A differential absorption LiDAR for multi-species greenhouse gas detection

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.

The applications for our dual DIAL are many and numerous. The first market for a dual DIAL would be the atmospheric science community who need complementary methods to Tall Tower and satellite platforms to inform them better of what is actually happening to CO2 and CH4 in the planetary boundary layer and a few km above. Better range-resolved profiles of CO2 and CH4 will improve both the testing and validation of climate models and crucially also the inverse mesoscale models that will actually determine the sources and sinks for these gases - still a major uncertainty in carbon cycle science.

The issue of fracking for shale gas is unlikely to go away given the geopolitics involved but it is likely that any drilling rig on land will have to demonstrate that fugitive emissions of CH4 are low (current estimates suggest as much as 6% of the CH4 mined is actually lost to the air at the wellhead). It is likely that a monitoring system around an individual rig or series of rigs will be required before planning permission is granted and to allay fears of local residents. Our dual DIAl would be able to scan in the horizontal (with some modification) to routinely monitor fracking sits to quantify any emissions.

If a carbon market is ever to be established (and the Chair of IPCC 2013 5th report suggests this is the only way forward to avoid dangerous emissions of carbon dioxide and methane) then the dual DIAL could be deployed to routinely measure carbon exchange over a range of natural ecosystems and to act as early warning sensors for possible loss of CH4 from frozen hydrate reserves. Equally, a network of DIAL systems could be used to measure the inflow and outflow of C from major carbon stores such as the tropical forests; this in turn could be used to protect the forests and to provide finance to developing nations. If a carbon price is set on emissions then owners of large industrial plants would wish to make sure that they only pay for what they emit - current inventory methods may be imprecise but the dual DIAL could make absolute measurements of emissions.