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The Saharan dust in London last week affected the atmosphere, as well as causing irritation to the many people who suffer from breathing difficulties. Just as with the smog of the 1950s and of Dickens’ day, which was caused by soot from coal burning, the cloud of dust particles was dense enough that sunlight was reduced.
When this happens, polluting gases and particles of all kinds are not dispersed upwards, and are concentrated in layers below the tops of the tallest buildings. When finally the sun broke through last week, the dust was dispersed and for those suffering from its effects there was some relief.
But London is fortunate compared with some other places in southern Europe, for example, because such meteorological events are very unlikely to occur in the height of summer when they could coincide with a heat wave.
Still Britain should pay increasing attention to the more severe, frequent events being experienced in large Asian cities. Here huge sandstorms, travelling over thousands of kilometres, can envelop urban areas with populations of tens of millions of people.
In Xi’an and Beijing, where dust from coal combustion adds to dust from industrial sources, the sun sometimes cannot penetrate at all, and in winter it can be quite dark at midday - as older Londoners may recall from the 1950s.
The concentrations of particulates can be more than 10 times greater than World Health Organisation recommended limits. The combination of this and emissions from motor vehicles, which are concentrated along highways, can lead to health effects so severe that an eight-year old girl living near a busy road in China’s Jiangsu Province was reported in November to have developed particle-related lung cancer.
In South East Asia, coastal cities have other air pollution problems due to the burning of forests and agricultural areas, combined with diesel soot and nitrogen oxides emitted from the high volume of shipping. Future policies to deal with this were discussed last week at the International Maritime Organisation in London.
STREET BY STREET FORECASTS
Scientific research is enabling detailed forecasts of air pollution in these different cities, as well as in the local areas within cities where pollution levels are highest. Some cities rely on a statistical approach to forecasting and regulating air pollution, using previous data from a network of air pollution measurements.
But such methods are limited because they cannot predict future changes, due to their dependence on past data. City-centre pollution measurements and past trends have been used in Italy and France to regulate traffic entering central areas.
Some cities, including London, also make use of ‘bottom-up’ computer modelling of the relevant scientific processes and estimates of future data trends. Two scales are used: firstly, at international and national meteorological centres (the European Medium Range Weather Forecasting Centre at Reading and the Met Office), data are collected and forecasts computed of regional-scale weather and atmospheric chemistry.
These forecasts take account of events such as sandstorms, forest fires, volcano emissions and movement of national-scale pollution (e.g. from mainland Europe to Britain). They make allowances for urbanisation and the effect of changes in climate on the natural environment, such as desertification or deforestation, which in turn feed into estimates of changing trends in the occurrence of sand storms and forest fires.
The next stage is to forecast the environment and pollution within and around urban areas, so as to inform communities, organisations and people about detailed implications for their operations and for human health. With recent developments in urban environmental modelling - for example the ADMS-Urban modelling system developed by UK research groups - forecasts of pollutants street by street are possible, based on meteorology, atmospheric chemistry and projections of traffic and other emissions.
The highest concentrations are regularly predicted along narrow traffic-laden streets in London where there are many children and vulnerable adults in schools, hospitals and social housing. Here, prior warning of high pollution can help; the airTEXT service delivers free forecasts to Londoners by text, email and voicemail (sign up at www.airtext.info or text ‘airtext’ to 78070).
As global cities collaborate and compete with each other to improve environments, they all want to ensure they have the most up-to-date methods for monitoring, forecasting and communicating on air pollution, especially during major events. For example, methods were tested during the Olympic Games in Beijing and London. Now as the central areas of London become more like Hong Kong, British urban modelling is benefiting from collaboration with research in those cities.
But last week’s London pollution demonstrated again - as in studies of urban heat-waves like those in Europe in 2003 - that forecasts and warnings need to be related more precisely to health impacts.
The warnings of bad air pollution, which are based on forecast concentrations of gases and small dust particles (less than 10 and 2.5 microns), do not allow for the differing impact on individuals – some of whom are sensitive to ozone, others to humidity plus particles, and others to the daily number of hours of high temperature in a heat wave.
Across an urban area, these factors depend on regional-level meteorological conditions but also, as studies in London’s Olympic area showed, on features of the local landscape such as parks and water areas, or streets lined with tall buildings.
With more research to define these classes of impacts in urban areas, it will be possible to provide more focused warnings to the people and communities vulnerable to urban air pollution, which regrettably continues to worsen year by year in some of the world’s largest cities.
Julian Hunt is former director-general at the UK Met Office and visiting professor at Delft University of Technology. Amy Stidworthy is principal consultant at Cambridge Environmental Research Consultants.