THE EFFECT OF REFRACTIVE INDEX ON SIZE DISTRIBUTIONS AND LIGHT SCATTERING COEFFICIENTS DERIVED FROM OPTICAL PARTICLE COUNTERS

Abstract The effect of refractive index on particle size distributions measured by optical particle counters is examined. Similar to previous investigations, it is found that optical counters undersize ambient particles because the refractive index of these particles is generally lower than that of the latex particles commonly used for the calibration of optical counters. The maximum undersizing is found to occur when particle sizes are comparable to the wavelength of light used in the measurement. A new approach for modeling the effect of refractive index on the sizing of optical counters is presented. Previously derived optical response functions are compared and a generalized formulation is proposed which includes the existing response functions as special cases. Algorithms are presented for correcting size distributions measured by optical counters for the difference between the refractive index of ambient and calibration particles. Data collected by a Passive Cavity Aerosol Spectrometer (PCASP) and by an integrating nephelometer are compared. Light scattering coefficients calculated from the optical probe data uncorrected for the effect of refractive index differ from those measured by the integrating nephelometer by a factor of 2. An iterative procedure that adjusts the PCASP-measured size distribution for the effect of refractive index is used to derive the best agreement between calculated and observed light scattering coefficients. The refractive indices of aerosols at wavelength of 0.45 μ m that best fit the data vary between 1.3 and 1.5, with an average of 1.41. The relative importance of the underestimation of light scattering coefficients calculated from the PCASP-measured size distributions due to the refractive index and the size truncation effect are evaluated. The former is found to be more important than the latter. Implications of this study for addressing aerosol shortwave radiative forcing and potential uncertainties relevant to this study are discussed.

[1]  Peter H. McMurry,et al.  Optical counter response to monodisperse atmospheric aerosols , 1991 .

[2]  M. Mishchenko,et al.  Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids , 1997 .

[3]  H. V. Hulst.  Light Scattering by Small Particles , 1957 .

[4]  Y. Liu,et al.  Anomalous diffraction theory for arbitrarily oriented finite circular cylinders and comparison with exact T-matrix results. , 1998, Applied optics.

[5]  M. Hartmann,et al.  Light scattering by small particles. Von H. C. VANDE HULST. New York: Dover Publications, Inc. 1981. Paperback, 470 S., 103 Abb. und 46 Tab., US $ 7.50 , 1984 .

[6]  Annmarie Eldering,et al.  An air monitoring network using continuous particle size distribution monitors: Connecting pollutant properties to visibility via Mie scattering calculations , 1994 .

[7]  H. Auvermann,et al.  Response characteristics of knollenberg light-scattering aerosol counters , 1979 .

[8]  Measured aerosol light scattering coefficients compared with values calculated from EAA and optical particle counter measurements: Improving the utility of the comparison , 1988 .

[9]  K. T. Whitby,et al.  Multiwavelength nephelometer measurements in Los Angeles smog aerosol. I. Comparison of calculated and measured light scattering , 1972 .

[10]  C. V. Mathai,et al.  Estimation of atmospheric aerosol refractive index , 1980 .

[11]  B. G. Schuster,et al.  Detection and sizing of small particles in an open cavity gas laser. , 1972, Applied optics.

[12]  N. Kreisberg,et al.  Atmospheric Size Distributions Measured by Differential Mobility Optical Particle Size Spectrometry , 1998 .

[13]  D. Garvey,et al.  Response Characteristics of the Particle Measuring Systems Active Scattering Aerosol Spectrometer Probe (ASASP–X) , 1983 .

[14]  Robert J. Charlson,et al.  Performance Characteristics of a High-Sensitivity, Three-Wavelength, Total Scatter/Backscatter Nephelometer , 1996 .

[15]  S. Schwartz.  The whitehouse effect—Shortwave radiative forcing of climate by anthropogenic aerosols: an overview , 1996 .

[16]  S. Gassó,et al.  Aerosol measurements in the Arctic relevant to direct and indirect radiative forcing , 1996 .

[17]  Jost Heintzenberg,et al.  Design and Applications of the Integrating Nephelometer: A Review , 1996 .

[18]  H. Hansson,et al.  Electro-optical Detection of External Mixtures in Aerosols , 1990 .

[19]  R. Charlson,et al.  Optical characteristics of atmospheric aerosols , 1981 .

[20]  P. Quinn,et al.  Closure in tropospheric aerosol-climate research : A review and future needs for addressing aerosol direct shortwave radiative forcing , 1996 .

[21]  W. Arnott,et al.  Particle size distribution retrieval from multispectral optical depth: Influences of particle nonsphericity and refractive index , 1999 .

[22]  S. F. Marshall,et al.  Comparison of measured and calculated aerosol properties relevant to the direct radiative forcing of tropospheric sulfate aerosol on climate , 1995 .