Not as Concentrated as Pollutants in Rain : Dew Found to Contain Acidic Elements

First there was acid rain to spoil forests, kill aquatic life and destroy soil nutrients. Now there’s acid dew.

Acid dew is not new, but only recently have researchers identified it as another potentially harmful environmental stress caused by the emission of sulfur dioxide and nitrogen oxide from industries and motor vehicles.

“The concept of what I call acid dew occurs when little drops of water form on the leaf and then acidic species get incorporated into the drops through the night,” said Dr. William Chameides, a professor of geophysical sciences at Georgia Tech who recently published the first acid dew research in the Journal of Geophysical Research.

“Then, they (acidic compounds) oxidize in the moisture to form an acidic solution which increases in acidity as the water evaporates in the heat of the day,” Chameides said.

Content in Dew Smaller

In contrast, acid rain forms when sulfur and nitrogen combine in the atmosphere with oxygen, hydrogen and solar energy. The acids are then dissolved in the clouds before falling to earth in the form of acidic precipitation.

“The acidity in dew is much more small than what is brought to the earth’s surface in rain drops,” Chameides said. “You may wind up with these little spots with very high acidity. The question is whether these spots harm plants.

“They may cause a hole to form in the cuticle, the outside part of the leaf, which would be an entry point for insects or fungi,” he said.

Chameides used a sophisticated computer model to study the formation of acid dew and his computer predictions have been supported by limited field studies.

More Research Needed

Although Chameides is positive acid rain is more harmful than acid dew, he said more field work is needed to document the extent of the problem. In fact, he refused to speculate on specific problems acid dew can cause.

“When you start talking like that, you become an alarmist,” he said. “There are a number of things affecting agriculture and forests that might be related to pollution--like acid rain and ozone damage. All these things have an effect.

“What happens when a tree dies or crops fail is related to a number of stresses--including natural stresses like drought or insect attacks. It’s possible acid dew is just one more way pollutants can harm plants.”

Vehicle, Industry Sources

Sulfur dioxide and nitrogen oxide--the two main pollutants in acid deposition--originate mainly from motor vehicles and industries that burn fossil fuels, such as coal-fired electrical generating stations and metal and petroleum refineries.

Chameides’ study of the dry deposition of pollutants has important implications for costly efforts to reduce industrial and motor vehicle pollution in the United States and Canada.

Current regulatory efforts assume a reduction in emissions will have a direct, or linear, effect on the level of pollutants such as acid rain. Ideally, a 50% reduction in emissions should reduce the problem by one-half.

Chameides disagrees and said emissions control “may not be as efficient or effective as it seems.”

“The calculations I did . . . suggest the dry deposition process is not linear, meaning if you decrease emissions by 50%, you might not get a 50% reduction in the pollution,” he said.

Chameides plans further study on the problem of acid dew and hopes to expand his research on dry deposition to other pollutants such as ozone.