Spaceborne imaging spectrometry of methane plumes: Quantifying the benefit of aerosol lidar

Manuel Queisser , Sergio Thomás, David Vilaseca, Errico Armandillo, Daria Stepanova

Column averaged mixing ratios of trace gases, such as methane (XCH4), from spaceborne pushbroom spectrometers can be used to detect corresponding plumes and retrieve enhancements (ΔXCH4), i.e., the difference between plume and background XCH4. Over the global dust belt, however, significant scattering by dust aerosols may cause biased XCH4 that may propagate into biased ΔXCH4. To correct this, a co-aligned lidar could measure the aerosol profile in the middle of the scene (~20 km across track by 6 km along track). However, the vertical aerosol distribution may vary between the plume, covering hundreds of meters, and the background. Due to signal-to-noise requirements, only a single lidar profile per scene could be acquired, so either the plume or the background aerosol layer would be missed. Aerosol-induced bias would not be corrected equally for plume and background. A single lidar sounding in the center of the swath may, therefore, not improve the ΔXCH4 bias. To test this hypothesis, end-to-end simulations in Monte Carlo mode were performed, simulating 2000 retrievals for randomized aerosol profiles and observation conditions. The use of a scattering radiative transfer model (RTM) constrained with aerosol profile data from a lidar at the swath center led to a reduced mean absolute bias of ΔXCH4 (~0.5 ppb vs. ~2.6 ppb for an unconstrained RTM), with highest improvements for low albedo and strong plume/background albedo contrasts. This suggests that a pushbroom-lidar doublet could enable more accurate mass fluxes from point gas emitters, such as man-made greenhouse gas sources or volcanoes.