PERFORMANCE-CHARACTERISTICS OF A HIGH-SENSITIVITY, 3-WAVELENGTH, TOTAL SCATTER BACKSCATTER NEPHELOMETER/

As designed in the 1940s by Beuttell and Brewer, the integrating nephe lometer offers a direct method of measuring light scattering by airbor ne particles without assumptions about particle composition, shape, or physical state. A large number of such instruments have been deployed ; however, only a limited number of validation experiments have been a ttempted. This paper reports a set of closure experiments in which a g as-calibrated nephelometer is used to measure the scattering coefficie nt of laboratory-generated particles of known size and refractive inde x. Specifically, it evaluates the performance of a high-sensitivity, t hree-wavelength, total scatter/backscatter integrating nephelometer (T SI, Inc., model 3563). Sources of uncertainty associated with the gas- calibration procedure, with photon-counting statistics, and with nonid ealities in wavelength and angular sensitivity are investigated. Tests with particle-free gases indicate that noise levels are well predicte d by photon-counting statistics and that the nephelometer response is linear over a wide range of scattering coefficients. Tests with partic les show average discrepancies between measured and predicted scatteri ng of 4%-7%. Error analysis indicates that these discrepancies are wit hin experimental uncertainty, which was dominated by particle generati on uncertainty. The simulation of nephelometer response, which is vali dated by these tests, is used to show that errors arising from nephelo meter nonidealities are less than 10% for accumulation-mode or smaller particles (i.e., size distributions for which the volume mean diamete r is 0.4 mu m or less) and that significant differences exist between the total scatter and backscatter uncertainties. Based on these findin gs, appropriate applications of the model 3563 nephelometer are discus sed.