Modeling and measurements of angular truncation for an aerosol albedometer

Abstract

In this work, we examine the angular truncation behavior and present correction factors for the aerosol albedometer previously developed in our laboratory. This new instrument makes simultaneous measurement of extinction and scattering coefficients (b ext and b scat) on dispersed aerosol samples. The aerosol extinction coefficient is measured with cavity ring-down spectroscopy (CRDS), and the scattering coefficient is determined through the integrating sphere nephelometer. However, all nephelometers are not able to collect light scattered from an aerosol sample very near the forward (0˚) and reverse (180˚) directions, due to the geometrical constraints. This can result in systematic underestimation of scattering coefficient known as truncation error. In order to account for this problem and describe scattering by aerosols more precisely, correction factors (C) for this angular non-ideality have been theoretically developed. Truncation angles (θ) were calculated upon consideration of the geometry of the sphere nephelometer. As truncation error largely depends on particle size and refractive index, C values were computed for a series of spherical, homogeneous aerosol particles with different known particle sizes and refractive indices by Lorenz-Mie theory. Measurements on size-selected, laboratory generated aerosols of known size and composition allowed empirical measurement of truncation correction factors to compare with the Mie model results. Results indicate the model we built overestimates the fraction of light not collected by the sphere. Empirically observed correction factors of ≤ 1.12 for particles with size parameters (α) < 6 were determined. In addition, the effect of number of particles within the probe beam on the suitability of correction factors was also examined. Observations support the hypothesis that particles are rapidly transported / mixed through the probe beam, and measurement integration times as short as 52 s yield data that is indistinguishable from the probe region being homogeneously filled with aerosol, even at very low particle concentrations.

Single Scattering Albedo Monitor for Airborne Particulates

“…The model assumes spherical particles and makes the additional assumption that light scattered from particles along the optical path within the scattering volume can be detected with equal efficiency except for light scattered into the entrance and exit of the scattering cell. We found that this approach over-predicts the truncation correction, something also noted by Qian, et al (2012) who employ a CRD system with similar geometry. Once we allowed the effective pathlength to slightly increase (10% or 1 cm) past the nominal boundaries of the scattering volume, the agreement between modeled and predicted truncation, shown in Figure 6, improved.…”

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

“…This operation assumes that the aerosol particles in the instrument are homogeneously distributed along the central z axis of the glass sampling tube, which is reasonable assumption to make for the aerosol number concentrations typically observed in the atmosphere (Qian et al, 2012). For concentrations much lower than this longer averaging times could be used to avoid any noise issues related to inhomogeneity.…”

Atmospheric black carbon can exhibit enhanced light absorption at high relative humidity

“…Left -Hygroscopic growth factor (G.F.) model employed in this study (red) compared to select observational data from the literature (Pan et al, 2009;Swietlicki et al, 2008;Massoli et al, 2009;Santarpia et al, 2005) and predicted behavior for pure ammonium sulfate (Dp = 390 nm). Right -Correction factor for integrating sphere nephelometer derived from hygroscopic growth model and Qian et al (2012). Scattering coefficients observed were corrected by this multiplier to account for angular truncation effects.…”