Use of dispersion modelling to assess road-user exposure to PM2.5 and its source apportionment

Atmospheric dispersion modelling was used to assess exposure of road users to PM2.5 in London. The measured exposure along one of the routes was similar to the modelled concentration at the most polluted points on that route. The model indicated that the nearest road is the dominant source of PM2.5 at these points, while imported transboundary secondary PM2.5 is more dominant along the rest of the route. Control of exposure is therefore easier to achieve by local air quality management measures than control of air quality monitored at fixed locations. This provides an incentive to develop model capability for exposure assessment. To achieve this, the high spatial resolution of the model output in street canyons was shown to be important. The modelʹs systematic underprediction of measured exposure was attributable to very short-range transient variability in PM2.5 concentration associated with traffic queuing, signal control, and plumes from individual vehicles, consistent with observations of the elemental carbon fraction of the PM2.5 samples. This therefore needs to be studied further.