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London Air Quality Network - Pollution Guide

Monitoring air pollution

Key Points:

There are two distinct types of air pollution monitoring generally in use - automatic and non-automatic.
Non-automatic monitoring methods are generally cheaper and easier to operate but do not give as much accuracy or resolution as automatic methods.
The most common type of monitoring is by diffusion tube (non-automatic). These typically provide a fortnightly or monthly mean concentration of nitrogen dioxide or benzene.
Continuous (automatic) analysers produce high-resolution measurements for each of the major pollutants.
Analysers are maintained to strict QA/QC procedures to ensure reliable results, but purchase and operation is expensive.
There are currently over 200 continuous analysers in use in the London Air Quality Network, located at over 100 monitoring sites.
Data is stored within the analyser and may be download by modem and disseminated to the public in many different ways.

There are two distinct types of air pollution monitoring in use in the two counties - automatic and non-automatic. Non-automatic methods are generally cheaper and easier to install and maintain but do not give as much accuracy or resolution as automatic methods.

Passive Sampling (non-automatic)

The most commonly used passive sampler is the diffusion tube. These provide a simple and inexpensive method of screening air quality in an area, to give a general indication of average pollution concentrations over a period of weeks or months. The sampler consists of a small plastic tube open at one end and an absorbent packed at the other. The absorbent used depends on the pollutant gas to be monitored; nitrogen dioxide being the most common, then benzene, sulphur dioxide and ozone. Tubes are usually exposed for two to four weeks then sent to a laboratory for analysis.

The low cost per tube permits sampling at a number of points in the area of interest; this is useful in highlighting 'hotspots' of high concentrations where more detailed studies may be needed. Recent comparisons of nitrogen dioxide diffusion tube measurements with simultaneous measurements from a co-located automatic nitrogen dioxide analyser found that the diffusion tubes tended to overestimate ambient nitrogen dioxide by approximately 10%. Other comparative studies have cited analysis laboratory, tube location, tube quality and meteorology as variables each affecting the accuracy of this method.

Each local authority positions their diffusion tubes at locations they feel pollution levels require further investigation. In recent years some districts have also used sulphur dioxide and benzene diffusion tubes.

Active Samplers (non- or semi-automatic)

Active sampler methods collect pollutant samples either by physical or chemical means for subsequent analysis in a laboratory. Typically, a known volume of air is pumped through a collector such as a filter or chemical solution for a known period of time, which is then removed for analysis. Samples can be taken each day, thereby providing measurements for shorter periods of time, but at a relatively low capitol cost compared with automatic monitoring methods. They do, however, require high labour costs. Sulphur dioxide/smoke bubblers have been used across the county for many years.

Continuous Analysers (automatic)

These produce high-resolution measurements for pollutants such as ozone, oxides of nitrogen, sulphur dioxide, carbon monoxide and PM10 particulates. Hydrocarbons can also be automatically measured, but the costs are extremely high.

This is the most expensive method of air quality monitoring routinely employed. In order to ensure that the data produced are accurate and reliable, strict maintenance, operational and quality assurance/control procedures are often required. These are usually followed to an agreed protocol to allow comparability between monitoring sites.

The sample is analysed on-line and in real-time. Data is stored within the analyser, or a separate logger and may be downloaded remotely by modem. It is the high resolution of such methods that allows pollution episodes to be analysed in detail and related to traffic flows, meteorology and other variables. By frequently downloading data from automatic analysers, information can be relayed to the public while it is still relevant. Data from automatic analysers forms the daily, weekly and monthly network reports provided to local authorities and all of the monitoring data contained within this site.

The number of continuous analysers in use in London has grown rapidly in recent years and currently stands at over two hundred; almost all are part of the LAQN.