Something in the air

London skyline

3 August 2012 by Sylvia Bohnenstengel

Remember the headlines about air pollution in Beijing before the last Olympics? We might think London's air is sweeter, but how can we be sure? Meteorologist Sylvia Bohnenstengel explains how she and colleagues will be monitoring the capital's air quality in the run-up to London 2012.

I like going for a run after work and I enjoy living in a big town. The two do not always go well together. I worry about the polluted air I'm breathing in when my route hits the rush-hour traffic, but I'm lucky enough to work as an urban meteorologist at Reading University, so I get to take a closer look at exactly how air pollution affects us.

In the run-up to the Olympics it's not just hobby runners like me who are interested in pollution; top athletes are concerned about how it might affect their Olympic performance. The Chinese government took drastic measures to improve local air quality during the Beijing Olympics in 2008, including regulating traffic and even stopping building work.

But our performance in sports is not the only reason we should care about air quality. Over 60 per cent of the world's population lives in cities, and poor air quality can cause serious discomfort and damage health. It affects each of us in different ways every day - like during my run. For others it could be a life-or-death issue.

I'm working on a two-year field campaign to measure air quality in London, called ClearfLo - Clean air for London. Why London? It's one of the biggest cities in Europe and traffic is certainly having an impact on its atmosphere, so London needs well-informed air-quality regulations. And the Olympics offers a once-in-a-lifetime chance to see how local changes to London's traffic routing might make a difference.

Over 60 per cent of the world's population lives in cities, and poor air quality can cause serious discomfort and damage health.

London's air has been monitored for more than 60 years, so you might wonder if this hasn't all been done before. But existing measurements don't tell us about all the pollutants out there, or very much about why they are there.

ClearfLo brings together scientists from all over the UK, Europe and US, with backgrounds in atmospheric chemistry, medicine and meteorology, to get a clearer picture of what's going on. The more we know about these pollutants, and the physical and chemical processes they're involved in, the better equipped we'll be to mitigate their effects.

We have set up air-quality monitoring stations across the city to measure the solid particles, ozone, nitrogen oxide and heat pollution over the city. Most of the stations are at ground level but a few are elevated, for example on top of the BT tower.

With some readings being taken up to ten times per second we'll have a good insight into how pollutants are mixed up in London's atmosphere and what happens when polluted air is blown into the city.

Our monitoring sites include areas with lots of traffic - in Marylebone Road and The Strand - as well as urban locations in North Kensington which have less traffic. To see the differences between urban and rural pollution we are taking measurements in Detling, east of London, and Harwell and Chilbolton to the west. This will help us work out what's in the air blowing into and out of London.

We'll be installing even more instruments during the Olympics to capture information on a wider range of pollutants. The Olympics will act as a real-life experiment, since the traffic flow will change considerably while the games are taking place. So we'll be able to see how far air pollution can be mitigated by regulating the movement of vehicles around the capital.

Because we can't monitor every point in London we'll fill in the gaps by running computer simulations and using remote- sensing techniques such as lidar (light detection and ranging) to measure vertical changes in pollution and wind. All this will help us build a 3D picture of the atmosphere and how it changes over time.

Setting up the equipment

Setting up the equipment in North Kensington

In particular, we'll be looking at three aspects of air pollution which have a strong bearing on human health.

The first is the lifecycle of ozone. Long-term exposure to ozone causes lung damage and short-term, acute increases in its concentration can be fatal for vulnerable people. We already know that nitrogen monoxide and dioxide affect ozone concentrations, but there are other gases involved too, especially in polluted urban areas.

One of these is the OH (hydroxyl) radical, which is strongly reactive and can indirectly either form or destroy ozone by kicking off various chemical reactions. One of the sources of OH in urban areas is nitrous acid (HONO).

Ironically we expect that HONO concentrations will increase as a by-product of modern emission-control technologies such as catalytic converters and particle filters. So we're monitoring HONO at street level to see how OH is formed and lost, how it can change in the atmosphere, and how exactly it influences ozone.

Particulate matter (PM) poses different problems. These tiny particles, ranging from 0·0005mm to 0·001mm across, can penetrate our lungs - the smaller the particle the deeper it can go - and can cause various problems in the lungs and heart. ClearfLo will help identify the mechanisms that make these particles grow and shrink; for instance our measurements already suggest that evaporation and condensation of water onto the particles might be an important factor.

We need to understand more about the sources of PM too. It can come directly from burning fossil fuels, for example in cars and factories, as well as from natural sources such as volcanoes and vegetation. It can also form from volatile organic compounds (VOCs) which readily release molecules into the air. There are thousands of types of VOCs, both natural and man-made, that are currently not well understood or even known, and our monitoring will help to identify these sources.

Thirdly, we'll be looking at the influence of the weather. Pollutants are mixed in the first one to two kilometres of our atmosphere, in a layer which is topped by a sharp increase in temperature that acts like a lid, trapping pollution below. What happens in this layer is partly controlled by the energy released from the ground.

This energy determines the height of the lid and also produces a distinct daily cycle; so during the day when the ground is warm the layer is deep and well mixed, which dilutes pollution, but at night as the ground cools the layer becomes shallower and pollution concentration increases.

We'll combine wind, air temperature and other measurements with a weather forecast model to understand as much as we can about these day/night variations and seasonal variations in different weather, so we can see what conditions make pollution worse.

After two years of monitoring, ClearfLo will have produced an enormous body of data. Our comprehensive 3D picture of London's atmosphere and air pollution will improve our computer simulation tools for weather forecasting.

It will also mean we can give policy-makers the detailed and comprehensive information they need to create effective guidelines for keeping London's air clean. And I might be able to find a healthier route for my evening run.


Sylvia Bohnenstengel works in the Department of Meteorology, University of Reading.