CALIBRATION OF INTEGRATING NEPHELOMETERS IN THE POST-HALOCARBON ERA

The Integrating Nephelometer is a widely used instrument which permits the determination of the scattering coefficient of aerosols, having c oncentrations from background levels to high pollution. The traditiona l calibration is done by filling the instrument with a gas having a hi gh refractive index and using the Rayleigh scattering of the gas molec ules as a reference. Mostly halocarbon gases are used. Since one calib ration of the instrument has the potential of destroying 40-80 tons of ozone, alternate methods for calibration have been investigated. Most gases with a high refractive index are difficult to handle or are tox ic; also they have a completely different wavelength dependence of the scattering coefficient as the aerosol, thus gases appear unsuitable f or calibration. As an alternative, a calibration with an aerosol conta ining particles which have no light absorption has been developed. The light extinction coefficient of the aerosol can be measured easily in a 1 m transmission cell, and with a subsequent dilution an aerosol ha ving a light scattering coefficient in the measuring range of the Neph elometer is obtained. By varying the dilution many calibration points can be obtained. Since the calibration aerosol can be adjusted to have similar optical properties to the aerosol to be measured, the truncat ion errors and deviations due to different wavelength dependence of th e calibration medium and the aerosol can be avoided. As a secondary st andard an improved version of the original magnesium oxide disk calibr ation is suggested.