Estimating cloud water deposition to subalpine spruce-fir forests—II. Model testing

Cloud water and throughfall deposition of mercury and trace elements in a high elevation spruce-fir forest at Mt. Mansfield, Vermont

As part of the Lake Champlain Basin watershed study of mercury (Hg) and pollutant deposition, cloud water and cloud throughfall collections were conducted at the south summit (1204 m) of Mt. Mansfield, Vermont between August 1 and October 31, 1998, for multi-element chemical analysis. A passive Teflon string collector was deployed during non-precipitating events to sample cloud/fog water at timberline, while three sets of paired funnels collected cloud throughfall under the red spruce-balsam fir canopy. Samples were analyzed for concentrations of Hg, major ions, and 10 trace elements. Ultra-clean sampling and analysis techniques were utilized throughout the study. Six events were sampled for cloud water alone and four events were sampled for both cloud water and cloud throughfall. Cloud throughfall chemistry showed substantial modification from incident cloud water. Much higher concentrations of Hg (2.3 x), base cations (Ca2+, K-, Mg2+; 3-18 x) and certain trace elements (Ni, Cu, Mn, Rb, Sr; 2-34 x) were observed in throughfall than in cloud water. These results confirm that cloud water can leach a wide variety of elements from tree foliage and wash off dry deposited elements. Cloud water deposited an average of 0.42 +/- 0.12 mm of water per hour. Estimated cloud water deposition of Hg was 7.4 microg m(-2) for the period August 1-October 31, approximately twice that deposited by rain during this period at a nearby low elevation Hg monitoring site. Our results indicate that cloud water and Hg deposition at Mt. Mansfield are likely to have considerable ecological effects.

Cloud water deposition at a high-elevation site in the Vosges Mountains (France)

The importance of cloud water deposition fluxes to a high-elevation site in the Vosges Mountains (France) has been studied. An existing one-dimensional cloud water deposition model has been adapted to site conditions. The adaptation of the main parameters has been tested and is discussed. These tests illustrate the small influence of the surface area index (SAI) and the roughness length on cloud water deposition and the negligible contribution of sedimentation compared to impaction of cloud droplets. Model input data, including cloud water composition, liquid water content (LWC) and wind speed, have been collected over a 1-year period and are used to evaluate annual cloud deposition fluxes of major ions and trace metals. The calculations indicate that the hydrologic input by cloud water (55.5 mm year(-1)) is negligible compared to the input by rainwater (1265 mm year(-1)). However, due to the higher concentrations in cloud water, the deposition fluxes of cloud water represent 10-28% of the total wet deposition (rain + cloud deposition) for major ions and 50% or higher for trace elements.