Multispecies mid-IR laser absorption spectroscopy for drone-based measurements of ship emissions

The accentuated global market substantially increased worldwide shipping and thus the related greenhouse gas (GHG) emissions. Without effective global measures, emissions from maritime transport will soon undermine any attempts to mitigate climate change. Therefore, the International Maritime Organization committed to new targets for GHG emission reduction. Similarly, the EU commission aims to regulate the emissions from all large ships starting from 2024 [1]. This includes now also CH₄ and N₂O emissions besides CO₂ and SO₂. For any market-based measure, however, a robust monitoring, reporting and verification (MRV) system is a prerequisite. Furthermore, air surveillance techniques that are able to assess smoke plumes, are effective approaches to verify compliance and to identify potential violations.

Within the framework of the Eurostars 3 project, ZEPHir, we support this effort by creating key enabling technologies for efficient monitoring of ship emissions. Leveraging on our recent advances in compact optical cells [2], laser driving schemes [3], and data-acquisition solutions [4], we target a multi-compound laser absorption spectrometer that can be carried aboard unmanned aerial vehicles (UAVs), such as drones, providing fast, in-situ, and high precision GHG measurements of vessel's exhaust plumes. For this purpose, two custom-made DFB quantum cascade lasers (QCLs) operated in a time-multiplexed regime (intermittent continuous wave operation) are coupled into a segmented circular multipass cell (SC-MPC) with an optical path length of 57 m. As each laser is selected to cover at least two different target species, we are able to assess all the regulated GHG compounds. As shown previously [5], the fastest analytical response is achieved in open-path configuration. However, open-path and thus at atmospheric pressure limits the analytical selectivity. Therefore, we identified a closed-path design at a pressure of 0.3 atm as the best compromise between selectivity and responsivity. Preliminary results indicate a noise equivalent absorbance in the range of 3 to 13 x10^-9 cm^-1 for all above GHGs within one second of averaging, well suited for measuring typical concentrations found ship plumes.

The final aim of this ongoing work is to provide a portable spectrometer to be systematically deployed in maritime ports and harbors for providing quick, flexible and reliable estimates of ship emissions.

 

References

• https://emsa.europa.eu/reducing-emissions/mrv-changes.html
• M. Graf, L. Emmenegger, and B. Tuzson, "Compact, circular, and optically stable multipass cell for mobile laser absorption spectroscopy", Opt. Lett. 43, 2434-2437 (2018)
• M. Fischer, B. Tuzson, A. Hugi, R. Brönnimann, A. Kunz, S. Blaser, M. Rochat, O. Landry, A. Müller, and L. Emmenegger, "Intermittent operation of QC-lasers for mid-IR spectroscopy with low heat dissipation: tuning characteristics and driving electronics", Opt. Express 22, 7014-7027 (2014)
• C. Liu, B. Tuzson, P. Scheidegger, H. Looser, B. Bereiter, M. Graf, M. Hundt, O. Aseev, D. Maas and L. Emmenegger, "Laser driving and data processing concept for mobile trace gas sensing: Design and implementation", Rev. Sci. Instrum. 1 June 2018; 89 (6): 065107
• Tuzson, B. and Graf, M. and Ravelid, J. and Scheidegger, P. and Kupferschmid, A. and Looser, H. and Morales, R. P. and Emmenegger, L., "A compact QCL spectrometer for mobile, high-precision methane sensing aboard drones", Atmos. Meas. Tech., 13, 4715–4726