The good news is that the pledge made by the Games organisers, and often repeated by politicians including London Mayor Boris Johnson, to make these the "first public transport Games" (pictured top) looks like it's being upheld. The bad news is that London already suffers some of the worst air quality in Europe and, despite the efforts to make transport around the city during the Games as sustainable as possible, all those extra journeys will inevitably mean extra pollution.

Those 11 million visitors will, between them, make something like 300 million trips around London. It is a measure of the success of the public transport ethos that barely one per cent of those will be by car – but, even so, that equates to about three million extra car journeys. All those dedicated transport lanes aren't there for nothing, after all. 

The city was once notorious for its pea-souper smogs, which caused death and disruption right up to the mid-20th century. Specific smog episodes like the one in December 1952 caused thousands of deaths and hundreds of thousands of illnesses. A move to cleaner fuels greatly improved the air quality in the English capital, but it is still among the most polluted cities in Europe. A 2010 survey of 2,800 air monitoring stations across the continent placed London at the bottom of the capital cities for the harmful pollutant nitrogen dioxide (NO2).

A CityScan instrument

The extra pollution caused by the Games may have been minimised by the concentration on public transport, but how will we know just how much effect the Games has on the city? A new technology developed by space scientists at a UK university is being deployed around London this week to find out exactly that.

The idea of monitoring conditions on Earth from a satellite in space is nothing new, and we've all become familiar with the weather images shown daily on television. Satellites are used to monitor all kinds of changes at ground level, including how well crops are growing and how quickly ice caps are melting.

Measuring air quality from space, though, presents a few problems. Although satellites can see through cloud at some wavelengths, these aren't the wavelengths that are helpful for this pollution monitoring. In fact, orbiting sensors tend to operate in ultraviolet (UV) wavelengths, while the cloud-penetrating beams are at the other end of the spectrum in the infra-red (IR).

The conventional way of resolving this limitation is to combine satellite data with ground-based measurements using a sensor located somewhere at street level. This way, the broad-brush picture of air quality taken from space can be backed up with specific readings taken on the ground. On cloudy days, only the ground-based information is available, but at least this gives some information on the overall quality of the air.

Working with a CityScan instrument

The new approach taken by researchers at the Department of Physics and Space Science at the University of Leicester provides a big improvement on that method, and is able to measure air quality across a wide area and in three dimensions. Standard ground-based sensors can make only a single point measurement.

The CityScan instruments developed by Dr Roland Leigh and his colleagues operate by gathering the sunlight scattered through the atmosphere, and building up a 3D picture over a radius of six to seven kilometres (about four miles).

The instruments are spectrometers, using a technique called Differential Optical Absorption Spectroscopy (DOAS). A spectrometer looks at the spectrum of light and measures the strengths of the different colours. In the case of absorption spectroscopy, the idea is to see which colours are missing (or reduced) from the full spectrum of sunlight, because it is known that certain materials absorb light at certain colours.

In much the same way as a human eye detects a pair of jeans to be blue, because denim absorbs most of the red light that hits it and reflects mostly the blue, an absorption spectrometer can detect the presence of nitrogen dioxide in the atmosphere because of the wavelengths of light – the colours – that are reduced or missing.

And in just the same way that the human eye can detect the difference in colour between quite similar blues – royal blue, navy blue, Prussian blue – the differential optical absorption spectroscope can detect very subtle differences in the light it receives from the atmosphere. The scientists use the instrument to construct a graph called an absorption spectrum or absorption curve that shows how much each individual wavelength is reduced and take that information to calculate a precise quantity of a pollutant in the atmosphere.

In the case of nitrogen dioxide, for example, they look for specific curves in the spectrum at around 200um and 400um wavelength, giving air tainted with NO2 gas a distinctive reddish-brown tint.

Spectrometers use a technique called Differential Optical Absorption Spectroscopy

The CityScan instruments have been deployed on three rooftops across the Greater London area, and will monitor the atmosphere over a wide area until well after the 2012 Olympiad is over. Each instrument is an unassuming grey box, with a rotating turret marked by a single vertical slit facing diagonally upwards to the sky.

In operation, the instruments slowly rotate so that their detector scans across the entire sky four times each hour. As well as spinning through 360 degrees horizontally, the detector measures 90 degrees vertically so that each rotation of the device takes in a full sweep of the atmosphere in three dimensions. The light they absorb is measured and a picture of the atmospheric quality across an entire hemisphere of around 6,000 to 7,000m diameter is drawn up.

The location of the devices is carefully chosen to build up a more complete picture of the air quality across the metropolis – and, perhaps surprisingly, none is located close to the main Olympic venue in Stratford. The sites are a 30-storey tower in North Kensington, a 14-storey building in Chelsea, and a 17-storey residential block in Westminster.

We asked lead researcher Dr Leigh about the choice of locations, and learned that this was the more practical solution. "Getting sites in the East End of London is very difficult at the moment, as you can imagine," he told insidethegames. "The most important thing is to have sites that give us overlapping coverage, so we can build up a picture of a large portion of the atmosphere."

The Nasa satellite Aura features an Omni instrument

Difficulties in finding sites in the east of London are probably not helped by the intense security presence for the Games. It could add an interesting twist to a routine day monitoring a scientific instrument to find that your every move is being followed by a marksman on an adjacent rooftop!

It is worth noting that two or three CityScan sensors, each covering a radius of six to seven kilometres, give information across about the same area as a single pixel on a satellite sensor – roughly 13x13km. Where the satellite gives a single point of data, the CityScan instrument measures the spectrum of scattered sunlight in 180,000 different directions for each 360-degree rotation, creating a 3D map of the concentration on nitrogen dioxide in the atmosphere with a resolution of 50m and a time interval of 15 minutes. This is an unprecedented quantity of information on the changing dynamic composition of the air in the urban environment.

Acting together with this detailed ground-based information, the researchers will collect data from the Omni instrument onboard the Nasa satellite Aura. Combing a broad-brush image from space with the detailed picture in three dimensions from ground level, the levels of pollutants in London will be more closely monitored during the 2012 Games than probably any city has ever seen before.

This new capability for wide-area pollution monitoring at ground level means that even if the poor weather that has marked the spring and early summer in the UK continues, the important job of keeping tabs on pollution can carry on. The three instruments in the CityScan network went live on July 18.

Ironically, the better weather that everybody is hoping for will mean that pollution levels will increase, regardless of the high level of sustainability built into the London 2012 transport system. As Dr Leigh explained, sunny weather has a number of negative effects on air quality, including the creation of low-level ozone at potentially hazardous levels: "The air quality is actually better in nasty weather, as the rain will wash nitrogen dioxide and aerosol particles like soot out of the atmosphere. Even so, it would be nice to see some sun for the Olympics."

Russ Swan is a freelance writer on science and engineering, and editor of labhomepage.com.

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