Precipitation Chemistry and Air Pollution
203
compounds, and aerosols. As a result, rural sites inland are exposed to maritime air-masses containing 0 3 concentrations around 60 to 80 ppb during the
sea-breeze period of the day, and the EU air quality standards for vegetation
protection may be exceeded. Furthermore, this situation can persist for several months on the Spanish Mediterranean coast.
The mean annual concentrations of S02 and NOx at the rural stations of
the Valencian network (10 and 20 ppb, respectively) are far below those permitted by legislation. However, high concentrations up to 40 and 29 ppb, respectively, were reported with the entry of the sea breeze (Millan et al. 1992;
Millan and Sanz 1993). Also, S02 peaks of hundreds of ppb during up to several hours were found in forested sites under the influence of major point
sources. The high levels of 0 3 and the possible synergy between 0 3 and other
pollutants, such as S02 and NOx, have to be considered. In fact, there is evidence of damage to cultivated plants sensitive to 0 3 in the area (Reinert et al.
1991). Similar evidence has been found in natural Mediterranean vegetation
only for Pinus halepensis (Gimeno et al. 1992) and Pinus sylvestris (Sanz et al.
1995). Although the genus Quercus seems quite insensitive to 0 3, the species
Q. alba and Q. gambelis were found to be sensitive in the USA. Adaptations to
drought, like epicuticular waxes and the closure of stomata at noon, minimize the exchange of gases and could be a cause of the lack of symptomatology because the maximum concentration of contaminants occurs when stomata are closed. However, within the Mediterranean range of holm oak,
many plant species are active in the humid and warm season (spring) when
stomata should be open at moments of high 0 3 concentrations.
14.6 Long-Term Trends in Precipitation Chemistry
Similarly to the widespread trend of decreasing S deposition in precipitation
reported for the northern hemisphere (Hedin et al. 1987, 1994; Berge 1988;
Buishand et al. 1988) sol- concentration and deposition in bulk precipitation has decreased significantly at Prades and Montseny since the early 1980s
(Fig. 14.3; Avila 1996). Linear regressions of VWM SOl- concentration on
year were used to smooth the interannual variability. Estimates from these
regressions yielded a reduction of 36% in the annual SOl- concentration at
Montseny between 1983 and 1994, and a reduction of 39% in sol- deposition. The corresponding figures for Prades were 60 and 59% between 1981
and 1994. Paralleling the sol- decrease, the frequency of acidic rains also
decreased at Montseny (Avila 1996). The general declining trend for S deposition in Europe and North America has been attributed to the reduction of S
emissions resulting from abatement strategies. Between 1980 and 1992, S02
emissions decreased around 37% in Europe and 30% in Spain (Agren 1994).
203
compounds, and aerosols. As a result, rural sites inland are exposed to maritime air-masses containing 0 3 concentrations around 60 to 80 ppb during the
sea-breeze period of the day, and the EU air quality standards for vegetation
protection may be exceeded. Furthermore, this situation can persist for several months on the Spanish Mediterranean coast.
The mean annual concentrations of S02 and NOx at the rural stations of
the Valencian network (10 and 20 ppb, respectively) are far below those permitted by legislation. However, high concentrations up to 40 and 29 ppb, respectively, were reported with the entry of the sea breeze (Millan et al. 1992;
Millan and Sanz 1993). Also, S02 peaks of hundreds of ppb during up to several hours were found in forested sites under the influence of major point
sources. The high levels of 0 3 and the possible synergy between 0 3 and other
pollutants, such as S02 and NOx, have to be considered. In fact, there is evidence of damage to cultivated plants sensitive to 0 3 in the area (Reinert et al.
1991). Similar evidence has been found in natural Mediterranean vegetation
only for Pinus halepensis (Gimeno et al. 1992) and Pinus sylvestris (Sanz et al.
1995). Although the genus Quercus seems quite insensitive to 0 3, the species
Q. alba and Q. gambelis were found to be sensitive in the USA. Adaptations to
drought, like epicuticular waxes and the closure of stomata at noon, minimize the exchange of gases and could be a cause of the lack of symptomatology because the maximum concentration of contaminants occurs when stomata are closed. However, within the Mediterranean range of holm oak,
many plant species are active in the humid and warm season (spring) when
stomata should be open at moments of high 0 3 concentrations.
14.6 Long-Term Trends in Precipitation Chemistry
Similarly to the widespread trend of decreasing S deposition in precipitation
reported for the northern hemisphere (Hedin et al. 1987, 1994; Berge 1988;
Buishand et al. 1988) sol- concentration and deposition in bulk precipitation has decreased significantly at Prades and Montseny since the early 1980s
(Fig. 14.3; Avila 1996). Linear regressions of VWM SOl- concentration on
year were used to smooth the interannual variability. Estimates from these
regressions yielded a reduction of 36% in the annual SOl- concentration at
Montseny between 1983 and 1994, and a reduction of 39% in sol- deposition. The corresponding figures for Prades were 60 and 59% between 1981
and 1994. Paralleling the sol- decrease, the frequency of acidic rains also
decreased at Montseny (Avila 1996). The general declining trend for S deposition in Europe and North America has been attributed to the reduction of S
emissions resulting from abatement strategies. Between 1980 and 1992, S02
emissions decreased around 37% in Europe and 30% in Spain (Agren 1994).