The Acid Rain Challenge
Table of Contents
Acid deposition, more commonly known as acid rain, occurs when emissions of SO2 and NOX from power plants, vehicles, and other sources react in the atmosphere (with water, oxygen, and oxidants) to form various acidic compounds. These acidic compounds then fall to earth in either a wet form (rain, snow, or fog) or a dry form (gases and particles) and can harm aquatic and terrestrial ecosystems (particularly forests); affect human health; impair visibility; and damage automotive finishes, buildings, bridges, monuments, and statues.
Key Commitments of the Acid Rain Annex
Both the United States and Canada have been successful in reducing SO2 and NOX emissions and thus, mitigating the impact of acid rain on each side of the border. Despite these achievements, however, further efforts are needed to restore all damaged ecosystems to their pre-acidified conditions.
Commitments and Progress: SO2 Emission Reductions
- In 2006, Canada’s total SO2 emissions were 2 million tonnes,1 or about 38 percent below the national cap of 3.2 million tonnes (Figure 1).
- SO2 reductions represent more than a 55-percent decrease from Canada’s total SO2 emissions in 1980 and a 35-percent decrease from the 1990 emission level.
- SO2 emissions in the seven easternmost provinces were 1.4 million tonnes in 2005, or nearly 40 percent below the (now expired) eastern Canada cap of 2.3 million tonnes.
- Canada is committed to further reducing acidifying emissions through the more recent Canada-wide Acid Rain Strategy for Post-2000.
Figure 1. Canadian SO2 Emissions from Acid Rain Sources, 1980–2006
Source: Environment Canada, 2008
1 One tonne is equal to 1.1 short tons.
- The United States succeeded in meeting its commitment to reduce annual SO2 emissions by 10 million tons from 1980 levels by 2000.
- In 2007, emissions of SO2 from the electric power sector fell below the 2010 national emission cap of 8.95 million tons for the first time, achieving the U.S. commitment three years early (Figure 2).
- National SO2 emissions from all sources have fallen from nearly 26 million tons in 1980 to less than 13 million tons in 2007 (see <www.epa.gov/ttn/chief/trends>).
- Most of the reductions in SO2 emissions are due to the Acid Rain Program (ARP), which requires major reductions of SO2 and NOX emissions from the electric power sector.
Figure 2. U.S. SO2 Emissions from Acid Rain Program Electric Generating Units, 1980–2007
Source: EPA, 2008
Preventing Air Quality Deterioration and Protecting Visibility
Under the Acid Rain Annex, Canada and the United States have recognized the importance of preventing air quality deterioration and protecting visibility from sources that could cause significant transboundary air pollution. In October 2007, a joint U.S.-Canada visibility workshop was held in Research Triangle Park, North Carolina. The U.S. Environmental Protection Agency (EPA), the U.S. Federal Land Managers, and Canadian government representatives came together to review the history of the U.S. visibility program and to share information and lessons learned from joint analyses, discuss international transport, and investigate future collaboration.
Commitments and Progress: NOX Emission Reductions
- Surpassed its NOX emission reduction target at power plants, major combustion sources, and metal smelting operations by 100,000 tonnes below the forecasted level of 970,000 tonnes.
- Recently passed stringent standards for NOX emissions from on-road and off-road sources, effective from 2004 to 2009.
- Emissions of NOX from all NOX program-affected units were 3 million tons, and total NOX emissions from all sources covered by the ARP were 3.3 million tons (Figure 3).
- This level is 4.8 million tons less than the projected NOX levels for 2000 without the ARP, or more than double the NOX emission reduction goal under the Acid Rain Annex.
Figure 3. NOX Emission Trends for All Acid Rain Program Units, 1990–2007
Source: EPA, 2008
Consultation and Notification Concerning Significant Transboundary
Air Pollution Since 1994, Canada and the United States have regularly notified each other concerning potential new sources and modifications to existing sources of transboundary air pollution within 62 miles (100 km) of the U.S.–Canada border. Since publication of the 2006 United States–Canada AQA Progress Report, Canada has notified the United States of eight additional sources, for a total of 52 Canadian notifications. The United States has notified Canada of nine additional sources, bringing the total number of U.S. notifications to 56. More information is available on the government Web sites of each country at:
Acid Deposition Trends
Both nations use wet deposition (rain or snow) data to assess how the atmosphere is responding to decreasing or increasing emissions of sulfur and nitrogen. Figures 4 and 5 show the U.S.–Canada spatial patterns of wet sulfate and wet nitrate deposition, respectively, for 1990 and 2005. The pattern from 1990 to 2005 illustrates that significant reductions occurred in wet sulfate deposition in both the eastern United States and much of eastern Canada. Reductions in wet nitrate deposition have generally been more modest than for wet sulfate deposition.
Figure 4. Annual Sulfate Wet Deposition
Figure 5. Annual Nitrate Wet Deposition
Source: National Atmospheric Chemistry (NAtChem) Database (www.msc-smc.ec.gc.ca/natchem/index_e.html) and the National Atmospheric Deposition Program (NADP)