Car emissions types and impacts

transport emissions air quality climate change

Emissions from petrol and diesel engines include carbon dioxide (CO2), carbon monoxide (CO), hydrocarbons (HCs) such as methane (CH4), particulate matter (PM) and nitrogen oxides (NOx), all of which have significant environmental impacts.

To find out what emissions are emitted from specific vehicles for a particular mileage use the Car Emissions Calculator.

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Road transport emissions

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 transport are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).

Road transport is the third largest source of UK greenhouse gases and accounts for over 20% of total emissions. Of the total greenhouse gas emissions from transport, over 85% are due to CO2 emissions from road vehicles.

Road transport also remains the main source of many local pollutants including benzene, 1,3-butadiene, carbon monoxide (CO), nitrogen oxides (NOx) and particulates (PMs).

Within urban areas, the percentage of contributions due to road transport is particularly high – in London road transport contributes almost 80% of particulate emissions. There is a growing body of evidence to link vehicle pollutants to human ill health including the incidence of respiratory and cardio-pulmonary disease and lung cancer.

Based on modelled concentrations of fine particulate air pollution (particulates of 2.5 microns diameter or less), the UK Government has estimated that air pollution has a "significant public health impact in the UK, with an effect equivalent to 29,000 deaths a year". While the conclusion relates to pollution from all sources, the findings identify vehicular traffic as responsible for around 35% of the particulates assessed.

Sources: Environmental Protection UK, Accessed 2014, Source: Public Health Impacts and Local Actions, Defra 2013

EU vehicle emissions standards

European directives have been instrumental in reducing what are known as the 'regulated emissions'. These include carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HCs) and particulate matter less than 10 microns in size (PM10). First introduced in 1992 (Euro 1), these form a set of rolling regulations designed to become more stringent year on year. Currently limits for new cars and light-duty vans must conform to Euro 6 standards.

The effect of tighter Euro standards on vehicle emissions has been to accelerate the introduction of greener vehicle technologies. For petrol 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 regulated pollutants; so much so, that a car manufactured today emits an order of magnitude fewer emissions than a car made the 1970s. That said, there is evidence that, Euro 1 to 4 saw the greatest reductions and that, more recently, the rate of reduction has slowed. In addition, there is now clear evidence that some key pollutants, including NOx and particulates, have not in fact measurably improved since Euro 4.

In contrast to the legislation for regulated pollutants, there was initially no current EU law that limited the amount of carbon dioxide produced by cars. However, in 2009, the European Parliament passed new car CO2 legislation which set an emissions cap of 130 g/km averaged over all new vehicles produced by each manufacturer by 2015. A subsequent limit has been agreed of 95 g/km for 2021. Model specific CO2 limits permit higher emission for heavier vehicles; however, average figures for each manufacturing group must comply with the overall target.

As part of the CO2 legislation, manufacturers exceeding targets from 2012 have to pay a penalty for each car registered, which amounts to €5 for the first g/km of over the limit, €15 for the second g/km, €25 for the third, and €95 for each subsequent gram. From 2019, stricter penalties will be introduced; every exceeding gram costing €95. Conversely, ultra-low emission vehicles count as 'super-credits' which can be used to lower manufacturers' overall emissions.

European tailpipe emissions standards for passenger cars (in g/km)

Euro Standard Implementation date* CO
Euro 1 July 1993 2.72 - - - 0.97 0.14
Euro 2 January 1997 1.00 - - - 0.70 0.08
Euro 3 January 2001 0.64 - - 0.50 0.56 0.05
Euro 4 January 2006 0.50 - - 0.25 0.30 0.025
Euro 5 September 2010 0.500 - - 0.180 0.230 0.005
Euro 6 September 2015 0.500 - - 0.080 0.170 0.005
Euro 1 July 1993 2.72 - - - 0.97 -
Euro 2 January 1997 2.20 - - - 0.50 -
Euro 3 January 2001 2.30 0.20 - 0.15 - -
Euro 4 January 2006 1.00 0.10 - 0.08 - -
Euro 5 September 2010 1.000 0.100 0.068 0.060 - 0.005**
Euro 6 September 2015 0.100 0.100 0.068 0.060 - 0.005**
* Market placement (or first registration) dates, after which all new engines placed on the market must meet the standard. EU emission standards also specify Type Approval dates (usually one year before the respective market placement dates) after which all newly type approved models must meet the standard.
** Applies only to vehicles with direct injection engines.

Sources: DieselNet, Accessed 2015, International Council on Clean Transportation

EU vehicle type approval

The key impact of emission (and cleaner fuel) standards has been to accelerate the introduction of emission control technologies including, for petrol cars: the three-way catalytic converter, and fuel injection systems; and for diesels: Lean-NOx catalysts, Selective Catalytic Reduction (SCR) and diesel particulate filters (DPFs).

The emissions standards form an important part of the system of ‘type approval’, the regulatory mechanism for ensuring that cars and vans sold within the EU meet minimum environmental and safety standards. The process involves the testing of a representative production vehicle and component parts at an accredited facility.

Once approved, a European Community Whole Vehicle Type Approval (ECWVTA) certificate is issued which permits vehicle sales and use in all EU Member States. An ECWVTA certificate also allows manufacturers to issue a Certificate of Conformity to buyers and users of each vehicle.

Type approval emissions tests are conducted in a laboratory on a rolling-road dynamometer. During the test, the vehicle is ‘driven’ on a simulated route using (currently) the New European Driving Cycle (NEDC), which consists of one extra-urban and four urban cycles. Measurements are averaged to produce ‘official combined’ figures for CO2, energy use and regulated pollutants.

In response to evidence of increasing discrepancies between real-world driving emissions and official test results (based on NEDC in the EU and test cycles used in other jurisdictions), a new Worldwide harmonized Light vehicles Test Procedures (WLTP) has been developed within the UNECE World Forum for Harmonization of Vehicle Regulations for introduction in 2017.

A second response to the deficiency of the NEDC is the introduction of a Real Driving Emissions (RDE) test using Portable Emissions Measurement Systems (PEMS) to check that the test cycle sufficiently represents the real-world. The RDE test will have a binding impact on the type approvals from September 2017 for all newly approved types of vehicles (from September 2019 for all new vehicles).

Find out about Continuous Vehicle Emissions Monitoring (CVEM)
Dynamometer (rolling road) and real driving vehicle emissions testing

Images show 'rolling-road' (dynamometer) and Real Driving Emissions (RDE) testing. Images courtesy of European Commission Joint Research Centre (left) and WhatCar? 2014 sourced from Newspress photo library (right).

Environmental impacts of vehicle emissions

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, 2% of which originates from the United Kingdom.

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)degC. This temperature is predicted to increase by 1.4-5.8degC 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, transport 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 (petrol) engine. At its peak, road transport was responsible for three quarters of airborne lead in the UK. However, due to the introduction of unleaded petrol and the elimination of leaded fuels in Europe in 2000, the amount of lead emitted has fallen by over 80%.

Ben Lane

Author:Ben Lane
Date Updated:10th Mar 2015

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