Acid dew: what it does.

Acid dew: What it does

Since Scandinavian scientists observed that acidification was killing off life in Norwegian and Swedish lakes, public scrutiny has focused on the environmental threat of acid rain. But atmospheric chemists have also been studying similar phenomena--among them acid fog, acid snow and, most recently, acid dew.

While the few previous studies on acid dew have shown that it does not rival acid rain as an environmental menace, new findings suggest that it can be harmful and that its acidification process may be more complex than was suspected. "There is a possibility that dew in fact plays a very important role in harming trees,' says William L. Chameides of the Georgia Institute of Technology in Atlanta.

Acid dew forms when dewdrops absorb nitric acid and sulfur dioxide--atmospheric chemicals that mostly originate as the exhaust of automobiles and coal-burning plants. Other chemicals then oxidize the sulfur dioxide to form sulfuric acid, which along with the nitric acid increases the acidity of the dewdrop.

Although these drops are usually not harmfully acidic at night, they begin to evaporate after the sun rises, which serves to concentrate the acid in the drop. In the morning, "it's possible that these tiny acidified drops cause damage to the leaf surface,' says Chameides, whose chemical study on acid dew will appear in the Oct. 20 JOURNAL OF GEOPHYSICAL RESEARCH.

Acid dew is an example of what scientists call dry deposition of trace atmospheric particles or gases. In contrast to fog or rain, which bring dissolved atmospheric chemicals to the earth, dry deposition occurs when particles or gases settle to the earth. In the case of acid dew, they happen to alight on a wet surface. According to Chameides, roughly a third of the sulfur dioxide produced in the United States reaches the ground via dry deposition.

Chameides's calculations are revealing that the mechanics of dry deposition may not be as simple as scientists had previously assumed--a finding that could have implications for emissions-control policies. Most models of dry deposition assume that "if you decrease the amount of sulfur dioxide in the atmosphere by 50 percent, you'll decrease the rate of sulfur deposition to the surface by 50 percent,' says Chameides. However, his work suggests that complex chemistry could undermine a 50 percent emissions reduction so that "deposition might only decrease by 25 percent or 10 percent.'