The effect of refractive index on size distributions and light scattering coefficients derived from optical particle counters

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 mu 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. Published by Elsevier Science Ltd.