Modeling and measurements of angular truncation for an aerosol albedometer

F. Qian; L. Ma; J. E. Thompson

doi:10.2971/jeos.2012.12021

Journal of the European Optical Society - Rapid publications, Vol 7 (2012)

Modeling and measurements of angular truncation for an aerosol albedometer

F. Qian, L. Ma, J. E. Thompson

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 (bext and bscat) 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 (0Â°) and reverse (180Â°) 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 (q) 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 â‰¤ 1.12 for particles with size parameters (a) < 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.

Full Text: PDF

Citation Details

Cite this article

References

J. E. Thompson, N. Barta, D. Policarpio, and R. DuVall, "Development of a Fixed Frequency Aerosol Albedometer," Opt. Express 16(3), 2191-2205 (2008).

K. Dial, S. Hiemstra, and J. E. Thompson, "Simultaneous Measurement of Optical Scattering and Extinction on Dispersed Aerosol Samples," Anal. Chem. 82, 7885-7896 (2010).

J. E. Thompson, "Aerosol Albedometer: A Tool for Measuring Optical Scattering and Extinction of Dispersed Aerosols," G.I.T. Laboratory Journal , 9-10 (2010).

H. Redmond, and J. E. Thompson, "Evaluation of a Quantitative Structure Property Relationship (QSPR) for Predicting Mid-Visible Refractive Index of Secondary Organic Aerosol (SOA)," Phys. Chem. 13, 6872-6882 (2011).

J. M. Haywood, and K. P. Shine, "The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget," Geophys. Res. Lett. 22(5), 603-606 (1995).

J. Haywood, and O. Boucher, "Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review," Rev. Geophys. 38(4), 513-543 (2000).

J. D. Smith, and D. B. Atkinson, "A portable pulsed cavity ring-down transmissometer for measurement of the optical extinction of the atmospheric aerosol," Analyst 126, 1216 (2001).

J. E. Thompson, B. W. Smith, and J. D. Winefordner, "Atmospheric Aerosol Measurements by Cavity Ringdown Turbidimetry," Aerosol Sci. Tech. 37(3), 221-230 (2003).

T. J. A. Butler, D. Mellon, J. Kim, J. Litman, and A. J. Orr-Ewing, "Optical-feedback cavity ring-down spectroscopy measurements of extinction by aerosol particles," J. Phys. Chem. A 113(16), 3963-3972 (2009).

N. Lang-Yona, Y. Rudich, E. Segre, E. Dinar, and A. Abo-Riziq, "Complex refractive indices of aerosols retrieved by continuous wave-cavity ring down aerosol spectrometer," Anal. Chem. 81(5), 1762-1769 (2009).

J. E. Thompson, B. W. Smith, and J. D. Winefordner, "Monitoring Atmospheric Extinction Through Cavity Ringdown Turbidity," Anal. Chem. 74(9), 1962-1967 (2002).

V. Bulatov, M. Fisher, and I. Schechter, "Aerosol analysis by cavityring- down laser spectroscopy," Anal. Chim. Acta 466, 1-9 (2002).

R. Varma, H. Moosmuller, and W. P. Arnott, "Toward an ideal integrating nephelometer," Opt. Lett. 28(12), 1007-1009 (2003).

J. Heintzenberg, A. Wiedensohler, T. M. Tuch, D. S. Covert, P. Sheridan, J. A. Ogren, J. Gras, et.al, "Intercomparisons and Aerosol Calibrations of 12 Commercial Integrating Nephelometers of Three Manufacturers," J. Atmos. Ocean. Tech. 23, 902-914 (2006).

T. L. Anderson, and J. A. Ogren, "Determining aerosol radiative properties using the TSI 3563 integrating nephelometer," Aerosol Sci. Tech. 29, 57-69 (1998).

H. E. Redmond, K. D. Dial, and J. E. Thompson, "Light scattering and absorption by wind blown dust: Theory, measurement, and recent data," Aeolian Research 2(1), 5-26 (2010).

L. Ma, D. Hsieh, D. Holder, T. Zobeck, C. Morgan, and J. E. Thompson, "Optical Properties of Aeolian Dusts Common to West Texas," Aeolian Research 3, 235-242 (2011).

G. Mie, "Beitrage zur Optik trüber Medien spezieller kolloidaler Metallösungen," Ann. Phys. 25, 377-445 (1908).

H. C. de Hulst, Light Scattering by Small Particles (Dover, New York, 1981).

R. T. Wang, and H. C. van de Hulst, "Rainbows: Mie computations and the Airy approximation," Appl. Optics 30, 106-117 (1991).

C. F. Bohren, and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley Interscience, New York, 1983).

philiplaven.com/mieplot.htm

J. E. Thompson, and K. Myers, "Cavity Ring-Down Lossmeter using a Pulsed Light Emitting Diode Source and Photon Counting," Meas. Sci. Technol. 18, 147-154 (2007).

S. Zadoo, and J. E. Thompson, "Rayleigh Scattering Measurements of Several Fluorocarbon Gases," J. Environ. Monitor. 13(11), 3294- 3297 (2011).

A. Pettersson, E. R. Lovejoy, C. A. Brock, S. S. Brown, and A. R. Ravishankara, "Measurement of aerosol optical extinction at 532 nm with pulsed cavity ring down spectroscopy," J. Aerosol. Sci. 35, 995-1011 (2004).

A. E. Siegman, Lasers (University Science Books: Mill Valley, California, 1986).

F. Qian, Truncation Correction for the Aerosol Albedometer: Development and Laboratory Validation of Scattering Correction Factors (M.S. Thesis, Texas Tech University, 2011).