An eye on emissions

A UK university team has developed laser radar technology to produce a reliable estimate of aviation pollution around airports

As EU limits on air quality threaten airport expansion, a UK university team has developed laser radar technology to produce a reliable estimate of aviation pollution and better understand how it acts on and around the runways.

Dr Michael Bennett at Manchester Metropolitan University’s Centre for Air Transport and the Environment is leading the project. His team replicated technology used in the US to create their own ‘eye-safe’ Light Detection and Ranging (LIDAR) device to track aircraft emissions. The team is the first in Europe to use this method to measure airport aircraft emissions.

The LIDAR reflects beams of light off nitrogen oxides and particulates in the air, measuring their shape and position, but not their concentration. It also has the ability to rapidly scan in elevation and azimuth, enabling it to track an aircraft and its emissions.

‘We have a backscatter LIDAR where you shoot out beams at certain wavelengths and you look for the same wavelengths coming back.

‘This particular system was developed as a mobile system to measure dispersion at power stations. Originally we put in a crystal which doubled the frequency and took it into the infrared range which was visible but very far from ‘eye-safe’. Ideal for measuring around power stations but not exactly something we could point at an aircraft as it took off and landed,’ said Bennett.

‘I then heard of someone in the US who tripled the frequency using two crystals. This took the wavelength to 355nm, in the UV range. At this range all of the optics work but it doesn’t penetrate the eye.’

One of the main issues the team had to take into account were the vortices formed when an aircraft takes off and lands. When on the ground and the engines are running ready for take off, the heat of the emissions mean they rise, but the vortices formed as the aircraft lifts or descends pushes the emissions downwards, potentially increasing their impact.

The vortices are streaks of air which form from the tips of the wings and follow the aircraft, forming a large area of descending air. This air then drives the emissions downward, increasing the ground levels in the local area.

The team was originally funded by the Department of Transport to take measurements at Heathrow and is now carrying out further tests at Manchester airport backed by the EPSRC.

One of the obstacles the team came up against may have shocked the residents of the recent ‘climate camp’ at Heathrow.

‘If we had done this study 15 years ago it would have been much easier. The main problem we had was that modern aircraft are too clean,’ said Bennett. ‘Having developed our system for power stations we could pick up almost nothing. We had to work quite hard to discriminate the aircraft plume above the background scatter.’

The DoT has forecast a five to seven per cent rise in air traffic needed to meet demand, but EU statutory limits on emissions could prove to be a major barrier to airport expansion. Bennett believes levels of NO2 and particulates, such as PM10, could be a ‘show stopper’ for the third runway at Heathrow and a second at Manchester.

Background emissions, such as those from the M4 near Heathrow, can have a large impact on air quality, leaving some areas marginal or even over current limits. The increased use of public transport is just one of the ways airports are trying to improve air quality by reducing the indirect surrounding emissions.

After using the LIDAR for various projects Bennett believes it has the potential for measuring other factors. He believes that with some delicate calibration the device could also measure emission concentration.

‘You could argue we’re halfway through a 10-year programme,’ said Bennett. ‘If we could get it to work with CO2 that would be a major breakthrough. There are many types of LIDAR. A Raman system, for instance, could give not only the amounts of emissions but where they are.

‘Also have you ever seen a jumbo jet land? I want to look at the smoke that comes off the tyres. We know how much rubber is lost, but we don’t know how much is left on the runway and how much is in the air.’