Long-term changes in aerosol radiative properties at Armilla (Spain)

The radiative characteristics of atmospheric aerosol over a suburban environment on the outskirts of a non-industrialized, medium size city in southeastern Spain (Granada, 37.18°N, 3.58°W, 660 m a.m.s.l.) are investigated using 5 years (1994–1998) of continuous spectral photometric measurements in the range 368–778 nm. From these measurements, the aerosol optical depth has been determined. In this paper we analyse the aerosol radiative properties derived from the aerosol size distributions, retrieved in a previous study, using Mie theory. The asymmetry factor values are representative of locations with moderate contamination, varying from 0.664 to 0.742 at 500 nm. The single scattering albedo evidenced a seasonal pattern, with lower values associated with winter conditions. This parameter varies from 0.928 to 0.966, with mean value of 0.948±0.011 at 500 nm. On other hand, the analysed series shows a pronounced change in aerosol radiative characteristic since 1996, which can be associated with urban encroachment on the suburban area. The relationship between g and the aerosol optical depth and its spectral dependence were analysed. Thus, different dependences of g on the Angstrom exponent were encountered for each wavelength analysed, which is of interest for the parametrization of aerosol properties.