The National Atmospheric Deposition Program collects rainfall samples weekly from more than 250 stations across the United States and analyzes them for pollutants. The program recently released a report detailing trends in acidic rainfall frequency and concentration over 25 years, from 1984 to 2009.
According to the report, acidic precipitation – rain or snowfall with a pH value of 5.0 or less – decreased in both frequency and concentration over the 25-year span.
The researchers largely attribute the decrease to the amendments to the Clean Air Act in 1990 regulating emissions of sulfur dioxide and nitrogen oxide, the gases that become sulfuric and nitric acid when mixed with rain water.
"What goes up does come down," Lehmann said. "Rainfall chemistry directly correlates with air pollution. When we looked at the magnitude of the trend, we found it compared very well to the magnitude of the decrease in emissions reported by the EPA (Environmental Protection Agency)."According to EPA data, sulfate emissions dropped more than 50 percent during the period covered by the study and nitrate emissions dropped more than 30 percent. The National Atmospheric Deposition Program report found that concentrations of corresponding acid ions in rainwater have dropped by similar magnitudes. In addition, frequency of acidic precipitation has decreased across the U.S.
The deposition program continues to monitor sulfur and nitrogen compounds in rain. Although acidic precipitation has decreased, it has not disappeared, particularly remaining prevalent across the eastern U.S. In addition, the program has expanded its screening and monitoring other problematic pollutants such as ammonia and mercury.
"We still have acid rain," Lehmann said. "Yes, the trend is down, and we should celebrate that, but it's still a problem. There is still progress to be made, and there are new regulations coming along to continue to reduce emissions of sulfur and nitrogen compounds."
The Illinois State Water Survey is a unit within the Prairie Research Institute at the U. of I.
Editor's notes: To reach Chris Lehman, call 217-265-8512; email firstname.lastname@example.org.
To reach David Gay, call 217-244-0462; email email@example.com. Data are available in the public domain at the National Atmospheric Deposition Program website.
Liz Ahlberg | University of Illinois
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