Seven years of measurements of aerosol scattering properties, near the surface, in the southwestern Iberia Peninsula

Abstract. Aerosol scattering properties, near the surface (at about 10 m height), were measured during a period of seven years (2002–2008) at Évora, Portugal. The average (and median) scattering and backscattering coefficients, at the wavelength of 550 nm, were found to be 42.5 Mm⁻¹ (29.9 Mm⁻¹) and 5.9 Mm⁻¹ (4.4 Mm⁻¹), respectively. Also, the average and median scattering Ångström exponent (1.4 and 1.5) indicate that scattering was, in general, dominated by sub-micrometer particles. Both seasonal and daily cycles are shown, which were related to local production and transport of particles from elsewhere. Summer and winter average values of the scattering coefficient, at the wavelength of 550 nm (47 and 54 Mm⁻¹, respectively), correspond to a significant increase in the aerosol particle concentration when compared with spring and fall (35 and 37 Mm⁻¹, respectively). Also, the average increase in the Ångström exponent for summer and winter seasons is consistent with the input of sub-micrometer particles from anthropogenic origin in winter and forest fires in summer.

Back-trajectory analysis indicated that the site was regularly under the influence of air masses from the 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 at the site during European air masses influence.