Megacities are hotspots for emissions that have serious implications for air pollution and climate change, as well as for vital resources such as water and soil quality. These impacts are felt not just locally, but regionally and globally. For this reason, the environmental impact of megacities is a phenomenon that needs to be addressed by policymakers. But for that we need a better scientific understanding of the exact processes at work.
Developing that understanding was the objective of CITYZEN, the first European Union (EU)-funded research project of its kind. The project brought together a range of important, but previously disparate, areas of research in order to provide an unprecedented, comprehensive and detailed picture of the mechanisms by which megacities impact the environment. One key focus area for CITYZEN was the complex relationship between air quality and climate change – a subject that has not previously been well understood, with limited interaction between the two areas of research.
Specifically, the key issues considered by the CITYZEN team, using a mix of satellite and ground-based observations and computer models of the atmosphere, were: the reductions in air quality and health effects resulting from air pollution in megacities; ecosystem damage caused by emissions from megacities; the transport of air pollution on regional and global scales; the influence of air pollution on weather and climate; and the effects of climate change on emissions and air quality in and around megacities.
Using the results of this research to model future scenarios, the aim of the project was to provide robust, scientifically-based advice to policymakers on the best options to reduce the environmental impacts associated with megacities at all levels, from local to global. Given the global nature of the issue, an important feature of the consortium was its inclusion of participants from non-EU countries – Turkey, Egypt and China – as well as EU partners. Intensive case studies were conducted at several sites around the world, including the Eastern Mediterranean area, the Po Valley in Italy, China’s Pearl River Delta, and the Benelux region.
“One important success for CITYZEN,” says Project Coordinator Dr Michael Gauss of the Norwegian Meteorological Institute, “was the improvement of measurement data sets.” CITYZEN was able to develop a way of refining satellite observation down to spatial resolutions that are fine enough to detect patterns of air pollution at the megacity level and to analyse their trends for the first time. The analysis was also supported by ground-based observations, including using motorcycles to travel around various locations in Turkey to analyse air samples and detect the ‘Istanbul signature’ in particulate matter (the tiny particles of pollutants in the atmosphere) - and thus determine how widely air pollution was being transported.
The consortium also broke new ground in the area of ‘emission inventories’, distinguishing between the factors affecting air pollution and climate change to identify possible ‘co-benefits’ – reductions in emissions which contribute to both pollution and climate change and which need to be policy priorities. “For example,” explains Dr Gauss, “black carbon particles have a warming effect and are also bad for air quality. By contrast, sulphates are a pollutant but they have a cooling effect, so reducing these would not be a co-benefit.”
Another major achievement of CITYZEN was its success in bringing together scientists involved in air quality modelling and those involved in climate change modelling. “As these two groups tend to operate on different scales, local and global respectively, they have traditionally been separated,” says Dr Gauss. “CITYZEN was one of the first projects that gathered them together. This is extremely important since everything is connected in the actual atmosphere,” he adds.
The results of CITYZEN’s wide-ranging work have already been put to use in advising policymakers around the world, including in China and Germany. Most notably they have formed part of the ongoing review of Europe’s air quality policy. As the global trend towards urbanisation and megacities continues, it is clear that the innovative work of the CITYZEN team will continue to be a vital resource.