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 concentrations from background levels to high pollution. The traditional calibration is done by filling the instrument with a gas having a high refractive index and using the Rayleigh scattering of the gas molecules as a reference. Mostly halocarbon gases are used. Since one calibration 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 toxic; also they have a completely different wavelength dependence of the scattering coefficient as the aerosol, thus gases appear unsuitable for calibration. As an alternative, a calibration with an aerosol containing 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 having a light scattering coefficient in the measuring range of the Nephelometer 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 truncation errors and deviations due to different wavelength dependence of the calibration medium and the aerosol can be avoided. As a secondary standard an improved version of the original magnesium oxide disk calibration is suggested.