Local Air Quality Management: Pollution Modelling

Mathematical "dispersion" models are used to assess air quality in areas where monitoring data are not available. Dispersion modelling is a valuable tool in the air quality review and assessment process for the following reasons:

1. Pollutant emissions arising from different source types (for example, industrial chimneys, road traffic etc) can be taken into account in terms of their impact upon ground-level concentrations.

2. Concentrations across a wide geographical area can be predicted, and assist with the determination of the geographic boundaries of any exceedences of the air quality objectives.

3. Concentrations can be predicted for future years (i.e. the years in which the objectives are to be achieved), taking into account emission controls and new or changed source emissions.

4. Scenario testing can be effectively undertaken to determine source contributions and control strategies.

Dispersion Models

There is a plethora of dispersion models developed to meet the needs of a variety of end-users. For the purpose of the local air quality review and assessment, the Group uses the following models:

The Design Manual for Roads and Bridges (DMRB) screening model, which can be used to predict concentrations of nitrogen oxides, nitrogen dioxide, PM10, carbon monoxide, benzene and 1,3-butadiene through the use of empirical statistics derived from the analysis of monitored data in the UK. The emissions factors calculated within DMRB are derived directly from the Emission Factors Toolkit. DMRB is available for download from www.airquality.co.uk.

AEOLIUS suite of models for assessing likely concentrations of air pollutants in street canyons. Street canyons are relatively narrow streets with buildings lining up continuously along both sides. Concentrations are calculated for nitrogen oxides, nitrogen dioxide, PM10, carbon monoxide, benzene, and 1,3-butadiene. AEOLIUS is available for download from www.met-office.co.uk.

BREEZE ROADS is designed to estimate carbon monoxide, particulate matter (PM) and nitrogen dioxide (NO₂) and other inert pollutant concentrations from motor vehicles at roadside locations. The model includes the CALINE4, CAL3QHC and CAL3QHCR line source dispersion models and a traffic algorithm for estimating vehicular queue lengths at signalised intersections. ROADS will estimate the total air pollution concentrations from both moving and idling vehicles.

BREEZE AERMOD is an advanced new-generation dispersion model. It is designed to predict pollutant concentrations from a variety of sources, including continuous point, flare, area, and volume sources. This model enables the user to estimate concentrations from any type of source emitting a non-reactive pollutant. In addition, it includes a special option for NO_x - NO₂ conversion based on ozone concentrations.

Model validation, verification, adjustment and uncertainty

The validation of these models has been carried out by the model developers. The Group is committed to performing further validation of the models, by comparing model predictions with available monitoring data at relevant locations. The results of such comparisons have been included in the review and assessment reports and are referred to as "model verification".

A number of local air quality management (LAQM) tools have been provided by DEFRA to help manage the data collected for review and assessment. They are available for download from www.airquality.co.uk.