200
A. Escarre, A. Carratala, A. Avila, J. Bellot, J. Piiiol and M. Millan
Another approach to define the origin of the ions in precipitation is
through correlation analysis between the ions. Single or multiple linear regressions between sol- and N03 - and H+ have been used to explain the precipitation acidity (Gorham 1955; Cogbill and Likens 1974; Gorham et al.
1984). At Prades and Montseny, correlations between sol- and N03 - with H+
were not significant, while these anions were significantly correlated with
Ca 2 + and NH4 + (A. vila 1996), ions which contribute to neutralizing the precipitation acidity. The neutralization capacity of the atmosphere at our sites
can also be inferred from the increase in acidity in successive rain events, a
fact that suggests the important role of washout in providing alkaline species
which become depleted in successive rains (Bellot and Escarre 1989; Carratala 1993). This finding substantiates the correlational evidence from the
Valencian network (Sect. 14.3).
A more general approach relies on multivariate methods as principal
component analysis. Different data sets of precipitation chemistry from this
Mediterranean area usually produce a first principal component representing
the degree of mineralization. Rains of high ion concentrations, mostly corresponding to African red rains, are well separated by this component. The
second principal component reflects the marine influence since it is mainly
correlated with Na+ and CI-. The third component is associated with the rain
acidity. Similar results were obtained at Mont Lozere (southern France), except that the third component was defined by the acid anions sol- and N03and pH was not significantly correlated with the other elements (Durand
et al. 1992).
14.5 The Airshed
Precipitation chemistry depends strongly on the acquisition of gaseous and
particulate materials by moving air masses, and on the subsequent transformations of these materials. From an ecosystem perspective, the regions
where these airborne materials originate define the loose boundaries of the
airshed for a given ecosystem. In this section we first trace the movements of
air masses reaching holm oak forests in NE Spain in order to identify the
origin of elements delivered by bulk precipitation. We then analyze the particular meteorological conditions prevailing over Iberia in summer that are
involved in the buildup of secondary air pollutants above the Spanish Mediterranean coast.
14.5.1 Back Trajectories and Origin of Air Masses
The back trajectory of an air mass arriving at a given locality at a certain
time can be computed by tracing its movement through pressure fields or isentropic surfaces in the atmosphere. Using the Valencian precipitation net-
A. Escarre, A. Carratala, A. Avila, J. Bellot, J. Piiiol and M. Millan
Another approach to define the origin of the ions in precipitation is
through correlation analysis between the ions. Single or multiple linear regressions between sol- and N03 - and H+ have been used to explain the precipitation acidity (Gorham 1955; Cogbill and Likens 1974; Gorham et al.
1984). At Prades and Montseny, correlations between sol- and N03 - with H+
were not significant, while these anions were significantly correlated with
Ca 2 + and NH4 + (A. vila 1996), ions which contribute to neutralizing the precipitation acidity. The neutralization capacity of the atmosphere at our sites
can also be inferred from the increase in acidity in successive rain events, a
fact that suggests the important role of washout in providing alkaline species
which become depleted in successive rains (Bellot and Escarre 1989; Carratala 1993). This finding substantiates the correlational evidence from the
Valencian network (Sect. 14.3).
A more general approach relies on multivariate methods as principal
component analysis. Different data sets of precipitation chemistry from this
Mediterranean area usually produce a first principal component representing
the degree of mineralization. Rains of high ion concentrations, mostly corresponding to African red rains, are well separated by this component. The
second principal component reflects the marine influence since it is mainly
correlated with Na+ and CI-. The third component is associated with the rain
acidity. Similar results were obtained at Mont Lozere (southern France), except that the third component was defined by the acid anions sol- and N03and pH was not significantly correlated with the other elements (Durand
et al. 1992).
14.5 The Airshed
Precipitation chemistry depends strongly on the acquisition of gaseous and
particulate materials by moving air masses, and on the subsequent transformations of these materials. From an ecosystem perspective, the regions
where these airborne materials originate define the loose boundaries of the
airshed for a given ecosystem. In this section we first trace the movements of
air masses reaching holm oak forests in NE Spain in order to identify the
origin of elements delivered by bulk precipitation. We then analyze the particular meteorological conditions prevailing over Iberia in summer that are
involved in the buildup of secondary air pollutants above the Spanish Mediterranean coast.
14.5.1 Back Trajectories and Origin of Air Masses
The back trajectory of an air mass arriving at a given locality at a certain
time can be computed by tracing its movement through pressure fields or isentropic surfaces in the atmosphere. Using the Valencian precipitation net-