Title of Example

Example

Monitoring

In the UK the first stage of the Review and Assessment of air quality, which all Local Authorities were required to undertake, emphasized the need to focus on public exposure and where the concentrations of pollutants were likely to be highest (sometimes called “hot spots”). It is likely to be more cost effective to start by looking at worst-case locations and then work outward if exceedences are found, rather than take an unfocussed look at a large geographical area. If there is no exceedence at the most polluted location, there should be no exceedences elsewhere. This approach should also help ensure that potential areas of exceedence are not missed.

Before any complex modelling took place in Bristol there were several years of monitoring data which could be used in assessing the areas of highest concentrations.

8-port smoke and sulphur dioxide monitors are old technology but are still used in the UK. The amount of smoke by measuring the darkness of particles on a filter does not have a direct relationship to PM10. However, it can be useful for indicating local hot spots.

Passive diffusion tubes are used extensively in Bristol for a number of purposes. Long term surveys have given good long term averages and trends across the city both at background and roadside locations. Because they are cheap and easy to use they have also been used in shorter-term surveys. Where there is a small complex area such as a major road junction, many tubes can be located and can give good results. Many dispersion models will not be able to replicate the detail that this monitoring gives.

Passive diffusion tubes are also available to measure BTX (benzene, toluene and xylene) and also 1,3 Butadiene. Bristol has used BTX tubes in a survey which covered the whole of the city which showed that we were not experiencing a problem with these pollutants.

In addition to permanent continuous analysers in Bristol there is also a mobile monitoring station in a trailer. This is used for short-term surveys up to a year in areas where the models predict hot spots. The trailer contains analysers which monitor NOx, CO and PM10 as well as wind speed , wind direction, temperature and sun light.

Mapping

Mapping of concentration data or statistics, often using Geographical Information Systems (GIS), is invaluable in assessing spatial patterns of pollution and exposure, identifying ‘hot spots’ and assisting in monitoring network design. This technique is particularly appropriate for area surveys with diffusion tube samples.

By combining air quality model output with GIS it is possible to make some assessment of exposure. The GIS procedures used to assess exposure are relatively simple, once the data has been sourced. It is important to understand the limitations of the data and how best the data can be used to gain an accurate picture of the number of people exposed to pollution. In the UK the recent national census will prove to be a useful source of data for this purpose.

This map shows the two datasets needed for an assessment of exposure in Bristol. The population data (points) is called Addresspoint and is derived from the Post Office records on addresses. It includes residential, commercial and industrial properties, so some “cleaning” of the data is needed before it can be used directly. A simple GIS query can calculate the number of addresses within the Air Quality Management Area (AQMA) polygons.

When considering exposure other factors should be taken into account such as where people spend most of their time, how people travel, where they live and work and the climate.

There is still considerable uncertainty about exposure and further work is being conducted through the DAPPLE project.

See www.daple.org.uk

www.defra.gov.uk/environment/airquality/laqm/guidance/pdf/laqm-tg03.pdf

Many assumptions have to be made about exposure such as where most people spend the majority of their time. There is also more information required about the relationship of inside and outside air quality. In the domestic environment there may be significant sources of nitrogen dioxide if natural gas ins used for cooking and heating. In the UK the majority of the population spend 80% of the time indoors.

Last Updated

13th January 2005