Vertical fogwater flux measurements above an elevated forest canopy at the Lägeren research site, Switzerland

During the winter of 2001/2002 wet and occult deposition measurements were performed at the Lägeren research site (690 m a.s.l.) in Switzerland. Two types of fog were observed: radiation fog (RF) and fog associated with atmospheric instabilities (FAI). The deposition measurements were performed above the forest canopy on a 45 m high tower. Occult deposition was measured by means of the eddy covariance method. Due to the large differences of microphysical properties of the two fog types, the liquid water fluxes were much higher (6.9 mgm−2 s−1) during RF than during FAI (0.57 mgm−2 s−1). Fogwater concentrations were considerably enhanced during RF compared with FAI. The comparison of fog and rain revealed that fogwater nutrient concentrations were 3–66 times larger than concentrations in precipitation. The considerably larger water fluxes and nutrient concentrations of RF resulted in much higher nutrient deposition compared with FAI. In winter when RF was quite frequent, occult deposition was the dominant pathway for nitrate and ammonium deposition. Daily fluxes of total inorganic nitrogen were 1.89 mgm−2 d−1 by occult and 1.01 mgm−2 d−1 by wet deposition. The estimated contribution of occult deposition to total annual nitrogen input was 16.4% or 4.3 kgNha−1 yr−1, and wet deposition contributed 26.5% (6.9 kgNha−1 yr−1). As a consequence, critical loads of annual N-input were exceeded, resulting in a significant over-fertilization at the Lägeren site.