Nested modelling to assess the significance of local and regional pollution sources to air quality in Norwich, a city in eastern England

Although urban areas predominantly suffer from pollution generated within their own environments, experience in the United Kingdom has shown that for certain pollutants (specifically particulates and ozone) the pattern of urban air quality is significantly affected by regional pollution levels. The UK National Air Quality Strategy (1997) originally ruled out the possibility of successfully tackling ozone pollution at a local urban level, relying instead upon national and European level policy instruments. Following the submission of UK local authorities' Stage 1 Review and Assessments and a report from the Airborne Particle Expert Group the UK government has decided to relax the targets set for PM₁₀ so that only those urban areas that have very large local emissions of particulates are likely to have to carry out remedial measures.

In order to further investigate the extent to which transboundary particulates affect air quality, the city of Norwich has been chosen as the focus of a modelling case study. Norwich is the eastern most city in the UK exposing it to easterly, pollution laden winds from the European mainland around 300km away. Although the city itself has relatively few sources of pollution compared to many other UK cities, it lies within a large rural region of Eastern England allowing emissions within the city itself to stand out clearly against those from outside the surrounding area or from the continent.

The work builds on the work done by APEG looking at the effect of regional pollution sources on levels of particulates in the United Kingdom. The UK Meteorological Office's NAME model (a lagrangian multiple particle model) is used to model sulphur dioxide, nitrogen oxides and particulates at local, national and regional scales. Particulates are dealt with in two forms. Firstly, primary particulates (those predominantly emitted from combustion processes) are modelled at all three scales. Then, at the European/regional level, the NAME model's sulphate chemistry module is used to calculate the formation of secondary sulphate particles using emissions of sulphur dioxide and ammonia. The model has been set up in such a way as to allow the categorisation of pollution into local (traffic and industrial), UK or European sources.

Another model, ADMS-Urban, is being used at an Urban city scale to estimate pollution from explicitly modelled sources (e.g. point and line sources rather than from gridded emission inventories). Due to the inability of this model to calculate secondary particulates and other transboundary pollution, the data from the NAME model will be used to create a variable background field, thus allowing fine local variations in pollution to be interlaced with much larger regional patterns.