Truncation and angular-scattering corrections for absorbing aerosol in the TSI 3563 nephelometer

The nephelometer, the primary instrument used for accurate in-situ scattering measurements, suffers from small errors due to truncation and angular nonidealities in the light source. Because that error depends on particle size, it has been traditionally estimated from the wavelength dependence of scattering, which also depends on particle size. Absorption by particles alters the scattering wavelength dependence, so the traditional correction is in error by 1-5% for absorbing particles, particularly when that absorption is wavelength-dependent. Single-scattering albedos under 0.9 and absorption ngstrom exponents above 2 can have errors of over 2%. This problem will occur only very near sources, including laboratory measurements of combustion aerosol. For these situations, we suggest the correction should be calculated using Mie theory and an assumed refractive index, resulting in less than a 2% error. Scattering wavelength dependence for absorbing particles should not be used as a measurement of particle size.