Acid precipitation drops in United States.

Measurements of rainwater at selected locations across the United States reveal a significant decline in the pollution that causes acid rain, according to a report issued last week by the U.S. Geological Survey.

The study shows that between 1980 and 1991, the concentration of sulfate ions - a key component of acid precipitation - dropped at a statistically significant rate at 26 out of 33 rainwater i collection sites.

"The decreases are fairly substantial and they are widespread," says Timothy A. Cohn, who coauthored the report with William G. Baier, both at the USGS in Reston, Va. Although the national network of rain collection sites has some 200 stations, the two researchers focused on sites with the most extensive precipitation records.

The USGS study also found that the acidity of rainwater decreased over the same period at most of the sites. The pH of precipitation rose significantly at nine of the 33 stations, signaling decreasing acidity. An additional 16 stations showed an increase, but in these cases the changes were not large enough to be statistically significant.

Such trends represent the payoff from air pollution standards put in place in the 1970s during implementation of the Clean Air Act of 1970 and its 1977 amendments. Although not fashioned to address acid rain, the laws sought to improve air quality by reducing emissions of sulfur dioxide gas, which turns into sulfate in the atmosphere. These emissions come chiefly from power plants that burn fossil fuels, especially sulfur-rich coal.

Since 1970, U.S. emissions of sulfur dioxide have dropped by 30 percent, with most of the decline coming in the 1970s and early 1980s, says Derek Winstanley, director of the National Acid Precipitation Assessment Program in Washington, D.C. The 1990 amendments to the Clean Air Act require the nation to reduce its sulfur dioxide output some 40 percent from the 1980 level.

The act also calls for a 10 percent reduction in emissions of nitrogen oxides, which convert in the atmosphere into nitrate, another component of acid rain. Nitrogen oxides are found in power plant emissions and car exhaust.

The USGS study shows that nitrate concentrations in the rainwater dropped at most stations, but only three sites displayed statistically significant declines. Jack Pickering of the USGS says the absence of major nitrate reductions conforms to expectations, because releases of nitrogen dioxides have not declined as much as sulfur dioxide emissions.

While other studies have shown decreasing concentrations of sulfate in rainwater, the USGS is the first to show the increases in pH that researchers have expected to see as sulfur emissions fail. "If we didn't start to see an increase in pH as we reduce sulfur dioxide emissions, we'd begin to wonder if we had an effective program in place here. This gives us confidence that things are working in the direction that one would hope," says Winstanley.