Report on the Environment
Nitrogen Oxides 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 "Nitrogen Oxides 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
“Nitrogen oxides” (NOx) is the term used to describe the sum of nitric oxide (NO), nitrogen dioxide (NO2), and other oxides of nitrogen. Most airborne NOx comes from combustion-related emissions sources of human origin, primarily fossil fuel combustion in electrical utilities, high-temperature operations at other industrial sources, and operation of motor vehicles. However, natural sources, like biological decay processes and lightning, also contribute to airborne NOx. Fuel-burning appliances, like home heaters and gas stoves, produce substantial amounts of NOx in indoor settings (U.S. EPA, 2003).
NOx plays a major role in several important environmental and human health issues. Short-term and long-term exposures to elevated air concentrations of NO2 are associated with various acute and chronic respiratory effects (U.S. EPA, 1993). NOx and volatile organic compounds react in the presence of sunlight to form ozone, which also is associated with human health and ecological effects (the Ozone Concentrations indicator). NOx and other pollutants react in the air to form compounds that contribute to acid deposition, which can damage forests and cause lakes and streams to acidify (the Acid Deposition indicator). Deposition of NOx also affects nitrogen cycles and can contribute to nuisance growth of algae that can disrupt the chemical balance of nutrients in water bodies, especially in coastal estuaries (the Lake and Stream Acidity indicator; the Trophic State of Coastal Waters indicator). NOx also plays a role in several other environmental issues, including formation of particulate matter (the PM Concentrations indicator), decreased visibility (the Regional Haze indicator), and global climate change (the U.S. Greenhouse Gas Emissions indicator; the Greenhouse Gas Concentrations indicator).
This indicator presents NOx 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,” such as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, and others. Since a substantial portion of airborne NOx comes from fossil fuel combustion in electric utilities, this indicator includes the separate category for “selected power generators” in addition to the fourcategories presented in the other emissions indicators. The indicator also includes estimates of biogenic NOx emissions in 2005. Biogenic emissions were estimated using the Biogenic Emissions Inventory System Model, Version 3.12, with data from the Biogenic Landcover Database and 2001 annual meteorological data.
NOx 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, 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.
This indicator focuses on trends in NOx emissions from anthropogenic sources. However, NOx emissions from biogenic sources were estimated for 2005 to provide a sense of the relative contributions of natural versus anthropogenic emissions. Nationally, biogenic emissions were estimated to contribute approximately 1 percent to NOx emissions from all sources during 2005 (Exhibit 2-7, panel B).
According to the NEI data, estimated nationwide anthropogenic emissions of NOx decreased by 25 percent between 1990 and 2005 (from 25,160,000 to 18,775,000 tons) (Exhibit 2-7, panel A). This downward trend results primarily from emissions reductions at electric utilities and among on-road mobile sources. Although total nationwide anthropogenic NOx emissions decreased during this period, emissions from some sources (such as nonroad vehicles and engines) have increased since 1990.
Estimated anthropogenic NOx emissions in nine of the ten EPA Regions decreased between 1990 and 2005 (Exhibit 2-8). The percent change in emissions over this time frame ranged from a 45 percent decrease (in Region 2) to an 8 percent increase (in Region 10). The largest absolute reduction (1,502,000 tons) occurred in Region 5.
- Comparable NOx 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.
- NOx emissions from miscellaneous sources are not included in the total emissions.
- Though NOx emissions from most electric utilities are measured directly using continuous monitoring devices, NOx emissions data for most other source types are estimates. These estimates are generated using well-established approaches, but still have uncertainties inherent in the emission factors and emissions models used to represent sources for which emissions have not been directly measured.
- The methodology for estimating emissions is continually reviewed and is subject to revision. Trend data prior to any 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 NOx 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 EPA’s 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/net/2002inventory.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. 1993. Air quality criteria for oxides of nitrogen. EPA/600/8-91/049aF-cF. Research Triangle Park, NC.
|Nitrogen Oxides 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 nitrogen oxide (NOx) emissions data for 1990, 1996 to 2002, and 2005. NOx emissions (combined with atmospheric fate and transport processes) determine corresponding trends in concentrations of NO2 and other pollutants, affect ozone formation, and contribute to other important environmental issues.
The emissions data for this indicator come 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 NOx 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 type (i.e., anthropogenic or biogenic), source category, and EPA Region.
The nitrogen oxides (NOx) emissions data in NEI can be grouped many different ways, but are organized here by two categories: measured emissions and estimated emissions.
NOx emissions data for the "Fuel combustion: selected power generators" sector, which includes coal-, gas-, and oil-fired power plants, are primarily generated by continuous emissions monitors (CEMs) that provide real-time measurements of NOx stack emission rates with a high degree of accuracy. These measurements are widely considered to provide scientifically valid accounts of NOx emissions from these operations. The direct measurements for this sector account for more than one-third of the total nationwide NOx emissions presented in this indicator. Some of the emissions data for sources outside of this sector also come from direct measurements.
NOx 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 below 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 NOx 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 NOx 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 NOx for 1990, from 1996 to 2002, and for 2005 were generated using consistent methodologies. In terms of spatial coverage, 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 all NOx 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.
The indicator describes nationwide NOx emissions from 1990, 1996 to 2002, and 2005. Data were not extrapolated beyond the scope of data collection, and no statistical generalization was performed to generate the regional and national emissions trends presented in this indicator. The regional trends were computed by totaling all emissions data for individual facilities and counties within the corresponding EPA Region for the specific inventory years. Similarly, national data represent totals across all states and territories considered in the inventory.
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 NOx 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 NOx 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 NOx 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." The indicators on ambient concentrations of ozone, nitrogen dioxide, and particulate matter, however, provide more direct insights on the state of the environment that is affected by NOx emissions.
|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 NOx 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).