Report on the Environment
Carbon Monoxide 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 "Carbon Monoxide 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
Carbon monoxide (CO) gas forms primarily when carbon fuels are not burned completely. Mobile sources account for the majority of CO emissions (U.S. EPA, 2003). These sources include both on-road vehicles (e.g., cars, trucks, motorcycles) and nonroad vehicles and engines (e.g., farm equipment, construction equipment, aircraft, marine vessels). Consequently, high concentrations of CO generally occur in areas with heavy traffic congestion. In cities, as much as 95 percent of all CO emissions may come from automobile exhaust (U.S. EPA, 2003). Other sources of CO emissions include industrial processes, non-transportation fuel combustion, and natural sources, such as wildfires. Fuel-burning appliances also are a large source of CO releases in indoor environments. Undetected releases of carbon monoxide in indoor settings can present serious health risks to building occupants. The CO Concentrations indicator describes health hazards associated with inhaling CO.
This indicator presents CO emissions from traditionally inventoried anthropogenic source categories: (1) “Fuel combustion,” which includes emissions from coal-, gas-, and oil-fired power plants and industrial, commercial, and institutional sources, as well as residential heaters (e.g., wood-burning stoves) and boilers; (2) “Other industrial processes,” which includes chemical production, petroleum refining, metals production, and industrial processes other than fuel combustion; (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, aircraft, and others. The indicator also includes estimates of biogenic CO emissions in 2005. Biogenic emissions were estimated using the Biogenic Emissions Inventory System Model, Version 3.12, with data from the Biogenic Emissions Landcover Database and 2001 annual meteorological data.
CO 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 most fuel combustion sources and industrial sources, emissions 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 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 inventories from 1990, 1996 to 2002, and 2005.
This indicator focuses on trends in CO emissions from anthropogenic sources. However, CO emissions from biogenic sources were estimated for 2005 to provide a sense of the relative contributions of natural versus anthropogenic emissions (Exhibit 2-1, panel B). Nationally, biogenic emissions were estimated to contribute approximately 16 percent to the CO emissions from all sources during 2005.
Nationwide estimated anthropogenic CO emissions have decreased 46 percent between 1990 and 2005, the most recent year for which aggregate NEI emissions estimates are available (Exhibit 2-1, panel A). Almost the entire emissions reduction is attributed to decreased emissions from on-road mobile sources. In 2005, mobile sources (both on-road and nonroad sources combined) accounted for 89 percent of the nation’s total anthropogenic CO emissions. The CO emissions reductions are reflected in corresponding reductions in ambient concentrations (the CO Concentrations indicator).
Net estimated anthropogenic CO emissions declined in all EPA Regions between 1990 and 2005 (Exhibit 2-2). The largest decrease (12.22 million tons) occurred in Region 5, and the smallest decrease (2.28 million tons) occurred in Region 10.
- Comparable CO 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.
- CO emissions from “miscellaneous sources,” including wildfires, are not included in the total emissions. Yearly fluctuations in wildfire emissions have the potential to mask trends in anthropogenic emissions and therefore have been excluded from the trends graphics. Details on emissions from miscellaneous sources can be found on EPA’s emission inventory Web site (http://www.epa.gov/ttn/chief/eiinformation.html), by downloading NEI inventory data for the “nonpoint sector.”
- The emissions data for CO are largely based on estimates that employ emission factors generated from empirical and engineering studies, rather than on actual measurements of CO emissions. 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.
- 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 CO 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/
|Carbon Monoxide 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 carbon monoxide (CO) emissions data for 1990, 1996 to 2002, and 2005. CO emissions (combined with atmospheric fate and transport processes) determine corresponding ambient CO concentration levels.
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) (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). Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, based on raw CO emissions data in EPA's NEI.
The carbon monoxide (CO) emissions data in NEI are based largely on emissions estimates, not direct measurements.
Mobile sources of CO emissions include on-road vehicles (e.g., cars, trucks, buses, and motorcycles) and nonroad vehicles and engines (e.g., farm equipment, construction equipment, lawnmowers, chainsaws, marine vessels, snowmobiles, aircraft). Mobile sources account for the overwhelming majority of nationwide estimated CO air emissions (e.g., 89 percent in 2005). Emissions from on-road mobile sources are estimated using EPA's MOBILE emissions model, with input data on vehicle miles traveled based on estimates provided by the Federal Highway Administration. Emissions from nonroad mobile sources are estimated using EPA's NONROAD emissions model. Both the MOBILE and NONROAD emissions models are considered to be scientifically and technically valid, and many aspects of both models have been subject to external, independent peer review. Full documentation of these two models is publicly available (U.S. EPA, 2007c and 2007d). Additionally, documentation on how these models were used specifically to develop NEI data is publicly available (U.S. EPA, 2008).
Point sources of CO emissions include fuel combustion sources (e.g., coal-, gas-, and oil-fired power plants; industrial, commercial, and institutional sources; residential heaters and boilers) and other industrial processes (i.e., chemical production, petroleum refining, metals production, and processes other than fuel combustion). Point sources account for a relatively small fraction of the estimated nationwide CO emissions. Emissions from point sources are primarily 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, 2007e). See U.S. EPA (2007a and 2007b) for further information on approaches commonly taken to estimate air emissions from various 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, 2007b). For CO, this often means using the MOBILE and NONROAD models as discussed above. 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 CO 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 all CO emissions released in the U.S. The NEI characterizes emissions sources, not human populations or ecosystems. NEI data provide insights on emissions sources throughout the country, including localized areas that might be near sensitive populations or ecosystems, although the focus of this indicator is on regional and national trends.
The indicator describes nationwide CO emissions for 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 CO 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 CO 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, 2007b).
The concept of reference points does not apply to emissions indicators. There are no thresholds or ranges of values associated with "safe" levels of CO 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 (especially trends mediated by anthropogenic activities), but they do not provide direct insight 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 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 in mobile source emissions would be expected to account for a considerable portion of the spatial and temporal variability in the nationwide aggregate CO emissions. The indicator likely accounts for this variability through changing input parameters used in the underlying emission estimation models (e.g., in terms of vehicle fleets and fuels).
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 (U.S. Environmental Protection Agency). 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 (122 pp, 496K, About PDF).
U.S. EPA. 2007a. Clearinghouse for inventories and emissions factors. http://www.epa.gov/ttn/chief/.
U.S. EPA. 2007b. 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. 2007c. MOBILE6 vehicle emission modeling software. http://www.epa.gov/otaq/m6.htm.
U.S. EPA. 2007d. NONROAD model (nonroad engines, equipment, and vehicles). http://www.epa.gov/otaq/nonrdmdl.htm.
U.S. EPA. 2007e. 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).
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).