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Maryland Power Plants and the Environment (CEIR-18)

4.1.3 Impacts from Power Plant Air Emissions

Acid Rain

Acid rain occurs when precursor pollutants, NOx and SO2, react with water and oxidants in the atmosphere to form acidic compounds. These acidic compounds are deposited with precipitation (“acid rain”) or as dry particles (“dry deposition”), acidifying lakes and streams, harming forest and coastal ecosystems, and damaging man-made structures. Wet deposition does not only include precipitation as rain, but also includes snow, fog, or mist. Dry deposition occurs in areas where weather is dry and materials in the atmosphere stick to the ground, buildings, homes, cars, and trees. The runoff occurring from dry deposition when it does rain is more acidic since it is a combination of both dry and wet deposition.

EPA’s Acid Rain Program (ARP) was established under the CAA Amendments of 1990 with the goal of reducing acid rain by limiting NOx and SO2 emissions from power plants in the U.S. The program capped total SO2 emissions from power plants at 8.95 million tons nationally by 2000. The ARP for SO2 was the first federal cap and trade program and, in large part, the mechanics of current pollutant trading systems were established under this program. As with regional or national cap and trade programs, SO2 emissions are controlled with an “allowance” trading system, under which affected power plants are allocated a certain number of tons of SO2 annually. These plants must then either reduce emissions to stay under the allowance cap or purchase SO2 allowances from power plants that have over-controlled and banked excess SO2 credits. NOx emissions under the ARP are controlled with rate-based limits (in units such as pounds per million Btu, lb/MMBtu) applied to certain coal-fired electric facilities.

Efforts to reduce acid rain have been largely successful nationwide. At the end of 2015, according to EPA’s Air Markets Program Data, national SO2 emissions totaled 2.2 million tons, a level that represents a reduction of more than 87 percent from 1980 levels and 86 percent from the 1990 levels, and is well below the annual SO2 allowance of 9.5 million tons. Phase II of the ARP limited NOx emissions from affected facilities, which were either allowed to meet an emissions rate or comply with an emissions averaging plan. As of 2009, all 960 units covered by the ARP achieved compliance with the NOx emission limitation requirements. As of 2015, NOx emissions have been reduced from 1995 levels of 5.8 million tons to 1.3 million tons. The National Acid Deposition Program has been measuring deposition of oxidized nitrogen and sulfur species for over 20 years, and has noted a dramatic decrease nationally in deposition of sulfur species corresponding to the decrease in emissions, as well as a decreasing trend in deposition of oxidized nitrogen species over this time period.


The persistent ozone “smog” problem in many areas of the country has been one of the most important drivers for regulation of power plant NOx emissions over the past two decades. Ozone exists naturally in the upper levels of the atmosphere (from 6 to 30 miles above the Earth’s surface) and protects the Earth from harmful ultraviolet rays. Although ozone is helpful in the stratosphere, it is harmful when it occurs in the troposphere, the layer closest to the Earth’s surface. Ozone is an invisible and reactive gas that is the major component of photochemical smog. It is not emitted directly into the atmosphere in significant amounts but instead forms through chemical reactions in the atmosphere. Ground-level ozone is formed when the precursor compounds — NOx from both mobile and stationary combustion sources (such as automobiles and power plants, respectively), and VOCs from industrial, chemical, and petroleum facilities and from natural sources — react in the presence of sunlight and elevated temperatures. Ozone levels are consequently highest during the summer months when temperatures are higher, the hours of daylight are greater, and the sun’s rays are more direct.

Weather plays such an important role in the formation of ozone that EPA has established an “ozone season” for each of the states, and has developed regulations that require power plants to restrict NOx emissions during the summer months. Maryland’s ozone season extends from April through October.

Ground-level ozone is a problem, not only because it creates unsightly smog and inhibits visibility, but also because of the adverse human health effects it can cause. Breathing air with high ozone concentrations can cause chest pain, throat irritation, and congestion; it can also worsen pre-existing conditions like emphysema, bronchitis, and asthma. Children and the elderly are especially vulnerable to health problems caused by ground-level ozone. Recent action by EPA reduced the level of ozone standard (8-hour) from 75 ppm to 70 ppm, introducing additional challenges for states including MDE to develop a plan to achieve the standard.

Since the mid-1990s, there have been a series of federal NOx reduction regulations, implemented at the state level, that have resulted in significant reductions in summertime (“ozone season”) emissions of NOx from power plants in Maryland and surrounding states. One of the most significant — referred to as the “NOx SIP Call” because it called for affected states to update their State Implementation Plans (SIP) to address ozone issues —is based on a NOx cap-and-trade program that allows sources to acquire “allowances” to emit a certain quantity of pollutants; sources can actually reduce emissions or purchase allowances from plants who have reduced emissions below their caps. In some states, including Maryland, emissions exceeded statewide NOx allocations for many years in the first decade of the 2000s, meaning that some plants in these states were buying NOx allowances rather than reducing plant-level NOx emissions. The allocation exceedance in Maryland is likely attributable to the fact that not many sources had installed state-of-the-art controls such as SCR systems over the period. Several of the coal-fired generating units in Maryland, which are among the larger NOx sources in the state, have since installed SCR systems.

Visibility and Regional Haze

Fine particulate matter, or PM2.5, consists of particles (such as dust, soot, and liquid droplets) that are about 1/30th the diameter of a human hair. PM2.5 can be emitted directly from stacks or created when gases react to form particles during transport in the atmosphere. PM2.5 is different from many other air pollutants in that it is not a chemical compound itself, but is comprised of various compounds in particle form. Common sources include:

PM2.5 affects visibility, but is not the only contributor to decreased visibility and regional haze. Certain gases and larger particles can also interfere with the ability of an observer to view an object. In general, visibility refers to the conditions that can facilitate the appreciation of natural landscapes. The national visibility goal, established as a part of the CAA Amendments of 1977, requires improving the visibility in federally managed “Class I areas.” These areas include more than 150 parks and wilderness areas across the United States that are considered pristine air quality areas (see Figure 4-14 for Class I areas near Maryland). Since 1988, EPA and other agencies have been monitoring visibility in these areas.

Figure 4-14 Designated PSD "Pristine" Areas Near Maryland

Figure 4-14 Map of the mid-atlantic showing "pristine" areas

Source:  ERM,  “Mandatory Class I areas.” Air Program Class 1 Redesignation. Fond du Lac Resource Management. Accessed 10 March 2016.

Since 2004, PPRP has participated in a coordinated effort with the Northeast States for Coordinated Air Use Management (NESCAUM) and the State of Vermont to evaluate impacts of visibility-impairing sources in the eastern United States. The studies have evaluated the tools and techniques currently available for identifying contributions to regional haze in the Northeast and Mid-Atlantic regions. PPRP was involved with the application of a dispersion model, CALPUFF, for estimating visibility degradation in Class I areas. The model identified the contributions of sources in different states in the eastern United States to visibility impairment in various Class I areas in the region. PPRP continues to support and contribute to this ongoing work. PPRP also evaluates the impacts of new power plants on Class I visibility to ensure that growth in the electrical generating sector does not contribute to impairment in these important areas.

Nitrogen Deposition

The Chesapeake Bay (Bay) is the largest estuary in the United States. Protection and restoration of living resources in the Bay has been the goal of the Chesapeake Bay Program since its inception in 1983. The program is a regional partnership which comprises the states of Maryland, Pennsylvania, and Virginia, the Chesapeake Bay Commission; EPA; and other participating advisory groups.

Click to OpenMDE Maryland TMDL Data Center websiteReducing nitrogen input from controllable sources is a high priority because excess nitrogen is one of the major sources of eutrophication — caused by the increase of chemical nutrients, typically containing nitrogen or phosphorus — in the Chesapeake Bay. Eutrophication is a process whereby water bodies, such as lakes or estuaries, receive excess nutrients that stimulate excessive plant and algal growth and, ultimately, reduce the dissolved oxygen content in the water, thus limiting the oxygen available for use by aquatic organisms. The 1987 Chesapeake Bay Agreement established a goal of reducing controllable nitrogen by 40 percent compared to 1985 levels, and program participants reaffirmed that goal in their 2000 agreement. Although these goals were once again reaffirmed in the 2010 agreement, the Chesapeake Bay partners have acknowledged that the goals would not be met and EPA has initiated a process of developing a total maximum daily load (TMDL) target for the Bay. The Chesapeake Bay TMDL is a federal “pollution diet” that sets limits on the amount of nutrients and sediment that can enter the Bay and its tidal rivers to meet water quality goals.

On June 16, 2014, representatives from each of the watershed’s six states signed the Chesapeake Bay Watershed Agreement, committing to create a healthy Bay by accelerating restoration and aligning federal directives with state and local goals. This agreement contains ten interrelated goals that work toward advancing the restoration and protection of the Bay, its tributaries and the land that surround them.

The Chesapeake Bay Program estimates that approximately 30 percent of the nitrogen load to the Bay comes from atmospheric deposition and subsequent transport of nitrogen through the watershed. Much of this loading comes from NOx emissions from power plants, industrial sources, and mobile sources. Increased efforts have been devoted recently to the role of ammonia in deposition processes.

For more than a decade, PPRP has evaluated the regional sources of NOx emissions and their impacts on the Chesapeake Bay. As a part of this effort, advanced computer modeling systems are used to simulate the transport and subsequent deposition of emissions from these regional sources to the Chesapeake Bay. The actual loading to the Bay is calculated using a methodology similar to that used by the United States Geological Survey for its land-to-bay models. The model allows PPRP to evaluate the relative contribution of Maryland sources and other regional sources to deposition totals. As a part of this study, PPRP has developed a screening tool to evaluate the potential reductions in nutrient loading to the Bay waters due to different emission control policies in different states. Using this tool, regional and local planning agencies can better develop emission reduction strategies to meet Bay restoration goals.

EPA has developed an advanced nitrogen deposition source apportionment technique, based on the photochemical grid model CMAQ, which is a refinement of the screening tool developed by PPRP. While much of the work related to deposition estimates and source apportionment going forward will be based on the CMAQ-based methodology, the screening tool is still available and can be used for developing first cut estimates of the effects of emissions changes on nitrogen loading. PPRP continues to work on updates to the underlying model (CALPUFF) and investigations of the newer SCICHEM model, to improve the accuracy of the modeled deposition rates.

The National Atmospheric Deposition Program (NADP) has developed total deposition maps for nitrogen and total sulfur for use in critical loads and other ecological assessments. The total deposition estimates are determined from the sum of both wet and dry deposition. Wet deposition values are obtained from combining NADP/National Trends Network (NADP/NTN) measured values or precipitation chemistry with precipitation estimates from the Parameter-elevation Regression on Independent Slopes Model (PRISM). The PRISM model estimates precipitation across the US based on elevation and slope. Dry deposition values are obtained by combining air concentration data with modeled deposition velocities. Figure 4-15 is a national map of total nitrogen deposition in 2000 and 2013. As shown in this figure, while total nitrogen deposition increased in some parts of the country, in the eastern US it decreased significantly from 2000 to 2013.

Figure 4-15  Total Nitrogen Deposition in 2000 and 2013

Figure 4-15 Two maps of the United States showing Nitrogen Deposits as colored coded regions

“Total Deposition Maps.” National Atmospheric Deposition Program. Accessed 10 March 2016.

Mercury Impacts

The primary stationary sources of mercury in the U.S. are, in order of decreasing emissions, coal-fired power plants, gold mining, municipal waste combustors, chlor-alkali plants, medical waste incinerators, and cement plants. Emissions from some source categories — notably medical waste incinerators — have decreased dramatically in recent years due to stringent EPA regulations. Additionally, as shown in Figure 4-8, mercury emissions from power plants in Maryland have decreased significantly since the implementation of the Healthy Air Act (HAA).

Due to the significance of power plant mercury emissions (including emissions from out-of-state sources), PPRP plays a significant role in supporting scientific research on this topic. PPRP has been actively involved in the study of regional sources of mercury emissions and their impacts on Maryland and the Chesapeake Bay. In cooperation with the University of Maryland, PPRP has sponsored several deposition monitoring programs and continues to evaluate the impacts of toxic emissions from power plants in Maryland. PPRP has also supported a project to measure ambient air mercury concentrations at the Piney Run monitoring site in Garrett County, Maryland, using a continuous mercury monitoring instrument. This state-of-the-art monitoring effort provides valuable data to the mercury research community.

PPRP is also involved with other on-going projects related to the effects of mercury emissions. The first project involves working with the Smithsonian Environmental Research Center and the University of Maryland Center for Environmental Science – Chesapeake Bay Laboratory to investigate the biogeochemistry of the processes involved with the fate of atmospheric mercury and how it ends up in fish tissue. In another cooperative project with MDE, researchers are monitoring mercury tissue burden in young fish — a long-term effort that will lead to a better understanding of trends in mercury tissue burden in response to federal and state regulations aimed at reducing mercury releases to the environment. PPRP also participates in discussions and planning sessions with NADP regarding the Mercury Deposition Network (MDN) that measures wet deposition of mercury across the U.S. and Canada, and the new Atmospheric Mercury Network (AMNet) that collects data consisting of speciated mercury concentrations and meteorological data. AMNet is intended to supplement the wet measurement network to lead to more complete understanding of total (wet plus dry) mercury deposition patterns.

In 2002, Maryland issued a state-wide fish consumption advisory for lakes, reservoirs, and other impoundments due to high mercury levels in fish. This advisory is currently in effect. PPRP has been involved for many years in conducting complex modeling studies to estimate the quantity of mercury from Maryland and other regional sources that is deposited in water bodies throughout the state. The location of sources of mercury emissions close to Maryland, and the location of some of the water bodies and watersheds evaluated in PPRP’s study, are shown in Figure 4-16.

Figure 4-16 Location of Larger Watersheds (WS) and Mercury Sources within Maryland

Figure 4-16 Map of the Larger Watersheds in Maryland

Source: ERM “Garrison, Mark, Anand Yegnan and Jenifer Flannery. “Mercury in Maryland: Modeling to Assess Impacts and Effects.” Maryland DNR PPRP. June 2010.

As a part of the continuing effort to evaluate impacts of regional sources of mercury emissions on mercury loading to Maryland water bodies, PPRP conducted a study to determine the reduction in mercury loads to the state’s water bodies due to implementation of Maryland HAA mercury controls. This analysis was based on the projected reductions in emissions from Maryland power plants, which was approximately 90 percent from 2007 base year levels. This analysis predicted that Maryland’s HAA emission reductions would potentially reduce mercury deposition to these water bodies contributed by Maryland power plants by an average of more than 75 percent. The analyses also compared the reductions in loading to the total loading from regional sources of mercury and global background levels. The modeling analysis predicted that the reduction in emissions at Maryland power plants would potentially reduce the mercury load to water bodies by 1 to 28 percent, the lower estimate being for the western Maryland water bodies, which are predominantly influenced by sources from outside Maryland. An analysis of the reductions in load due to actual emissions reductions achieved is currently underway. PPRP is developing an updated mercury emissions inventory, and is working in cooperation with scientists from the National Oceanic and Atmospheric Administration (NOAA) to complete this analysis.

MDE Maryland TMDL Data Center website

The Maryland Department of the Environment (MDE) develops total maximum daily load (TMDL) thresholds for water bodies in the State that have been impaired by pollution from man-made activities. MDE also participates in the development and evaluation of TMDLs for the Chesapeake Bay, which is impacted by pollutants from several states. A TMDL is the maximum amount of a pollutant that a body of water can receive from all sources, while still meeting water quality standards. The MDE has developed the website Maryland TMDL Data Center,, which provides a TMDL Search tool, TMDL Maps, a Waste Load Allocation (WLA) Search tool, and guidance to assist stormwater permittees develop implementation plans required by municipal separate storm sewer system (MS4) permits. The approved TMDLs are searchable based on county, watershed or entire state for a specific pollutant or all available pollutants. The TMDL Maps provide the geographic extents of existing TMDLs based on pollutant. The WLA Search tool provides the stormwater or wastewater waste load allocations for an NPDES permit.

The TMDL Data Center also has a stormwater toolkit that assists in applying stormwater WLAs to regulated stormwater communities. The Stormwater Documents provide guidance documents to assist stormwater permittees in implementing TMDLs. And the P6 Bay Model Development provides information on the Chesapeake Bay Phase 6 Watershed Model data.

“Statewide Fish Consumption Guidelines for All Ages.” MDE Fish Consumption Advisory – Guidelines for Recreationally Caught Fish Species in Maryland. MDE. 17 March 2016.