Title of Example

Example

Bristol has been monitoring air quality since the 1960’s. Originally the emphasis was on monitoring the emissions from the industrial area at Avonmouth at the coast. In the last decade, there has been a change to concentrating more on the urban environment where the main source of pollution is from motorised transport. This change has led to differing types of technology and monitoring sites.

Technology

Frisbee dust collector

The simplest type of sampler used is a dust deposit sampler constructed from inverted aluminium Frisbees. The aerodynamic shape is important, as any particulate matter deposited in the Frisbee is not blown out again by the wind. Water or rain washes the dust into a tube in the centre and into a bottle. The sample is then analysed in the laboratory for heavy metals.

8 – port smoke and sulphur dioxide

This comparatively old technology is still used in some sites but is less important than continuous analysers. The apparatus consists of 8 filters and 8 Dreschel bottles containing a solution of hydrogen peroxide. A timer transfers the sampled air through each filter and then each bottle for every day of the week.

The darkness of the filter is an indicator of the amount of black smoke in the atmosphere and the solution is titrated to determine the soluble acidity in the air which can be used to calculate the concentration of sulphur dioxide.

Nitrogen dioxide diffusion tube

This is an acrylic tube approximately 7 cm long with a cap at one end that contains a small metal mesh impregnated with triethanolomine. This chemical is very good at absorbing nitrogen dioxide from the air. The tube is exposed for a two-week period and then analysed in the laboratory.

The advantage of this technique is that it is cheap enough to have many tubes across the city building up long-term trends and spatial variations.

Airborne heavy metal sampler (M-Type)

A pump draws air though a membrane filter which collects particulate matter. The filter is exposed for two weeks and then is analysed for heavy metals by atomic absorption spectroscopy.

Continuous analysers

These analysers run continuously at a number of sites monitoring oxides of nitrogen (NOx), carbon monoxide (CO), sulphur dioxide (SO₂), ozone (O₃) and particulate matter (PM10). Real time information is stored on a data logger. Most of the sites have a modem which enable the data to be downloaded to a central computer. These analysers are regularly calibrated using gas of a known concentration.

Meteorology

Bristol has four stationary sites with meteorological equipment. All measure wind speed and wind direction and two also measure temperature.

Survey types

Frisbee deposited dust monitoring

There is a network of twelve Frisbee collectors around the industrial area of Avonmouth extending south through Avonmouth dock and across the river Avon to Royal Portbury dock. These sites have been permanent for over thirteen years.

Diffusion tubes

There are over 200 NO₂ diffusion tubes around the city in different types of location both roadside and background. Two surveys have been running for over ten years using diffusion tubes. 21 sites around the city are located at roadside locations, usually attached to street lighting columns. The other long running survey has 35 sites in background locations, one in each ward or political area of the city.

Although NO₂ diffusion tubes are not as accurate as real time analysers the long-term survey has provided a good indication of the long-term trends of NO₂ over the last 13 years.

Diffusion tubes are also used for short-term surveys ranging between a few weeks to a year. Because they are not expensive several can be distributed in a small area or along an individual road. It is then possible to pick up the variations in average concentrations on either side of a busy road where, for example, the nitrogen dioxide monitored is found to be higher on the side of a road where traffic is going slowly up a hill compared with the other side where traffic is going down the hill with fewer emissions. Survey like these have been done to evaluate the effect of the change in a road design.

Experiments have also been conducted to determine the variations in a canyon street where diffusion tubes were located at different heights up the side of a building.

When planning applications are made by developers, they are often required to do some air quality monitoring if there is no information about the location available. Diffusion tubes are usually used in this case for a short-term survey as these can be compared with the other permanent sites.

Continuous analysers

Currently Bristol has 8 sites with continuous analysers within the city boundary and 4 sites in neighbouring authorities for work, which has been commissioned by industrial companies.

Over the years a number of different sites have been used for locating continuous analysers. If a suitable building was available then it was often quite easy to have the analyser in the building with a sample inlet tube passed out of a window or a hole in the wall. Electricity was easy to supply and a fixed telephone line for the modem could be easily installed.

Over the past decade there has been a requirement to locate analysers at roadside locations or “hot-spots” where there is no suitable building. Bristol has used roadside cabinets made from glass reinforced plastic which can hold two or three analysers. The cabinet has it’s own air conditioning unit as the heat from the pumps within the small box needs to be reduced. Getting permission to install a cabinet by the roadside and then get an electricity supply and a telephone line installed can take a long time. It has not been unusual for it to take over a year to get a site like this operating.

More recently the introduction of GSM modems, which work over a mobile phone network, has made it easier to locate roadside cabinets.

As part of the review and assessment process for air quality which Local Authorities in the UK are required to do, sites have been found which reflect the urban background to assess exposure. These are generally located away from the road side and closer to a building front as this is more representative of where people live and work.

Bristol has had a national monitoring site controlled by the Government in Bristol for 10 years. This site is located in an urban background location in the city centre. Bristol City Council officers are the local site operators for the calibration of the analysers. There is a City Council owned roadside site, which is affiliated to the national network. The data from both of these sites are collected by Cassella-Stanger on behalf of the Government and the results can be seen on the national archive at www.airquality.co.uk.

Some continues monitoring sites are located for different reasons. Bristol has an ozone analyser located in a park land area to the north of the city as this is more representative of the surrounding countryside where ozone concentrations are likely to be higher. This is because when oxides of nitrogen and other pollutants are moving away from the urban environment the photochemistry will produce more ozone than in the city centre.

Some analysers have been located down wind of some major industrial sources. If the pollution to be monitored is coming from a point source it is unlikely that a plumb will be detected all of the time. The best that can be done is to find a site which is downwind of the prevailing wind most of the time.

Further information

www.bristol-city.gov.uk/airquality

Last Updated

13th January 2005