h i g h l i g h t s < Study of an intensive and long-lasting aerosol event at regional level. < Characterization of the atmospheric thermodynamic structure during this event. < Drastic increase in the aerosol fine concentration during the stagnation episode. < Increase in modal radius and width of fine particles mode due to growth processes. < Unusual high single scattering albedo with neutral spectral dependence. a b s t r a c t Columnar and ground level aerosol properties as well as mass concentrations of some gaseous pollutants were measured at two urban sites (Granada and Málaga, South Spain) before, during and after an extended stagnation episode from 7 to 13 February 2011. This long lasting event was associated with a very strong and persistent blocking high-pressure system over the Iberian Peninsula, together with very intense and persistent temperature inversions near the ground level. The columnar aerosol load at Granada showed a significant increase during this stagnation episode as indicated by aerosol optical depth at 440 nm, reaching values four times higher (0.6) than before and after the event. A significant increase in aerosol load at night time was also evidenced by star photometer measurements. Similarly, pronounced enhancement in columnar aerosol load was observed at Málaga, indicating the regional extension of this event. Analysis of ground level measurements obtained at Granada showed a significant increase in aerosol scattering coefficients and aerosol number concentrations during the stagnation episode. Furthermore, the analysis of aerosol size distribution measurements has evidenced the large contribution of fine particles at ground level as well as in the atmospheric column during the stagnation period. The fine mode radius measured at Granada showed a large displacement towards larger sizes together with a pronounced increase in the geometric standard deviation of the fine mode during the high pollution event in the morning hours on 9 February. This was attributed to the growth of aerosol particles due to coagulation and condensation processes as a result of the high fine aerosol load next to the surface fav-oured by the high pressure system and thermal inversion on that day. This increases in the radius and width of the fine mode results in more efficient scattering in the 440e1020 nm spectral range which, in combination with nearly constant and low imaginary refractive index (0.002) leads to high single scat-tering albedo with neutral spectral dependency. During the persistent temperature inversion episode the daily European PM 10 mass concentration limit (50 mg m À3) was exceeded in Granada for most days and in Málaga for three consecutive days. Nevertheless, the mass concentrations of NO 2 , CO and O 3 were below the European thresholds with a noticeable decrease in O 3 mass concentration at both sites.