Laboratory Validation and Field Deployment of a Compact Single- Scattering Albedo (SSA) Monitor

An evaluation of the performance and accuracy of a Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor (CAPS PMssa, Aerodyne Res. Inc.) was conducted in an optical closure study with proven technologies: Cavity Attenuated Phase-Shift Particle Extinction Monitor (CAPS PMex, Aerodyne Res. Inc.); 3-wavelengh Integrating Nephelometer (TSI Model 3563); and 3-wavelength filter-based Particle Soot Absorption Photometer (PSAP, Radiance 10 Research). The evaluation was conducted by connecting the instruments to a controlled aerosol generation system and comparing the measured scattering, extinction, and absorption coefficients measured by the CAPS PMssa with the independent measurements. Three different particle types were used to generate aerosol samples with single-scattering albedos (SSA) ranging from 0.4 to 1.0 at 630 nm wavelength. The CAPS PMssa measurements compared well with the proven technologies. Extinction measurement comparisons exhibited a slope of the linear regression line for the full data 15 set of 0.96 (-0.02/+0.06), an intercept near zero, and a regression coefficient R 2 >0.99; whereas, scattering measurements had a slope of 1.01 (-0.07/+0.06), an intercept of less than +/-2×10 -6 m -1 (Mm -1 ), and a coefficient R 2 ~1.0. The derived CAPS PMssa absorption compared well to the PSAP measurements at low levels (< 70 Mm -1 ) for the small particle sizes and modest (0.4 to 0.6) SSA values tested, with a linear regression slope of 1.0, an intercept of -4 Mm -1 , and a coefficient R 2 =0.97. Comparisons at higher particle loadings were compromised by loading effects on the PSAP filters. For the SSA 20 measurements, agreement was highest (regression slopes within 1%) for SSA = 1.0 particles, though the difference between the measured values increased to 9% for extinction coefficients lower than 55 Mm -1 . SSA measurements for absorbing particles exhibited absolute differences up to 18%, though it is not clear which measurement had the lowest accuracy. For a given particle type, the CAPS PMssa instrument exhibited the lowest scatter around the average. This study demonstrates that the CAPS PMssa is a robust and reliable instrument for the direct measurement of the scattering and extinction 25 coefficients and thus SSA. This conclusion also holds as well for the indirect measurement of the absorption coefficient with the constraint that the accuracy of this particular measurement degrades as the SSA and particle size increases.


Introduction
Airborne aerosols impact climate directly though the interaction with incident solar light by scattering, generating a cooling 30 effect, or by absorbing it and reemitting infrared radiation, having a heating effect. According to Haywood and Shine (1995), the effect of aerosols on the atmospheric radiation budget in the visible spectral range depends on the aerosols optical depth (AOD), the single-scattering albedo (SSA), and the backscattered fraction (BF). The radiative forcing efficiency (RFE) describes the resulting aerosol direct forcing per unit AOD (Andrews et al., 2011;Haywood and Shine, 1995;Sheridan et al., 2012) and is widely used to describing the radiative impact of a given aerosol type. As an aerosol 35 Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2019-146 Manuscript under review for journal Atmos. Meas. Tech. Discussion started: 24 May 2019 c Author(s) 2019. CC BY 4.0 License.
intensive parameter the RFE value depends only on SSA and BF. As is stated in the latest IPCC report (Boucher et al., 2013), uncertainties in SSA and the vertical distribution of aerosol contribute significantly to the overall uncertainties in the direct aerosol radiative forcing, while AOD and aerosol size distribution are relatively well constrained.
The measurement of SSA requires the simultaneous but independent observation of two parameters since, by definition, the SSA is the ratio of the scattering to the extinction coefficient (where extinction is the sum of the scattering 40 and absorptionsee Equation (1) and (2); the index p refers to the contribution of aerosol particles to overall light extinction, which has also a contribution by gas molecules, identified by the index g not shown in the equation).
Measuring all three aerosol optical parameters independently allows for the closure of optical properties and thus the 45 determination of uncertainties of the involved instruments.
The aerosol optical parameters are typically measured in-situ by instruments such as Integrating Nephelometers (NEPH) for the scattering coefficient (Heintzenberg and Charlson, 1996); photoacoustic (see e.g., Lack et al. (2006) ;Arnott et al. (2006)) and filter-based methods such as the Particle-Soot Absorption Photometer (PSAP; Bond et al. (1999)), the Multi Angle Absorption Photometer (MAAP; Petzold and Schönlinner (2004)) and more recently the Tricolor Absorption 50 Photometer (TAP; Ogren et al. (2017)) for the absorption coefficient; and for the extinction coefficient, the Cavity Ring Down (CRD) technology (Moosmüller et al., 2005) or, since 2007, the Cavity Attenuated Phase Shift Particle Extinction Monitor (CAPS PM ex ) (Massoli et al., 2010). To measure the SSA using the optical closure approach involves separate instruments with different principles and uncertainties, leading to potential sources of significant errors and biases.
A novel instrument based on cavity attenuated phase-shift technology and incorporating an integrating sphere was 55 recently developed by Aerodyne Research, Inc. This novel instrument represents a major step forward in the observation of aerosol optical properties since it simultaneously measures two of the three aerosol optical parameters from the same air sample, reducing the potential sources of sampling biases (Onasch et al., 2015). The two main applications of the CAPS PM ssa instrument, apart from the direct measurement of scattering and extinction coefficients, are the indirect measurement of the aerosol absorption coefficient and the measurement of the single-scattering albedo. A few recent in-situ application 60 studies of the CAPS PM ssa instrument are already available (Corbin et al., 2018;Han et al., 2017). The present optical closure study intends to quantify uncertainties in the measurement of the primary aerosol optical properties and the resulting SSA by the CAPS PM ssa for several types of laboratory aerosol by applying a full set of established instrumentation for measuring the extinction (CAPS PM ex ), absorption (PSAP), and scattering (Integrating Nephelometer TSI Model 3563) coefficients at multiple wavelengths 65 2 Instruments and Methods

Instrumental Set-up
The laboratory study was conceived to evaluate the operational principle of the CAPS PM ssa and its performance and accuracy when compared to proven technologies. The instrumental set-up used is shown in Figure 1.
In this study, similar to previous work (Massoli et al., 2010;Petzold et al., 2013); two collision-type aerosol 70 generators (TSI Model 3076) were used; one containing a solution of deionized water and purely scattering aerosol, Ammonium Sulphate (AS), and a second containing absorbing aerosol, water-soluble colloidal graphite (Aquadag -ADfrom Agar Scientific) or Black Carbon (REGAL 400R Pigment Black -BCfrom Cabot Corporation). The SSA of the dispersed aerosol ranged from approximately 0.4 (pure AD or BC) to 1.0 (pure AS), with the modal value of the particle size distribution being below 100 nm in all cases. A drying tube filled with silica gel was positioned after each particle 75 generator in order to reduce the relative humidity below 30%. Once the samples were passed through the dryer, they entered a mixing chamber where effective ensemble particle SSA values of 0.4 < SSA < 1.0 could be produced by mixing aerosol flows containing both absorbing and scattering aerosols. The aerosol generation set-up specifications are shown in Table 1, whereas Table 2 compiles the information about the applied instrument and correction schemes.
Three mass flow controllers (MFC), one at each generator's head and a third after the mixing chamber, supplied 80 particle-free compressed air to the sample to both reach the desired humidity and particle number concentration and to make-up the flow required by the instruments. The particle number concentration was measured by a condensation particle counter (CPC).    To ensure an isoaxial, isokinetic sampling by all instruments, special sampling tips made of stainless steel were designed such that the sample air extraction tips were each concentrically placed along the centre line of the sample tube of 1 inch inner diameter. The inlet nozzles diameters are dimensioned such that the flow velocities in the sample tube and inside extraction tip nozzles match. Distances between the extraction points for the different instruments were 20 cm. 100 All scattering instruments were calibrated using CO 2 (high span gas) and particle-free air (low span gas), before starting the experiments. This procedure includes also, as recommended by the manufacturers, the calibration of scattering channel of the CAPS PM ssa , against the extinction channel of the instrument. For the filter-based absorption instruments, no calibration is necessary since they both operate with a blank filter in parallel as reference (see description in the subsections below). 105 The optical instruments were placed downstream of the generation system, as shown, and will be described in more detail in the following subsections.

Integrating Nephelometer
In this optical closure study, an integrating nephelometer (NEPH) of the type TSI Model 3563 was used. The NEPH collects scattering measurements both in the forward and backscatter directions at three wavelengths 450, 550, and 700 nm 110 (Heintzenberg et al., 2006). The NEPH data was corrected for truncation angle effects using the approach proposed by Massoli et al. (2009) for strongly light-absorbing aerosol and the approaches proposed by Anderson et al. (1996) and Müller et al. (2009) for predominantly light-scattering aerosols.

Particle-Soot Absorption Photometer
The PSAP is a filter-based three wavelength (467, 530, 660 nm) instrument, manufactured by Radiance Research, that 115 provides continuous measurement of the light absorption coefficient. The instrument uses two spots on a quartz fibre filter; one receives the particle containing sample, and the second clean air. The instrument measures then the difference in the transmission of light between a loaded and a blank filter spot (Bond et al., 1999). Absorption coefficient data were determined using the approach proposed by Ogren (2010

2.1.3
The CAPS PM ex 120 The CAPS PM ex system, described in detail and assessed in several studies, such as Massoli et al. (2010), Petzold et al. (2013) and Perim de Faria et al. (2017) measures light extinction by determining the change in signal phase shift caused by the introduction of particles into an optical cavity. The use of high reflectivity mirrors (reflectivity approx. 99.99%) in the optical cavity creates the long measurement path of approx. 2 km required to measure very low values of light extinction (LOD of 1-2 Mm -1 in 1 second sample period). 125

The CAPS PM ssa
The CAPS PM ssa (Onasch et al., 2015), uses the same principle to measure light extinction as the CAPS PM ex , but it also contains, located at the centre of the measurement cell, a 10 cm diameter integrating sphere capable of measuring light scattering on the same aerosol sample, as shown in Figure 2. The integrating sphere acts as an integrating nephelometer, which measures the scattering of light by particles at all angles, only excluding the near 0 and near 180° 130 angles since at these directions the opening of the extinction chamber is located, allowing the sample and light beam to pass through. The sphere shows 98-99% Lambertian reflectance efficiency due to its high reflectivity coating (Avian D from Avian Technologies). The usage of an integrating sphere increases the collection of scattered light at the photomultiplier compared to a traditional cosine corrected detector arrangement.
The scattering channel is calibrated against the extinction channel using small particles (<250 nm) that have 135 SSA=1.0, in this case ammonium sulphate, and set equal to the extinction measurement. Thus, the monitor should be thought of as providing separate extinction and SSA values with the scattering channel a derived measurement. This calibration procedure also allows the user to prove monitor linearity over a wide range of optical extinctions without the limitation of using individual gases with sometimes not particularly well-known Rayleigh scattering coefficients.
As in the CAPS PM ex , the sample flow is set to 0.85 lpm and is controlled by a critical orifice. The measurement 140 sample enters the chamber in one end and exits through an opening located in the other end flowing through a glass tube inside the integrating sphere ( Figure 2). The mirrors are kept particle-free by a continuously flowing purge flow (25 cm 3 min -1 ).

145
The baseline determination system is identical to the one used in the CAPS PM ex , in which filtered and thus particle-free sample air fills the measurement chamber and is used to quantify contributions of gas molecules to the instrument response by Rayleigh scattering and potential absorption of light, and to determine interferences of system

Data Treatment
All multi-wavelength instruments were adjusted to match the other instruments' wavelengths for the intercomparison by using the Ångström exponent approach; see Equation (3) and (4), where å is the Ångström exponent, σ is the optical property measured (extinction, scattering or absorption coefficient), x and y are the operating wavelengths of the instrument, and w refers to the wavelength, to which the property should be adjusted. For a better understanding of the wavelength adjustment, the complete description is given in Figure 3 from 160 Petzold et al. (2013).
All instruments provide 1 second resolution data. Data was collected over 5 minutes for each experimental point to remove any effect of differences in response times and fluctuations in the aerosol generation system. The data was averaged for each extinction/scattering/absorption level, and the standard deviation was calculated from the mean.
Standard linear regression analysis was performed for the mean values of each level. For the cases with the 165 standard deviation of the intercept value being higher than the value itself, the regression model interception was forced to zero intercept, since the intercept value shows no significant difference to zero.

Results and Discussion
In this section, we present the results and relevant discussion of findings for the optical closure study. All the measurements presented here were corrected to the CAPS PM ssa operational wavelength of 630 nm. 170

Extinction Coefficient
The extinction coefficient measured by the CAPS PM ssa was analysed in comparison with proven technologies. On the direct measurement of σ ep , we compared the two CAPS systems for AS and AD (Petzold et al., 2013). The direct measurement of σ ep from the CAPS PM ssa was also compared with the indirect measurement given by the sum of the absorption coefficient measured by the PSAP with the scattering coefficient measured by the NEPH for BC, AD, and MIX 175 (as defined in Table 1). For AS with the measured SSA value of 1.0, extinction coefficients provided by the CAPS extinction channels and scattering coefficients provided by the CAPS scattering channel and the NEPH instrument are used for the evaluation of the light scattering measurements in the next subsection. The time series for the extinction channels are shown in Figure 3 and the averages and standard deviations for each test point are shown in Table A1 in the supplemental information. The higher variability observed in the last plot of the figure is due to particle load fluctuations 180 from generation system when operating at very high loads.  185 Figure 4 shows the scatter plot of the measured extinction coefficient for the two CAPS systems for AD and AS and the comparison with the sum of the NEPH and PSAP for AD and BC. The best results for the AD and BC were found when applying the Massoli et al. (2009) correction with the assumption, that no particle size cut has been used for the inlet system (no-cut approach) to the NEPH data, and Virkkula (2010) for strongly light-absorbing aerosols AD and BC to the 190 PSAP data. For the mixture, the applied corrections were Anderson et al. (1996) for the NEPH data and Ogren (2010) for the PSAP data. The extinction channels from the two CAPS and the sum of the NEPH and PSAP (PSAP-NEPH) signals show a good agreement for all aerosol types, with linear regression slopes (m) between 0.94 and 1.02 and correlation coefficients above 0.99 (all regression analysis data for the averaged values of each level is presented in Table 3 together with their standard deviation). For the linear regression analysis of the full data set including all types of aerosols, the slope 195 found was 0.96 (R 2 =0.99) for the comparison of the CAPS PM ssa extinction data with the sum of NEPH and PSAP data, and 0.97 (R 2 =1.00) for the comparison of the CAPS PM ssa and CAPS PM ex extinction data. The slopes of the regression analysis and their standard deviation are shown in Figure 5 as a function of the sampled aerosol single-scattering albedo. As it can be seen there is no systematic difference in the slope with increase or decrease of the aerosol SSA.  It is worth noting that for the particular instruments used in our study, the standard deviation for the extinction data of the CAPS PM ssa is larger than for the extinction data provided by the CAPS PM ex (horizontal error bars). This 205 finding is shown in the histogram of the extinction channel from one measurement level (in this case the used dataset refers to the 25 Mm -1 target-level for AD aerosol) for both equipment ( Figure 6). Thus, the precision of this particular CAPS PM ssa is lower than the precision of the CAPS PM ex . Regarding the precision of the CAPS PM ssa in comparison with proven technologies, the standard deviation found in this study for both cases are comparable. The precision in the CAPS PM ex and PSAP-NEPH extinction measurements found in this study are very similar to the one found by Petzold et al. (2013). 210

Scattering Coefficient
The scattering channel of the CAPS PM ssa was evaluated in comparison to the NEPH measurements for AD, BC, AS, and MIX (Table 1). The time series of scattering coefficient data for the various aerosol runs is shown in Figure 7. 215 Supplemental Table A2 shows the average and 1-σ standard deviation obtained for the targeted scattering coefficient levels.
There is no systematic error found neither in the average nor in the standard deviation of the measured values. The precision of both instruments for the measurement of scattering coefficient is very similar. 220 Figure 8 shows the scatter plot of the 1-second average and standard deviation of the CAPS PM ssa against NEPH.
As it can be seen from Figure 8 and the data compiled in Table 4, the agreement with the NEPH measurements is excellent, with less than 8% difference in the slope, offset smaller than 2.00 Mm -1 and correlation coefficient of 1.00 for all aerosol types. The slope value and standard deviation as a function of SSA is shown in Figure 9. For the AD, BC and Mix cases, 225 the NEPH data was corrected with the Massoli et al. (2009) approach. For the AS case both the Anderson et al. (1996) and Müller et al. (2009) were applied and the results given were practically the same, less than 2% in the slope and less than 1.00 Mm -1 difference in the offset. For the overall measurement linear regression model, including all types of aerosols, the slope found was 1.01 (R 2 =1.00) for the comparison of the CAPS PM ssa with the NEPH.    Table 4.

Absorption Coefficient 235
In spite of the fact that the CAPS PM ssa is not capable of directly measuring the absorption coefficient, the values can be derived as the difference of the extinction and the scattering coefficients; see Equation (1). From the difference of the two CAPS PM ssa channels the calculated absorption coefficients were compared to the direct measurement by the PSAP. In this analysis, when operating with a mixture of AS and AD, the PSAP data were treated using the correction from Ogren (2010). The time series for the measurement of the different aerosols are shown in Figure 10 whereas Supplemental Table  240 A3 shows the average and 1-σ standard deviation obtained for the targeted absorption coefficient levels. The scatter plot for the average measured values from both methods for all levels is shown in Figure 11, whereas the results of the linear regression analysis are compiled in Table 5. The agreement between the methods is good, with deviations below 11% in the slope, and offsets less than 2.0 Mm -1 . The correlation coefficient is above 0.98 for all cases.
For the full data set of CAPS PM ssa and PSAP absorption coefficient data including all types of aerosols, the slope is 0.91 250 with a correlation coefficient of R 2 =0.98. Figure 11 demonstrates that for higher absorption coefficients, the two methods deviate more strongly than for lower absorption coefficients. This is mainly caused by the correction algorithm applied to the PSAP data (also seen on Figure 10); filter loading corrections are significantly larger for higher absorption coefficient levels than for lower absorption coefficient levels. If the three data points for higher absorption coefficient data ( ap > 70 Mm -1 ) are removed from the regression analysis, the slope value increases to 1.00 (R 2 =0.97), although with an offset of -255 3.64. This finding proves that, although the CAPS PM ssa cannot directly measure aerosol light absorption, it provides a rather reliable measurement of the absorption coefficient of the sampled aerosol, at least for the small particle sizes and intermediate SSA values sampled in this study. The accuracy of absorption measurements by the two channels of the CAPS PM ssa may be significantly reduced for weakly absorbing but large-sized and irregularly shaped mineral dust particles. 260 Table 5. Linear regression parameters including standard deviation of the mean, intercept, standard intercept, and R 2 for the comparison of the CAPS PM ssa and the PSAP instruments.

Aerosol
Reference

Single Scattering Albedo Measurement
The ultimate property targeted by the CAPS PM ssa is the aerosol single-scattering albedo. Figure 12 shows the average and standard deviation of the SSA measured by the CAPS PM ssa and the applied proven technologies for each aerosol type containing a light-absorbing fraction, at the different extinction coefficient levels. The values for each level are also 270 compiled in Supplemental Table A4. Analysing the error propagation for the measured parameters (extinction and scattering coefficients), the increase of the uncertainty at the lower extinction coefficient levels is also visible for both CAPS PM ssa and proven technologies; see Table 6 for details. From the experimental set-up, it was observed that the particle generation system was lightly unstable 285 when operating at lower extinction/scattering levels, resulting in higher variations of the absolute values, which could explain such higher error propagation. This supports the previous findings that the CAPS PM ssa accuracy is very good and comparable to the proven technologies.

Summary and Outlook
An optical closure study has been performed using different types of aerosols (pure scattering, strongly absorbing, and mixture) to evaluate the performance and accuracy of the recently launched Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor.
The results from the instrument intercomparison with proven technologies (CAPS PM ex , NEPH, and PSAP) show 295 a very good agreement for all aerosol types, with accuracy of 96% and 99% for the extinction coefficient and scattering coefficient channels, respectively, for all aerosol types. The small deviation of 4% observed in the extinction channel between the CAPS PM ssa and PSAP-NEPH combination originates from the applied correction algorithm to the PSAP data, since it is a logarithmic function of the filter transmission leading to deviations in the dataset. For the evaluation of the performance for each aerosol individually, the extinction channel shows accuracy between 94% and 98%; and the 300 scattering channel, between 94% and 98%. These values are very similar to those found by Petzold et al. (2013) for the CAPS PM ex .
Regarding the application of the CAPS PM ssa for the measurement of the absorption coefficient and singlescattering albedo, the instrument has shown good performance on both sides. The accuracy of the absorption coefficient measurement by the CAPS PM ssa in comparison with the PSAP was 91%, as obtained for the linear regression analysis for 305 all investigated aerosol types and aerosol loadings. The large difference observed here comes from the correction scheme applied to the PSAP data at high loadings, as stated earlier. It is possible to observe that the higher deviations occur at high absorption coefficient, also where the transmission of the filter has a steeper decrease. Once the linear regression analysis excludes the points where the average absorption coefficient was higher than 70 Mm -1 , the slope approaches 100% agreement between the two technologies. For the measurement of SSA, the CAPS PM ssa showed a very good stability for 310 all measured σ ep levels, better than the PSAP-NEPH combination. The measured values are within what is expected for the different types of aerosols (0.4 for strongly absorbing aerosols and 1.0 for purely scattering aerosols).
The results reported from our study demonstrate that the CAPS PM ssa is a very robust and reliable instrument for the direct measurement of the scattering and extinction coefficient, as well as for the indirect measurement of the absorption coefficient and single scattering albedo. 315

Author Contributions
JP, UB, and AP designed the study and prepared the manuscript, with contributions from all co-authors. AF and TO provided technical details of the instrumentation and contributed to the interpretation of the study results.