8.1. Setting the Context

Analyzing large-scale data patterns is essential for identifying pollution hotspots, understanding source-receptor relationships, and evaluating the effectiveness of environmental policies. By investigating the geographic distribution and long-term changes in precipitation chemistry using NADP/NTN data, we can see how pollution sources, regional geography, and policy interventions have shaped the acid rain landscape across the United States.

8.2. Spatial Patterns (1990–1993)

An analysis of four years of data from 180 NADP/NTN sites reveals distinct geographic patterns for key chemical constituents, which can be linked to their primary sources.

• Sulfate: The highest concentrations are found in a region covering Illinois, Indiana, Ohio, and Pennsylvania. This pattern corresponds directly to the location of high sulfur emissions from coal-burning electrical power plants in the Ohio River Valley.
• Nitrate: High concentrations appear in the Northeast and in Southern California. This distribution is consistent with emissions from high-temperature combustion sources, such as power plants and automobiles, which are prevalent in these densely populated areas.
• Calcium: Elevated levels are found across the Midwestern, plains, and western states. This is attributed to wind-blown dust from soils and unpaved roads, which are more significant sources in arid regions with less protective vegetation.
• Ammonium: High concentrations are centered over the central U.S. This pattern is linked to agricultural sources, particularly ammonia emissions from livestock feedlots and the application of fertilizers.
• pH: This final map of precipitation pH is not a map of a single pollutant, but rather a composite map of chemical geography. Its pattern can only be understood as the net result of the overlapping spatial distributions of the acidic anions we’ve discussed—sulfate and nitrate—and the neutralizing basic cations—calcium and ammonium. The lowest pH values (most acidic rain) are found in a region centered over Pennsylvania and New York, where high levels of acidic anions coincide with lower levels of neutralizing bases.

8.3. Temporal Patterns

Seasonal Variation

A study by Bowersox and Stensland (1985) revealed a consistent seasonal pattern in precipitation chemistry. As shown in Table 3 of the source, concentrations of sulfate, hydrogen ion, and several other species are consistently higher during the warm-period months (May-September) than during the cold-period months (November-March) across most of the U.S.

Long-Term Trends and Policy Impacts

Analyzing long-term trends is challenging due to historical inconsistencies in sampling methods. However, the high-quality NADP/NTN dataset has provided crucial insights:

• An analysis of NADP/NTN data from 1980 to 1992 showed widespread declines in sulfate concentrations, consistent with reductions in sulfur dioxide emissions. However, these were often accompanied by decreases in base cations, meaning that the acidity of the rain did not always decrease as much as expected.
• The impact of policy became clear following the 1990 Clean Air Act Amendments. In 1995, after Phase I of the act mandated emission reductions, sulfate and hydrogen ion concentrations in the eastern U.S. decreased much more than predicted by prior trends. This provided strong evidence that the legislation was effective. Further reductions were required by Phase II in the year 2000, promising additional improvements.

8.4. Concluding Transition

The clear link between industrial emissions and acid rain in the United States raises an important question: what does precipitation chemistry look like in remote regions of the globe, far from these pollution sources?