by Andrew Speak, PhD candidate Geography
Remember the hole in the ozone layer? It’s amazing how little discussion is given now to what seemed like a major global crisis just 20 years ago, but such is the nature of the media. A huge hole in the Earth’s protective ozone layer was discovered over the Antarctic in the mid 1980s which prompted a ban on refrigerators and aerosol sprays that contained the culprit – CFCs. It was another example, of which there are many in recent years, of how the whole of the developed and developing world was forced to wake up to the consequences of altering the composition of our atmosphere, and the media was full of cartoons of giant hairspray cans burning a hole in the planet. Well the good news is that the hole is repairing itself since the CFC phase-out, albeit very, very slowly but a whole generation are coming along now with no idea that it ever existed. This brings home the fact that environmental issues, that no longer seem important, can get neglected from media coverage and thus escape the attention of the majority of people. So what about urban air pollution?
The UK has a long history of urban air pollution, with laws introduced as far back as the 13th century to regulate the use of coal in London in a bid to reduce smoke pollution. In Manchester in 1792, the town hall emphasised the need for industrial chimneys to reduce smoke from coal combustion. Many of Manchester’s buildings were covered in a layer of soot and grime, which undoubtedly found its way into the lungs of Mancunians.
View from Blackfriar’s Bridge over the River Irwell, 1870’s. Engraving by Charles Roberts (Evans Picture Library)
The Clean Air Act of 1956 was introduced as a reaction to the Great London Smog, caused by burning low grade, sulphur-rich coal in a winter temperature inversion period, which caused an estimated 12,000 excess deaths. Since then, the quality of the air in our cities has gradually improved thanks to a switch from coal to gas, industries moving out of city centres, and improvements in the technologies that reduce emissions. So now, air pollution feels like something that happens far away in LA with its daily photochemical smog cycle, or in the permahaze-shrouded megacities of China where coal is still a major energy source and car ownership is increasing exponentially. Well don’t breathe too deeply when walking down Oxford Road just yet. The reason is road traffic, which is responsible for most of the main urban air pollutants – carbon monoxide (CO), benzenes, oxides of nitrogen (NOx) and particulate matter (PM). The latter consists of fine particles, smaller than 10 micrometres in diameter (a human hair is about 50 micrometres wide) which can penetrate deep into the lungs, causing inflammation and allowing harmful substances present such as lead and copper to exert effects.
Illustration of the size of particulate matter fractions, PM10 and PM2.5
Car ownership in the UK is still increasing (DfT, 2009), despite efforts to convince people to cycle or use public transport, and this is offsetting the impact of vehicular emission controls. These emissions are linked to 50,000 premature deaths a year in the UK, and shorten our life expectancy by an average of seven to eight months. They have also been linked to childhood asthma and even type II diabetes. So the effects of air pollution are a bit more profound than just a blackening of bogies, personally experienced in London, a place not lightly called ‘The Big Smoke’. Just because it is invisible, and only infrequently mentioned in the newspapers, does not mean the problem has gone away.
Monitoring stations have been set up around Manchester to provide information on the urban background air quality. Ever wondered what that little building is in Piccadilly Gardens? The website www.greatairmanchester.org.uk provides online access to these air quality data. Most days the air quality is acceptable but there are spatial and temporal patterns to be aware of. For instance, there is a peak twice daily coinciding with rush hour traffic, and this appears to be strongest in the city centre and on a Monday morning. Also, roadsides are places to avoid being in for long periods of time. Interestingly, a study in Lancaster found that if you are walking on an inclined road it’s better to walk on the side of the road that cars goes downhill because trees on the side of the road next to cars driving uphill were found to have higher loads of PM pollution on their leaves from the increased emissions of cars struggling up a slope (Maher et al., 2008).
Urban air quality monitoring station in Manchester city centre
This particle-capturing property of vegetation is being exploited to improve the air quality in cities. Strategic roadside tree planting can remove a large amount of pollution by trapping it on the leaf surface where it is subsequently washed off by rains. This is yet another example of the benefits of urban greenspace, which include keeping people cool in heatwaves, reducing the risk of surface flooding, and simply lifting people’s spirits and making them feel better. Tree-planting schemes are hindered, however, by a general lack of space within cites, and the fact that there is a considerable dollar sign attached to urban land. For example, the site of the old BBC would make a lovely park but we all know it is destined to be a glass and steel multi-purpose hotel/supermarket/student accommodation/leisure complex. Or something. So it would appear that air pollution is here to stay, as technologies to reduce vehicle pollution at its source seem to have stalled.
One solution is to use the space afforded by rooftops and install green roofs. Recent research in Manchester has shown that they can make a not-insignificant dent in the PM concentrations in the city centre, with 0.2 tonnes being removed a year in a scenario that involved all flat roofs getting a sedum green roof (Speak et al., 2012). Larger plants, such as grasses and shrubs, would have a bigger impact, but are a bit more expensive to install and maintain than an ‘extensive’ sedum green roof. See A (Green) Roof Above Your Head? for my other blog on green roofs in the UK.
An extensive green roof on the Number One First Street building, Manchester
A helping hand also comes from that seemingly permanent fixture in Manchester – the rain. The rain droplets scavenge pollutants from the air as they fall, and recharge the capture efficiency of urban surfaces by giving them a good wash. This, along with frequent strong westerly winds, means Manchester’s air quality isn’t as bad as in some other European cities, especially those in central and southern Europe. However, the European Commission recently gave the UK a final warning over failures to meet limits for PM in London. Perhaps it’s time to see urban greenspace as more than just an optional design feature for our city centres. When combined with a decrease in car usage, maybe then we can ‘safely’ forget about this invisible threat.
DFT 2009. Department for Transport: Transport Trends 2009 Edition. London: HMSO.
MAHER, B. A., MOORE, C. & MATZKA, J. (2008) Spatial variation in vehicle-derived metal pollution identified by magnetic and elemental analysis of roadside tree leaves. Atmospheric Environment 42: 364-373.
SPEAK, A. F., ROTHWELL, J. J., LINDLEY, S. J. & SMITH, C. L. (2012) Urban particulate pollution reduction by four species of green roof vegetation in a UK city. Atmospheric Environment 61: 283 – 293.