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Monitoring & Modelling Air Pollution

Introduction

Both the monitoring and modelling of air pollution is essential to provide a picture of the damage humans are doing to the environment, and to enable pollution problems to be discovered and dealt with. Since the introduction of the National Air Quality Strategy in 1997, local authorities are being encouraged to make assessments of local air quality (a local air quality review), through the monitoring of air pollutant emissions and atmospheric concentrations, and through the modelling of air pollution dispersion and deposition (as acid rain).

Monitoring Local Air Quality

Most local authorities to some degree have undertaken air quality monitoring. This has traditionally included a smoke and sulphur dioxide "bubbler", and perhaps some roadside lead monitoring and nitrogen dioxide diffusion tube monitoring. In terms of the automatic monitoring of air quality various sites have been monitoring atmospheric concentrations of number of pollutants over many years. However, in the past 10 years there has been more extensive monitoring of air quality, both as part of the DEFRA automatic urban network and as "stand-alone" sites. Clearly an assessment of the monitoring data needs to take place in each authority to determine if any of the national standards are likely to be currently exceeded. The more inexpensive monitoring techniques, such as NO2 diffusion tubes, may be employed to locate potential pollution "hot spots". Clearly, in areas where no monitoring takes place it will be appropriate to use surrogate data in the air quality review. However, for authorities where it is expected that standards are exceeded more detailed data is required for the assessment process.

Monitoring Networks

Co-ordinated air quality monitoring throughout the UK has been carried out since the early 1960s, and now there are nine networks with a total of 1565 sites. The aim of all of these networks is to provide reliable and high quality measurements of air quality throughout the UK. The networks, which are automatic, utilise instruments that monitor the air continuously. Through the use of data loggers and modems the data are interrogated every hour and are used to provide information to the public via television and radio weather forecasts, CEEFAX and TELETEXT, newspapers and the Internet.

The primary objectives of this network are to provide the public with rapid and reliable information on urban air quality, and to satisfy the statutory requirements of EC Directives on ozone, nitrogen dioxide and sulphur dioxide. There are currently 104 sites.

The aim of this network is to monitor 25 hydrocarbon species in urban air on a continuous basis. The compounds measured have been selected because of their photochemical oxidant formation potential. In addition, 2 known carcinogens are measured, benzene and 1,3-butadiene.

The overall objective of this network is to provide information on photochemical pollution across the UK. There are 18 sites on this network, all of which monitor ozone, with NOx and SO₂ monitored at 3 sites.

In 1986 and 1991 Warren Spring Laboratory (now NETCEN) undertook diffusion tube measurements at 360 sites at existing smoke and SO₂ sites throughout the country on behalf of the Government. As a result of a significant increase in concentrations in 1991 a network of 1200 sites was established in 1993. This survey is operated in conjunction with local authorities that identified one kerbside, one intermediate and two background sites in their areas.

This network has been in operation since the 1960s and was implemented following the Clean Air Acts. Before 1981 all sites were referred to as the 'National Survey', but since then a restructure has been carried out owing to decreasing concentrations, so that remaining sites were classified as either part of the 'Basic Urban Network' or 'EC Directive Network'. There are 165 sites in this network.

Eight important trace elements (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) have been monitored since 1976 at five urban sites. In addition, lead is monitored under four different networks: i) Multi-element - five urban sites since 1976; ii) Lead in Petrol - 8 sites (2 rural, 3 urban and 3 kerbside) since 1983-4 (currently 6 of these remain with one addition); iii) Industrial Area - monitoring occurs at hot spot industrial areas (9 sites in 3 areas); and iv) Rural network - 4 long term trend sites in rural areas.

Precipitation composition is monitored at 32 sites throughout the country to provide an accurate measurement of pollutant deposition in rain and snow to assist in implementing a critical loads approach to environmental protection. All precipitation samples are analysed for conductivity, pH, cations (NH⁴⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (NO^3-, Cl^-, SO₄^2- and PO₄^3-).

A network of 38 sites is maintained with the primary aim of mapping rural SO₂ concentrations, using volumetric apparatus.

Measurements of toxic organic micropollutants (dioxins and polycyclic aromatic hydrocarbons or PAHs) are undertaken at 17 sites, 15 of which are urban sites.

Making Emissions Estimates

A number of local and regional air pollutant emission inventories have been developed in the UK. These inventories are invaluable in the air quality review process. Emissions of the key pollutants are provided on a 1-km by 1-km basis. These pollution maps will enable identification of "hot spots". Less developed emissions inventories monitor point (e.g. combustion plants), line (e.g. roads and railways) and area (e.g. domestic and agricultural emissions) sources for the purposes the air quality review. Emission estimates can be calculated from production rates, vehicle flows, population figures and assigned an appropriate emission factor. The level of detail required in an emissions inventory depends upon local circumstances.

Air Quality Dispersion Modelling

Dispersion modelling of air quality enables prediction of air quality in a given scenario and/or for a future year. Dispersion modelling requires specialist software and expertise, and in many circumstances, is a one off task. The Department for Environment, Food & Rural Affairs has not recommended a particular model to use for compliance with the Environment Act. However, current scientific thinking in the UK leads to the recommendation of ADMS-Urban for the modelling of local authority areas. This model will provide contour plots showing levels of pollution for both the present scenario and 2005. ADMS-Urban is the only commonly used model that enables a direct comparison with national standards.