Laboratory validation of a compact single-scattering albedo (SSA) monitor
2021
Abstract. An evaluation of the performance and relative accuracy of
a Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor (CAPS
PM SSA ; Aerodyne Research, Inc.) was conducted in an optical-closure study with proven technologies: Cavity Attenuated Phase-Shift Particle Extinction Monitor (CAPS PM ex ; Aerodyne Research, Inc.), three-wavelength integrating nephelometer (TSI Model 3563) and three-wavelength filter-based Particle Soot Absorption Photometer (PSAP; Radiance Research Inc.). 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 PM SSA with the independent measurements. Three different particle types were used to generate aerosol samples with single-scattering albedos (SSAs) ranging from 0.4 to 1.0 at 630 nm wavelength. The CAPS PM SSA measurements compared well with the
proven technologies. Extinction measurement comparisons exhibited a slope of the linear regression line for the full dataset between 1.05 and 1.01, an intercept below ± 1.5 × 10 - 6 m −1 ( ±1.5 Mm −1 ), and a regression coefficient R2>0.99 , whereas scattering measurements had a slope between 0.90 and 1.04, an intercept of less than ± 2.0 × 10 - 6 m −1 (2.0 Mm −1 ), and a coefficient R2>0.96 . The derived CAPS PM SSA absorption compared
well to the PSAP measurements for the small particle sizes and modest (0.4
to 0.6) SSA values tested, with a linear regression slope between 0.90 and
1.07, an intercept of ± 3.0 × 10 - 6 m −1 (3.0 Mm −1 ), and a coefficient R2>0.99 . For the SSA measurements, agreement was highest (regression slopes within 1 %) for SSA =1.0 particles at extinction levels of per tens of inverse megameters and above; however, as extinctions approach 0, small uncertainties in the baseline can introduce larger errors. SSA measurements for absorbing particles exhibited absolute differences up to 18 %, though it is not clear which measurement
had the best relative accuracy. For a given particle type, the CAPS
PM SSA instrument exhibited the lowest scatter around the average. This
study demonstrates that the CAPS PM SSA is a robust and reliable
instrument for the direct measurement of the scattering and extinction
coefficients and thus SSA. This conclusion also holds for the
indirect measurement of the absorption coefficient with the constraint that
the relative accuracy of this particular determination degrades as the SSA
and particle size increases.
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