Report on the Environment
Sulfur Dioxide Emissions
What are the trends in outdoor air quality and their effects on human health and the environment?
The above question pertains to all 'Outdoor Air' Indicators, however, the information on these pages (overview, graphics, references and metadata) relates specifically to "Sulfur Dioxide Emissions". Use the right side drop list to view the other related indicators on this question.
- Avoid spilling gasoline, and don't "top off" the tank.
- Check daily air quality forecasts
- Use paints, stains, finishes, and paint strippers that are water-based or low in volatile organic compounds.
- On ozone action days, refuel your vehicle after dusk.
- Avoid burning leaves, trash, and other materials that cause particle pollution when incinerated.
- Acid Deposition
- Air Toxics Emissions
- Ambient Concentrations of Benzene
- Ambient Concentrations of Carbon Monoxide
- Ambient Concentrations of Lead
- Ambient Concentrations of Manganese Compounds in EPA Region 5
- Ambient Concentrations of Nitrogen Dioxide
- Ambient Concentrations of Ozone
- Ambient Concentrations of Particulate Matter
- Carbon Monoxide Emissions
- Concentrations of Ozone-Depleting Substances
- Lake and Stream Acidity
- Lead Emissions
- Mercury Emissions
- Nitrogen Oxides Emissions
- Ozone Injury to Forest Plants
- Ozone Levels over North America
- Ozone and Particulate Matter Concentrations for U.S. Counties in the U.S./Mexico Border Region
- Particulate Matter Emissions
- Percent of Days with Air Quality Index Values Greater Than 100
- Regional Haze
- Sulfur Dioxide Emissions
- Volatile Organic Compounds Emissions
Click to enlarge exhibit
Sulfur dioxide (SO2) belongs to the family of sulfur oxide (SOx) gases. These gases are formed when fuel containing sulfur (mainly coal and oil) is burned (e.g., for electricity generation) and during metal smelting and other industrial processes. High concentrations of SO2 are associated with multiple health and environmental effects (U.S. EPA, 2003). The highest concentrations of SO2 have been recorded in the vicinity of large industrial facilities. Although relatively few people live in areas where SO2 concentrations exceed the National Ambient Air Quality Standards (NAAQS), SO2 emissions are an important environmental issue because they are a major precursor to ambient PM2.5 concentrations: many more people live in PM2.5 non-attainment areas, which has several documented human health and ecological effects (the PM Concentrations indicator).
Health effects associated with SO2 depend on the exposure concentrations and durations, and on the susceptibility of exposed populations. Asthmatics are much more susceptible to SO2 exposure than people who do not have asthma (U.S. EPA, 1986). Effects associated with longer-term exposures to high concentrations of SO2, in conjunction with high levels of PM, include respiratory illness, alterations in the lungs’ defenses, and aggravation of existing heart or lung disease. The most susceptible populations under these conditions include individuals with cardiovascular disease or chronic lung disease, children, and older adults (U.S. EPA, 1982).
Many other environmental concerns are associated with high concentrations of SO2. For example, airborne SO2, along with NOx, contributes to acidic deposition (the Acid Deposition indicator); SO2 is a major precursor to PM2.5 (the PM Concentrations indicator); and SO2 contributes to impaired visibility (the Regional Haze indicator). SO2 exposure also can harm vegetation by increasing foliar injury, decreasing plant growth and yield, and decreasing the number and variety of plant species in a given community. Finally, SO2 can accelerate the corrosion of materials (e.g., concrete, limestone) that are used in buildings, statues, and monuments that are part of the nation’s cultural heritage (U.S. EPA, 1982).
This indicator presents SO2 emissions from traditionally inventoried anthropogenic source categories: (1) “Fuel combustion: selected power generators,” which includes emissions from coal-, gas-, and oil-fired power plants that are required to use continuous emissions monitors (CEMs) to report emissions as part of the Acid Rain Program (ARP); (2) “Fuel combustion: other sources,” which includes industrial, commercial, and institutional sources, as well as residential heaters and boilers not required to use CEMs; (3) “Other industrial processes,” which includes chemical production and petroleum refining; (4) “On-road vehicles,” which includes cars, trucks, buses, and motorcycles; and (5) “Nonroad vehicles and engines,” which include farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, and others. Because a substantial portion of airborne SO2 comes from fossil fuel combustion in electric utilities, this indicator includes the separate “Fuel combustion: selected power generators” category in addition to the four categories presented in the other emissions indicators.
SO2 emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite of data from many different sources, including industry and numerous state, tribal, and local agencies. Different data sources use different data collection methods, and many of the emissions data are based on estimates rather than actual measurements. For major electricity generating units, most data come from CEMs that measure actual emissions. For other fuel combustion sources and industrial processes, data are estimated using emission factors. Emissions from on-road and nonroad sources were estimated using EPA-approved modeling approaches (U.S. EPA, 2008).
NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. territories of Puerto Rico and Virgin Islands, and some of the territories of federally recognized American Indian nations. Data are presented only for 1990, from 1996 to 2002, and for 2005. Data are available from 1991 to 1995 and from 2003 to 2004, but these data have not been updated to be comparable to the more recent inventories from 1990, 1996 to 2002, and 2005.
National estimated SO2 emissions decreased 36 percent between 1990 and 2005 (from 23,064,000 to 14,701,000 tons) (Exhibit 2-27). This downward trend resulted primarily from emissions reductions at electrical utilities. Between 1990 and 2005, air emissions from electric utilities have consistently accounted for roughly two-thirds of the nationwide SO2 emissions.
Net SO2 emissions declined in all EPA Regions between 1990 and 2005 (Exhibit 2-28). During this time frame, the largest percent reductions in SO2 emissions were seen in Regions 1 (55 percent), 2 (55 percent), and 5 (48 percent), and the smallest reductions were observed in Regions 6 (18 percent) and 9 (13 percent).
- Though emissions from most electric utilities are measured directly using continuous monitoring devices, SO2 emissions data for other source types are based on estimates that employ emission factors generated from empirical and engineering studies. Although these estimates are generated using well-established approaches, the estimates have uncertainties inherent in the emission factors and emissions models used to represent sources for which emissions have not been directly measured.
- Comparable SO2 emissions estimates through the NEI are available only for 1990, 1996-2002, and 2005. Data for 1991-1995 and 2003-2004 are not provided due to differences in emissions estimation methodologies from other inventory years, which could lead to improper trend assessments.
- SO2 emissions from “miscellaneous sources” are not included in the total emissions. Details on emissions from miscellaneous sources can be found by downloading 2002 NEI inventory data for the “nonpoint sector” (http://www.epa.gov/ttn/chief/eiinformation.html).
- The methodology for estimating emissions is continually reviewed and is subject to revision. Trend data prior to these revisions must be considered in the context of those changes.
- Not all states and local agencies provide the same data or level of detail for a given year.
Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, based on biogenic and anthropogenic SO2 emissions data in the NEI. The most recent data are taken from Version 2.0 of the 2005 NEI (U.S. EPA, 2009). These and earlier emissions data can be accessed from EPAs emission inventory Web site (http://www.epa.gov/ttn/chief/eiinformation.html). This indicator aggregates NEI data by source type (anthropogenic or biogenic), source category, and EPA Region.
U.S. EPA (United States Environmental Protection Agency). 2009. Data from the National Emissions Inventory, Version 2.0. Accessed 2009. http://www.epa.gov/ttn/chief/eiinformation.html
U.S. EPA. 2008. Documentation for the 2005 mobile National Emissions Inventory, Version 2. ftp://ftp.epa.gov/EmisInventory/2005_nei/mobile/2005_mobile_nei_version_2_report.pdf
U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. EPA/454/R-03/005. Research Triangle Park, NC. http://www.epa.gov/air/airtrends/aqtrnd03/
U.S. EPA. 1986. Second addendum to the air quality criteria for particulate matter and sulfur oxides (1982): Assessment of newly available health effects information. EPA/450/S-86/012. Research Triangle Park, NC.
U.S. EPA. 1982. Air quality criteria for particulate matter and sulfur oxides. EPA/600/P-82/020a-c. Research Triangle Park, NC.
|Sulfur Dioxide Emissions|
|2.||ROE Question(s) This Indicator Helps to Answer|
|This indicator is used to help answer one ROE question: "What are the trends in outdoor air quality and their effects on human health and the environment?"|
This indicator presents regional and national sulfur dioxide (SO2) emissions data for 1990, 1996 to 2002, and 2005. SO2 emissions (combined with atmospheric fate and transport processes) determine corresponding ambient SO2 concentration levels and contribute to acid deposition.
This emissions data for this indicator was obtained from EPA's National Emissions Inventory (NEI). The NEI is a composite of data from many different sources, including industry and numerous state, tribal, and local agencies.
EPA makes the complete underlying data set (i.e., the NEI) and all data dictionaries available through its Web site named, "Clearinghouse for Inventories and Emission Factors" (CHIEF). Summary data in this indicator were provided by EPA's Office of Air Quality Planning and Standards, based on raw SO2 emissions data in EPA's NEI (2005 data: http://www.epa.gov/ttn/chief/net/2005inventory.html; 2002 data: http://www.epa.gov/ttn/chief/net/2002inventory.html; pre-2002 data: http://www.epa.gov/ttn/chief/net/critsummary.html). This indicator aggregates the raw NEI data by source category and EPA Region.
The sulfur dioxide (SO2) emissions data in NEI can be grouped into two categories: measured emissions and estimated emissions.
SO2 emissions data from the "Fuel combustion: selected power generators" sector, which includes coal-, gas-, and oil-fired power plants, are primarily generated by continuous emissions monitors (CEMs), which provide real-time measurements of SO2 stack emission rates with a high degree of accuracy. These measurements are widely considered to provide scientifically valid accounts of SO2 emissions from these operations. The direct measurements for this sector account for roughly two-thirds of the total nationwide SO2 emissions presented in this indicator. Some of the emissions data for sources outside of this sector also come from direct measurements.
SO2 emissions data emissions data are estimated using emission factors for the following source categories: (1) "Fuel combustion: other sources," which includes industrial, commercial, and institutional sources, as well as residential heaters and boilers not required to use CEMs; (2) "Other industrial processes," which includes chemical production and petroleum refining; (3) "On-road vehicles," which includes cars, trucks, buses, and motorcycles; and (4) "Nonroad vehicles and engines," such as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, and aircraft. For sources without measured emission rates, the emissions data in NEI are estimates generated by using emission factors, models, or other estimation methodologies. Though the estimated emission rates have inherent uncertainties, the approaches used to estimate these emissions are well documented (e.g., U.S. EPA, 2004), widely accepted as technically valid, and have been peer reviewed. Moreover, efforts are made to update and improve the estimation methodologies periodically (U.S. EPA, 2007c). The references listed at the end of this section identify the range of emissions sources covered by the inventory, which include a broad array of stationary and mobile sources.
The NEI is a composite of data from many different sources. State and local agencies and other parties provide much of the data to EPA. Although these original data are accompanied with little or no documentation on the specific methods used to estimate emissions, state and local agencies and other parties generally follow procedures documented in an emission inventory guidebook on acceptable methods for estimating emissions (U.S. EPA, 2007a). For SO2 emissions data generated using continuous emissions monitoring devices, analytical procedures and performance specifications are documented in multiple reports available from EPA’s Emissions Measurement Center — an online clearinghouse of documents pertaining to emissions measurement methodologies (U.S. EPA, 2007b). After EPA receives emissions data reported by state and local agencies and other parties, the Agency processes the data according to procedures outlined in the NEI Preparation Plan (e.g., U.S. EPA, 2004). Taken together, these references describe the preferred approaches that state and local agencies and other parties follow to generate SO2 emissions data and the approach EPA takes to compile and organize these data.
In some cases, the data provided by state and local agencies and other parties are absent or incomplete. When this occurs, EPA fills the gaps using various data extrapolation methods, such as using data from previous years or inferring data for a given county based on data from other counties believed to have common properties that influence emissions (e.g., population density, daily low and high temperatures). Steps taken to fill these data gaps have been applied consistently over the years and have been subject to independent peer review.
NEI emissions data for SO2 are available for 1990, 1996 to 2002, and 2005 using a consistent methodology. Emissions data in the inventory cover all 50 states, the District of Columbia, Puerto Rico, and the U.S. Virgin Islands. Thus, NEI data are meant to capture an estimate of SO2 emissions released in the U.S. The NEI characterizes emissions sources, not human populations or ecosystems. Therefore, the concept of "sensitive populations or ecosystems" does not apply to this indicator. It should be noted that NEI data do provide insights on emissions sources throughout the country, including localized areas that might be near sensitive populations or ecosystems, though the focus of this indicator is on regional and national trends.
No transformation or model was needed to generate the indicator data from the NEI data because the NEI data already cover the entire nation. The indicator data simply reflect regional and national summations of individual entries in the NEI database for SO2. In other words, the indicator was developed by adding together all of the relevant entries that were submitted by state and local agencies and other parties.
Reproducing the entire NEI database would require reproducing tens of thousands of emissions estimates or measurements that state and local agencies and other parties submit to EPA. Reproducing these figures would be an extremely daunting and time-consuming task, as populating the NEI database requires a large level of effort and access to data generated by hundreds of different parties. Note, however, that key aspects of NEI development and implementation are subject to independent peer review to ensure that the data are scientifically sound and technically accurate. While reproducing the entire NEI database would be difficult, reproducing the SO2 emissions data that this indicator reports for different EPA Regions and different source categories is more straightforward. This can be accomplished by first downloading the entire database of SO2 emissions (which can be accessed as text files from the links documented in "Data Availability"). The indicator data can then be verified by importing the text files into some type of database or spreadsheet software and then running queries to verify the national and regional totals.
|9.||Quality Assurance and Quality Control|
The data in the NEI are gathered from numerous sources. Though the quality of the original data submitted to EPA can vary, several quality assurance (QA) and quality control (QC) measures are in place to ensure that data of acceptable quality enter the inventory and are processed correctly. It is presumed that state agencies supplying emissions data have QA plans, but EPA does not systematically obtain information on QA practices from the states. The EPA contractors who support the Agency on inventory development operate under general contract-wide QA plans, which can be made available on request. In addition, EPA's more recent QC practices performed during the blending and merging of data from numerous sources are publicly available (U.S. EPA, 2007a).
The concept of reference points, thresholds, and ranges of values "that unambiguously reflect the state of the environment" do not really apply to emissions indicators. There are no thresholds or ranges of values associated with "safe" levels of SO2 emissions across an entire region or nation. The air quality impacts associated with a given regional or national emissions total depend on the distribution of emissions among individual sources and the release parameters (e.g., stack heights, exit velocities) at these sources. Emissions data can provide general insights on air quality trends, but cannot be used alone to gauge "the state of the environment" (i.e., ambient air concentrations of sulfur dioxide). The indicator on ambient sulfur dioxide concentrations, however, provides more direct insights on the state of the environment.
|11.||Comparability Over Time and Space|
NEI data have been collected since 1990. Data are presented for those years (i.e., 1990, 1996 to 2002, and 2005) in which NEI data have been fully updated using consistent methodologies. Assuming the providers of the data abide by these consistent measurement and estimation methodologies, the emissions data should be reasonably comparable over both time and space.
|12.||Sources of Uncertainty|
Content under review.
|13.||Sources of Variability|
Variability across emissions sources most likely does not impact this indicator's conclusions. As the indicator shows, the decrease in nationwide SO2 emissions between 1990 and 2005 resulted largely from reduced emissions from the "Fuel combustion: selected power generators" sector. It is unlikely that the trend between 1990 and 2005 for this sector is somehow an artifact of uncertainty or variability because this sector's emissions data are primarily from direct measurements from CEMs, which have relatively low uncertainty. It is far more difficult to evaluate the role of uncertainty and variability for source categories characterized by emissions estimates, rather than direct measurements.
This indicator presents a time series of regional and national emissions estimates. No special statistical techniques or analyses were used to characterize the long-term trends or their statistical significance.
Limitations to this indicator include the following:
U.S. EPA (United States Environmental Protection Agency). 2007a. Emission Inventory Improvement Program technical report series. Volumes 1-10. Updated October 18, 2007. http://www.epa.gov/ttn/chief/eiip/techreport/.
U.S. EPA. 2007b. Emissions Measurement Center. http://www.epa.gov/ttn/emc/.
U.S. EPA. 2007c. What is the Emission Inventory Improvement Program (EIIP)? http://www.epa.gov/ttn/chief/eiip/whatis.html.
U.S. EPA. 2004. 2002 National Emission Inventory (NEI) preparation plan. Final report, August. ftp://ftp.epa.gov/EmisInventory/2002finalnei/general_information/2002neiplan_081004final.pdf (89 pp, 232K, About PDF).
U.S. EPA. 2000. Performance specification 2—Specifications and test procedures for SO2 and NOx continuous emission monitoring systems in stationary sources. February. http://www.epa.gov/ttn/emc/perfspec/ps-2.pdf (11 pp, 111K).
U.S. EPA. 1996. Evaluating the uncertainty of emission estimates. Volume VI: Chapter 4. July, 1996. http://www.epa.gov/ttn/chief/eiip/techreport/volume06/vi04.pdf (55 pp, 304K).