Improving Air Quality in the United Kingdom

DOUGLAS R. MIDDLETON

1 Introduction

The concentration of pollutants in urban areas from sources near the ground has become of increasing concern in the UK, particularly since the London pollution episode of December 1991. During this episode from 11 to 15 December 1991, NO2 was unusually high, with values from 350 to 400ppb recorded at several sites and reaching 423 ppb at Bridge Place. These values were well above the standard which recommended a maximum hourly average concentration for NO2 of 150ppb. A study by South East Institute of Public Health mapped contours of air pollution measurements in the form of annual average concentrations of nitrogen dioxide (NO2). During 1995, these contours in much of London were above the NO2 Standard that appears in the UK National Air Quality Strategy. This Standard is 21 ppb for an annual mean. Since in London some 76% of the emissions of oxides of nitrogen are associated with road transport, measures for improving air quality will have to address transport planning. Looking forward, the report shows a projection of NO2 contours in London for the year 2000. It suggests that by that date in central London, NO2 will still be likely to exceed 40 µg m-3 (i.e. 21 ppb).

The Environment Act 1995 has increased the powers for local authorities to manage air quality and to consult with a wide range of bodies. The decisions to be taken in managing air quality will cross traditional boundaries. It will be necessary, for example, for Environmental Health Officers and environmental scientists in local government to liaise quite closely with traffic planners and highway engineers, as well as with managers of local industry and the Environment Agency inspectors, in order to assemble the emissions databases upon which the modelling of air quality for reviews and assessments will rely. The National Strategy for Air Quality sets out Government policy with regard to improving ambient air quality in the UK. It looks to the year 2005 and is relevant to both statutory requirements and to further voluntary action.

The Environment Act 1995 requires Local Authorities to review past and assess future air quality within their areas of jurisdiction. It also requires them to identify areas where levels of pollutants are high and, if necessary, designate them to be local air quality management areas. Such areas will be defined using the Standards and Objectives which appear in Table 3.1 of the Strategy. Management areas might be based upon administrative or other boundaries, but will need to contain locales where air quality will be high to the year 2005. Computer models provide not only a means of forecasting air pollution events, but also ways to investigate the contribution of actual or planned pollution sources. Similarly, monitoring data can be used as a means of projecting forward in order to assess the extent of control that is needed. Monitoring data are costly to acquire, and do not easily lend themselves to consideration of future controls on individual sources. There will therefore be more use of emissions inventories and modelling to complement the monitoring of pollutants.

We have here an example of how the new moves to improve air quality through local management are providing a stimulus to scientific research. As well as encouraging the development of simple assessment techniques, it has led to the need to develop simple monitoring methods for pollutants such as buta-1,3-diene, along the lines of the diffusion tubes for benzene. There is also work now in progress by a number of local authority groupings funded by the Department of the Environment (DOE) to validate dispersion models against each other and against measurements. Finally, we shall see below that the UK National Air Quality Strategy presents Standards and Objectives that will serve as a yardstick against which to judge air quality improvements.

2 Pollution in Street Canyons

Much monitoring has focused on the background level of airborne substances such as Pb, NOx, CO, O3, particles and organic compounds. However, it is at kerbside locations where the general public may suffer exposures to the highest concentrations of pollutants. This is particularly true in a street canyon, a relatively narrow street between buildings that line up continuously along both sides. The combination of large vehicle emissions and reduced dispersion in these circumstances can lead to high levels of pollution. A well-trafficked street canyon therefore represents an important facet of air quality management.

Recognizing that some authorities may not have the resources to run elaborate models, but will need a quantitative method, Buckland and Middleton have produced a simple method. The result is AEOLIUS, a selection of nomograms and charts that has been devised along similar lines to volume 11 of the Design Manual for Roads and Bridges. AEOLIUS is designed with one purpose in mind: to estimate the likely maximum concentrations from traffic in urban street canyons. It does not include the additional background concentrations from sources outside the street; they should be added by the user as necessary. AEOLIUS and other models are being tested by some local authorities during the trials cited earlier.

Only a brief outline of canyon dispersion principles is made here. A fuller description of canyon models appears in Buckland and the papers cited therein. When the wind blows across a street canyon a vortex is typically generated, with the wind flow at street level opposite to that above roof level. A consequence is lower concentrations of pollutants on the windward side of the street compared with those of the leeward. The windward side is here defined as the side the roof wind blows to whilst the leeward side is the side the roof wind blows from. The quantity of pollutant that a monitor directly receives from vehicle emissions is calculated using a simple Gaussian model. The contribution from air recirculated by the vortex is calculated using a simple box model. The principle is that the inflow rate of pollutant into the volume of recirculated air is equal to the outflow rate and that the pollutants are well mixed inside this volume.

The canyon concentration is proportional to the total emission rate Q from all vehicles, which will reflect changes in the vehicle fleet emission factors. For N vehicles an hour (with all vehicle types combined) and a combined emission factor of q grams km-1 vehicle-1, the total emission rate Q µg m-1 s-1 is given by the equation: Q = Nq/3.6, which converts grams to micrograms, km-1 to m-1 and vehicles hr-1 to vehicles s-1. This equation means that in order to improve air quality by reducing motor vehicle emissions Q, it is necessary to reduce...