Sahara dust, ocean spray, volcanoes, biomass burning: pathways of nutrients into Andean rainforests

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+, NH+, Ca2+ʹ Mg2+, Cl-, SO2-, NO-, PO3-). About 35% of the weekly samples had very low ion contents, with pH mostly above 5 and conductivity below 10 ^S/cm. 10-days back trajectories (FLEXTRA) showed that respective air masses originated in pristine continental areas, with little or no obvious pollution sources. 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 ^S/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. Enhanced SO4- and NO+ 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 , about 10% of the total input, could be identified to originate from active volcanoes, whose plumes were encountered by about 10% of all trajectories.