Particulate matter in the Royal Borough

• Monitoring
• Additional particulate monitoring at North Kensington
• Long term trends

Monitoring

The measurement of particulates is different from gaseous pollutants, and is generally less straightforward as they are a complex mixture of chemical substances of different sizes.

For information on the sources and health effects of particulate matter, see Sources and health effects of air pollution

There are a number of techniques for measuring particulates in the Royal Borough, however, the main method we use is the TEOM.

TEOM (Tapered Element Oscillating Microbalance):

Air is sucked in through the sampling head which restricts the size of the particle entering the device (for instance a PM₁₀ sampling head will only allow particles with a diameter less than or equal to 10 micro-metres). Some of the air then passes through the filter within the tapered element oscillating microbalance and as the number of particles deposited increases the natural frequency of the vibration of the element decreases. There is therefore a direct relationship between the change in the vibrating frequency and the mass on the filter.

The TEOM measures the mass of the particles and allows 15 minute average readings which is very useful for tracking pollution incidents as it allows real time analysis. However, it tends to under-read the particle levels monitored compared with the Partisol Plus Unit as it is heated to prevent water entering the device, and therefore many of the volatile particles are evaporated.

The Royal Borough uses a TEOM at the North Kensington and Cromwell Road sites. You can get information about TEOM data at these sites from www.airquality.co.uk.

Partisol Plus Unit

The Partisol Plus Unit is a gravimetric method which is similar to the TEOM in measuring the mass of the particles collected on the filter.

A sampling head is used to restrict the size of the particles entering the device. The air is sucked into through the sampling head over a period of 24 hours and passes over the pre-weighed filter which is then weighed after the sampling period and the difference is then measured.

This method allows for normal temperature and humidity conditions. The Partisol Plus Unit provides a 24 hour average concentration of particulates, unlike the real-time analysis of the TEOM. The Borough has a partisol at a roadside location on the Earls Court Road. DEFRA uses a PM10 and a PM2.5 partisol at the North Kensington monitoring site.

top of page

Additional particulate monitoring at North Kensington

At the beginning of 2000 the DETR chose the North Kensington air quality site as a "super site" for further research into the monitoring of particulates. Since then three new pieces of equipment have been installed:

• a Condensation Particle Counter
• a Partisol Plus Unit to monitor sulphate
• a continuous carbon monitor

The Partisol Plus Unit monitoring sulphate and the continuous carbon monitor are used to monitor the chemical make-up of the particulates which will provide data for research into the biological potency of PM₁₀ being carried out by Napier University, Edinburgh.

Both sulphate and carbon are secondary particles formed through the chemical reactions of gaseous pollutants from for example cars. The Condensation Particle Counter monitors the number of particulates as opposed to the weight of the particles (monitored by the TEOM and Partisol already installed at the site).

This information will help new research into very small particles (less than PM_0.1) which are the greatest in number, but make a negligible contribution to the mass measured through the conventional methods already installed at the site. The very small particles are therefore underestimated using the conventional methods already installed.

Long term trends

Particulates have been monitored using TEOMs at two locations in the Borough since 1995 and 1998, however continuous monitoring for particulates is relatively recent, and therefore the long term data is fairly limited. Monitoring using a gravimetric method began in 2002.

Data collected shows that the annual mean objective level for 2004 (40 µg/m3) has not been exceeded at either the Cromwell Road kerbside location or the North Kensington urban background site, however it was exceeded at the kerbside location in Earls Court in 2003 and in 2004. Also the new objective for 2010 is currently being exceeded at all monitoring locations.

The 2004 objective allows for the level to be exceeded 35 times. Breaches of this occurred in 1999, 2002 at the Cromwell Rd monitoring site and in 2003 and 2004 at the more recently installed Earls Court site. The new objective for 2010 allows for the level to be exceeded only 10 times. This objective is currently not being met.