Intense winter atmospheric pollution episodes affecting the Western Mediterranean Original Research Article

The geographic location of the Western Mediterranean Basin and its peculiar topography, the climatic conditions and the intense anthropogenic and natural emissions of atmospheric pollutants are key factors necessary to interpret the atmospheric aerosol phenomenology over this area. During the cold season it is common to have severe atmospheric particulate matter (PM) pollution episodes (of an anthropogenic origin) affecting this region, not only in the urban and industrial areas but also in the regional and rural sites. During these episodes, the midday hourly PM1 levels at regional background sites are in many cases higher than those at urban areas. Around 10% of the days under winter anticyclonic conditions registered similar PM1 levels at the regional background than at the urban area and, sporadically the daily PM1 levels at the regional background sites may exceed those at urban sites. Furthermore, the very high hourly PM1 levels measured at regional background sites during these episodes are not regularly attained in the closest urban areas, which leads to the hypothesis that an important formation of secondary aerosols occurs during the transport of the polluted air masses towards the elevated rural sites.