UK air quality data ‘more robust’ than in Europe

The approach used to measure levels of nitrogen dioxide in the UK may have resulted in a larger number of areas in the country exceeding EU air pollution limits than in other European countries, according to a Defra report published last week (February 15).

However, the report concludes that the UK’s approach potentially provides a more detailed analysis of air pollution data for EU compliance assessment compared to other member states.

Defra's Nobel House headquarters in London - the department published a report on air modelling on February 15

Defra’s Nobel House headquarters in London – the department published a report on air modelling on February 15

Currently the UK supplements measured data with modelled predictions for its EU compliance assessments, which is the equivalent to monitoring everywhere across the UK.

Measured air quality is based on actual readings at monitoring sites, whereas modelling uses various mathematical and scientific methods to calculate concentrations. Modelling is used to estimate pollution levels from specific sources or in areas that do not have monitoring sites, for example.

Only the UK and Netherlands currently submit modelling data alongside monitoring data to the EU, while other large countries such as France and Germany only submit measured data.

The combination of existing measurements and model predictions simulates the effect of the UK having a larger measurement network, which the country would need if it were to use a monitoring-only approach. This approach is permitted under the EU Air Quality Directive (2008/50/EC), as monitoring can be expensive and is unable to cover the whole territory of a member state.

Modelling

However, the modelling approach has resulted in the UK reporting that 40 out of 43 (93%) zones in the country exceeded the annual mean nitrogen dioxide limit value for 2010.

This proportion is much higher than other large member states with similar concentrations and distributions of nitrogen dioxide, such as France (30%), Italy (36%) and Germany (62%).

According to the report, if the UK had a similar number of measurement sites to these other large European countries (400 to 500 sites) the expected number of zones exceeding would be 22 (range 14 to 32 for 400 sites) or 26 (range 16 to 34 for 500 sites), respectively.

As a result, the report states, “the number of zones exceeding the limit value is far fewer than the current UK situation of relying on all modelled data to make up the number of sites”.

It concludes that the UK and Netherlands’ approach “potentially reveals much more information about the totality of concentrations across the territory of a member state and hence more information on potential exceedances of the limit value than from monitoring alone, even if the monitoring requirements of the Directive are complied with.”

Furthermore, the report also concludes that the use of modelling “potentially gives a more robust assessment of the time period for which one or more parts of a zone are in exceedance than a monitoring approach alone.”

The report, ‘An investigation of different approaches to compliance assessment in the air quality Directive 2008/50/EC’, was commissioned by Defra in order to compare the effects on compliance of using both modelling and monitoring data with the approaches used by other large EU member states.

It was prepared by Dr David Carslaw and Professor Martin Williams of Kings’ College London and John Stedman of Ricardo-AEA on behalf of Defra, the Welsh Government, the Scottish Government and the Department of the Environment for Northern Ireland.

The EU uses air monitoring data submitted by member states to make sure stipulated air quality limits are complied with. Exceedances of EU pollution limits by a member state can result in large fines.

The UK is required to report air quality data on an annual basis under two European Directives on air quality: the European Council Directive on ambient air quality and cleaner air for Europe (2008/50/EC) and the Fourth Daughter Directive 2004/107/EC under the Air Quality Framework Directive (1996/62/EC). More information on air quality legislation is available on the EC website.