Technology: Infrared beam gets the measure of air pollution

日期:2019-02-28 04:08:10 作者:瞿尼痊 阅读:

By JUDITH PERERA BRITAIN’S National Physical Laboratory is developing an infrared laser that can detect atmospheric pollutants in minute concentrations up to 3 kilometres away. It is one of a range of infrared detectors – including some which are not based on lasers – that NPL hopes will extend the use of sophisticated remote monitoring systems in industry. NPL, located at Teddington, Middlesex, developed the laser with support from British Petroleum (BP). John Bannister of BP’s Loss and Environmental Control Unit says that the detector ‘takes our understanding of and ability to measure emission much further than is possible with any other system now operating’. Until now, researchers at NPL and elsewhere have restricted their efforts to developing systems that use ultraviolet and visible wavelengths, because suitable lasers which could be tuned over the range were readily available. But the pollutants that these can detect are limited. The infrared laser extends the range dramatically, so that many common pollutants such as methane, carbon monoxide, carbon dioxide and hydrogen sulphide can now be measured. Each pollutant absorbs light at a different, characteristic wavelength. Sulphur dioxide, for example, absorbs light at 300 nanometres (ultraviolet), nitric oxide at 446 nanometres (visible spectrum) and carbon monoxide at 4.5 micrometres (infrared). NPL’s system uses a tunable dye laser, pumped by another solid-state laser which normally produces a pulsed beam of light in the visible and ultraviolet ranges. A system of nonlinear optics converts the light into infrared so that it can operate in the range from 2.5 to 6 micrometres. Researchers have made their system portable by setting it up in a large articulated lorry. The method of detecting pollutants is called differential absorption lidar (light detecting and ranging), or DIAL. This involves sending out two pulses of laser light – one at the wavelength which will be absorbed by a particular pollutant and one, close to it, which will not. Light scattered back from both beams as they pass through the atmosphere is collected and measured. The second beam acts as a reference by providing a picture of the light scatter from the atmosphere that is unaffected by the pollutant. The two beams provide information that enables researchers to measure the range and concentration of the pollutant with great accuracy. With lasers, scientists can monitor industrial pollution far more efficiently than by conventional methods, which depend on collecting and analysing samples of air at fixed points on the ground or attaching instruments to chimney stacks. Such methods require separate instruments for each pollutant. They have proved useless for checking toxic emissions from flares used to burn off wastes. A single laser system, however, could do all these tasks from a central point. It could track a plume of smoke as it travelled across the countryside, monitoring any chemical reactions taking place. Lasers can also check leaks from valves and pipes – something which has until now been done by placing bags around selected valves or pipes to collect any gases for analysis. ‘It is a whole different ball game,’ says Bannister. ‘We will be able to measure right across a plant and pinpoint exactly where emissions are coming from, and it will also help with loss control.’ Peter Woods of NPL’s Division of Quantum Metrology says that there are plans to extend the infrared range beyond 6 micrometres. The laboratory is also hoping to replace the dye laser with one of the newly developed tunable solid-state lasers made from titanium and sapphire, which should be easier and more reliable to operate. NPL has also developed a smaller, cheaper, and simpler version of this system, which uses a low-power continuous wave tunable diode laser mounted in a van. It can detect pollutants up to 1 kilometre away, at concentrations of parts per million, compared with parts per billion for the larger system. The smaller device does not use the DIAL method, but gives readings of the total or average amount of a pollutant along the whole path of the beam which is reflected back by a special mirror. With a set of mirrors positioned at different places, the whole area can be monitored far more thoroughly than with conventional methods. This technique has worked, for example, in monitoring the build-up of gas from landfill sites. Within two years, the laboratory will have developed an even cheaper monitoring device – this time, the size of a suitcase. The detector will produce an infrared beam, but one that is not generated by a laser. It works on the same principle as the diode laser, using a mirror up to a kilometre away to reflect the beam back along the same path to a detector for analysis. The laboratory’s work on this detector was sponsored by a group of potential users, including BP, British Coal, Elf (UK),