This Policy Position Statement outlines the technical challenges
involved in the assessment and management of future concentrations
of nitrogen dioxide (NO2) in the atmosphere. It notes that NO2 is
the pollutant responsible for most breaches in European standards
for air quality in the UK and that, as a result, a robust evidence
based approach is required to assess future concentrations of this
pollutant to allow good policy making.
CIWEM's Position on Nitrogen Dioxide in
- The main cause of the exceedance of the annual mean air quality
objective for NO2 is emissions from road vehicles. These contribute
directly to local concentrations of pollutants but also to the
general background concentrations of this pollutant. As a result,
most Air Quality Management Areas (AQMAs) have been declared in
large urban areas where there are high levels of traffic, or where
there are 'street canyons' together with congested traffic.
- Considerable progress has been made to reduce emissions of
nitrogen oxides (typically nitric oxide and NO2) from motor
vehicles, but the increasing proportion of NO2 in vehicle exhaust
means the UK is likely fail to comply with the European limit
- CIWEM notes that there are numerous small towns that have
recorded high concentrations of NO2 that are not easily explained
by our current understanding of vehicle emissions and dispersion of
pollutants. This is a topic that needs further research so that we
can assess the measures needed to reduce concentrations in these
- Forecasting of NO2 concentrations in the future relies on good
information regarding vehicle emissions and robust air quality
monitoring. CIWEM is concerned that the emission information used
to derive current emission factors does not include sufficient
information on the most recent vehicle types. The emissions of NO2
directly from vehicles (known as "primary NO2") requires further
research to be able to assess future concentrations of this
pollutant more accurately.
- A new approach is required, that may require giving local
authorities new powers to introduce low-emission strategies, to
ensure that the EU limit values are achieved by 2015. This
will be necessary to ensure that the EU grants the UK government a
time extension from the current requirement for compliance by
The Chartered Institution of Water and Environmental
Management (CIWEM) is the leading professional body for the people
who plan, protect and care for the environment and its resources,
providing educational opportunities, independent information to the
public and advice to government. Members in 98 countries include
scientists, engineers, ecologists and students.
Exposure to NO2 can decrease lung function and increase a
person's susceptibility to allergens and affect asthmatics. Air
quality standards were set to protect vulnerable members of the
Nitrogen dioxide (NO2) is a gas formed by the reaction of
nitrogen and oxygen in combustion and also by various chemical
processes in the atmosphere. Combustion of fuels such as petrol and
diesel produce a mixture of nitrogen oxides, principally nitric
oxide (NO) and a smaller amount of NO2 known as primary NO2. Once
emitted into the atmosphere, nitric oxide reacts with ozone and
other oxidants to form NO2. Further reactions can create and
destroy NO2 but it is the initial reaction of nitric oxide with
ozone and the primary emissions of NO2 that are most important
contribution to total NO2 concentrations. The amount of conversion
of NO to NO2 is a key factor in the assessment of future
concentrations of NO2 and a range of concentrations are observed at
various monitoring sites in the UK.
European standards have been set for concentrations of NO2 in
the atmosphere. These set an annual mean limit value of 40µg/m3 and
an hourly mean concentration of 200µg/m3 that can be exceeded up to
18 times a year. A date for compliance with these regulations of 1
January 2010 was set for the European Standards.
The UK introduced a system of air quality management through its
National Air Quality Strategy. The Strategy has set air quality
objectives based on the European limit values. The air quality
objectives are normally the same concentrations as the European
limit values but sometimes with earlier target years. For NO2 the
objectives were set at the same values as the European limit values
but the target year for compliance was 31 December 2005.
The UK Air Quality Strategy requires that local authorities
review air quality in their area and assess whether they will meet
air quality objectives by the relevant target years. If there is a
risk of exceedance then the local authority must declare an Air
Quality management Area (AQMA). By February 2009 208 local
authorities had declared at least one AQMA for exceedence of the
The air quality objectives and the EU limit values are widely
exceeded. A 2008 EU Directive allows an extension for the
compliance date for achieving the EU limit values from 2010 to 2015
provided an action plan is provided. Department for
Environment, Food and Rural Affairs (Defra) modelling predicts that
in 2010 exceedences of the annual mean objective will occur along
nearly 2,500 km of roads, and the one hour objective along about
330 km, and by 2015 that exceedences will still occur along
850 km and 25 km of roads respectively. Even by 2020
exceedences are predicted to continue in some places.
This modelling assumes an "average" year in meteorological terms,
and therefore these predictions may be optimistic.
Emissions of nitrogen oxides result in local increases in
concentrations but the pollutant is also transported further
resulting in an increase in background concentrations at distant
locations. All combustion of fossil fuels results in some emissions
of nitrogen oxides but in urban areas that main source of this
pollutant is road vehicles. For instance in London it is currently
estimated that 42% of the emissions are from motor vehicles.
Assessment of NO2 concentrations
Future trends in NO2 concentrations are assessed by air quality
modelling. This process takes information regarding the quantities
of pollutant emitted from each source, the existing background
concentrations, the location of the source and local meteorology.
This processes this information to predict pollutant
concentrations. Modelling air pollution is difficult, and the
models themselves contain a number of uncertainties, which are not
considered in this PPS. However, of key importance to the accuracy
of the models is the emission data used, i.e. the quantities of
pollutants emitted from each source and the fact the models first
calculate total nitrogen oxides concentrations and then assess the
likely conversion to NO2 using various approaches.
The emission data used for most vehicles is based on emission
testing of vehicles. We note with concern that although the
emission data provided by Defra/Department for Transport (DfT) is
based on tests carried out on over a thousand vehicles, the
majority of tests are on older vehicle types that are becoming less
common in the current vehicle fleet. Some emission data for recent
vehicles have been obtained from very few tests. There is an
urgent requirement to work with our European Community partners to
obtain a more robust dataset. For vehicles meeting future
emission standards it is assumed that emissions will drop in direct
proportion to the improvement in the emission limits, which has
historically provide to be an over simplistic approach. It is,
therefore, hard to have confidence in this information when
projecting forward into the future.
One area where there is little information is the quantity of
NO2 directly emitted from vehicles (primary NO2). As noted above,
most nitrogen oxides are emitted in the form of nitric oxide and
this converts in the atmosphere to NO2. It is generally estimated
that about 5-10% of the nitric oxides are emitted in the form of
NO2. However, there is increasing evidence that the proportion of
primary NO2 is increasing on more modern vehicles (Air Quality
Expert Group, 2007, Trends in Primary NO2 in the UK) and this will
have profound impacts on our estimates of future concentrations.
There is an important need to understand this issue further and to
commission further research to obtain suitable information.
NO2 concentrations in smaller towns
Most air quality studies have concentrated on areas where it was
expected that air quality objectives would be exceeded i.e. where
there are high levels of traffic in our major towns and cities.
However, the experience of CIWEM Air Panel members is that there
are numerous locations in smaller towns with lower traffic volumes
where observed NO2 concentrations are much higher than expected and
often well above the objective level. At present, it is hard to
identify the reasons this is occurring. Possible causes are
the type of monitoring being carried out, emission factors not
accounting for local operating conditions and a change in primary
NO2 emissions in the local area. Given the very high concentrations
being observed it is essential that further research is carried out
to fully understand why this is occurring and to devise mitigation
methods to reduce concentrations.
Meeting the EU Limit value
Defra analysis of the reasons for future exceedences of the EU
limit values suggests that different approaches will be needed in
different locations to ensure compliance. The main causes of
the exceedences are emissions from heavy goods vehicles and buses,
and high background concentrations. It is clear that new measures,
at both national and local levels, will be required. The high
levels of NO2 measured in small towns must be adequately taken into
account when drawing up these new measures.
In deciding the most effective polices and measures to introduce
the Government use cost benefit analysis in which the cost of
measures are compared to their benefits. Some benefits are
difficult to quantify and therefore may not be fully accounted
for. There is concern that the approach used by Defra could
conclude that it will be cheaper not to achieve compliance but pay
any fines imposed by the European Commission.
More AQMAs have been declared for exceedence of the NO2
objectives than for any other pollutants. Concentrations in
several parts of the UK still remain well above the relevant
objectives and limit values and action is required at a national
and local level to improve air quality.
However, there are limitations in the information being used to
determine future concentrations of NO2, particularly in terms of
the emission data being used and the level of primary NO2 emissions
where information is based on only a few vehicle emission tests.
Action is needed at a national and European level to address these
information gaps and to therefore have more confidence in our
predictions of future air quality and the effectiveness of
Although emissions of nitrogen oxides at a national level have
declined by almost half since 1990, a corresponding improvement in
measured NO2 concentrations has not been observed. This is
because of the complex relationship between nitric oxide, NO2,
ozone and other substances in the air. In some locations
concentrations are increasing whereas in others levels appear to be
constant or falling. Uncertainties result both from limited
understanding of the processes giving rise to high concentrations
and the changing nature of emissions of nitrogen oxides.
Pollution control technologies such as diesel particulate filters
and the increase in the popularity of diesel cars in recent years
have both affected the proportion of nitrogen oxides emitted as
NO2. Greater understanding of the in-use emissions from both
light and heavy duty diesel vehicles, and how they change as
vehicles age, are required.
Although poor air quality is normally associated with highly
trafficked large urban areas we are observing several locations in
smaller towns with much higher than expected NO2 concentrations. At
present, it is not possible to understand why this is happening,
and there is an urgent need to examine these areas in more detail
to identify the cause and hence provide appropriate mitigation
measures that will improve air quality.
There is concern that the current cost benefit analysis approach
used by Defra will conclude that it will be cheaper for the UK to
not achieve compliance but pay any fines imposed by the European
Note: CIWEM Policy Position Statements (PPS) represent the
Institution's views on issues at a particular point in time. It is
accepted that situations change as research provides new evidence.
It should be understood, therefore, that CIWEM PPS's are under
constant review, and that previously-held views may alter and lead
to revised PPS's.