Seven years of measurements of aerosol scattering properties, near the surface, in the southwestern Iberia Peninsula
1Évora Geophysics Centre, Évora, Portugal
2Physics Department of the University of Évora, Évora, Portugal
Abstract. Aerosol scattering properties, near the surface, were measured over a period of seven years (2002–2008) at Évora, Portugal. This long time series provides valuable information on light scattering by aerosols, namely related with their climatology and with their direct effect on climate. The average (and median) scattering coefficient, at the wavelength of 550 nm, and the scattering Ångström exponent were found to be 42.5 Mm−1 (29.9 Mm−1) and 1.4 (1.5) respectively, indicating that scattering is, in general, of moderate magnitude and dominated by fine particles. Both seasonal and daily cycles were shown, which were related to local production as well as with transport of particles from elsewhere. The average summer and winter values of the scattering coefficient (47 and 54 Mm−1 respectively), at the wavelength of 550 nm, correspond to a significant increase in the aerosol particle concentration when compared with spring and fall (35 and 37 Mm−1 respectively); also the simultaneous increase of the Ångström exponent, from 1.2–1.4 towards 1.4–1.6, is consistent with the input of fine particles from anthropogenic origin in winter and forest fires in summer.
Back-trajectory analysis indicate that the site was regularly under the influence of clean air masses from Atlantic area, with low particle loads (low scattering coefficients) but as the influence of transport from the continent (Iberia Peninsula) increased, the aerosol particle load was observed to increase as well as the relative importance of fine particles over coarse ones, approaching the features observed during European air masses influence.