Margin of safety

MQT on 2010-10-17
Session date: 
October 17, 2010
Question By: 
Mike Tuffrey
Liberal Democrats
Asked Of: 
The Mayor


In paragraph 5.6.25 on page 33 of the 'Revised Mayor's Transport Strategy Integrated Impact Assessment: Report on the Assessment of the Proposal to Defer Low Emission Zone Phase 3 from 2010 to 2012' you stated in relation to the Government's reapplication for a time extension to comply with PM10 daily limit value: 'However, there was no margin of safety. This is a significant risk to compliance with EU limit values given the variations in air quality year by year due to changes in weather conditions and the contribution from external sources'. What is the maximum in percentage terms that the typical annual mean concentration of each of PM10 and NO2 has varied in Greater London since 1999 due to each of: (i) weather conditions; (ii) the contribution from external sources; and/or (iii) weather conditions and the contribution from external sources together?


Answer for Margin of safety

Answer for Margin of safety

Answered By: 
The Mayor

It is not technically possible to determine variations and contributions as percentages in a meaningful way for Greater London.

The contribution of sources both within and outside London, and the role that meteorology plays in the overall concentrations that we measure are extremely difficult to disaggregate as the relative contributions of each will vary during the day, week, and month, and by location (such as background, roadside and kerbside). The London Air Quality Network Air Quality Index, based on monitoring undertaken in London, provides a useful picture of overall trends in London's air quality. The Index shows a downward trend for both N02 and PM10 over the last decade, though with a peak in 2003 which was largely due to weather conditions.