202
A. Escarre,A. Carrataia,A. Avila, J. Bellat, J. Piiial and M. Millan
nean (Fig. 14.2c,d). In this situation, S02 and other pollutants emitted from
power plants in eastern and central Europe would be transported southwestwards for distances greater than 1500 km, reaching the Mediterranean
area through the Triest Gap (Camuffo et aI. 1991). The most acid rains reported in the Valencian network were associated with this continental flow.
African air masses were associated with a synoptic situation of high pressure over central Europe and low in SW Spain, producing southern air flows
from Africa to reach eastern Spain (Fig. 14.2b). This African influence has
been widely reported for the western Mediterranean and, as mentioned in
Sect. 14.2, they are of special importance to Spain due to its proximity to the
Sahara. Rains produced in these conditions had a very high alkalinity and
high concentrations of most ions. At Montseny, the African origin of such air
masses has been confirmed by back-trajectory analysis and by the mineralogical composition of the dust contained in red rains (.Avila et al. 1997).
14.5.2 The Iberian Low System, Stratification and Recirculation Processes
The summer meteorology in the western Mediterranean basin differs considerably from that in central and northern Europe (Millan et al. 1992, 1996,
1997). The high coastal mountain ranges surrounding the warm Mediterranean Basin, the region's high insolation and the inland soil aridity favour the
formation of convective processes ranging from local, through meso to synoptic scales with marked diurnal cycles. In summer, a thermal low is formed
almost every day and is reinforced in the middle hours of the day as insolation increases. At night, the low-pressure system weakens, producing a net
flux to the sea. This oscillating movement of air masses favours the incorporation of many anthropogenic and biogenic pollutants into the air masses,
remaining there and recirculating for several days (Millan et al. 1996). A
similar process has been described for the Italian Peninsula, Greece (Lalas
et al. 1983), Los Angeles Bay (USA) (Blumenthal et al. 1978), and Japan
(Finlayson-Pitts and Pitts 1986). Recent experimental evidence supports the
hypothesis that organized links exist between the circulation systems at different spatial and time scales, in which the aged pollutants emitted in central
Europe could enter the central and western Mediterranean and contribute to
the pollution climate there (Millan et al. 1996). As much as 60% of the photooxidant concentration levels at the coast may result from aged air masses
transported in layers from "upstream" source areas. This considerably broadens the spatial extent of ecosystem boundaries compared to the conventional
water-divide criterion used in small-catchment studies.
14.5.3 Secondary Pollutants and Their Potential Effects
The recirculatory nature of the coastal flows under strong insolation converts these flows into "large natural photo-chemical reactors" where most of
the NOx emissions and other precursors are transformed into oxidants, acidic
A. Escarre,A. Carrataia,A. Avila, J. Bellat, J. Piiial and M. Millan
nean (Fig. 14.2c,d). In this situation, S02 and other pollutants emitted from
power plants in eastern and central Europe would be transported southwestwards for distances greater than 1500 km, reaching the Mediterranean
area through the Triest Gap (Camuffo et aI. 1991). The most acid rains reported in the Valencian network were associated with this continental flow.
African air masses were associated with a synoptic situation of high pressure over central Europe and low in SW Spain, producing southern air flows
from Africa to reach eastern Spain (Fig. 14.2b). This African influence has
been widely reported for the western Mediterranean and, as mentioned in
Sect. 14.2, they are of special importance to Spain due to its proximity to the
Sahara. Rains produced in these conditions had a very high alkalinity and
high concentrations of most ions. At Montseny, the African origin of such air
masses has been confirmed by back-trajectory analysis and by the mineralogical composition of the dust contained in red rains (.Avila et al. 1997).
14.5.2 The Iberian Low System, Stratification and Recirculation Processes
The summer meteorology in the western Mediterranean basin differs considerably from that in central and northern Europe (Millan et al. 1992, 1996,
1997). The high coastal mountain ranges surrounding the warm Mediterranean Basin, the region's high insolation and the inland soil aridity favour the
formation of convective processes ranging from local, through meso to synoptic scales with marked diurnal cycles. In summer, a thermal low is formed
almost every day and is reinforced in the middle hours of the day as insolation increases. At night, the low-pressure system weakens, producing a net
flux to the sea. This oscillating movement of air masses favours the incorporation of many anthropogenic and biogenic pollutants into the air masses,
remaining there and recirculating for several days (Millan et al. 1996). A
similar process has been described for the Italian Peninsula, Greece (Lalas
et al. 1983), Los Angeles Bay (USA) (Blumenthal et al. 1978), and Japan
(Finlayson-Pitts and Pitts 1986). Recent experimental evidence supports the
hypothesis that organized links exist between the circulation systems at different spatial and time scales, in which the aged pollutants emitted in central
Europe could enter the central and western Mediterranean and contribute to
the pollution climate there (Millan et al. 1996). As much as 60% of the photooxidant concentration levels at the coast may result from aged air masses
transported in layers from "upstream" source areas. This considerably broadens the spatial extent of ecosystem boundaries compared to the conventional
water-divide criterion used in small-catchment studies.
14.5.3 Secondary Pollutants and Their Potential Effects
The recirculatory nature of the coastal flows under strong insolation converts these flows into "large natural photo-chemical reactors" where most of
the NOx emissions and other precursors are transformed into oxidants, acidic