Preprints
https://doi.org/10.5194/amt-2021-205
https://doi.org/10.5194/amt-2021-205

  20 Jul 2021

20 Jul 2021

Review status: this preprint is currently under review for the journal AMT.

Remote sensing of methane plumes: instrument tradeoff analysis for detecting and quantifying local sources at global scale

Siraput Jongaramrungruang1, Georgios Matheou2, Andrew K. Thorpe3, Zhao-Cheng Zeng1, and Christian Frankenberg1,2 Siraput Jongaramrungruang et al.
  • 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
  • 2Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA
  • 3NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Abstract. Methane (CH4) is the 2nd most important anthropogenic greenhouse gas with a significant impact on radiative forcing, tropospheric air quality and stratospheric water vapor. Remote-sensing observations enable the detection and quantification of local methane emissions across large geographical areas, which is a critical step for understanding local flux distributions and subsequently prioritizing mitigation strategies. Obtaining methane column concentration measurements with low noise and minimal surface interference has direct consequences for accurately determining the location and emission rates of methane sources. The quality of retrieved column enhancements depends on the choices of instrument and retrieval parameters. Here, we studied the changes in precision error and bias as a result of different spectral resolutions, instrument optical performance and detector exposure times by using a realistic instrument noise model. In addition, we formally analysed the impact of spectrally complex surface albedo features on retrievals using the Iterative Maximum a Posteriori- Differential Optical Absorption Spectroscopy (IMAP-DOAS) algorithm. We built an end-to-end modelling framework that can simulate observed radiances from reflected solar irradiance through a simulated CH4 plume over several natural and man-made surfaces. Our analysis shows that complex surface features can alias into retrieved methane abundances, explaining the existence of retrieval biases in current airborne methane observations. The impact can be mitigated with higher spectral resolution and a larger polynomial degree to approximate surface albedo variations. Using a spectral resolution of 1.5 nm, an exposure time of 20 ms, and a polynomial degree of 25, a retrieval precision error below 0.007 mole m−2 or 1.0 % of total atmospheric CH4 column can be achieved for high albedo cases, while minimizing the bias due to surface interference such that the noise is uncorrelated among various surfaces. At coarser spectral resolutions, it becomes increasingly harder to separate complex surface albedo features from atmospheric absorption features. Our modelling framework provides the basis for assessing trade-offs for future remote-sensing instruments and algorithmic designs. For instance, we find that improving the spectral resolution beyond 0.2 nm would actually decrease the retrieval precision as detector readout noise will play an increasing role. Our work contributes towards building an enhanced monitoring system that can measure CH4 concentration fields to determine methane sources accurately and efficiently at scale.

Siraput Jongaramrungruang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-205', Anonymous Referee #1, 23 Aug 2021
    • AC1: 'Reply on RC1', Siraput Jongramrungruang, 21 Sep 2021
  • RC2: 'Comment on amt-2021-205', Anonymous Referee #2, 23 Aug 2021
    • AC2: 'Reply on RC2', Siraput Jongramrungruang, 21 Sep 2021

Siraput Jongaramrungruang et al.

Siraput Jongaramrungruang et al.

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Short summary
This study shows how precision error and bias in column methane retrieval change with different instrument specifications, and the impact of spectrally complex surface albedos on retrievals. We show how surface interferences can be mitigated with an optimal spectral resolution and a higher polynomial degree in a retrieval process. The findings can inform future satellite instrument designs to have robust observations capable of separating real CH4 plume enhancements from surface interferences.