441
Observatory. Nevertheless, the equatorial oceanic air masses have very different
background levels of greenhouse gases due to the latitudinal gradients of these species.
o Curved trajectories (CT, hereafter). Air masses beginning in sectors 1 and 2, but then
describing a curve and arriving Izafia via other different sectors. They are influenced first by
the westerlies and then by the easterlies.
o Marine local trajectories (MLT, hereafter). Trajectories being within a 2ooo-km radius
circle and air masses flowing mainly over the ocean.
o Continental local trajectories (CLT, hereafter). Trajectories being within a 2ooo-km radius
circle and being located mainly over the African continent. Air masses with these
trajectories can transport airborne crystal particles from the coastal region of the Sahara
Desert
However, the emphasis in this paper is on the categories of trajectories reflecting
long-range transport. This classification scheme avoids a source of error that would result if
the wind speed were not taken into account and trajectories were categorized merely by
direction.
o No reported (NO, hereafter). Trajectories that were not calculated because of missing data.
3. Five-year climatology of back-trajectories
In Figs 2a and 2b, histograms of all 850- and 700-HPa 0000 Z trajectories to Izafia
are represented for the period of study (1983-1987). For the 850-HPa level, sector 1 has the
highest frequency of occurrence (36.6%), more than twice the frequency of the next sector
(18.2% in sector 2). Likewise for 700-HPa trajectories, sector 1 dominates. Air masses
arriving from sector 1 (North Atlantic) are generally clean and moist. The frequency of
occurrence of trajectories from Europe (sector 2) drops from 18.2% at 850 Ha. Trajectories
arrive from Africa (sector 3) with roughly the same frequency of occurrence (12%) for both
levels. Thus, for the 850-HPa level, the European trajectories are more frequent than the
African ones, whereas for the 700-HPa level, the opposite occurs. A higher percentage of
trajectories in sector 4 at the 700-HPa level than at the 850-HPa level is observed. A
significant number of local trajectories (CUT and MLT) were noted for both levels (about
18%). Marine local trajectories are clearly more frequent than continental local trajectories.
Finally the number of curved trajectories represents percentages of about 11 % at the 850-
Observatory. Nevertheless, the equatorial oceanic air masses have very different
background levels of greenhouse gases due to the latitudinal gradients of these species.
o Curved trajectories (CT, hereafter). Air masses beginning in sectors 1 and 2, but then
describing a curve and arriving Izafia via other different sectors. They are influenced first by
the westerlies and then by the easterlies.
o Marine local trajectories (MLT, hereafter). Trajectories being within a 2ooo-km radius
circle and air masses flowing mainly over the ocean.
o Continental local trajectories (CLT, hereafter). Trajectories being within a 2ooo-km radius
circle and being located mainly over the African continent. Air masses with these
trajectories can transport airborne crystal particles from the coastal region of the Sahara
Desert
However, the emphasis in this paper is on the categories of trajectories reflecting
long-range transport. This classification scheme avoids a source of error that would result if
the wind speed were not taken into account and trajectories were categorized merely by
direction.
o No reported (NO, hereafter). Trajectories that were not calculated because of missing data.
3. Five-year climatology of back-trajectories
In Figs 2a and 2b, histograms of all 850- and 700-HPa 0000 Z trajectories to Izafia
are represented for the period of study (1983-1987). For the 850-HPa level, sector 1 has the
highest frequency of occurrence (36.6%), more than twice the frequency of the next sector
(18.2% in sector 2). Likewise for 700-HPa trajectories, sector 1 dominates. Air masses
arriving from sector 1 (North Atlantic) are generally clean and moist. The frequency of
occurrence of trajectories from Europe (sector 2) drops from 18.2% at 850 Ha. Trajectories
arrive from Africa (sector 3) with roughly the same frequency of occurrence (12%) for both
levels. Thus, for the 850-HPa level, the European trajectories are more frequent than the
African ones, whereas for the 700-HPa level, the opposite occurs. A higher percentage of
trajectories in sector 4 at the 700-HPa level than at the 850-HPa level is observed. A
significant number of local trajectories (CUT and MLT) were noted for both levels (about
18%). Marine local trajectories are clearly more frequent than continental local trajectories.
Finally the number of curved trajectories represents percentages of about 11 % at the 850-