Road transportation emissionsReturn to top

Vehicle emissions contribute to the increasing concentration of gases that are leading to climate change. In order of significance, the principal greenhouse gases associated with road transportation are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). In the US, transportation is the second largest source of greenhouse gases and accounts for around 28% of total emissions. Of the total greenhouse gas emissions from transport, almost 60% are due to CO2 emissions from the use of motor gasoline.

Road transportation also remains the main source of many criteria air pollutants including carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs) and particulates (PMs). Within the US as a whole, the percentage of contributions due to transportation is particularly high – 68% of CO, 57% of NOx and 34% of VOCs – the percentages are even higher in urban areas. There is a growing body of evidence to link vehicle pollutants to human ill health including the incidence of respiratory and cardiopulmonary disease and lung cancer.

Impacts of vehicle emissionsReturn to top

Most vehicles use a combustion engine to provide motive power. In use, within a combustion engine, chemical reactions take place between the hydrocarbons of the fossil fuel, the fuel additives and the gases that naturally occur in the atmosphere.

These processes include oxidation of the fuel, which produces carbon dioxide (CO2) and carbon monoxide (CO). Nitrogen from the air is also oxidised to nitrogen oxides (NOx). Partially burnt and unburned fuel is present in the exhaust gases forming a complex cocktail of hydrocarbons (HCs) such as methane (CH4) and other volatile organic compounds (VOCs) including benzene and 1,3-butadiene. Particulate matter (PM) is also produced and is especially prevalent in diesel exhaust. Some pollutants are also produced away from the vehicle – for example, ground-level (tropospheric) ozone (O3) is formed by the chemical action of sunlight on emitted VOCs.

Carbon Dioxide (CO2)

While carbon dioxide is non-toxic, its main environmental effect is as a greenhouse gas. Each year an estimated 30 billion tonnes of carbon dioxide are emitted due to human activity, around 20% of which originates from the United States.

To illustrate the scale of the impact of these emissions as a result of human activities, the atmospheric concentration of carbon dioxide (from all sources) has increased by 31% since 1750. The present concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years. Over the last two decades, about three-quarters of the anthropogenic emissions of carbon dioxide have been a result of burning of fossil fuels, the rest being predominantly due to land-use change (eg deforestation).

By enhancing the greenhouse effect, greenhouse gas emissions are leading to increases of the Earth's atmospheric, land and sea temperatures. During the 20th century the global average surface temperature (the average of near surface air temperature over land and sea surface temperature) increased by 0.6 (+/-0.2)°C. This temperature is predicted to increase by 1.4-5.8°C by 2100 (1990 baseline). Based on palaeo-climate data, the projected rate of warming is very likely to be without precedent during at least the last 10,000 years. The concomitant rises in sea levels and resulting climatic change will be of great (and as yet unknown) significance to all patterns of life on Earth.

Carbon Monoxide (CO)

Produced during the incomplete combustion of carbon compounds such as fossil fuels, this gas is known to be deleterious to human health. During respiration it readily combines with haemoglobin in the blood thus hindering the body's ability to take up oxygen. It is thought therefore to aggravate respiratory and heart disease.

Carbon monoxide also contributes to global warming to a small degree. This it does indirectly after first taking part in chemical reactions within the atmosphere. One such reaction would be with oxygen, forming carbon dioxide and thus contributing to the enhanced greenhouse effect.

Nitrogen Oxides (NOx)

As a result of the high temperatures occurring during combustion, nitrogen combines with oxygen from the air forming oxides of nitrogen (NO, NO2, N2O etc.). These gases are known to be responsible for acid deposition via the formation of nitric acid. Nitrogen dioxide (NO2) is toxic even in small concentrations and is known to cause and aggravate human respiratory diseases. Nitrous oxide (N2O) also contributes directly to global warming and is responsible for around 7% of the enhanced greenhouse effect.

Particulates (PMs)

Particulates, commonly known as 'black smoke', are fine particles produced by incomplete combustion, the burning of lubrication oil and by the presence of impurities within the fuel. Typically with a dimension of the order of 10 microns or less (known as 'PM10'), they are known to cause and aggravate human respiratory diseases and are thought to be carcinogenic. The World Health Organisation has issued a report stating that there are no concentrations of airborne micro-sized particulate matter that are not hazardous to human health.

Volatile Organic Compounds (VOCs)

Volatile organic compounds consist of a number of different chemicals including hydrocarbons (eg methane), which are released during the production, refining, storage and combustion of fossil fuels. The largest environmental risks of VOCs are due to the presence of benzene and 1,3-butadiene, which are both carcinogens and are easily inhaled due to their volatile nature. Other chemicals in this category are responsible for the production of tropospheric ozone, which is toxic even in low concentrations.

Methane is a significant greenhouse gas and is released during the drilling for oil and gas and during the combustion of petroleum products. Around 5% of methane emissions are due to the production and use of fuels used for road transport.

Tropospheric Ozone (O3)

In the stratosphere, ozone absorbs ultraviolet light, therefore reducing the number of harmful rays reaching living organisms at the Earth's surface. However, at ground level (the troposphere), ozone is toxic to animals and plants. Ozone is thought to be responsible for aggravating human respiratory disease and is known to reduce crop yields.

While the concentration of stratospheric ozone is being depleted by the action of chlorofluorocarbons and other chemicals, exhaust emissions from road vehicles are increasing the concentration of ozone at ground level. Although there are a number of sources of man-made tropospheric ozone, transportation is known to be a major contributor of emissions through the action of sunlight on emitted VOCs.

Lead (Pb)

Lead is known to affect the mental development of young children and is known to be toxic. It was originally introduced into petroleum products as an 'anti-knock' additive to improve combustion in a spark-ignition (gasoline) engine. Standards to phase out leaded gasoline were first implemented in 1973. By 1995, leaded fuel in the US only accounted for 0.6% of total gasoline sales. From 1 January 1996, the Clean Air Act banned the sale of leaded fuel for use in on-road vehicles.

Federal and California emissions standardsReturn to top

US regulatory Vehicle Emissions Standards have been instrumental in reducing 'certified' emissions. These include carbon monoxide (CO), nitrogen oxides (NOx), non-methane organic gases (NMOG) and particulates (PM). First introduced in 1991, these form a set of rolling regulations designed to become more stringent over time.

In the US, national emissions standards are set by the Environmental Protection Agency (EPA). However, due to the historically poor air quality in the Los Angeles metropolitan area, the State of California is allowed special dispensation from the federal government to set its own vehicle emissions standards, which it does through the California Air Resources Board (CARB). Over time, several other states have chosen to adopt the more stringent California standards in place of the federal regulations. These now include: Maine, Vermont, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Maryland, Oregon, Washington, and Arizona.

For light-duty vehicles, two sets, or Tiers, of federal emission standards, defined in grams per mile, were originally set following the Clean Air Act Amendments of 1990. Tier 1 standard was in operated from 1994 to 1997, and Tier 2 standards, which apply to cars and light trucks, have been phased in during the period from 2004 through 2009. Within the current Tier 2 emissions limits, vehicles are allocated to 'Bins' ranging from 1 to 10, with 1 being for zero-emission vehicles and 10 being for the most polluting vehicles allowed for use on US roads.

In parallel with the national Tier 2 legislation, California introduced the Low Emission Vehicle (LEV) II Program. Under LEV II regulations, existing standards for Low Emission Vehicles (LEVs), Ultra Low Emission Vehicles (ULEVs), and Super Ultra Low Emission Vehicles (SULEVs) were made more stringent, and new vehicle categories were introduced including the Zero Emission Vehicle (ZEV) and the Partial Zero Emission Vehicle (PZEV), which applies to some plug-in hybrid technologies, some natural gas cars and gasoline cars that have systems to eliminate evaporative emissions from the fuel system and which have 150,000-mile/15-year warranties on emission-control components.

The effect of the US emissions standards on vehicle emissions has been to accelerate the introduction of greener vehicle technologies. For gasoline cars, this has been achieved in part through the use of the three-way catalytic converter and the move to fuel injection systems. For diesels, NOx and particulate emissions have been reduced through the development of direct injection engines and diesel particulate filters (DPFs). These technological advances, together with the cleaner fuels that made these developments possible, have led to a dramatic reduction in certified pollutants; so much so, that a car manufactured today produces many tens of times fewer pollutants than a car made in 1970. Car manufacturers are well aware that future cars will have to conform to yet tighter regulations – indeed, tighter standards are already being planned for 2010 an beyond.

In contrast to the standards for certified pollutants, there has been no comparable regulation of carbon dioxide produced by cars. However, in May 2009, President Obama announced a new national fuel economy and emissions policy that will incorporated greenhouse gas emissions. This will be achieved through the use of average fleet fuel economy targets for cars of 42 mpg, and trucks will have to average 26 mpg by 2016.