By mapping the concentration of key ions across the country, we can visualize the geographic relationship between emission sources and their downwind impact. The data from 1990-1993, just before the implementation of major new regulations, provide a clear baseline of the regional nature of the acid rain problem.

4.1. The Geography of Acidic Deposition

• Sulfate: The highest concentrations of sulfate were found in a region centered over the Ohio River Valley and extending into the Northeast. This pattern directly corresponds with the high density of coal-burning electrical power plants in that area, the primary source of sulfur emissions.
• Nitrate: High concentrations of nitrate were observed in the Northeast and in Southern California. This pattern aligns with major sources of high-temperature combustion, including both power plants and the high density of automobiles in these populated areas.

4.2. The Geography of Neutralizing Deposition

• Calcium: Elevated levels of calcium were measured across the Midwest and the plains states. These concentrations are attributed to dust generated from agricultural soils and unpaved roads, which are more prevalent in the generally more arid conditions of these regions.
• Ammonium: The highest concentrations of ammonium were found in the central United States, a pattern strongly linked to agricultural sources, particularly emissions from livestock feedlots.

4.3. The Integrated pH Landscape

The national map of precipitation pH is the result of the overlapping patterns of both acidic and neutralizing ions. Critically, the area of lowest pH (most acidic rain) was centered over Pennsylvania and New York, located slightly east of the peak sulfate concentrations in the Ohio Valley. This occurs because this region receives high concentrations of acidic sulfate and nitrate ions from upwind sources while having very low concentrations of the neutralizing, dust-related calcium and agriculture-related ammonium ions that are common in the Midwest.

These spatial patterns clearly demonstrate that regional precipitation chemistry is directly linked to specific, geographically concentrated human activities, setting the stage for targeted policy action.