Mineral dust transport to the Sierra Nevada, California: Loading rates and potential source areas
Résumé
The transport and deposition of aeolian dust represents an important material input pathway for many marine and terrestrial ecosystems and may be an ecologically significant source of exogenous phosphorus (P) to alpine lakes. In order to assess the abundance and elemental composition of atmospheric mineral dust over the Sierra Nevada of California, we collected size-fractionated atmospheric particulate matter (PM) samples during July 2008 to March 2009 at a mixed conifer site located in Sequoia National Park. PM concentrations were at their highest levels during the dry season, averaging 8.8 +/- 3.7 and 11.1 +/- 7.5 mu g m(-3) for the coarse (1 mu m < D-a < 15 mm) and fine (D-a < 1 mu m) fractions, respectively, while winter months were characterized by low (< 1 mu g m(-3)) PM concentrations in both size fractions. Using Al as a diagnostic tracer for mineral aerosol, we observed a significant and uniform contribution (50-80%) from aeolian dust to the total coarse PM load, whereas submicron particles contained comparatively little crustal material (7-33%). The mass concentrations of elements (Fe, Ca, Mg, P, and V) in the coarse PM fraction were significantly correlated with Al throughout the study, and coarse PM exhibited elemental signatures that were temporally consistent and distinguishable from those of other sites. Conversely, higher elemental enrichments were observed in the fine PM fraction for Fe, V, and P, indicating a greater contribution from anthropogenic emissions to the fine particle load. Fe/Al and Fe/Ca ratios suggest a mixture of mineral dust from regional agricultural activities and long-range transport of mineral dust from Asia. Asian sources comprised 40-90% of mineral dust in July 2008 and then declined to between 10 and 30% in August and early September.
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