Carbon monoxide (CO) gas forms primarily when carbon fuels are not burned completely. Mobile sources account for the majority of CO emissions. 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 motor vehicle exhaust (U.S. EPA, 2008). Other sources of CO emissions include industrial processes, non-transportation fuel combustion, and natural sources, such as forest wildfires. 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, and aircraft. The indicator also includes estimates of biogenic and forest wildfire CO emissions in 2014. Biogenic emissions were estimated using the Biogenic Emissions Inventory System Model, Version 3.61, with data from the Biogenic Emissions Landcover Database, Version 4.1, and 2014 annual meteorological data. The emissions trends indicator excludes CO estimates of prescribed burning, forest wildfires, and other miscellaneous sources because those data were either not readily available in the 1990 inventory or are small contributors to the total inventory and because the emissions from prescribed burning and forest wildfires are highly variable over time.
CO emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite of data from many different sources, with CO data coming primarily from EPA models as well as from state, tribal, and local air quality management agencies. Different data sources use different data collection methods, and many of the emissions data are based on estimates rather than actual measurements. For mobile sources, the data are based on mobile source models for on-road and nonroad vehicles, often using state-supplied model inputs (U.S. EPA, 2018a). Emissions from forest wildfires in 2002 through 2014 come primarily from an estimation process that bases fire activity and location on satellite detection (U.S. EPA, 2018a). For most fuel combustion sources and industrial sources, emissions are from the state, local, and tribal air quality management agencies and are estimated primarily using emission factors.
NEI data have been compiled 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, 1996, 1999, 2002, 2005, 2008, 2011, and 2014. With the exception of 1993, the NEI data are published on a triennial cycle, thus an annual trend is not readily available. The NEI data are the basis of the national and regional air pollutant emission trends shown in this indicator (U.S. EPA, 2018c).