Atmospheric Deposition of Nutrients and Pollutants in North America: An Ecological Perspective

Research on air pollution and acidic deposition during the last 15 yr has greatly increased our knowledge of the rates and the processes of atmospheric deposition. The invigoration of the field has been a direct result of interchange and cooperation among ecosystem ecologists, micrometeorologists, and plant physiological ecologists who each approach the study of atmospheric deposition from different perspectives. This has led to the widespread realization among ecologists of the importance of dry and cloud deposition and the introduction of new methods to estimate these fluxes. In this paper I summarize the current understanding of atmospheric deposition processes, measurement methods, and patterns of deposition in North America. Dry deposition measurements are still highly uncertain in most cases, and methodology is still an active area of research and debate, but it seems clear that ecologists will require a suite of different methods to evaluate dry deposition fluxes of the various elements important to ecosystems. Standard model formulations have been developed for estimating dry and cloud deposition, and these models are finding wide use in flux estimation at sites where direct measurements are unavailable. National monitoring networks for wet and dry deposition have been established and are providing information on continental— and regional—scale patterns. Research has demonstrated that deposition rates are increased substantially at high—elevation sites by enhancement of wet, dry, and especially, cloud deposition. Patterns of O₃ exposure are also different at high—elevation sites. The deposition of O₃, while mechanistically similar to that of other gases, has been treated differently in the ecological and botanical literature because of its direct phytotoxicity at ambient concentrations in industrialized areas. Current efforts in ozone exposure research involve determining appropriate exposure indices for interpreting biological responses and coupling models of atmospheric transport with models of ozone disposition within the plant.