Comparison of telephotometer and nephelometer measurements of atmospheric attenuation and its relationship to aerosol size distribution

This paper presents the results of a six week study of light attenuation by the atmospheric aerosol in the Calgary urban area during the winter of 1979–1980. A Vista Ranger-3030 multiwavelength telephotometer measured the apparent contrast of a dark high rise building at the city center, 6 km away, from which the extinction coefficient was deduced. At the midpoint of this 6 km observation path, simultaneous measurements of the scattering coefficient and the aerosol particle size distribution were also made using an integrating nephelometer (MRI 1550) and an active scattering aerosol spectrometer (ASAS-300). The measured extinction and scattering coefficients give a fair agreement with one another even though the former was obtained from a long path measurement while the latter was from a point measurement. The observed scatter between telephotometer and nephelometer results is attributed to some absorption in the aerosol studied. An examination of the Angström exponent, α in the empirical wavelength dependence relation of the scattering coefficient, a (λ) = k λ −a and the power law exponent, 1 in the aerosol particle size distribution, n( R) = cR − v shows that the theoretically predicted relation α = v −3 holds good for Calgary aerosols ( k and c being constants). The average values of α, θ and ( v − α) are 0.96±0.27. 3.94±0.29 and 3.00 ±0.20 respectively and are in agreement with values reported by other workers.