field datasets to demonstrate and summarize thepotential and the limitations of using the CAPS PMssa for measuringabsorption. The first example uses mobile measurements on a highway road tohighlight the excellent responsiveness and sensitivity of the instrument,which enables much higher time resolution measurements of relativeabsorption than is possible with filter-based instruments. The secondexample from a stationary field site (Cabauw, the Netherlands) demonstrateshow truncation-related uncertainties can lead to large biases in EMS-derivedabsolute absorption coefficients. Nevertheless, we use a subset of fine-mode-dominated aerosols from the dataset to show that under certain conditionsand despite the remaining truncation uncertainties, the CAPS PMssa can stillprovide consistent EMS-derived absorption measurements, even for atmosphericaerosols with high SSA. Finally, we present a detailed list ofrecommendations for future studies that use the CAPS PMssa to measureabsorption with the EMS method. These recommendations could also be followedto obtain accurate measurements (i.e., errors less than 5 %–10 %) of SSA and scattering and extinction coefficients with the instrument.

atmospheric measurement techniques

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