Sulfur Dioxide Emissions

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.

Measured 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.

Estimated Emissions

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.

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.

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.

Content under review.

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:

1. 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.
2. 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.
3. SO₂ emissions from "miscellaneous sources" are not included in the total emissions. Details on emissions from miscellaneous sources can be found by downloading 2005 NEI inventory data for the "nonpoint sector" (http://www.epa.gov/ttn/chief/net/2005inventory.html).
4. 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.
5. Not all states and local agencies provide the same data or level of detail for a given year.

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).