A differential absorption LiDAR for multi-species greenhouse gas detection

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There is a compelling need to understand the transport and atmospheric concentrations of the main greenhouse gases. We are developing a laser-based device that can measure the vertical profile of greenhouse gases in the lower atmosphere to a height of about 5 km and at heights separated by about 50 m. This will give unprecedented information about the variability in atmospheric concentration that in turn can be used to infer the sources and sinks of those greenhouse gases. For the first time, climate scientists and government regulators will be able to address the challenge of 'how can you hope to manage something if you don't measure it'. Satellite platforms do not provide the height resolution to be useful in the lower atmosphere yet this is precisely where land-atmosphere interaction takes place and where most of the important information lies. Other ground-based remote sensing devices only provide column-integrated values and do not operate during the night. Our novel instrument will work in all weathers and at all times of the day. Our novel instrument uses a method known as DIAL (Differential Absorption Lidar) in which a laser is fired into the atmosphere at a wavelength that is known to be absorbed by one of the greenhouse gases. By repeatedly firing the laser at this wavelength but alternating with a laser wavelength that is not absorbed by any other greenhouse gas, DIAL can be used to measure the vertical profile of greenhouse gases. Our existing LiDAR system can range to a distance of about 5 km and our tests suggest that at the sampling frequency we intend to use, our vertical resolution for greenhouse gas detection will be about 50 m in the vertical. This research will add a methane channel to the laser system and be integrated into the laser assembly and optical pathway of our telescope-based system.The instrument we are building uses components from the telecommunications industry and thus we can take advantage of the miniaturisation in laser and electronics technology that has developed in that industry over the past 5 years. Previous instruments using earlier technology have been expensive (millions of pounds to purchase) and so large and heavy they require a truck to support them, thus further limiting their scope for deployment to remote locations. Our DIAL has been designed to be an order of magnitude less expensive to build by the use of mainly off-the-shelf components and to be field portable and eye-safe.