An integrating sphere spectral system to measure continuous spectra of aerosol absorption coefficient

An integrating sphere spectral system has been developed that allows the measurement of continuous spectra of light absorption coefficients, σ a ( λ ) of atmospheric aerosols. This system is based on the filter attenuation method covering the spectral wavelength range 320–800 nm with a variable spectral resolution. The sample filters had to be diluted in chloroform previous to the light attenuation measurements and hence the system cannot measure continuously but only on the basis of daily sampling. The system was calibrated using known amounts of absorbing and non-absorbing industrial substances at relevant atmospheric absorption levels. The resulting values of σ a ( λ ) were compared with those delivered by a commercial photometer (particle soot absorption photometer) in the laboratory and also with field measurements, with a maximum discrepancy of 12.5% for atmospherically relevant values. The measurement uncertainty of the integrating sphere system is about 10% maximum. The importance of obtaining continuous spectral data is the possibility to determine the Ångström absorption exponent, α a , in different spectral ranges, including the UV. This exponent can then be used to help in obtaining the aerosol composition information.