Blue sky research
A number of projects within the Natural Environment Research Council's URGENT programme are concerned with the chemical reactions that occur between pollutants and the natural components of urban air, and with developing models that will enable better management of air quality. Mike Welch, University of Birmingham, explains how.Atmospheric pollutant concentrations are a complex function of emissions strength, distribution, meteorology and its influence on dispersion, and atmospheric chemistry. Ground-level concentrations of pollutants emitted from roads and elevated point sources can be reasonably well predicted, but currently the tools for estimating concentrations of pollutants where there are many contributory sources are in need of further development.
The model, developed in collaboration with the Meteorological Office, is based on three main components. The first is a predictive model of meteorological factors such as wind speed, direction and atmospheric temperature profiles. This is coupled to a dispersion model in which pollutant emissions are described as parcels of air which carry pollutants, and which become dispersed through atmospheric motions. The third part, which has been the main focus of the project, has been the incorporation of detailed chemistry describing the formation and removal of secondary pollutants.
In order to test the predictive capabilities of the model, air pollutant concentration data have been collected in summer and winter campaigns. Data collection has gone far beyond the conventional air pollutants measured in monitoring networks to include a far wider range of volatile organic compounds, oxygenates and short-lived reactive free radical species. These can be used to test not only the final predictions of the model, but also the suitability of the chemical mechanisms within the model. One unexpected finding has been that the oxidising capacity of the urban atmosphere is far higher in the winter than had previously been expected. These studies, involving the Universities of Birmingham, Cambridge, Leeds and East Anglia have led to major advances in understanding urban atmospheric chemistry at a fundamental level.
Particles originating from vehicle exhausts and road surface dust can penetrate deep into the lungs and are known to cause serious health effects and be implicated in asthma episodes. While emissions from exhausts are well defined, the characteristics of particles on the surface of roads, the emission rates from tyre/road interactions and the re-suspension of road dust is not well understood. Another URGENT project at the University of Birmingham is studying these interactions.
Work at the University of Leeds has resulted in the development of a model that provides a more accurate picture of the concentrations of emissions from chimneys in urban areas, or elsewhere, taking into account land use and the local topography. User-friendly software has been developed that can be used by people in industry with only a limited knowledge of atmospheric modelling who may need to increase their understanding of what happens to their emissions.
Although many studies have shown that there is a clear relationship between increased levels of airborne particles and adverse health effects, the underlying mechanisms are poorly understood. Since particulate air pollution is a complex mixture of components, highly heterogeneous and often site specific, it is a reasonable assumption that different cocktails of airborne particles at different sites are likely to possess different degrees of toxicity in the lungs. A project at the University of Cardiff has set out to test this by characterising and determining the toxicity of material collected from a number of sites in South Wales, including Cardiff City (urban), Port Talbot (industrial/urban), and an opencast coal mine (industrial/rural). One of the interesting conclusions so far from this work is that the dust generated in the vicinity of the opencast mine shows negligible bioreactivity in the lung and does not appear to be of concern.
A joint project involving the University of Lancaster and the Centre for Ecology
and Hydrology in Edinburgh has shown that close to 100 tonnes per year of PM10
particulates are deposited on trees in the West Midlands, and that deposition
on woodland is between two and five times faster than to grassland, such that
a 25 per cent increase in the number of trees would increase deposition by 15
per cent. There is a balance to be struck between this benefit, which is very
much a local effect, and the emission of volatile organic compounds, (to varying
extents depending on species), which may result in the production of ozone and
particles at some distance downwind. Nevertheless, planting trees in pollution
hot spots may have more benefits than their potential to absorb carbon dioxide
and their aesthetic value.