Vehicle Emissions Information

Motor vehicles are the largest contributor of air pollutants and greenhouse gases in Washington State, accounting for over half of all regulated emissions. The impact of vehicle emissions threatens to increase further as a result of the expected population increase in the state and the fact that vehicle miles traveled are expected to increase as fast as, if not faster than, population. These vehicle emissions:

• significantly degrade air quality
• impair visibility
• contribute to global warming
• contain toxic contaminants and
• threaten public health and the environment.

Components of Vehicle Emissions
Hydrocarbons: Hydrocarbon (HC), also referred to as volatile organic compounds (or VOCs), emissions are the result of incomplete fuel combustion in the engine and evaporative losses from the fuel system in the case of gasoline-powered vehicles. Some of these hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground-level ozone, a major component of smog. Ozone results in decreased visibility and presents numerous health impacts, including eye irritation, lung damage and aggravation of respiratory problems. In addition, a number of the components of hydrocarbon emissions are toxic, with the potential to cause cancer.
Nitrogen Oxides: Nitrogen and oxygen atoms in the air react under the high pressure and temperature of vehicle engines, forming various nitrogen oxides known as NOx. Nitrogen oxides, like hydrocarbons, contribute to the formation of ozone. In addition, they are an element in the formation of acid rain.
Carbon Monoxide: Carbon monoxide (CO) is the result of the partial oxidation of carbon in fuel during incomplete combustion in the vehicle engine. Carbon monoxide is responsible for numerous health issues, such as diminishing the flow of oxygen in the bloodstream and is particularly dangerous to those with heart disease.
Carbon Dioxide: Carbon dioxide (CO₂) is formed when carbon in fuel is fully oxidized during the combustion process. Carbon dioxide is considered a ‘greenhouse gas’, trapping the earth’s heat and contributing to the potential of global warming.
Particulate Matter: Particulate matter (PM) consist of small, discrete solid or aerosol particles. Some particles are toxic or have toxic substances adhered to their surfaces. Particulate matter in the respiratory tract may produce injury by itself, or it may react with gases to increase the effects on the body. The elderly, those suffering from respiratory illness, and young children are especially prone to the harmful effects of particulate. Particulate matter is also responsible for degradation in visibility.
Toxic Contaminants: Gasoline and diesel fuels contain and release chemicals, many of which are listed by the Environmental Protection Agency as toxic contaminants, including known carcinogens. These toxic contaminants are most commonly associated with hydrocarbons and particulate matter.

Sources of Vehicle Emissions
Vehicle emissions are categorized as exhaust emissions, including both start and running emissions, and evaporative emissions. Emissions levels for each of these categories are dependent on several factors, including the vehicle model year, vehicle weight class, vehicle mileage, maintenance record and fuel type.

Exhaust Emissions:
Exhaust emissions are those pollutants emitted through the vehicle exhaust system, or tail pipe. These emissions can vary significantly depending on the model and age of the vehicle, as well as the choice of fuel. Exhaust emissions also vary depending on engine temperature. A cold engine produces more than twice as much pollution as a warm one. 

In general, alternative fuels such as natural gas, propane, ethanol and biodiesel have significantly reduced emissions compared to gasoline and diesel vehicles. Electric vehicles are zero-emission vehicles.

The following table provides a general comparison of gasoline vehicle exhaust emissions with those of natural gas, propane, ethanol and electric vehicles.

Start Emissions:
Start emissions are a sub-category of exhaust emissions, determined as part of the complicated federal test procedure for certifying compliance with overall vehicle emission standards. In addition to the other factors affecting vehicle emissions, start emissions vary depending on the length of time the vehicle has been turned off between starts, known as ‘soak time’. Cold-start and, to a lesser extent, hot-start emissions are disproportionately higher than exhaust emissions during operation, after the vehicle has been warmed-up. The ‘extra’ start emissions are due to the fact that emissions control equipment is not fully operational until a vehicle engine is warmed-up. Weather conditions and temperature also effect start emissions. For example, in warmer temperatures, the ‘extra’ start emissions occur for a shorter period of time. 

The following chart presents selected start data for gasoline powered light-duty vehicles. Emissions rates for the 12-hour and 10-minute soak are in grams. The composite emission rate is a weighted combination to represent two trips. The first trip is a cold-start trip after a 12 hour soak, and the other is a hot-start trip after a 10 minute soak. Composite emissions are expressed in grams per mile.

Attempts to test cold-start emissions for alternative fuel vehicles have been very limited. It is known that electric vehicles produce no start emissions and it is theorized that natural gas vehicles produce fewer start emissions than gasoline vehicles. This is due to the gaseous nature of the fuel which does not require additional fuel to be released upon starting the vehicle nor the vaporization of liquid fuels associated with gasoline engines.

Evaporative Emissions:
Vehicle gasoline tanks and engines also produce evaporative emissions. These evaporative losses can account for a majority of total hydrocarbon pollution on hot days when ozone levels are the highest. In contrast, natural gas and electric vehicles have insignificant or no evaporative emissions. Evaporative emissions occur in several ways:

Diurnal: As engine and ambient temperatures increase, the fuel tank becomes heated, venting gasoline vapors.
Running losses: The hot engine and exhaust system can vaporize gasoline when vehicles are running.
Hot soak: Gasoline evaporation continues after the vehicle is turned off, while the engine remains hot for a period of time.
Refueling: Gasoline vapors in fuel tanks are forced out when the tank is filled with liquid fuel.

The regulation of diurnal, running loss and hot soak evaporative emissions in light-duty vehicles began in 1971. Under a phase-in program beginning in 1998, light-duty vehicles are required to include on-board vapor recovery systems to address evaporative emissions during refueling.

Heavy-Duty Vehicle Emissions
Although heavy-duty trucks and buses account for under six percent of the miles driven by highway vehicles in the United States, they are responsible for one-quarter of smog-causing pollution (NOx in particular) from highway vehicles, over half the soot (PM) from highway traffic, the majority of the cancer threat posed by air pollution in some urban areas, six percent of the nation's global warming pollution, and over one-tenth of America's oil consumption.

In an effort the address the impacts, particularly those resulting from the use of diesel fuel, the federal government has developed new standards and test procedures for heavy-duty trucks and buses. The new standards will become effective in 2006 and 2007.  Diesel vehicles over 8,500 pounds will be required to comply with new emission standards.  Heavy-duty gasoline engines will also be required to meet new, more stringent standards. In addition, the new regulations will require a reduction in the sulfur content of on-road diesel fuel, with a maximum 15 parts per million sulfur content.

The Future of Vehicle Emissions
Vehicle emissions are being reduced with the introduction of new technology and the provisions of the 1990 Clean Air Act. These measures include much cleaner gasoline and diesel fuels, tighter exhaust emissions standards, increased durability, improved control of evaporative emissions, and computerized diagnostic systems that identify malfunctioning emission controls.

These efforts, however, are not enough to address the impacts of vehicle emissions, particularly in the short-term. Alternative fuel vehicles represent a significant means to reduce the impact of vehicle emissions, particularly in the interim period until new federal standards and new technologies are implemented and until older vehicles are replaced with the newer, cleaner vehicles. Further, continuing increases in the number of vehicles and miles traveled threaten to offset much of the emission control efforts.

Alternative fuel vehicles have other significant advantages as well:

• Limited or no evaporative emissions
• Reduced start emissions
• Significantly reduced greenhouse gas (CO₂) emissions
• Significantly reduced levels of toxic contaminants
• Reduce dependence on imported oil and susceptibility to the price and supply fluctuations that this dependence creates
• Reduced spill hazards
• Renewable energy source

Alternative fuel vehicles also present an economically viable option to address air quality and global warming issues, particularly when compared with the threats and costs that vehicle emissions pose to the state’s economic vitality, environment and public health.

The Coalition is grateful to the U.S. EPA for their assistance in compiling this information.

The U.S. Environmental Protection Agency has additional information on motor vehicle emissions.  http://www.epa.gov/otaq/  The Agency has also developed a ranking of many new U.S. car models based on their tailpipe emissions.  http://www.epa.gov/autoemissions/.  Unfortunately, many of the available alternative fuel vehicles are not included.