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A draft report investigating unexpected trends in roadside nitrogen dioxide (NO2) concentrations and possible links...
A draft report investigating unexpected trends in roadside nitrogen dioxide (NO2) concentrations and possible links with changes in vehicle technologies has been published today for comment by the Air Quality Expert Group (AQEG).

The report was commissioned by Defra in January 2006 to consider why, despite significant falls in annual mean concentrations of nitrogen oxides (NOx) in urban areas over recent years, concentrations of NO2 (a component of NOx) at roadsides have not declined as expected.

AQEG analysed monitoring and emissions data and carried out modelling studies to determine the extent of this trend and investigate the possible explanations.

Trends in primary nitrogen dioxide in the UK is now published for consultation, and views on the AQEG conclusions are invited.

The draft AQEG report found that:

* the most likely explanation of the observed trend in NO2 concentrations is a change in the percentage of road traffic NOx emissions directly emitted as NO2. Total emissions of road traffic NOx have decreased at the same time

* emissions measurements indicate that the increased proportion of primary NO2 from road transport is related to the increasing number of light-duty diesel vehicles, especially Euro III cars fitted with oxidation catalysts and the fitting of diesel particulate filters to heavy-duty vehicles, particularly London buses

* the increase in the fraction of NOx emitted as NO2 by road traffic is likely to have implications for the attainment of the Air Quality Strategy Objectives for NO2. Levels are still expected to fall in the future, but not as fast as had previously been expected.

AQEG highlights the importance of future choices of vehicle abatement technologies, particularly with regard to particulate matter (PM) reduction methods that may increase primary NO2 emissions. The group draws attention to the conclusions of its first report (Nitrogen Dioxide in the United Kingdom) which highlighted that NO2 should not be considered in isolation from other pollutants, and that trade-offs would be likely.

Diesel oxidation catalysts and particulate filters are increasingly fitted to motor vehicles to comply with tighter vehicle emissions standards and to ensure compliance with air quality standards for PM10.

The best current evidence suggests that health effects from exposure to particulate matter are much more significant that those from NO2. Long-term exposure to PM10 is linked to respiratory and cardiovascular diseases and symptoms amongst patients suffering from asthma. The observed changes in NO2/NOx ratio are localised and have no climate change implications.

The deadline for comments on the draft report is 7 November 2006. The final report is expected to be published in spring 2007. For more information click here.

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