Forests in Decline

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It should be fairly clear by now that thedeclineofforests in North America and Europe can't be specifically pinned on acidrain. A collectionofatmospheric contaminants, including acidic deposition, is probably to blame. Does this mean that the hullabaloo over acid rain in recent years hasbeen overinflated or that the current federal policyofwaiting for the resultsoffurther research before taking action is warranted? Let's take a brief look at the state ofknowledge about acid rain asofthis writ ing (late 1984); then you decide.

There's nearly unanimous agreement that acid rain does exist and that it's at least a local problem. Furthermore, few scientists doubt that the emissionofsulfur dioxide and nitrogen oxides from power plants, industry, and vehicles can and does cause acid deposition. From those two points, however, opinions diverge. Perhaps the most important and perplexing problems are these: [1] Can the emission of pollutants from a particular source be linked to acidic products falling outofthe sky hundreds of miles away? [2] How do the gaseous emissions become acidic products, and what controls the degreeoftheir formation? [3] What effect does acid deposition have on living things?

[1] Despite the fact that the complexities of the transportation and dispersion of clouds of pollutants are not well understood, scientists have been able to develop models that accurately predict where these plumes will go. They have theories that can be experimentally verified, but they lack a complete explanationofwhy the theories work. (This is called a heuristic model by the scientific community.) Of course, it's not very satisfying for the researchers to say, "We know what happens, but we don't know why. " But for the purposes of being able to say just who's polluting what, the evidence is satisfactory. The emissions from a coal fired power plant in the Ohio Valley can be followed to the AdirondackMountains . . . or to Canada. So when you read or hear that more research is needed to determine how acid rain is transported, you might agree. But don't conclude that the need for more study should prevent regulators from pointing an accusing finger!

[2] Sulfur dioxide and nitrogen oxide gases become acidic products through complex reactions with oxidants, such as hydroxyl radical (OH,) and hydrogen peroxide (H 2 O 2 ), in the atmosphere. Without oxidants, SO 2 and NO x would not become acid precipitation. The concentrationsof oxidants in the air appear to be a function of ozone and sunshine levels, bothofwhich vary significantly. It's already known, for example, that the pH of precipitation in winter is higher than in summer . . . even if the same amounts of sulfur dioxide and nitrogen oxides are present in the atmosphere. The suspicion, therefore, is that raised or lowered levels of pollutants do not dictate parallel changes in the acidity of precipitation. And to confuse matters further, nitrogen oxides themselves are involved in the production of ozone, suggesting that there may be a feedback loop between nitrogen oxides, ozone, and acid precipitation.

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