observations, are typically subject to uncertainties of a factor of three or more. The development of new techniques for more representative determination of wet as well as dry deposition fluxes, perhaps from a low-flying airborne platform, must therefore also be considered a high priority.

In some instances, such as in high-altitude forests and foggy regions, the deposition of cloud droplets may be the primary avenue by which toxics and nutrients are delivered to the Earth's surface.26 It is extremely difficult to measure such fluxes because the droplets are so transient that their flux is easily altered by the presence of measuring devices. Thus, new methodologies need to be developed to assess the importance of droplet deposition and to provide reliable flux measurements.

In the recent past, deposition monitoring networks have proven useful in determining the ecological impacts of atmospheric deposition (e.g., the National Acid Deposition Program/National Trends Network). However, these networks have mostly been limited to monitoring the deposition of a specific chemical or class of compounds (e.g., acid deposition, ozone). For this reason these networks have provided very limited information on the stresses and benefits experienced by an ecosystem from atmospheric deposition and thus on the long-term effects of this deposition as well. With the development of new deposition measurement techniques, it should be possible to design more comprehensive atmospheric deposition/exposure monitoring networks. Implementation of these networks for key ecosystems would provide a long-term record of atmospheric deposition; with colocated ecological monitoring, this record would no doubt prove useful in establishing causal relationships between atmospheric deposition and ecosystems' vitality and succession.

Toxics and Nutrients: Processes and Mechanisms Leading to Deposition

Even with reliable and fully evaluated deposition measurement techniques, it will never be possible to measure dry and wet fluxes for all species of interest over all ecosystems of interest, over all time. Consequently, process-oriented field studies, which make observations of fluxes under carefully selected ranges of conditions, must be undertaken to identify the factors that control fluxes. With these factors identified, algorithms and parameterizations describing deposition fluxes could be developed, tested by further observations, and incorporated into regional and global atmospheric chemistry models, as well as integrated atmospheric/biospheric response models.

NOTES

1. NRC (1992).

2. Farman et al. (1985).

3. Anderson et al. (1991).



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