Abstract
Abstract. Methane is the second most important anthropogenic greenhouse gas in the Earth climate system but emission quantification of localized point sources has been proven challenging, resulting in ambiguous regional budgets and source category distributions. Although recent advancements in airborne remote sensing instruments enable retrievals of methane enhancements at an unprecedented resolution of 1–5 m at regional scales, emission quantification of individual sources can be limited by the lack of knowledge of local wind speed. Here, we developed an algorithm that can estimate flux rates solely from mapped methane plumes, avoiding the need for ancillary information on wind speed. The algorithm was trained on synthetic measurements using large eddy simulations under a range of background wind speeds of 1–10 m s−1 and source emission rates ranging from 10 to 1000 kg h−1. The surrogate measurements mimic plume mapping performed by the next-generation Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-NG) and provide an ensemble of 2-D snapshots of column methane enhancements at 5 m spatial resolution. We make use of the integrated total methane enhancement in each plume, denoted as integrated methane enhancement (IME), and investigate how this IME relates to the actual methane flux rate. Our analysis shows that the IME corresponds to the flux rate nonlinearly and is strongly dependent on the background wind speed over the plume. We demonstrate that the plume width, defined based on the plume angular distribution around its main axis, provides information on the associated background wind speed. This allows us to invert source flux rate based solely on the IME and the plume shape itself. On average, the error estimate based on randomly generated plumes is approximately 30 % for an individual estimate and less than 10 % for an aggregation of 30 plumes. A validation against a natural gas controlled-release experiment agrees to within 32 %, supporting the basis for the applicability of this technique to quantifying point sources over large geographical areas in airborne field campaigns and future space-based observations.
Highlights
Methane is the second most important anthropogenic greenhouse gas in Earth’s atmosphere, with additional indirect impacts as it affects both tropospheric ozone and stratospheric water vapor
We showed that Gaussian plume modeling cannot be used for a meaningful comparison with observed methane plumes from a point source
Large eddy simulations (LESs) were used to generate realistic synthetic measurements of methane plumes under different background wind speeds and source flux rates. This allowed a comparison of the performances of two considered instruments, one measuring in the shortwave infrared (AVIRIS-NG) and the other in the thermal infrared (HyTES), resulting in widely different vertical sensitivities towards methane enhancements
Summary
Methane is the second most important anthropogenic greenhouse gas in Earth’s atmosphere, with additional indirect impacts as it affects both tropospheric ozone and stratospheric water vapor. Our understanding of global and regional CH4 budgets has remained inadequate due to the fact that the strength and distribution of CH4 emissions from various source types are not well-constrained (Houweling et al, 2017; Turner et al, 2017). Estimates of CH4 emissions from point sources (e.g., at facility scale) are uncertain, since space-based observations lack sufficiently fine spatial resolutions while in situ measurements are too sparse and mostly representative of large-scale background concentrations. Improved estimates of the CH4 emissions at this point-source scale are critical in guiding emission mitigation efforts. S. Jongaramrungruang et al.: CH4 quantification from 2-D plume imagery
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