Molecular Level Understanding of New Particle Formation in the Urban Atmosphere: Contribution of Local Pollutants

Airborne particles are made up of tiny specks of solid matter or liquid droplets floating in the air, too small to be seen individually by the naked eye. Gaining a good understanding of these particles is important for a number of reasons including the following:

- breathing high concentrations is bad for human health, having been associated with increased hospital admissions and reduced life expectancy. More deaths occur globally due to airborne particle exposure than from malaria and AIDS combined;

- small particles influence the formation of clouds, and their concentration in the atmosphere has a direct and indirect influence on climate by affecting the amount of sunshine reflected back to space;

- high concentrations of particles in the air cause a loss of visibility and the hazes that can be seen on polluted days.

One of the most pressing current scientific questions in atmospheric science is the formation of new particles in the atmosphere from gases by processes known as nucleation. Recent advances in instrumentation have allowed direct observation of nucleation processes as gas molecules join together to form new particles. Up until now these processes have been studied mostly under very controlled conditions in reaction chambers and in clean mountain-top air. The processes occurring in heavily polluted air are different from these and our proposal is to make measurements of particle nucleation processes in polluted atmospheres in cities where the composition is very well characterised. This will allow us to evaluate the contribution of pollutant emissions to particle formation processes and their contribution to new particle production in the urban atmosphere, as well as processes unconnected with road traffic emissions.

Field measurements will be made in Beijing and Barcelona, cities where new particle formation occurs frequently, and where two sampling sites will be established, one at the kerbside of a major highway and the other at a background location. These will be equipped with highly sophisticated state-of-the-art instruments which will identify the times when new particles are forming in the atmosphere and will determine the chemical characteristics of the molecules which are condensing into clusters to form the new particles. Both cities have high sunshine intensity (needed for frequent new particle formation), but widely differing pollution climates.

The outcome of the work will be a much better understanding of the processes responsible for new particle formation through nucleation in polluted air. Such knowledge will allow better design of mitigation strategies for reduction of ultrafine particle concentrations as well as providing the necessary knowledge to improve climate prediction models where currently one of the largest uncertainty relates to the role of airborne particles in affecting climate. In the future, it is expected that reductions in pollutant emissions and improved air quality may affect new particle formation. Lower concentrations of emitted particles will favour increased new particle formation, while reductions in sulphur dioxide and organic compounds are expected to be unfavourable. Well-designed numerical models are needed to predict the net future impacts upon new particle formation, and hence both urban particle concentrations and cloud condensation nuclei in more remote locations.

The results are likely to prove of considerable significance to the user community in two specific areas.

Atmospheric aerosol is a large contributor to radiative forcing in the atmosphere both through direct and indirect processes. Aerosol provides one of the greatest uncertainties in climate prediction as assessed by the IPCC. Research which enhances understanding of the sources of particles in the atmosphere and especially of cloud condensation nuclei can contribute importantly to the improvement of climate models. Improved parameterisations of nucleation in polluted atmospheres which will be generated in the project will feed directly into atmospheric models leading ultimately to adoption in weather forecasting and climate prediction models and in air quality models.

The second major area is that of human exposure to airborne particles and effects upon human health. Several studies have associated adverse human health effects in urban populations with the particle number count. The latter is driven by primary emissions from sources such as road vehicles and from atmospheric nucleation processes. Consequently, a better understanding of particle formation from nucleation processes, and the influence of the changes in precursor emissions and of the condensation sink provided by pre-existing particles is critical to predicting future trends in atmospheric particle number concentrations. This is highly relevant to predicting future health impacts of breathing polluted air, and to the design of mitigation strategies. The new parameterisations which will be developed in the course of this work will give a much improved predictive capability.