Variability of dust inputs to the CANIGO zone

Deep-Sea Research II 49 (2002) 3455–3464

Variability of dust inputs to the CANIGO zone ! M.E. Torres-Padron*, M.D. Gelado-Caballero, C. Collado-Sa! nchez, V.F. Siruela-Matos, P.J. Cardona-Castellano, J.J. Herna! ndez-Brito Department of Chemistry, Facultad de Ciencias del Mar, Universidad de Las Palmas de GC, 35017 Las Palmas, Spain Received 25 July 2000; received in revised form 1 June 2001; accepted 6 June 2001

Abstract The variability of dust inputs in the Canary Islands Azores Gibraltar Observations (CANIGO) area is discussed for the period 1997–1998. The samples were collected daily at ‘‘Pico de la Gorra’’, at the top of Gran Canaria Island (1980 m). A seasonal pattern of Saharan dust events was observed during this period with maximum fluxes in winter and summer. The intensity of events was higher in 1998 than in 1997. Seasonal patterns, composition, and relationship of dust inputs with meteorological data can help define the effects of episodic dust inputs on the surface oceanic reservoir and their correlation with the sinking particulate flux, trace metal distributions in the water column and sediment chemical composition in the CANIGO region. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The influence of mineral dust in the atmosphere has been observed at many geographical regions. In particular, arid regions such as the Sahara Desert, and semiarid regions, such as North Africa or Sahel, are susceptible to the mobilization of dust particles which are then transported within the NE trade winds system over the Atlantic Ocean for long distances (Duce et al., 1991; Prospero, 1996; Moulin et al., 1997). The exchange of these particles across the air–sea interface produces an important impact on the biogeochemistry of the oceans, e.g., the biological availability, fate and residence time of trace elements within the water column (Spokes et al., 1994; Jickells, 1999). A knowledge of atmosphere–ocean transfer is *Corresponding author. Fax: +34-9284-52922. E-mail address: [email protected] ! (M.E. Torres-Padron).

necessary to understand the geochemical cycles of the elements associated with these particles and to evaluate their impact on the chemical and biological processes in surface oceanic waters. Many publications have focused on the Atlantic Ocean. For example, Chester and Johnson (1971) presented the dust loading data off the west African coast. Other authors, Schutz . (1980) and Prospero and Carlson (1981), have shown that large amounts of soil dust are mobilized by winds and that substantial quantities can be carried great distances, such as Sargasso Sea (e.g. Jickells, 1999). Concentrations of mineral aerosols published by different authors (Prospero and Carlson, 1972, 1981; Prospero et al., 1989; Chester et al., 1984; Chester and Murphy, 1990; Talbot et al., 1986) in North Atlantic region in the last years, between 651 and 401N, vary according to location, altitude, and sampling conditions. Saharan atmospheric pulses appear as intermittent increases in concentrations within the average

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! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

loading concentration of the atmosphere. An average maximum value related to Saharan inputs oscillates between 35 and 160 mg m
3, according to Prospero and Carlson (1972), in an area near Barbados. Chester et al. (1984) obtained values lower than 1 mg m
3 of material in non-Saharan periods, increasing upto 700 mg m
3 in dust storms from western Africa. But the estimates of the atmospheric loading across the Atlantic Ocean, occasionally reach 103 mg m
3. They vary considerably due to the strong pulses or storms. Data from Barbados present concentrations higher than 375 mg m
3 (Talbot et al., 1986) during Saharan dust events, whilst the background concentrations in the same area are only around 10 mg m
3. This transport presents a seasonal variability which depends on the Intertropical Convergence Zone (ITCZ). There is a clear annual cycle with maximum dust concentrations in the summer and a winter minimum in Bermuda and Barbados (>100 mg m
3 and o1 mg m
3). However, mineral * aerosol concentrations in Izana (Tenerife, 281300 N, 161500 W) do not appear to present this marked variability (Arimoto et al., 1995). Mineral aerosol measurements at Sal Island (161450 N, 221570 W) by Chiapello et al. (1997) have shown a similar variability to that obtained in the present study, with maximum values during the winter months. The present study focuses on the interannual and seasonal variability of dust loading in the Canary Islands during a sampling period of 2 year. In addition, we have measured the particle size distribution during different periods to estimate dust deposition variability in the ocean surface.

161250 W), at the top of Gran Canaria Island (1980 m) to avoid local anthropogenic contamination (Fig. 1). Glass fiber 8  10 in (Whatman GF/ A) and paper (Whatman 41) filters were used to collect the material. Hydroscopic effects were not observed. Aerosol concentration was obtained by the difference of weight between the dried filters (801C during 12–24 h) before and after the collection. Replicate samples were collected simulta-

2. Methods 2.1. Sampling and analytical techniques Dust concentrations in the air were measured by using five high volume capture systems (CAV-A) from MCV, S.A., equipped with a rectangular filter heads made of PVC. The device was operated for 12 h daily using a flow rate of 60 m3 h
1. The samples were taken daily during the period of 1997 and 1998 at ‘‘Pico de la Gorra’’ (271550 N,

Fig. 1. Location of sampling place in Gran Canaria island (Canary Islands).

! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

neously, with different collection equipment having cumulative errors less than 5%. Blanks of filters were used to determinate the precision of the method (LD 3 mg m
3 with a SD of 1.4%). Particle size distribution of the dust was measured in Saharan events using a cascade impactor. The system consisted of 6 stages where particles are deposited according to their aerodynamic diameter: the top stage collect particles above 10 mm; stages 2–5 gather particles of 10–4.9, 4.9–2.7, 2.7– 1.3, 1.3–0.61 mm, respectively, and the bottom stage receives particles smaller than 0.61 mm in diameter. Particles >10 mm are not observed in the filters. For dry and wet deposition measurements, cubic containers with a surface area of 660 cm2 equipped with a rain sensor were used. Dry deposition was sampled with rectangular containers of a surface area of 1800 cm2. Deposition flux was expressed in mg m
2 d
1. Experimental data of dry deposition were obtained during the 1993– 1996 period. These data were used here for checking the consistency of estimates from the input flux models.

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pressure favours the Saharan dust inputs as a result of the thermal depression in the Sahara Desert.

3. Results and discussion 3.1. Results 3.1.1. Transport of Saharan mineral dust over the Canary Islands At the altitude of collection site (D2.0 km), a maximum dust particle concentration should be expected as described by Schutz . and Leber (1981) for the vertical distribution of mineral aerosol from several transport distances from Saharan sources. The variability of daily concentrations of mineral dust in air during the 1997–1998 period (Fig. 2) is clearly highlighted. The highest values occurred by the intrusion of air masses from the African continent, in the form of ‘‘pulses’’, each lasting an average of 3–8 days. During the sampling, the highest values of dust concentration were measured in winter. In 1998, the inputs were more intense and frequent, even in the summer. Average annual concentrations were quite different: 27 mg m
3 in 1997 and 70 mg m
3 in 1998. These differences are observed in Saharan dust days: during 1997, a maximum concentration of dust was 340 mg m
3, compared to a maximum of 1300 mg m
3 in 1998. These differences are a good indicator of the strong annual variability experienced during the sampling period.

2.2. Meteorological conditions Synoptic meteorological analysis has given two main conditions during the principal events of the transport of Saharan dust. In winter, the Azores high pressure penetrates up to the western Mediterranean Sea, producing east–west isobars on the African continent up to the Canary Islands. During summer, the remoteness of the Azores high

1400 1997

1998

Dust loading (µg/m3)

1200 1000 800 600 400 200 0 D

E

F

M

A

M

J

J

A

S

O

N

D

E

F M

A

M

J

J

A

S

O

N

D

Sampling date

Fig. 2. Dust loading (in mg m
3) variability during 1997–1998 period in Gran Canaria island (Canary Islands).

! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

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We have classified the sampling in terms of ‘‘clean days’’ and ‘‘Saharan days’’, according to the dust concentration in the atmosphere. We chose the dust concentration greater than 100 mg m
3 to distinguish the Saharan outbreak periods. The variability of dust concentration in the air during ‘‘non-Saharan days’’ is quite low, whilst it is much higher during ‘‘Saharan days’’ (see Table 1). Moreover, during 1998, there were important variations in dust concentrations. A monthly distribution of Saharan dust inputs (Fig. 3) shows the presence of Saharan events during January–March (winter) and July–September (summer), whilst small inputs are apparent during late spring. Similar features are repeated during the sampling period. Seasonal distributions (Fig. 4) also indicate that the highest frequency of Saharan dust inputs in the Canary Islands is produced during winter (42%) and summer (29%). These events decrease during spring (6%).

3.1.2. Particle size distribution It is also important to study the particle size distribution in Saharan dust, as the particle size plays a major role in the control of the dry deposition of mineral particles. Considering the whole sampling period, it is evident that the main fractions of the particles during Saharan events are in the intermediate size range (0.6–5 mm), whilst particles with diameter >10 mm are present in a smaller proportion (5–8%). Particles with diameter smaller than 0.61 mm contribute upto 10% of the total aerosol population. Particle distribution is presented in Fig. 5 for both Saharan and non-Saharan periods.

% days >100 µg/m3 Fall 23%

Winter 42%

Table 1 Average mineral aerosols (in mg m
3) during the studied period Sampling year

Non-Saharan days

Saharan days

1997 1998

18.6712.7 21.2713.7

126.3754.2 199.37209.1

Summer 29%

Spring 6%

Fig. 4. Seasonal distribution of Saharan dust inputs obtained of dust loading data during 1997–1998 period.

Fig. 3. Monthly distribution of Saharan dust events during 1997–1998 period.

! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

(a) 60

27/01/97

12/02/98

28/02/98

(%) 20

0 10 µm

4.9 µm

2.7 µm

1.3 µm

0.6 µm

0.01 µm

0.6 µm

0.01 µm

(b) 60 29/11/96

(%)

40

20

0 10 µm

1st approach: Calculation of the daily dry flux was carried out using the expression: Fp ¼ Vd  Cpa ;

40

4.9 µm

2.7 µm

1.3 µm

Fig. 5. Size distribution of atmospheric particles during (a) ‘‘Saharan’’ days and (b) ‘‘non-Saharan’’ days.

3.1.3. Saharan dust deposition in the Canary Islands region In order to estimate fluxes, collection site data of Pico de la Gorra and Tafira have been extrapolated over adjacent marine areas. It is known that this extrapolation from coastal sites will inevitably be rather uncertain due to variability of the atmospheric conditions, different meteorological regimes, different scavenging processes and different aerosol particle sizes and concentrations in the marine areas. This extrapolation is a practical approach due the difficulties in the collection of long time series of measurements of atmospheric particle concentrations and direct wet deposition flux at sea.

3.2. Dry deposition Different approaches for estimating the dry deposition flux of the atmospheric material were carried out in the study area. Subsequently, a comparison was made of the different adopted approaches.

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ð1Þ

where Vd is the dry deposition velocity and Cpa is the daily concentration of mineral dust. Using the Tegen and Fung approximation (1994) and assuming that the 100% of the atmospheric particles are in the range between 0.73 and 6.1 mm, the calculated deposition velocity is of 1.2 cm s
1. To distinguish between the ‘‘nonSaharan’’ and ‘‘Saharan’’ events, the daily deposition fluxes were calculated using data from Table 1. During 1997, there were a total of 51 days with the presence of Saharan dust in the Canary region. If the average dust loading was of 126.3 mg m
3 during the Saharan days and 18.6 mg m
3 during the clean days, the annual flux during 1997 was 30 mg m
2 d
1. During 1998, 113 Saharan days occurred, with a daily average particle concentration of 199.3 mg m
3; average particle concentration during clean days was 21 mg m
3. According to these data, the average daily flux was of 80 mg m
2 d
1 during 1998. From these data, it can be observed that the material deposition flux during the Saharan events is 10 times higher than the flux without these events. 2nd approach: The second approach entailed the estimation of dry deposition velocity from experimental data of dry deposition and the average dust concentration for the period 1997–1998. It is known that the dry deposition estimates on plates has some difficulties when one tries to extrapolate to dry deposition flux (Arimoto et al., 1987, 1989). However, we have used these experimental values in order to obtain a more realistic dry deposition velocity and to compare this estimate with the theoretical velocity derived from a reported model (see Tegen and Fung, 1994). The dry deposition velocity was calculated by using the atmospheric fluxes in both Saharan and non-Saharan periods. Experimental flux obtained during the ‘‘non-Saharan’’ days varied between 8 and 14 mg m
2 d
1 for the study period. During Saharan events, the average daily flux varied between 85 and 225 mg m
2 d
1. Experimental deposition velocity was calculated from Eq. (1). For ‘‘non-Saharan’’ days, with using an average

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! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

particle concentration of 20 mg m
3, an average deposition velocity was 1.7 cm s
1. For the Saharan periods, with average atmospheric concentrations of 126.3 and 199.3 mg m
3, dry deposition velocity was 0.9 and 1.4 cm s
1, respectively. With these data, we used 1.2 cm s
1 as the average experimental velocity. Estimates of the deposition velocity made in ‘‘non-Saharan ‘‘days will have a lower precision due to the small quantities of deposited material. It is important to point out that the sampling devices, may also have been influenced by local anthropogenic sources. During the Saharan input periods, the atmospheric deposition was higher and, it is likely that the local pollution was less important. However, these estimations, based on trays collection, must be considered with caution since surrogate plates do not mimic the characteristics of natural waters (Arimoto et al., 1987; Noll et al., 1988; Dulac et al., 1989; Hall et al., 1994). 3.3. Wet deposition Wet deposition was estimated using the expression: Fw ¼ S  ðCÞair  P  r
1

ð2Þ

which depends on the scavenging factor, S; that is defined according to the expression S ¼ ðCrain  rÞ=Cair ;

ð3Þ

where Crain is the aerosol concentration in rain in grams of dust per kilogram of rainwater, Cair is the aerosol concentration in air in units of grams of dust per kilogram of air, P is the precipitation rate (l m
2 yr
1) and r is the air density (kg m
3). Factors known to affect the S relationship include the size of particles being scavenged, their physical and chemical form and the cloud properties including droplet size, temperature, and cloud type. In particular, the scavenging ratios may present (a) systematic variations (e.g. nitrate), partly at least as a function of particle size, (b) no-variation (e.g. Fe and Mn) or (c) considerable variations (e.g. Zn). Owing to the complexity of processes affecting scavenging, there is no satisfactory theoretical treatment available to predict

the S values. An indirect empirical approach could utilize concentrations in collected rain and, simultaneously, in measured air concentrations at ground level; the ratio provides an S value for the substance under consideration. In this work, we applied the parameterization of scavenging ratios to the ‘‘aerosol particles’’ and not to individual chemical species. In this sense, it is assumed that (a) the particle size spectra are the same in the sampled air and rain and (b) the rainfall at sea is similar to the adjacent land area. This latter assumption is probably an important uncertainty. Precipitation data by the National Institute of Meteorology (INM) will be assumed to represent the oceanic area for the 1980–1997 period. We will assume the proposed model by Tegen and Fung (1994) with a linear relationship between Crain and Cair : A series of approaches will be applied in order to simplify the data treatment: 1st approach: Using a range of S values between 100 and 2000 (see e.g. Duce, 1995), an average annual precipitation of 140 l m
2 yr
1 and an average of 20 annual days of rain (Marzol Jae! n, 1993) in the Canary region, we estimate the annual wet flux in g m
2 d
1 to be 0.011 g m
2 d
1 (0.22 g m
2 yr
1) for S ¼ 100; increasing to 0.023 g m
2 d
1 (4.50 g m
2 yr
1) for S ¼ 2000: 2nd approach: Using experimental data of Crain and Cair ; annual wet deposition can be estimated by using the average precipitation and the calculated S value. The average daily concentration of particles in rain, obtained during clean days in 1996, was 0.04 g m
2 d
1 rising to 0.23 g m
2 d
1 during Saharan days. If the average daily concentration of particles in the air is 20 mg m
3, and we ignore wet deposition during Saharan days, the S experimental value would be 2635. Thus, the annual wet deposition would be of 4.0 g m
2 yr
1. However, important precipitation events can occur after dust events, and if we use the average loading value and air dust concentration for Saharan days, the S value is 1690. Both approaches can be compared with the wet deposition flux calculated directly from measurements of particle concentrations in the rain (1.02 g m
2 yr
1). Results are shown in Table 2.

! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron Table 2 Wet deposition fluxes obtained using different approaches Approach

Wet deposition flux

S estimated

11

0.22 4.50

100a 2000a

21

4.00 3.20

2635b 1690b

Experimental

1.02

456c

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Table 3 Estimated input fluxes in the Canary region Fluxes

(g m
2 yr
1) (1997)

(g m
2 yr
1) (1998)

Dry Wet Total Canary Islands region (tons yr
1)

10.90 1.02 11.92 1.0  106

29.20 1.02 30.22 2.4  106

a

Duce et al. (1991). Present work estimated from Tegen and Fung model (1994). c Present work from experimental deposition measurements. b

3.4. Total deposition Annual estimates of total deposition permits us to evaluate the quantity of atmospheric material deposited on the oceanic surface. If we assume that the total deposition is the sum of the dry and wet deposition, and that the wet deposition is the same in the study period (1.02 g m
2 yr
1), the total flux produced varies from 10.9 to 29.2 g m
2 yr
1. For a surface of 80,000 km2 in the study region (Ratmeyer et al., 1999), total deposition is 1.0  106 tons yr
1 during 1997 and 2.4  106 tons yr
1 during 1998. Estimations of dust deposition are summarized in Table 3. 3.5. Discussion Average concentration of dust obtained in this study appears to be in good agreement with published values by other authors (Table 4). Interannual variability during collection time was significant (Table 1). In 1998, Saharan dust inputs were nearly three times higher than in 1997. Interannual variations in dust transport in the Atlantic Ocean are well correlated with the climatic variability defined by the North Atlantic Oscillation (NAO)(Hurrell, 1995; Moulin et al., 1997). The winter NAO index (Jones et al., 1997; Osborn et al., 1999) (December–March) for 1996/ 1997 and 1997/1998 were +0.18 and +0.80, respectively, and correspond to increased average dust concentrations in the 1997 winter from 41 mgm
3 to 88 mgm
3 in 1996/1997 and 1977/

1998 respectively. A longer temporal series in the Canary Islands region, however, is needed to confirm this relationship. Saharan events prevail during winter, in good agreement with statistic analysis of meteorological parameters (humidity, visibility, rainfall, etc.) by Dorta Antequera (1999) in the area during the decadal period 1982–1991. Therefore, the meteorological conditions which produce the Saharan outbreaks inputs are usual in the early winter. One of the main differences observed for the seasonal pattern with opposite to records in the North Atlantic Ocean is the absence of Saharan events during spring period (Fig. 4). The latitudinal movement of the ITCZ appears to be the main factor in determining the seasonal variability for the collection sites. The zone of maximum dust transport off the North African coast is located around 51N in winter and 201N in summer, associated with the seasonal migration of the ITCZ (Moulin et al., 1997). Canary region is located at 281N in the outer boundary of ITCZ. It would be expected a lower dust transport in this area than southern latitudes. It is possible, moreover, that local meteorological conditions (i.e. cyclones) could affect in a different way the observed seasonal pattern in the Canary region than other Atlantic areas. In any case, Saharan inputs are important in that they represent more than 60% of annual flux of input in surface ocean, in the Canaries region despite the fact that they occur only 25% of the time (Fig. 6). Deposition processes in the oceans are produced by wet and dry deposition. It is difficult to make a general statement on the comparative importance of the dry and/or wet deposition of atmospheric

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! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

Table 4 Dust concentration (mg m
3) published by different authors Location

Authors

Dust (mg m
3) (min–max)

African coast Barbados Sal Island North Atlantic (22–641N) North Atlantic (0–281N) Bermuda (S) Barbados North Atlantic (65–401N) * Canary region (Izana) Canary region (Gran Canaria)

Chester and Johnson (1971) Prospero and Carlson (1972) Savoie and Prospero (1977) Prospero et al. (1979) Prospero et al. (1979) Chen and Duce (1983) Talbot et al. (1986) Chester and Murphy (1990) Arimoto et al. (1995) Gelado-Caballero et al. (1996)

10.50 35.60–160 10.00–180 1.30 36.60 4.20 27.00–350 0.50–2.50 1.0–100.0 20.0–250.0

100

% Days

80 60 40 20

0 00

>1 00

-1

75

5

-7

0

-5

50

25

25

0-

µg/m3

% annual flux

100 80 60 40 20 0 00

>1

00

-1

5 -7

0 -5

75

50

25

25

0-

particles in the ocean. In accordance with Prospero et al. (1989), the most important regions for the production of particulate material are, probably, those that meet in arid and humid zones of transition, with an annual precipitation between 100 and 200 mm. This situation could coincide with the position occupied by the Canary Islands. According to the present study dry deposition accounts for more than 80% of the total, while the wet deposition supposes a 5% of the total for 1997 and a 3% for 1998. Duce et al. (1991) reported that wet deposition of dust might dominate over dry deposition, but Jickells et al. (1998) suggest that globally dry deposition exceeds wet. Over the North Atlantic dry deposition appears to dominate due to the higher concentrations of coarse mode aerosol present so close to sources (Duce et al., 1991; Jickells et al., 1998). However, the punctual impact of the wet deposition, after a Saharan dust event, could be more important than the dry deposition. This could be confirmed with the result of some samples obtained during 1999. For example, on January 8th 1999, a strong dust input of Saharian material occurred in the Canary Islands region. High volume samplers could not register these inputs due to an intense precipitation that occurred in the sampling area but the analysis of rain samples gave dust concentrations of 0.16 g m
2 d
1, calculated by filtration. Estimates of annual dust inputs along this work (1.0  106 tons yr
1 in 1997 and 2.4  106 tons yr
1 in 1998) using average atmospheric loading and the previously discussed calculations of the velo-

µg/m3 Fig. 6. Annual percentage and estimation of Saharan dust events in annual atmospheric inputs.

city of dry deposition and S (factor of wet deposition). These estimates seem to be close to the estimates by Schutz . and Leber (1981)

! et al. / Deep-Sea Research II 49 (2002) 3455–3464 M.E. Torres-Padron

(2.0
8.6  106 tons yr
1) in the Canary region. These estimations suggest that the total mass transported from the Sahara westward (260  106 tons yr
1, see Schutz . (1980)), only 1% would be deposited in the study area.

Acknowledgements

3.6. Conclusions

References

Saharan dust inputs in Canary region are produced during winter and summer periods, lasting an average of 3–8 days. As such, during summer, Saharan dust events in 1998 were higher than 1997. Important interannual and seasonal components control the Saharan dust inputs to the Canary region which could be related with NAO variations. Experimental dry deposition velocity is about 1.2 cm s
1 as inferred from the experimental dry deposition, similar to that obtained for 0.73– 6.1 mm size range of Tegen and Fung approximation (1994). Annual average dry flux could therefore range from 0.03 to 0.08 g m
2 d
1, and the dry inputs to Canary region would be 1.0  106– 2.4  106 tons yr
1. With respect to wet deposition, the S value varies depending of meteorological conditions during the non-Saharan and Saharan events. Experimental S values obtained (500–1500) appear to be in good agreement with those reported (200– 2000) by Duce et al. (1991) and Duce (1995). From the experimental data fluxes, we could conclude that the dust deposition is mainly due to the dry deposition in the study area, except on days of precipitation following strong Saharan dust events which could result in an important inputs of lithogenic material to the oceanic surface. We must resolve the effects of these dust inputs on the surface ocean, particularly in the design of sampling strategies to obtain more mineralogical data and relate them with different global climatological conditions. Saharan dust inputs may produce a significant impact in the biogeochemical cycles of trace elements like Al, Fe and Mn (Jickells, 1999) and they must be studied in the near future.

3463

This work was supported by the CANIGO Project. We thank the National Meteorology Institute for their contribution with meteorological data. We dedicate this paper in memory to our friend, Antonio Mesa.

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