Elsevier

Atmospheric Environment

Volume 30, Issue 8, April 1996, Pages 1317-1325
Atmospheric Environment

The weathering of aeolian dusts in alpine snows

https://doi.org/10.1016/1352-2310(95)00309-6Get rights and content

Abstract

The chemistry of precipitation (snow and rainfall), snow cover and meltwaters was studied at a French alpine site during the winter-spring seasons of 1986–1987 and 1987–1988. Both acid ( < pH 5.6) and alkaline ( > pH 5.6) deposition events occurred. The strong-acid anions, SO4 and BO3, contributed to the acidity of precipitation but NO3 was the principal anion associated with acidic snowfall. Many alkaline snowfalls are associated with airborne calcareous dusts from regional sources. The most alkaline snowfall, however, was associated with dust from the Sahara Desert. The weathering of dusts in the snow cover during melt leads to the consumption of acidity and an increase in the pH of meltwaters. The results of both field and laboratory experiments show that inputs of calcareous dusts by local aeolian erosion and transport can contribute significantly to the neutralization process. The laboratory experiments also show that the size and distribution of dusts in the snow cover have an effect on the degree of neutralization. Dust in the lower strata of snow cover is more efficient in neutralizing the acidity of meltwaters than dust in the upper strata. The relationship between the distribution of dust and its efficiency to neutralize the acidity of meltwaters can be explained by the kinetics of calcite dissolution under conditions of progressive decreases in the acidity of leached snow and the partial pressures of CO2 within the snow column during the melt process.

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Cited by (21)

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    Drought can cause increases in dust generation as well, due to its effect on soil moisture and vegetation coverage (Prospero and Lamb, 2003; Munson et al., 2011). Studies in France, Italy, and Switzerland have documented high pH, Ca2+, and alkalinity concentrations in “red” rain and snow from southerly storms tracking from the Sahara Desert, and the distinctive chemistry has been attributed to aeolian dust entrained by desert winds (Loye-Pilot et al., 1986; Schwikowski et al., 1995; Delmas et al., 1996; Rogora et al., 2004). Similar chemistry in rain and snow in the Rocky Mountains and Tien Shan (Asia) has been attributed to aeolian deposition as well (Williams et al., 1992; Clow and Ingersoll, 1994; Clow et al., 2002; Dong et al., 2009; Rhoades et al., 2010; Brahney et al., 2013).

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    While Aeolian dust contained eighteen percent of calcite at the beginning of the experiment, no calcite has been found in the residual dust at the end of snow melting, suggesting a total dissolution of calcite from Sahara. They carried out another experiment with the aim of calculating dissolution rates under controlled conditions and found out that Ca release reached 2 and 3 g m−2 day−1 at pH 4.80 and 4.35, respectively (Delmas et al., 1996). This is much higher than earlier results focusing in the French Alps: Deangelis and Gaudichet (1991) computed a particulate Ca release of 6.7 10−4 g m−2 day−1, for which 94% were found out to come from Sahara, after analysis of a 70 m long ice core drilled in the Mont Blanc covering the period 1955–1985.

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    Elements that are soluble and/or exchangeable may quickly become available for biological uptake, chemical reactions, or export. Delmas et al. (1996) studied the chemistry of snow cover and melt-waters in France and found that Saharan dust that had accumulated in the snowpack quickly weathered releasing base cations, which raised the pH of the melt-waters. Similarly, surveys of snowpack and precipitation chemistry in the Rocky Mountains revealed patterns in snowpack solute chemistry and alkalinity attributable to dust inputs (Turk et al., 2001; Clow et al., 2002), which may influence stream chemistry in this region (Campbell et al., 1995; Clow et al., 1997).

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