Summary

This sub-project report outlines work undertaken during the first year. This includes an initial case study in modelling the relative contributions to concentrations of primary PM10 at an urban location from sources over different distance scales. Initial progress towards nested modelling has concentrated on implementation of MIMO as a local scale model, with application to exploratory 2-D studies. Preliminary consideration has also been given to the production and use of source-receptor matrices in exploring abatement strategies to improve urban air quality. Initial data has also been obtained from the Silwood Park Atmospheric Research Station, SPARS, to the West of London, and looks very promising for studying pollutant fluxes entering and leaving London.
 
 

Aim of the research

In developing strategies to reduce exceedance of air quality standards it is important to have a thorough understanding of the relative contributions of different sources to the atmospheric concentrations. Thus this project explores methods of source apportionment, to differentiate contributions to pollutant concentrations at any location from , for example, a local street, sources in the surrounding area and out to city or regional scale, and background pollutant concentrations imported into the city (including transboundary air pollution). In addition to the spatial breakdown it is also important to distinguish contributions from different types of source, both stationary and mobile (e.g diesel versus petrol etc). The project will address selected pollutants, including CO, NO2, and PM10, and will concentrate particularly on London. It includes both modelling studies; and pollutant measurements and chemical analysis of particulate samples.
 
 

Activities during the year

In order to apply models to source apportionment it is necessary to implement models which can simulate dispersion over different distance scales. For initial case studies some very simple Lagrangian models have been assembled, and applied to a case study considering the relative contributions from sources outside London, sources across the city area, and from a local street. The results from this case study are described below.

In the longer term the project aims to couple more complex Eulerian models, specifically the MIMO and MEMO models for microscale and city/mesoscale applications respectively . The first step towards this has been taken in implementing the MIMO model at Imperial College, with application to 2-D street canyons of varying geometries. Initial consideration has been given to allowing for the effect of relatively rapid NOx/O3 chemistry on NO2 concentrations, and to the treatment of neighbouring streets.

On the measurement side the Silwood Park Atmospheric Research Station, SPARS, is now operational and is accumulating its first measurements, including data from 2 trial campaigns in summer and winter conditions. SPARS is located in a rural are just to the west of London, where it can analyse pollutant fluxes exported from London to commuter towns downwind in easterly winds, and contrast these with cleaner air entering the city in westerly winds. The overall aim of SPARS is detailed study of the chemistry of the city plume, but already the initial data looks very promising for the purposes of the SAP project, with marked directional and temporal dependence of pollutant concentrations.

The current concerns about PM10 have emphasised the need to distinguish the secondary and primary particulate fractions, and use chemical analysis to differentiate the contributions from different sources. This is a large topic to which the SAP project can only make a modest contribution, but some analysis of particle composition has been initiated contrasting samples collected in London in dry periods, and following rain when longer range secondary particulate contributions have been depleted.
 
 

Principal results

The principal results obtained so far are those contributed to the UK Atmospheric Particulates Expert Group’s report as a case study on the relative contributions to primary PM10 concentrations from local roads, other sources across the London area, and longer range contributions in air imported into London. This applied to severe air pollution episode conditions, with anticyclonic conditions and low wind speeds giving residence times over the city of 12 hours or more below a persistent inversion at 200 metres.

Emissions of primary particulate over Europe were based on the PM10 inventory compiled by TNO, subdivided into 3 size categories, and European scale transport was modelled with a simple Lagrangian trajectory model. This implied imported concentrations of primary PM10 over London in the range 15 to 30 mg.m^-3.

The contribution from London emissions was calculated using the URBPM model (similar to the URBNOX model of O’Keeffe, 1997) to simulate dispersion and concentration profiles above the city, resulting in an additional 30 to 40 mg.m^-3 of primary PM10.

The total of these imported and city scale contributions may be compared with the contribution close to a major road such as the Cromwell Road, with traffic flows of the order of 10,000 vehicles per hour. Here a local scale model such as the CALINE 4 model, predicts road-side concentrations of around 12 mg.m^-3 in average meteorological conditions. But more complex CFD modelling of street canyons and junctions implies localised hot-spots of concentration up to an order of magnitude higher (see sub-project on microscale urban dispersion- R Colvile (PI) et al.).
 
 

Main conclusions

The above results apply only to primary PM10 and ignore secondary particulate contributions. However they clearly illustrate how even primary PM10 alone can lead to episodes of high concentration in excess of air quality standards. They also emphasise the potential importance of the background contribution, both imported from outside the city, and generated from emissions within a large city such as London, in addition to local sources within a particular district.
 

Aim for the coming year

A major aim for the coming year on the modelling side will be to apply and assess simulations with the MIMO model to examine the contribution of a major road to neighbouring back streets. This will be done in collaboration with other partners in SATURN, and make use of experimental data from wind-tunnels. In addition we plan to start including local scale chemistry. The next step will be to implement the MEMO model and address the problems of coupling the two models efectively.

A new component has been added to the modelling side of the project, and will start to contribute over the next year (including collaboration with D Carruthers and use of the ADMS model). This is concerned with the use of model results in investigating effective abatement strategies, combining the use of source receptor models derived from a combination of models. It will build on integrated assessment modelling techniques as developed in the Abatement Strategies Assessment Model (ASAM) and used by Imperial College in work for UN ECE Task Forces under the Convention on Long-Range Transboundary Air Pollution.

On the measurement side further results will be obtained by the Silwood Park Atmospheric Research Station, including a major international campaign in the summer ( the PRIME campaign centred on measurement of RO₂ radicals but with extensive additional data). Further work will also be done on source apportionment of PM10 and PM2.5.
 
 

Acknowledgements

This work would not be possible without the collaboration with other participants in the SATURN project, and in particular Prof N Moussiopoulos (with a shared student working partly in Thessaloniki). In particular we are grateful for the models which have been made available, and the provision of experimental data for model testing. Other members of the Air Pollution Group at Imperial College have also helped in addition to those named on this project report, including Jason Bowsher, Helen Adams, Rachel Warren and members of the Atmospheric Chemistry Research Unit.
 
 

References

The SAP project provided material for:- "Source Apportionment of Airborne Particulate Matter in the United Kingdom" (January 1999) Report of the UK Airborne Particles Expert Group.