Regional differences of column aerosol parameters in western Iberian Peninsula

Abstract This study presents a characterization of aerosols columnar properties measured at three different AERONET sites in the western part of the Iberian Peninsula, namely Evora and Cabo da Roca, in Portugal, and Caceres, in Spain, during the period from 2005 to 2010. AERONET level 2.0 products have been analyzed. The comparison of these three stations has great interest because it has not been conducted yet and it allows to characterize the aerosols of a wide region in western Iberian Peninsula by a long-term analysis of their aerosol properties. In addition, it allows analyzing the possible differences in these properties between the three sites located at different distances from the coast. The results show differences between the aerosol optical depth at 440 and 1020 nm at the three stations, being the mean values in Cabo da Roca at 1020 and 440 nm (0.08 and 0.16) slightly higher than in the other two stations (Evora: 0.06 and 0.15; Caceres: 0.05 and 0.14). Greater differences among the three stations are found for the Angstrom exponent values. Thus, Caceres shows the highest mean value (1.33), followed by Evora (1.14) and, finally, by Cabo da Roca (1.00) which exhibits the smallest median α values due to the presence of coarse sea salt particles and high atmospheric humidity. These values are consistent with the values of volume size distribution, exhibiting a greater value of large particles at Cabo da Roca. The mean values of the single scattering albedo ( ω ) have been also analyzed, obtaining higher results as the coast is approached: Cabo da Roca: ω (440) = 0.95; ω (1020) = 0.96, Evora: ω (440) = 0.93; ω (1020) = 0.95 and Caceres: ω (440) = 0.88; ω (1020) = 0.83. The differences between the three stations are explained in terms of the distance to the coast and to the occasional arrival of diverse air masses transporting different aerosol types to each station. One classification method proposed for the study region has been applied to multi-year measurements performed at the three sites and verified by means of daily 120-h back-trajectory analyses of air masses. The most frequent situations observed at all sites include the clean, maritime and continental situations, which account for about 80% or more. The remaining percentage is accounted by the more episodic situations of desert dust (10–14%) and forest fires emissions (3–5%). The probability density plots calculated for the different classified aerosol situations show significant differences consistent with the different paths of the air masses corresponding to each aerosol situation.