Beyond acid rain: A biogeochemical perspective on ecosystem/atmosphere exchange

Background/Question/Methods

Although Angus Smith (Air and Rain: The beginnings of a chemical climatology) recognized precipitation as a source of nutrients and acidity at local scales in the late 1800’s, it wasn't until publication of Eville's paper, On the acidity and salinity of rain, in 1955 that the magnitude of atmospheric inputs to ecosystems and potential for long-distance transport that allowed anthropogenic influences to extend across continents began to be recognized. Even so, several decades passed before the full impact of acid rain was realized; promoting a period of intense research on atmospheric transport, atmospheric chemistry, and terrestrial and aquatic ecology, and eventual passage of legislation to control pollutants that caused the problem. The details are the subject of innumerable publications and launched many graduate student careers. A full review is beyond the scope of this talk, but the basic elements of the acid rain problem are simple. Elements that were tied up in relatively inert forms (sulfur as sulfides in coal and mineral ores and nitrogen as atmospheric N₂) are converted to gaseous compounds that could be transported long distances before oxidants in the atmosphere or in cloud and fog droplets transformed them to harmful forms (sulfuric and nitric acid). The same transformations occur in natural weathering of sulfides and fixation of dinitrogen by lightning, but at a much smaller magnitude and reduced spatial extent. Other examples of ecosystems affected by atmospheric inputs include widespread contamination by mercury, pesticides and organic pollutants. Biosphere/atmosphere exchange is bidirectional. Vegetation emits a vast array of organic compounds that oxidize and condense to form aerosol, but in the presence of excess nitrogen oxides from human activity they fuel formation of toxic ozone. Atmospheric transport and deposition are neither universally harmful nor always anthropogenic. Saharan dust blown across the Atlantic may be the main source of new phosphorus in the Amazon, where it would otherwise be scarce in the highly weathered soils.

Results/Conclusions

In order to learn from these examples and prevent the next incidence of long-distance ecosystem damage it is important to recognize the full biogeochemical cycle of a substance. Harmful emissions may have analogs in natural biogeochemical cycles, but differ in magnitude and spatial scale. Establishing the threshold where the harm will be unacceptable requires a broad-based synthesis linking chemistry and ecology as exemplified by Eville’s early work highlighting the impact coal combustion on distant lakes.