Explanation of Indicator
Carbon monoxide (CO) is a gas that is emitted into the atmosphere by natural and man-made sources. Elevated carbon monoxide levels are most likely to occur in urban areas as a result of emissions from motor vehicles. This indicator measures the emissions from mobile sources based on vehicle miles traveled (VMT) data and the average emission factor, as well as the exceedances of the ambient standard for carbon monoxide. The average emission factor varies according to vehicle speeds, local meteorology, the type and model year mix of the vehicles, and the year of the analysis. The ambient standard for carbon monoxide is 9 parts per million (ppm) for an 8-hour averaging time and 35 ppm for a 1-hour averaging time. Neither of these standards may be exceeded more than once per year at the same monitor.
Two measures of carbon monoxide are presented: emissions and concentration exceedances. Emissions are a release of a substance into the atmosphere. An exceedance occurs when emissions are concentrated in the air and surpass a set maximum standard. This concentration could be due to either increased emissions or atmospheric conditions. Emissions are monitored and reported by dischargers, while monitoring stations report hourly CO values.
Increased uptake of CO reduces the blood's ability to transport oxygen to tissues. Exposures to high-level concentrations of CO cause physical and pathological changes. An eight-hour exposure to a CO concentration of 100 parts per million will cause dizziness, headaches, and lethargy in most people. Additional exposure may result in death.
Emissions data to support this indicator can be found in National Air Pollutant Emissions Trends Report, 1900-1993 prepared by U.S. EPA, Office of Air Quality Planning and Standards, Emissions, Monitoring and Analysis Division, Research Triangle Park, North Carolina 27711. Exceedance data can be acquired from this same organization. For further information and assistance call Jonathan Miller at (919) 541-3330 or Warren Freas at (919) 541-5469. This information for individual states can be obtained by contacting the air program in the states environmental protection agency.
Data are available in a hard copy format for associated retrieval and printout costs. Monitoring data are available in summary format at no charge.
Information is summarized annually for the states.
Emissions data generally are of good quality. Emissions are usually calculated indirectly using general emission factors and source activity data and, thus, are only estimates and not direct measurements. Emissions data are limited by the accuracy of these factors and activity information and are minimally quality controlled.
The ambient monitoring data that serve as the foundation for the determination of exceedances are also limited. Monitoring systems can vary significantly between states, creating results that are not completely comparable. First, the number of monitors in each state may vary, depending on the resources and interest of each state. States with more monitors may detect more exceedances. Second, the siting criteria for monitors can lead to anomalous results. States that invest in more intensive monitoring -- such as placing more monitors in urban areas with known problems -- can generate multiple exceedances where states with less intensive siting might produce only one exceedance. Finally, since states are allowed to adopt standards that are more stringent than the federal standard, the possibility exists that states with tougher standards may generate more exceedances.
Sample Data Analysis: Florida
As depicted in the graphs below, carbon monoxide emissions from mobile sources have changed very little in 14 years. Interestingly enough, however, exceedances for carbon monoxide have decreased dramatically. In fact, the exceedances recorded in 1987 and 1988 were, in each case, a result of local, temporary activity near the monitoring site and not representative of ambient air in the general area. Therefore, it is safe to report that an exceedance has not been experienced in almost ten years.
There are several factors that have contributed to the reduction of exceedances for carbon monoxide. Higher concentrations of CO (or exceedances) are related to emissions density at localized areas rather than total emissions over a large (county-wide) area. As emissions of CO have been reduced for individual cars and the number of vehicles queued at individual intersections has remained the same, the concentrations have been reduced. Another factor has been the decision of many cities in Florida to adjust traffic patterns to discourage the build-up of carbon monoxide. Carbon monoxide emissions are higher when a vehicle is idling. Adjusting traffic flows in congested areas to reduce the amount of time a car must idle can significantly reduce the amount of carbon monoxide released into the air. Fewer carbon monoxide exceedances have also been attributed to the federal government's requirement for catalytic converters on vehicles. Therefore, even with Florida's increase in vehicle miles traveled, technology has mitigated their impact.