A study of the aerosol of Santiago de Chile III: Light absorption measurements

The light absorption coefficient of the aerosol of Santiago de Chile has been measured by means of the integrating plate method. Values ranging between 0.01 and 0.2 km−1 have been found. Variations of the absorption coefficient by a factor of 8 within a few hours have frequently been found and have their causes in the specific meteorology of the Santiago air basin. In the summer a minimum occurs between 1 and 4 a.m. and a maximum between 8 and 10 a.m., whereas in the winter a maximum is found during the night and minimum values are in the afternoon with a smaller amplitude. Similar variations of the light absorption coefficient have been found also inside uninhabited rooms, since the aerosol was transported there almost without any losses. The daily pattern is similar but delayed and has a lower mean value and amplitude. When converting the light absorption to mass of black carbon, concentrations of elemental carbon of 1.3–25 μg m−3 were obtained. This is similar to other large cities, where a considerable part of the mobile sources are equipped with diesel motors. The light absorption contributes to about 15% of the light extinction of the aerosol, and considering an equal scattering of the soot particles, the contribution of soot to light attenuation in the atmosphere is about 30%. The single scattering albedo of the aerosol is 0.85, thus the heating effect of the absorption dominates. The light absorption coefficient is wavelength dependent with an average relationship of σa = k(λλ0)−α, the exponent being α = 0.92. The exponent of the wavelength dependence of the extinction coefficient of the aerosol was 1.22.