|Abstract Title:||Preconcentration of atmospheric methane for high-frequency isotopic measurement|
|Session Choice:||Emerging technologies|
|Presenter Name:||Mr Chris Rennick|
|Company/Organisation:||National Physical Laboratory|
Abstract Information :
On the global scale methane (CH4) concentrations have more than doubled over the last 150 years, and the contribution to the enhanced greenhouse effect is almost half of that due to the increase in carbon dioxide (CO2) over the same period. Microbial, fossil fuel, biomass burning and landfill are dominant methane sources with differing annual variabilities; however, in the UK for example, mixing ratio measurements from a tall tower network and regional scale inversion modelling have thus far been unable to disaggregate emissions from specific source categories with any significant certainty.
The mean isotopic composition of CH4 is quantified by the isotopic ratios δ-13C and δ-D, and the specific combination of these quantities provides a signature of the emission source. The technical challenge is to measure the concentration of these rare isotopologues at a sufficient precision and sampling rate. Here, we explore the potential for isotope ratio measurements by diode laser spectroscopy, and describe current analytical work at the National Physical Laboratory that will realise deployment of such measurements.
Spectroscopic measurement in the infrared by quantum cascade laser (QCL) absorption is a well-established technique to quantify the mixing ratios of trace species in atmospheric samples and, coupled with a suitable preconcentrator, high-precision measurements of isotope ratios are possible. We simulate isotopic variations at the four UK greenhouse gas tall tower network sites to understand the levels of precision needed in both δ-13C and δ-D in order to detect particular scenarios of emissions.
The current preconcentration system under development at NPL is designed to make the highest precision measurements yet of the standard isotope ratios in atmospheric samples via a new large-volume cryogenic trap design and controlled thermal desorption into a QCL spectrometer.