Problems in comparing particulate spectrometer and visibility meter data

During the course of a study of vertical profiles of aerosol properties in low stratus clouds, data were acquired simultaneously with particulate spectrometers and with a visibility meter. Calculations made from the particulate data using Mie theory did not agree with extinction as inferred from the visibility meter data when the aerosol extinction was very high or very low. Several possible explanations for this discrepancy were investigated. It was found that the limited acceptance angle of the visibility meter, a form of integrating nephelometer, can contribute to the problem at high extinction values. In low extinction conditions a peculiarity of the electrooptical design of the particulate spectrometer may cause it to lead to serious errors in estimates of aerosol density.