2.0 ACID PRECIPITATION

2.1 Progress Toward Acid Rain Control in the United States

The National Acid Precipitation Assessment Program (NAPAP) is now completing work on the "1996 Integrated Assessment of NAPAP," as required by Congress under the Clean Air Act Amendments (CAAA) of 1990. The conclusions of the report include:

• In the first year of compliance with mandated controls under the CAAA (1995), the U.S. sulphur dioxide (SO₂) emissions were 40% below the allowable level, due primarily to a market-based approach which allowed application of least-cost options for reducing emissions from large point sources.
• In response to these SO₂ reductions, levels of sulphate in wet deposition measured downwind of the large power plants of the Ohio River Valley were lower than expected, given trends in precipitation chemistry. However, concentrations of nitrate in rain and snow were not reduced at these measurement sites. These long-term data on deposition chemistry point up the importance of continued support for the National Atmospheric Deposition Program (NADP).
• Lakes and streams in the northeast U.S. have shown decreasing sulphate concentrations over the past 15 years, but pH and ANC (Acid Neutralizing Capacity) measures have not changed significantly. It is suggested that the chemical and biological responses to decreases in emissions lag considerably.
• In the period since the last assessment (1990), knowledge of the effects of nitrogen species (NO₂ and others) deposition on both terrestrial and aquatic ecosystems, especially in upland areas throughout the U.S., has improved significantly. It is clear that nitrogen deposition plays an important role in episodic acidification of freshwaters and in the eutrophication of estuaries.
• Visibility in the eastern U.S. is expected to improve with a decrease in SO₂ emissions, since particulate sulphate is responsible for much of the regional haze there. Measurements at 13 sites show that sulphate particle concentrations are lower in the winter and fall and higher during spring and summer.
• It is expected that the reduction of sulphate particles in the eastern U.S. will have a positive impact on public health.

2.2 Need for Continuing Support of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) by the United States Geological Survey (USGS)

For nearly 20 years, the NADP/NTN has provided scientific information on spatial and temporal changes in the chemistry of precipitation throughout the nation. The USGS provides more than half of the total costs for coordination and data management, as well as all sample collection and chemical analysis costs for about 70 of the nearly 200 monitoring sites in the network.

This USGS ongoing commitment has guaranteed a stable financial platform from which nearly 100 other organizations have been able to build. Eleven (11) other federal agencies, along with 70 universities and state agricultural experiment stations, the Canadian government, and nine private companies provide additional financial support to the NADP/NTN program.

In the U.S., the amount of precipitation is measured daily at about 4000 weather stations. At 200 sites, weekly determination of the amount and diversity of chemical substances in precipitation are made. This continuous, uninterrupted record represents the only reliable source of critical data and information for:

• Monitoring precipitation chemistry as required under the Bilateral Air Quality Agreement between the Government of the United States and the Government of Canada;
• Verifying the effectiveness of the Clean Air Act Amendments of 1990;
• Determining degradation and recovery of air quality in Class I Wilderness Areas and National Parks;
• Identifying geographical patterns and temporal trends in acid deposition;
• Estimating the atmospheric component of the biogeochemical cycles of important mineral elements;
• Providing data for more than 600 scientific studies of the health of terrestrial and aquatic ecosystems;
• Developing cost-effective recovery plans for estuaries such as Chesapeake Bay;
• Serving more than 500 school and college science-education projects annually throughout the nation;
• Providing industry with data to assess materials damage from chemical deposition.

In the fiscal year (FY) 1998 budget category of the USGS, the funds previously designated for "Acid Rain" (including those that sustain the 70 stations in the NADP/NTN program) have been eliminated and the program merged with the National Water Quality Assessment Program (NAWQA). At the same time, the U.S. Department of Agriculture has proposed adding an 8% administrative charge on the operations of all the stations. These two actions, if implemented in the FY 98 budget cycle, will result in great uncertainty regarding the continued operation of this national, long-term network. Sustaining this unique federal, state, industry and binational cooperative program is important because routine measurement of the chemistry of rain and snow is one of the most cost-effective means of determining the general health of the atmosphere.

Recommendation:

The Board encourages the Commission to seek assurances from the U.S. government that the NADP/NTN network, and its associated consortium, will continue to be supported without any significant reduction in the generated data.

2.3 Trends in Acid Rain Precursors

The components (largely sulphate and nitrate) of what is commonly referred to as 'acid rain' are deposited in both dry and wet forms onto the earth's surface; these two forms require two different measurement technologies. In the United States, wet deposition associated with rain and snow is measured by the NADP, a consortium led by the USGS (see previous discussion).

Dry deposition is not measured directly, but is calculated from measurements of pollutant concentrations in air. Total deposition is determined through a combination of the measurements of wet and dry deposition.

The U.S. Environmental Protection Agency (EPA) has been operating a network of approximately 50 sites since the late 1980's called CASTNet (the Clean Air Status and Trends Network) to measure the dry deposition of ozone, sulphur and nitrogen compounds. This is a rural background network, located to minimize the direct impact of urban areas and individual industrial plumes on the measurements.

The material presented here focuses on seven-year trends for these air concentration measurements from ten of the CASTNet sites closest to the U.S./Canada border (see Figure 2). The time frame was from the fall of 1989 to September 1995.

The graphs (Figures 3, 4, and 5) show that, for the seven year period, sulphur compounds in the rural ambient air near the border have been reduced by about 30%, whereas nitrogen compounds (Figure 6) and ozone (Figure 7) have been reduced approximately 10%. These are direct measurements of the concentrations in ambient air of these gaseous materials. Figure 8 presents the sulphur dioxide concentration data in a different format. Smoothed trend data are presented for each of the 10 sites. This format shows the variability in the trends across the time period and from site to site.

Figure 2. Current CASTNet Sites near the U.S.-Canada Border with a 1989-1995 Trend Record

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Calculated trends in dry deposition or in total deposition are not available at this time. There is still much uncertainty in the calculation procedure which prevents scientifically valid trends from being produced for these two items.

These direct concentration measurements show that programs under the U.S. Clean Air Act are improving rural air quality. However, changes in the ecosystem need to be examined further to determine if a benefit is being exhibited at the receptor. The International Air Quality Advisory Board (IAQAB) will be acquiring further data to look at this issue and others associated with 'acid rain' in the near future.

2.4 U.S. EPA Nitrogen Oxides Emission Reduction Program - Final Rule for Phase II, Group 1 and Group 2 Boilers - (Coal Fired Utilities)

The Federal Register of December 19, 1996 contained the final rule implementing the second stage of the Nitrogen Oxides (NO_X) Reduction Program under title IV of the Clean Air Act Amendments of 1990. Reductions were to be accomplished by establishing NO_X emission limitations for certain coal-fired utility units and by revising earlier NO_X emission limitations for others.

2.4.1 Need for Reductions in NO_X Emissions

Emissions of nitrogen oxides discharged into the atmosphere from the burning of fossil fuels for electrical power generation have significant adverse effects on human health and the environment, contributing substantially to the formation of ozone, acid deposition, eutrophication of water bodies, inhalable fine particles, and visibility degradation.

Figure 9.

U.S. Federal Register, December 19, 1996 (Vol. 61, Number 245)-Rules and Regulations

As shown in Figure 9, electric utilities are a major contributor to the nationwide NO_X burden, emitting approximately 32% of total atmospheric NO_X in 1992 (1994 data confirm this distribution). Approximately 90 percent of electric utility NO_X comes from coal-fired power plants.

The reductions achieved by this rule, while substantial, represent only about 5% of total NO_X emissions. The U.S. EPA has determined that additional reductions are needed to maintain compliance with the current ambient ozone standard (120 ppm over one hour), to stabilize or, perhaps, to reduce the number of acidified lakes in the Northeastern States, and to control eutrophication in some regions of the country, such as Chesapeake Bay.

The EPA projections indicate reductions due to rules such as this will be more than offset as the number of sources and emissions from source categories, including transportation, increase. After the year 2002, when implementation of the NO_X control programs is largely complete, growth in source use is expected to continue and NO_X emissions will also increase for the foreseeable future.

2.5 Ozone

With regard to ozone, the EPA has projected that additional regional NO_X reductions of at least 50% (see Table 1) from current levels may be necessary over large portions of the United States to attain and maintain the current ozone ambient air quality standard of .12 ppm over one hour; modeling results indicate that reductions of approximately 75% could be necessary over much of the nation to reduce ozone concentrations to levels below the current standard.

Ozone Transport Assessment Group modeling simulations using an urban airshed model (UAM-V) confirm the estimate that broad NO_X emission reductions will decrease regional ozone, high ozone and ozone in non-attainment areas; similar reductions will be needed in particular subregions to lower ozone both in that subregion and others.

Further NO_X reductions needed to maintain the current standard have been estimated for several other specific areas of the country; these range from 50 to 75% measured against 1990 levels throughout the Northeast Ozone Transport Region, up to 90% in the Southeast and a combination of 75% reductions for NO_X and 25% for VOCs regionally in addition to local reductions of both pollutants in the New York region. Significant NO_X reductions will also be required in the Lake Michigan area, although these are as yet unquantified.

2.6 Acid Precipitation

Additional regional NO_X reductions of 40% (Table 1) appear necessary to mitigate acid deposition effects. Additional 40 to 50% reduction mandated in the current Clean Air Act may be required in the Eastern U.S. to maintain the number of acidified lakes in the Adirondacks at 1984 levels. Without these reductions, the number of acidic lakes are projected to rise by 40% by the year 2040.

Table 1. Estimated Regional Reductions Necessary to Mitigate Various Environmental Effects
	Ozone	Acid Deposition	Eutrophication
Regional NOX Reductions Necessary	>50%	>40%	up to 40

Note: No_x reductions achieved from the Final Rule as percentage of total No_x emissions ± 5%

2.6.1 First Stage of the NO_X Reduction Program

Title IV of the U.S. Clean Air Act specifies a two-part strategy to reduce emissions from coal-fired electric power plants. The first stage of the program, promulgated April 13, 1995 would reduce annual NO_X emissions in the United States by over 400,000 tons per year between 1996 and 1999 (Phase I), and by approximately 1.17 million tons per year beginning in the year 2000 (Phase II). These reductions are achieved by coal-fired dry bottom wall-fired boilers and tangentially fired boilers (Group 1).

2.6.2 Second Stage of the NO_X Reduction Program

In developing its requirements for a second stage of the title IV Program U.S. EPA has: (1) determined that more effective low NO_X burner (LNB) technology is available to establish more stringent standards for Phase II, Group 1 boilers (approximately 600 units) than those established for Phase I; and (2) established limitations for other boilers known as Group 2 (wet bottom boilers, cyclones, cell burner boilers, and vertically fired boilers - 145 units), based on NO_X control technologies that are comparable in cost to LNBs.

The final rule sets lower Group 1 emission limits and establishes emission limits for several other types of coal-fired boilers (Group 2) in Phase II. By the year 2000, the Phase II NO_X rule will achieve an additional reduction of 890,000 tons of NO_X annually.

2.6.3 Compliance and Deadlines

Phase I affected units were to meet the applicable limits by 1996; Phase II affected units are required to meet the applicable limits by the year 2000. The final rule relies upon target performance standards, but also allows emissions averaging and the use of alternative, higher emissions limits (AELs) where meeting the applicable limits is unfeasible. Utilities choose the method of compliance which best suits their needs.

2.6.4 Addition of Limited Cap and Trade Option⁽¹⁾

As a more flexible emissions trading approach, an option allowing a state (or group of states) to petition U.S. EPA to accept an emissions cap and trade program as a substitute for compliance with limits and reductions made final in this rule is available. Under this option, the Administrator can provide relief for boilers from emission limitations established in the final rule subject to a cap and trade program under Title I. The relief is contingent on the Administrator finding that alternative compliance through the cap and trade program will achieve lower total NO_X emissions from the Group 1 and Group 2 boilers in the state (or group of states) than if the new emission limitations remained applicable.

This added flexibility is meant to encourage states and utilities involved in the Ozone Transport Assessment Group region, where approximately 87% of the boilers covered by this rule are located, to move forward on a more cost-effective regional cap and trade program for NO_X reduction.

2.7 The Acid Rain Issue in Canada

The Eastern Canada Acid Rain Program, which began in 1985, has achieved many of its objectives. However, acid deposition continues to have the potential to cause damage to ecosystems in regions of Canada and additional reductions of sulphur dioxide and other acidifying emissions are needed.

2.7.1 Deposition

Wet sulphate deposition has diminished; the area in Canada receiving more than the target of 20 kg/ha/year (kilograms per hectare per year) was reduced by 46 percent in the decade since the early 1980's. Data on the changes in deposition resulting from the 1995 reductions in emissions are not yet available, but model predictions indicate that, by the year 2010, when the provisions relating to acid rain in the U.S. Clean Air Act are fully in place, virtually all of Canada will receive less than 20 kg/ha per year in sulphate deposition.

2.7.2 Environmental Response

There has been positive, albeit slow, improvement in the response of Canada's lakes to the reductions in sulphur dioxide emissions. Sulphate levels are declining in most lakes in Ontario and Quebec, and are stable in Atlantic Canada. Acidity levels remain high, in part because reductions in sulphur dioxide emissions are too recent in many regions for lakes to respond noticeably.

Neither NO_X emissions or nitrate deposition have changed significantly since the inception of the acid rain program. There are disturbing signs that, in time, this situation may undermine some of the benefits achieved through the control of sulphur dioxide emissions.

2.7.3 The Future

The target load of 20 Kg/ha/yr of sulphate in precipitation was an early objective of the Canadian efforts to reduce acid rain. The value was derived from limited data available in the early 1980's, based mainly on the loss of sport fish which occurs at approximately pH 5.3. It was recognized at that time that very sensitive areas would not be protected by this target and that further evaluation would be needed when more information became available. An SO₂ control plan that would lower the maximum deposition in any sensitive area to 20 Kg/ha/yr would mean that most other sensitive areas would receive substantially less wet sulphate deposition.

New information gathered in the past few years has been analyzed to determine the "critical load" for aquatic systems. The critical load is the highest deposition of acidifying compounds that will not cause chemical changes leading to long term harmful effects on the overall structure or function of a specific aquatic ecosystem. In the case of aquatic ecosystems, the critical load determined will be exceeded by up to 10 kilograms per hectare of wet sulphate in parts of central Ontario and southern Quebec. This same area is where sulphate and nitrate deposition most exceeds critical loads for forests. Models predict that forested and aquatic ecosystems in Ontario, Quebec, New Brunswick and Nova Scotia will continue to receive annual deposition amounts in excess of the critical loads.

This deposition rate appears to pose serious risks to forested ecosystems, both in terms of reduced productivity and decreased forest health. Field studies suggest that deleterious effects are already occurring but, for the moment, the symptoms are limited to forests that are subjected to other stresses such as harsh climatic and soil conditions. It appears that reductions in both sulphate and nitrate deposition will be needed to reduce the risk for those forested ecosystems most sensitive to acid deposition. Reductions in either sulphate or nitrate deposition may be sufficient elsewhere. In the case of aquatic ecosystems, the deposition rate after implementation of control programs is expected to result in approximately 10 percent of the lakes in central Ontario remaining acidic. This compares to the 20 percent of lakes in that region which were acidic before the Canadian and American controls on sulphur dioxide. Approximately 20 percent of lakes in Nova Scotia and New Brunswick are also expected to remain acidic, down from 25 percent, and 10 percent of the lakes in southern Quebec; down from 12 percent. As stated above, nitrogen deposition poses an additional threat and it may, in time, undermine the ecological benefits resulting from reductions of sulphur dioxide emissions.

1. U.S. Federal Register, December 19, 1996. (Vol. 61, Number 245) - Rules and Regulations

URL: http://www.ijc.org/rel/boards/iaqab/r23-ch2.html