Saharan dust

Base cations are mobilized by weathering and cation exchange reactions that neutralize acids in the watershed. They respond therefore indirectly to changes in sulphate and nitrate concentrations. In fact, if acid anion concentrations (mainly sulphate) decrease, base cations are also expected to decrease. However, in the last few years an increase of the occurrence of alkaline rain episodes (probably due to climatic effects) has been observed and it is likely that calcareous Saharan dust, rich in base cations, is responsible for it [27]. Accelerated weathering, resulting from recent climate warming may also contribute to higher base cation concentrations [28]. 

Because of decreased SO2 emissions, the importance of nitrogen for freshwater acidification becomes more and more determinant. Therefore, uncertainties in the evolution of nitrogen emissions make it difficult to predict future trends in freshwater recovery. On the other hand, climate warming may increase the number of alkaline rain episodes (containing Saharan dust) and enhance weathering reactions, which buffer better acid precipitation, increasing alkalinity and pH. 

Fig. 1 Climbers ascending Weissmies (4,023 m) in the Swiss Alps. The crevasse wall clearly shows the layering of snow. The most prominent layers confine the accumulation of individual years. The yellowish layer is a Saharan dust deposit, a phenomenon commonly occurring in the Alps. Photo Jtirg Alean, Glaciers online http //www.glaciers-online.net...

Fig. 4 Depth-age relationship of the ice cores from Fiescherhom glacier [12] and Colle Gnifetti [13, 14], Besides annual layer counting and radiocarbon ( C) dating, two types of time markers were used Saharan dust events (labeled by the year only) and volcanic eruptions (labeled by year and name of volcano). Depth is given in water equivalent. This is the amount of water contained in the ice core which is calculated using fim and ice density, respectively, both increasing with depth...

Prospero JM, Saharan dust transport over the North Atlantic Ocean and Mediterranean An overview, in Guerzoni S, Chester R. (eds.). The Impact of Desert Dust Across the Mediterranean, Kluwer Academic Publishers, Amsterdam, pp. 133— 151, 1996. 

Caquineau, S., A. Gaudichet, L. Gomes, M.-C. Magonthier, and B. Chatenet, Saharan Dust Clay Ratio as a Relevant Tracer to Assess the Origin of Soil-Derived Aerosols, Geophys. Res. Lett., 25, 983-986 (1998). 

In dry climates such as South European countries, the low and infrequent precipitations hamper the wash-out and the moistening of road surface, favouring road dust resuspension by traffic-induced turbulence. Moreover additional inputs of dust come from the urban soil resuspension due to the little vegetal covering and from sporadic intensive deposition of Saharan dust outbreaks or uncontrolled construction/demolition activities. 

Bruckmann P, Birmili W, Straub W et al (2008) An outbreak of Saharan dust causing high PM10 levels north of the Alps. Gefahrst Reinhalt Luft 68 490-498... 

Middleton NJ, Goudie AS (2001) Saharan dust sources and trajectories. Trans Inst Br GeogrNS 26 165-181... 

E Mitsakou C, Kallos G, Papantoniou N, Spyrou C, Solomos S, Astitha M, Housiadas C (2008) Saharan dust levels in Greece and received inhalation doses. Atmos Chem Phys 8 11967-11996... 

Papayannis A, Mamouri RE, Amiridis V, Kazadzis S, Perez C, Tsaknakis G, Kokkalis P, Baldasano JM (2009) Systematic lidar observations of Saharan dust layers over Athens, Greece in the frame of EARLINET project (2004e2006). Ann Geophys 27 3611-3620... 

Remoundaki E, Bourliva A, Kokkalis P, Mamouri RE, Papayannis A, Grigoratos T, Samara C, Tsezos M (2011) Composition of PM10 during a Saharan dust transport event over Athens, Greece. Sci Total Environ 409 4361 -372... 

SwapR, Garstang M, GrecoS, Talbot R, KallbergP (1992) Saharan dust in the Amazon Basin. Tellus 44B 133-149... 

Prospero, J.M., Nees, R.T. and Uematsu, M. (1987) Deposition rate of particulate and dissolved aluminium derived for Saharan dust in precipitation at Miami, Florida. J. Geophys. Res., 92, 14723-14731. 

Mircea M, D Isidoro M, Maurizi A, Tampieri F, Facchini MC, Decesari S, Fuzzi S (2007) Regional modeling of aerosols using the air quality model BOLCHEM Saharan dust intrusions over Italy. EGU General Assembly, Vienna... 

A number of studies have speculated on diazotroph response to mineral dust Fe fertilization in the Atlantic (Mahaffey et al, 2003), and Pacific Oceans (Johnson et al, 2003). Circumstantial evidence for dust stimulation of marine diazotrophic biomass has recently been reported. A 1999 Saharan dust event coincided with increases in dissolved Fe concentrations on the west Florida shelf and a 100-fold increase in Trichodesmium biomass. N2 fixation rates were not measured, but DON concentrations doubled, presumably due to exudation by N2 fixers (Lenes et al, 2001). A recent Saharan dust addition experiment to surface water samples collected along a west African cruise transect (35°W-17°W), found a minimal increase in CO2 fixation and a large stimulation in N2 fixation, suggesting that diazotrophs were co-limited by both P and Fe (Mills et al, 2004). Note however, analysis of aerosol dust shows that while providing Fe, it also supplies P and combined N (Baker et al, 2003 Mills et al, 2004 Ridame and Guieu, 2002). 

It is now evident that marine N2 fixation can be limited by either P or Fe, although the relative importance of these two nutrients is stiU highly debatable. There may be an interaction between iron supply and P limitation of primary production through N2 fixation. Recently, Mills et al. (2004) used a factorial nutrient addition experiment to show that Fe and P can co-limit N2 fixation in the eastern tropical North Atlantic. They used trace-metal clean techniques with the addition of N, P, Fe, or Saharan dust individually or in combination to investigate which nutrient limits N2 fixation. Their experiments demonstrated that at two out of three stations, N2 fixation was enhanced by 2-3 times by the addition of P and Fe together, but the addition of either P or Fe alone had less effect (Fig. 38.14). Added dust also enhanced N2 fixation, which they suggested could be due to mineral dissolution of both Fe and P. 

Croot, P. L., Streu, P., and Baker, A. R. (2004b). Short residence time for iron in surface seawater impacted by atmospheric dry deposition from Saharan dust events. Geophys. Res. Lett. 31, L23-S08. 

Schulz M., Balkanski Y. J., Guelle W., and Dulac F. (1998) Role of aerosol size distribution and source location in a three-dimensional simulation of a Saharan dust episode test against satellite-derived optical thickness. J. Geophys. Res. 103, 10579-10592. 

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