Section 3:
Scientific and Technical Research and Cooperation

Emission Inventories and Trends

JOINT EFFORTS

The United States and Canada have updated and improved their emission inventories and projections to reflect the latest information available. These emission inventories were also processed for U.S. and Canadian air quality models to support the technical assessment of air quality problems.

In the fall of 2003, the two countries held a workshop on Innovative Methods for Emission Inventory Development under the auspices of NARSTO. 20 As a followup to the workshop, an Emissions Inventory Assessment is underway that will provide recommendations to improve the quality, timeliness, comparability, and cost of compiling emission inventories in North America. A draft of the assessment report will be available by late 2004.

Emissions data for both countries for 2002 are presented in Figures 21, 22, 23, and 24 (see below). Figure 21 shows the distribution of emissions by source category grouping for SO2, NOx, and VOCs.

• SO2 emissions in the United States stem primarily from coal-fired combustion in the electric power sector; Canadian SO2 emissions stem mostly from coal-fired combustion in the industrial sector, with few emissions from the electric power sector, due to the large hydroelectric capacity in Canada.
• The distribution of NOx emissions is very similar between the two countries, except that emissions from the electric power sector are proportionately higher in the United States, again reflecting more coal combustion in that sector.
• VOC emissions are the most diverse of the emission profiles in each country. The most significant difference is that most VOCs come from the industrial sector in Canada. This is the result of the proportionately higher contribution of oil and gas production in Canada.

Figure 21. U.S and Canada National Emissions by Sector for Selected Pollutants (2002)

Source: EPA and Environment Canada

The emission trends, shown in Figures 22, 23, and 24, for NOx, VOCs, and SO2 show the relative contribution in emissions over the 1990-2002 period.

Figure 22. U.S.-Canada NOX Emissions

Source: EPA and Environment Canada

Figure 23. U.S.-Canada VOC Emissions

Source: EPA and Environment Canada

Figure 24. U.S.-Canada SO2 Emissions

Source: EPA and Environment Canada

In the United States, the major reductions in NOx emissions came from on-road mobile sources and electric power generation sources. For VOCs, the reductions came from on-road mobile sources and solvent utilization.

For SO2, the reductions were from electric power generation sources. For all three pollutants during this time period, the United States generated substantially more emissions than Canada. At the same time, while both countries have seen major reductions in SO2 emissions, the United States has shown greater emission reductions than Canada for VOCs and NOx.

Air Quality Reporting and Mapping

JOINT EFFORTS

Each country is responsible for ensuring calibration and routine comparability of ozone measurement data. Since 2001, the United States and Canada have collaborated on contributing to the EPA-led AIRNOW program (www.epa.gov/airnow).

Figure 25. AIRNOW Map Illustrating Real-Time Concentrations of Gound-Level Ozone

Source: EPA

This Web site provides real-time maps depicting ozone levels on a continental scale in season (see Figure 25) and, since 2003, year-round particulate levels in the United States.Canadian scientists have been experimenting with algorithms to improve the mapping effort, using a combination of real-time ozone data and information from Canada's operational air quality forecasting model, CHRONOS (see Figure 26 for an example).

Figure 26. Analysis of Ground-level Ozone for July 31, 2003

This analysis combines measurement data and predictions from the Canadian CHRONOS model to optimize the information presented.

Source: Environment Canada

CANADA

Environment Canada is expanding and refurbishing federal and provincial networks of monitoring stations across the country. Canada maintains two national ambient air quality monitoring networks. The National Air Pollution Surveillance (NAPS) Network is a joint federal, provincial, territorial, and municipal network established in 1969. It is primarily an urban network, with more than 240 air monitors at more than 136 sites. The federal Canadian Air and Precipitation Monitoring Network (CAPMoN) is a rural network with 23 air monitoring stations in Canada and one in the United States. Some provinces and municipalities operate CAPMoN networks that integrate the local NAPS sites.

The NAPS network gathers measurements on the components of smog (i.e., ozone, PM, SO2, CO, NOx, VOCs, ions, and metals). In 2002 and 2003, Environment Canada invested in new equipment for the NAPS network, including 18 new and replacement ozone monitors, 15 new and replacement NOx monitors, 34 continuous PM2.5 monitors (Tapered Element Oscillating Microbalances (TEOMs)), and three PM dichotomous samplers. In addition, Environment Canada started a chemical speciation sampling program in December 2002 to characterize PM. The agency also built two new laboratories to support this work and equipped them with an ion-coupled plasma-mass spectroscopy (ICP-MS) instrument for metals analysis and an organic carbon/elemental carbon analyzer.

In 2002, Environment Canada refurbished the ozone monitors in CAPMoN with new instruments. The agency purchased and tested new equipment for PM2.5, 10 mass measurements, and PM composition measurements. In addition, Environment Canada started PM mass measurements at one site and made preparations for PM equipment installation at other network sites.

At present, the ozone monitors in CAPMoN are gathering data in real-time, in support of the Air Quality Prediction Program, and for distribution to the U.S. AIRNOW program. PM2.5, 10 mass measurements, PM2.5 speciation measurements, and VOC measurements are being made at five CAPMoN sites (within 500 km of the border). Nitrogen compounds (including NOx) are being measured at two sites-the Centre for Atmospheric Research, Egbert (Ontario) and Kejimkujik (Nova Scotia).

UNITED STATES

EPA's ambient air quality monitoring program is carried out by state and local agencies and consists of three major categories of monitoring stations that measure the criteria pollutants: State and Local Air Monitoring Stations (SLAMS), National Air Monitoring Stations (NAMS), and Special Purpose Monitoring Stations (SPMS). Additionally, a fourth category of monitoring station, the Photochemical Assessment Monitoring Stations (PAMS), which measures ozone precursors (approximately 60 volatile hydrocarbons and carbonyl), is required by the 1990 Clean Air Act Amendments. Descriptions of these networks can be found at www.epa.gov/oar/oaqps/qa/monprog.html.

EPA also operates the Clean Air Status and Trends Network (CASTNET), a long-term monitoring program established to assess the effectiveness of SO2 and NOx emission reductions. CASTNET's objectives are to define the geographic distribution of pollutants and atmospheric deposition fluxes, detect and quantify trends in pollutants and deposition, and provide data on the dry deposition component of acid deposition and ground-level ozone concentrations in rural areas over broad geographic regions of the United States (www.epa.gov/castnet/).

To monitor ozone, the United States operates 856 SLAMS and 198 NAMS sites. Additionally, state, local, tribal, and other non-governmental agencies operate approximately 332 SPMS for ozone. There is little distinction among the state, local, or tribal SLAMS, NAMS, or SPMS sites; the data are all used for similar purposes. The PAMS networks measure ozone precursors in the most severe ozone nonattainment areas. These sites also provide information on pollutant transport and local meteorology. In 2003, approximately 89 PAMS were in operation in five regions of the United States: the Northeast, the Great Lakes area, Georgia (Atlanta), Texas (primarily Houston), and seven areas in California.

Ambient monitoring for PM2.5 is conducted at approximately 1,100 Federal Reference Method (FRM) PM2.5 sites, with approximately 267 continuous ambient monitors. These are particularly needed for public data reporting and mapping efforts currently being planned. EPA is focused on real-time data reporting through the AIRNOW system in the 36 metropolitan areas that are carried by various media sources, including USA Today.

Additionally, chemically speciated PM data are collected at 54 urban trends sites, 221 supplemental sites, more than 50 rural sites using IMPROVE protocols, and approximately 180 IMPROVE sites in Class 1 areas. EPA currently operates five urban sites that use continuous chemical speciation technologies for nitrates, sulfates, and carbon, and expects to add up to seven more urban sites in 2005. The Agency will use the results from these sites to consider whether to use this continuous measurement technology at other state locations. Tribal agencies are also providing additional fine PM data through the use of both filter-based FRM and the IMPROVE protocols.

Transboundary Particulate Matter Science Assessment

As an outgrowth of the Joint Plan of Action for Addressing Transboundary Air Pollution, signed in 1997, the governments of Canada and the United States have completed a joint science assessment report on PM. This document represents the first Canada-United States science assessment of an air pollution issue and is serving as a basis for decisionmaking for possible updates to the Air Quality Agreement. Results from three binational workshops between 1999 and 2003 identified several key objectives for a Canada-United States transboundary PM science assessment. This section outlines these objectives and findings, along with several figures from the report, as examples of supporting analyses.

• Objective 1: Identify whether a fine PM problem exists in the border region, based on current standards.
• Objective 2: Identify the extent of the problem; if standards are exceeded, where, when, and by how much?

Recent air quality monitoring data indicate that annual average levels of PM2.5 are as high as 18 µg/m 3 in the northeastern United States, but are consistently lower than 12 µg/m3 in the mid-continental states (see Figure 27). When Canadian hourly TEOM observations are included, a more detailed picture of ambient levels can be seen. The 98th percentile values for the years 2000 to 2002 are shown in Figure 28.

Figure 27. Mean Concentrations of PM2.5 at Canadian Dichotomous and U.S. FRM Monitors in the Border Region (2002-2003)

(Note: Canadian sites are years 2000-2002; all sites do not include three years of data.)

Figure 28. 98th Percentile PM2.5 Concentrations at Canadian TOEM and U.S. FRM Sites (2000-2002)

(Note: Canadian sites do not all include three years of data.)

Source: Figures adapted from "Transboundary Transport: Trends in and Analysis of Fine Inhable Particules in the Transboundary Region: Science Assessment." A Report by the Canada-U.S. Air Quality Committee, Subcommittee 2: Scientific Cooperation. November 2004.

The northeastern United States is again a region of high ambient PM levels, with 98th percentile values in excess of 30 µg/m3 at many of the sites. Canadian locations exhibit generally lower levels of PM2.5, although concentrations greater than 30 µg/m3 occur in several regions of the country for the years 2000 to 2002, particularly in the Windsor-Quebec City corridor.

• Objective 3: Describe the PM issue in terms of geographic regions (e.g., West, Central, East).

Current ambient levels of PM2.5 in the border regions exceed the standards set for PM2.5, primarily in the eastern portion of the border domain. Some sites in the Georgia Basin-Puget Sound airshed have elevated PM2.5 levels (with very few sites exceeding either standard for the time periods evaluated), but the levels are generally lower than in the East in Canada and the United States. Urban concentrations of PM2.5 (Figure 29) are higher than rural sites (Figure 30) in all regions of both Canada and the United States (note scale of embedded pie charts).

Figure 29. Summaru of Urban PM2.5 Speciation Data from EPA and NAPS Speciation Networks (September 2001-August 2002)

Figure 30. Summaru of Rural PM2.5 Speciation Data from U.S. IMPROVE and Canadian Networks (September 2001-August 2002)

• Objective 4: Identify PM precursors of concern on a regional or sub-regional basis.

PM2.5 in the border region consists of, in order of relative importance to annual PM2.5 levels: organic/black carbon, sulfate, nitrate, ammonium, soil dust, and trace elements. Secondary particulate (i.e., ammonium, nitrate, and sulfate) plays a key role under episodic conditions in Ontario. In the United States-Canada border region, carbon and sulfates are the dominant species of PM2.5 aerosols in spring, summer, and fall. In the United States, nitrates are a major species in the winter in the Northeast, and carbon is a major species in the winter in the Northwest.

Anthropogenic emissions of SO2, NOx, and ammonia are identified as PM precursors of concern in the East and Midwest United States. Comparison of urban and rural speciation and levels (Figures 29 and 30, noting difference in scale) indicate important natural sources of total carbonaceous material (TCM), and also anthropogenic sources, such as motor vehicles or solvent usage. Forest fires are a significant, though episodic, source of TCM.

• Objective 5: Describe source regions of PM and its precursors in the context of geographic regions (i.e., West, Central, East).

Emissions from the northeastern United States and southern Canada have an impact on PM2.5 levels in many areas of the two countries, including as far east as Nova Scotia and New Brunswick, particularly influencing the top 25th percentile of PM2.5 concentrations in these regions. Source-receptor analyses indicate that several areas contribute to elevated PM levels in eastern North America. These areas include, but are not restricted to:

• Air masses originating from a relatively large area from southeast Ohio to the western part of Virginia and western Kentucky to central Tennessee, which tended to result in relatively high PM2.5 concentrations over northeastern North America.
• The Windsor-Quebec City Corridor.
• The U.S. Midwest and Boston to Washington, D.C. corridor.
• The Ohio River Valley.
• Northern Alberta and Saskatchewan and central United States (e.g., Montana, North Dakota).
• Vancouver/Seattle, Oregon, and Northern California.

Figure 31 shows an example of the source determination work, illustrating a source-receptor analysis for ambient contaminants related to coal-fired plant emissions, using measurements at Toronto and sites in the eastern United States. The study identifies a coherent and plausible source region.

Figure 31. Source-receptor Analysis for Ambient Contaminants Related to Coal-fired Emissions

Using measurements at receptor sites in Toronto and in the eastern United States, based on air mass trajectories. The different particle constituents are indocated in different colors; nested contours indentify source regions with increasing probability. From Transboundary Particulate Matter Science Assessment.

Souce: Figures adapted from "Transboundary Tranport, Trends in and Analysis fo Fine Inhalable Particules in the Transboundary Region: Science Assessment." A Report by the Canada-U.S. Air Quality Committee, Subcommittee 2: Scientific Cooperation. November 2002.

• Objective 6: Describe emissions of PM precursors.

A common inventory of PM precursors SO2, NOX, and NH3 was created based on shared U.S. (1990 and 1996) and Canadian (1990 and 1995) emission information. Annual total emissions are presented in Figures 32, 33, and 34. Emissions of SO2 and NOX are concentrated in the industrial Midwest, northern United States, and southern Ontario. Emissions of NH3 are concentrated further west in the central Midwest region. The emissions of SO2, NOX, and NH3, and their contributions to PM2.5 levels, vary seasonally.

Figure 32. U.S.-Canada 1995-1996 SO2 Emissions

Figure 33. U.S.-Canada 1995-1996 NOX Emissions

Figure 34. U.S.-Canada 1995-1996 NH3 Emissions

Source: Figures adapted from "Transboundary Transport, Trends in and Analysis of Fine Inhalable Particles in the Transboundary Region: Science Assessment." A Report by the Canada-U.S. Air Quality Committee, Subcommittee 2: Scientific Cooperation. November 2004.

• Objective 7: Identify the effect of current and proposed emission reduction scenarios on fine PM levels in North America.

Projected PM2.5 reductions were estimated with model scenarios using shared emission scenarios for 2010 and 2020, which were developed based on the common U.S.-Canada Inventory. U.S. work used the model, REMSAD, focusing on annual levels, while the Canadian model, AURAMS, was applied to a winter and a summer episode of high particulate levels. U.S. and Canadian controls that are expected to be implemented were found to result in maximum annual reductions of PM2.5 of 2.3 µg/m3 in 2020 (see Figure 35).

Figure 35. Anticipated Reductions in Annual PM2.5 Concentration in 2020 from Expected U.S and Canadian Controls

Results were developed using the REMSTAD model with a full year model run (1996 meteorology). From Transbounday Particulate Matter Science Assessment. For Clarity, reults over Atlantic Ocean are not shown.

Source: Figures adapted from "Transboundary Transport, Trends in and Analysis of Fine Inhalable Particles in the Transboundary Region: Science Assessment." A Report by the Canada-U.S. Air Quality Committee, Subcommittee 2: Scientific Cooperation. November 2004.

The reductions vary temporally and spatially, with larger reductions in the eastern portion of the REMSAD modeling domain. Proposed additional SO2 and NOx emission reductions should provide additional reductions in ambient PM2.5 levels in eastern North America. The observed PM2.5 reductions might vary by season and will depend strongly on reductions in PM2.5 sulfate ion mass (Figure 36).

Figure 36. Anticipated Reductions in PM2.5 Concentration, and in Its Sulfate Composition from Additional U.S. and Canadian Controls

Results were developed using the Canadian model AURAMS, based on an 11-day summer ozone episode (July 8-18, 1995, meteorology.) From Transboundary Particulate Matter Science Assessment.

Health Effects

Canada and the United States generally collaborate on health effects research at the hands-on working level. Individual researchers or research groups share methodologies and datasets to advance understanding of the nature and extent of air pollution effects on human health. In this framework, Health Canada completed health science updates for PM2.5 and ozone in support of the Canada-wide Standards process.

As a result of the pace of toxicological and epidemiological research on these substances, the review considered progress in understanding health effects of these pollutants. The updates to the health science assessments for PM2.5 and ozone conclude that the new evidence gathered from clinical, toxicological, and epidemiological studies continues to support the standards.

Health Canada, in collaboration with EPA officials, initiated discussions for the development of possible surveillance mechanisms to monitor health and air pollution bilaterally. Health Canada hosted a bilateral federal (Canada/United States) science workshop in March 2003. The Tracking Public Health Impacts of Transboundary Air Pollution Workshop presented a suite of indicators to identify the health impacts of investments resulting in long-term air quality changes. Work towards the development of a valid air health indicator is ongoing.

Health effects of air pollution research in the United States has focused primarily on PM in recent years. EPA has a well-established PM health effects research program, consistent with the recommendations of the National Research Council's Committee on Research Priorities for Airborne Particulate Matter. Key findings of recent PM health effects research were presented in a draft Criteria Document and in a draft Staff Paper developed as part of EPA's regular review of its NAAQS for PM.

The results of some recent research related to PM health effects are provided below:

• Recently published epidemiologic studies have continued to provide evidence linking serious health effects with exposure to fine particles. Of particular note are the re-analysis and followup analyses completed using updated data from the American Cancer Society cohort that show long-term exposure to fine particles and sulfates (a fine particle component) to be associated with increased mortality from cardiovascular diseases. A full discussion of the new evidence is included in the Canadian and U.S. scientific review documents described previously.
• New total respiratory deposition data were obtained from healthy adults for ultra-fine particles, and an empirical formula was developed to estimate the total deposition in various breathing conditions. The new empirical formula will be useful for improving the health risk assessment process by relating exposure and activity information to the internal dose, which has a direct relevance to the biological responses. Studies determined that as many as 10 times more particles are deposited in certain regions of the lungs in people with pulmonary disease, which might indicate that their increased susceptibility is due to receiving an increased dose.
• Multiple hypotheses now exist describing the biological mechanisms by which very small concentrations of inhaled PM produce cardiovascular and pulmonary changes contributing to increased illness and death. Similarly, laboratory studies and animal models that mimic human disease have stimulated several theories about how the physicochemical properties of PM produce toxicity. No single attribute seems to exist that makes PM toxic, but size and certain chemical components (e.g., metals) appear to be involved. This finding is supported by both laboratory and field evidence.

Aquatic Effects Research and Monitoring

Research and monitoring for aquatic effects from air pollution involves numerous studies of water chemistry trends and biological recovery coordinated within the international scientific community. One such forum is the International Cooperative Program on Assessment and Monitoring of Acidification of Rivers and Lakes (http://www.iis.niva.no/icp-waters/icp_index.htm), established under the Convention on Long-Range Transboundary Air Pollution (LRTAP). As reported in the 2002 Progress Report, monitoring data show water chemistry improvements in response to steadily decreasing emissions of sulfur oxides, but this improvement is occurring in a complex pattern reflecting emissions of other substances, different lake characteristics, and climate interactions. Conspicuous recovery is only seen in limited areas that formerly had very high initial deposition levels.

These complex interactions are evaluated through the application of dynamic models developed using information from detailed process studies. Generalization to regional levels requires some calibration of key parameters, but such model applications help to understand the past and project future recovery. For example, application of the MAGIC (Model of Acidification of Groundwater in Catchments) model to lakes in Atlantic Canada suggests that the chemical condition of the lakes is much improved compared to the condition in the mid-1970s.

Further recovery requires additional reductions in acidic deposition (up to 50 percent), however, and will take decades to occur in some areas. A summary of such work for Canada will be published shortly in the 2004 Canadian Acid Rain Science Assessment.

EPA scientists recently concluded a 10-year analysis of water quality data in the United States to determine how U.S. waters are responding to the reduction in acid deposition that has occurred over that time period. 21 The analysis indicates that acid neutralizing capacity (ANC)-a measure of the ability to buffer acidity-in lakes in the Adirondacks, Appalachians, and Upper Midwest has begun to increase, a sign of greater capacity to withstand acidity and a sign of recovery.

Lakes in the northeastern United States, however, as well as streams in the Blue Ridge region of Virginia and West Virginia, do not yet show signs of recovery. All areas monitored by EPA's acidic surface water monitoring program, except for Blue Ridge streams, show that sulfate concentrations in lakes are declining. The analysis indicates that this reduction is occurring fastest in the most acid-sensitive waters and that the reductions in sulfate concentrations in water are a direct result of implementation of the U.S. Clean Air Act Amendments of 1990.

The Hubbard Brook Research Foundation continues to support comprehensive research and monitoring of the effects of acid deposition in the northeastern United States. One recent publication, Acid Rain Revisited: Advances in Scientific Understanding Since the Passage of the 1970 and 1990 Clean Air Act Amendments22, provides a concise overview of the most important research results over the last years.

The authors report that acid deposition is accelerating base cation leaching from soils and increasing aluminum concentrations in soil porewater. Acid deposition also increases the concentration of sulfur and nitrogen in soils, leaches calcium from red spruce needles and base cations from sugar maples, making them more susceptible to freezing, pests, drought, and other stresses, and, of course, acidifies lakes and streams. The study also reports positive progress as a result of implementation of the Clean Air Act but that more emission reductions are needed for full recovery to take place.

Forest Effects

Canadian and U.S. governments are involved in a joint Forest Mapping Project under the Acid Rain Action Plan endorsed by the NEG/ECP. The project involves applying a protocol developed and published in 1991 to assess forest sensitivity to atmospheric sulfur and nitrogen deposition. The maps covering Quebec and the Atlantic provinces, along with the New England states, will depict critical loads, or sustainable loads, for sensitive forest ecosystems. These "loads" depict the maximum deposition of atmospheric sulfur and nitrogen that a forest ecosystem can sustain without a net loss in soil reserves of plant nutrients.

Although the government of Ontario is not part of the NEG/ECP, the province has also carried out critical loads mapping through a contract with Trent University. Figure 37 is a Canadian product of the collaborative project, published in the 2004 Canadian Acid Rain Assessment. In this figure, critical loads depict a "no harvest" scenario. Depending on the harvesting practice used, additional soil nutrient losses could occur, thus reducing the critical load for a given forest ecosystem.

Figure 37. Critical Loads for Forest Soils in Canada

Critical loads for forest soils in Canada, representing the combined effects for both acidic sulfur and nitrogen deposition. For sulfur, 1000 eq/ha/yr; for nitrogen, to 14 kg.ha.yr. this figure us a preliminary result of the 2004 Canada Acid Rain Science Assessment. To obtain permission to reproduce this map in whole or in part, contact the Canadian Forest Service Atlantic Forestry Centre (afcpublications@nrcan.gc.ca).

U.S. and Canadian scientists continue to work at the U.S.-led Aspen Free-Air CO2 Enrichment (FACE) Project (http://aspenface.mtu.edu), established in 1997 on a site in northern Wisconsin. The free-air experiment is in its seventh year (1998-2004), studying trembling aspen, paper birch, and sugar maple exposure to elevated CO2 and ozone concentrations. Aspen FACE is the world's largest, open-air climate change research facility and the only FACE site where scientists can study the impact of the greenhouse gases carbon dioxide (CO2) and ozone on forest ecosystems.

These two gases act in opposing ways, and they can be harmful even at relatively low concentrations. Ozone offsets or moderates the positive responses induced by elevated CO2. Aspen FACE ozone exposure ranged between 78 and 93 ppb from 1998 to 2003, which is the fourth highest annual daily maximum 8-hour concentration. This exposure induced negative effects in aspen, which have cascaded through the ecosystem from gene expression to productivity. It has been found that ozone delays leaf-out and significantly accelerates leaf-drop in the fall, thus shortening the effective growing season for aspen (North America's most widely distributed forest tree species) between four and six weeks.

The U.S. Department of Agriculture (USDA)/U.S. Forest Service initiated the Forest Health Monitoring Program in 1991 (www.na.fs.fed.us/spfo/fhm) as a multi-agency cooperative effort to determine the status, changes, and trends of forest health indicators in all forest ecosystems in the United States. An analysis of spatial trends in average annual wet sulfate and inorganic nitrogen deposition (1994-2001) is presented in Figures 38 and 39. Sulfur and nitrogen deposition remains high in sensitive ecoregions.

Figure 38. USDA Evaluation of Annual Average Wet Sulfate Deposition by Ecoregion (1994-2001)

Figure 39. USDA Evaluation of Annual Average Wet Inorganic Nitrogen Deposition by Ecoregion (1994-2001)

Figure 40. USDA Ozone Bioindicator Expressed as a Biosite Index

Figure 40 shows the results of an ozone bioindicator study, which found that the oak-hickory forests of southern Illinois and Indiana are in the highest risk category for ozone damage. Most ecoregion sections in the north central and western United States had a biosite index of less than 5 (see Table 2).

The most recent U.S. research on the effects of acid deposition on forest ecosystems focuses on the effects of biogeochemical processes that affect plant uptake, retention, and cycling of nutrients within forested ecosystems. In particular, researchers now know that documented decreases in base cations (calcium, magnesium, potassium, and others) from soils in the northeastern and southeastern United States are at least partially attributable to acid deposition.23

Other research has shown that unpolluted temperate forests can become separated from historic sources of nutrients in bedrock and rely almost exclusively on atmospheric deposition for all necessary nutrients, providing a new picture of the sensitivity of forests to air pollution. 24 Finally, research on red spruce has indicated that calcium loss from needles as a result of acid deposition can make trees more susceptible to disease, frost, and drought.25

20 Formerly an acronym for "North American Research Strategy for Tropospheric Ozone, "the term NARSTO has become simply a wordmark signifying this tri-national, public-private partnership, which addresses the issue of tropospheric pollution, including ozone and suspended particulate matter.

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21 Stoddard, J.L., J.S.Kahl, F.A.Deviney, D.R.DeWalle, C.T.Driscoll, A.T.Herlihy, J.H.Kellogg, P.S.Murdoch, J.R.Webb, and K.E.Webster. 2003. Response of surface water chemistry to the Clean Air Act Amendments of 1990. EPA620-R-03-001, U.S.Environmental Protection Agency, Washington, DC.

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22 Driscoll,C.T., G.B.Lawrence, A.J.Bulger, T.J.Butler, C.S.Cronan, C.Eagar, K.F.Lambert, G.E.Likens, J.L.Stoddard, K.C.Weathers. 2001. Acid Rain Revisited: Advances in scientific understanding since the passage of the 1970 and 1990 Clean Air Act Amendments. Hubbard Brook Research Foundation. Science Links TM Publication, Vol.1 , No.1.

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23 Lawrence,G.W., M.B.David, S.W.Bailey and W.C.Shortle. 1997. Assessment of calcium status in soils of red spruce forests in the northeastern United States. Biogeochemistry 38:19-39; Huntington, T.G., R.P.Hooper, C.E.Johnson, B.T.Aulenbach, R.Cappellato and A. E. Blum. 2000. Calcium depletion in a southeastern United States forest ecosystem. Soil Science Society of America Journal 64:1845-1858.

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24 Kennedy,M.J., L.O.Hedin ,and L.A.Derry. 2002. Decoupling of unpolluted temperate forests from rock nutrient sources revealed by natural 87Sr/86Sr and 84Sr tracer addition. Proc .Natl. Acad. Sci.99:9639-9644.

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25 DeHayes,D.H., P.G.Schaberg, G.J.Hawley, G.R.Strimbeck .1999. Acid rain impacts calcium nutrition and forest health: Alteration of membrane-associated calcium leads to membrane destabilization and foliar injury in red spruce. BioScience. 49:789-800.

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