Origin of high summer PM10 and TSP concentrations at rural sites in Eastern Spain

Concentrations of airborne particulates undergo a seasonal evolution characterised by a summer maximum in rural areas in Eastern Spain. In the summer months the daily mean concentrations of PM10 and TSP (PM) experience wide variations. In 3-day periods, increases in the PM concentrations from 15 to 30–40 μg/m3 are frequently reported, and increases from 15 to 40–60 μg/m3 occur several times throughout the summer. These variations are simultaneously reported at rural stations throughout the flat Ebro basin (600 m a.s.l.) and at mountain sites located at high altitude (>1000 m a.s.l.). The origin of high and low PM episodes was investigated by correlating PM levels with the concentrations of gaseous pollutants, and making use of meteorological analysis and satellite observations. The highest PM events (daily concentrations in the range 40–60 μg/m3) were documented during outbreaks of African dust. The second highest PM events (daily concentrations in the range 20–45 μg/m3) were recorded during regional episodes associated with ozone events. These summer regional PM episodes were induced by the abrupt orography surrounding the Western Mediterranean and by the regional meteorology, which favour the ageing of polluted air masses into the basin. These regional events occur in a synoptic meteorological context characterised by a weak pressure horizontal gradient over the Western Mediterranean often associated with the development of the Iberian thermal low, when advection of air masses is not significant. In this meteorological context, the transport of particulate pollutants from urban/industrial to rural sites is brought about by the breeze circulation at the coastal (sea breeze) and mountain (mountain breeze) sites. The persistence of this breeze circulation for several days (periods of up to 2 weeks were reported) results in a low renovation of air masses leading to an accumulation of airborne particulates in the regional atmosphere. The lowest PM events (daily concentrations <20 μg/m3) were reported during abrupt entries of Atlantic air into the Mediterranean.