Abstract

A comparison between efforts to detect methane anomalies by a simple band ratio approach from the Airborne Visual Infrared Imaging Spectrometer-Classic (AVIRIS-C) data for the Kern Front oil field, Central California, and the Coal Oil Point marine hydrocarbon seep field, offshore southern California, was conducted. The detection succeeded for the marine source and failed for the terrestrial source, despite these sources being of comparable strength. Scene differences were investigated in higher spectral and spatial resolution collected by the AVIRIS-C successor instrument, AVIRIS Next Generation (AVIRIS-NG), by a sensitivity study. Sensitivity to factors including water vapor, aerosol, planetary boundary layer (PBL) structure, illumination and viewing angle, and surface albedo clutter were explored. The study used the residual radiance method, with sensitivity derived from MODTRAN (MODerate resolution atmospheric correction TRANsmission) simulations of column methane (XCH4). Simulations used the spectral specifications and geometries of AVIRIS-NG and were based on a uniform or an in situ vertical CH4 profile, which was measured concurrent with the AVIRIS-NG data. Small but significant sensitivity was found for PBL structure and water vapor; however, highly non-linear, extremely strong sensitivity was found for surface albedo error. For example, a 10% decrease in the surface albedo corresponded to a 300% XCH4 increase over background XCH4 to compensate for the total signal, less so for stronger plumes. This strong non-linear sensitivity resulted from the high percentage of surface-reflected radiance in the airborne at-sensor total radiance. Coarse spectral resolution and feedback from interferents like water vapor underlay this sensitivity. Imaging spectrometry like AVIRIS and the Hyperspectral InfraRed Imager (HyspIRI) candidate satellite mission, have the advantages of contextual spatial information and greater at-sensor total radiance. However, they also face challenges due to their relatively broad spectral resolution compared to trace gas specific orbital sensors, e.g., the Greenhouse gases Observing SATellite (GOSAT), which is especially applicable to trace gas retrievals over scenes with high spectral albedo variability. Results of the sensitivity analysis are applicable for the residual radiance method and CH4 profiles used in the analysis, but they illustrate potential significant challenges in CH4 retrievals using other approaches.

Highlights

  • The potent greenhouse gas (GHG), methane (CH4 ) is the second most important anthropogenic gas affecting the global radiative balance after carbon dioxide (CO2 )

  • Small but significant sensitivity was found for planetary boundary layer (PBL) structure and water vapor; highly non-linear, extremely strong sensitivity was found for surface albedo error

  • The scoping study identified a surprising difference in performance of the band ratio approach for similar strength sources from the Coal Oil Point (COP) marine seep field and for Kern Front oil field (Figure 1)

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Summary

Introduction

The potent greenhouse gas (GHG), methane (CH4 ) is the second most important anthropogenic gas affecting the global radiative balance after carbon dioxide (CO2 ). CH4 is 34 times stronger as Remote Sens. 60–70% of global CH4 emissions arise from anthropogenic sources including rice agriculture, fossil fuel industrial (FFI) production, waste handling, and domestic ruminants [2]. Given that CH4 ’s atmospheric residence time is only ~8.5 years [4]—i.e., far shorter than the century timescale of CO2 , there are advantages to addressing global warming through CH4 emission regulations compared to CO2 [5]. Mean XCH4 over subpixels in GOSAT pixel.

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