Advances in Geosciences www.adv-geosci.net 1680-7340 1680-7359 22 4th EGU Alexander von Humboldt Conference "The Andes: Challenge for Geosciences" 2009 10.5194/adgeo-22-85-2009 http://www.adv-geosci.net/22/85/2009/ http://www.adv-geosci.net/22/85/2009/adgeo-22-85-2009.html http://www.adv-geosci.net/22/85/2009/adgeo-22-85-2009.pdf 85 94 2009-10-13 Sahara dust, ocean spray, volcanoes, biomass burning: pathways of nutrients into Andean rainforests P. Fabian peterfabian@web.de R. Rollenbeck N. Spichtinger L. Brothers G. Dominguez M. Thiemens Technical University of Munich (TUM), TUM Life Science Center Weihenstephan, Am Hochanger 13, 85354 Freising, Germany Laboratory for Climatology and Remote Sensing (LCRS), University of Marburg, Germany Department of Chemistry, University of California San Diego (UCSD), USA Regular rain and fogwater sampling in the Podocarpus National Park, on the humid eastern slopes of the Ecuadorian Andes, along an altitude profile between 1960 and 3180 m, has been carried out since 2002. The samples, accumulated over about 1-week intervals, were analysed for pH, conductivity and major ions (K⁺, Na⁺, NH₄⁺, Ca²⁺, Mg²⁺, Cl^&minus;, SO₄^2&minus;, NO₃^&minus;, PO₄^3&minus;). <br><br> About 35% of the weekly samples had very low ion contents, with pH mostly above 5 and conductivity below 10 μS/cm. 10-days back trajectories (FLEXTRA) showed that respective air masses originated in pristine continental areas, with little or no obvious pollution sources. <br><br> About 65%, however, were significantly loaded with cations and anions, with pH as low as 3.5 to 4.0 and conductivity up to 50 μS/cm. The corresponding back trajectories clearly showed that air masses had passed over areas of intense biomass burning, active volcanoes, and the ocean, with episodic Sahara and/or Namib desert dust interference. <br><br> Enhanced SO₄^2&minus; and NO₃⁺ were identified, by combining satellite-based fire pixel observations with back trajectories, as predominantly resulting from biomass burning. Analyses of oxygen isotopes ¹⁶O, ¹⁷O, and ¹⁸O in nitrate show that nitrate in the samples is indeed a product of atmospheric conversion of precursors. Some SO₄^2&minus;, about 10% of the total input, could be identified to originate from active volcanoes, whose plumes were encountered by about 10% of all trajectories. <br><br> Enhanced Na⁺, K⁺, and Cl^&minus; were found to originate from ocean spray sources. They were associated with winds providing Atlantic air masses to the receptor site within less than 5 days. 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